Modified low-power fast spectrum melt fuel reactor design with improved neutronics

Abstract

A simple reactor is described in which the majority of the reflector material is outside the reactor vessel. The reactor vessel is a cylinder that contains all of the fuel salt and a displacement component (which may be a reflector) in the upper section of the reactor vessel. Unlike the displacement component, the reflector elements, including the radial reflector and the bottom reflector, are located outside the vessel. The salt flows around the outer surface of the displacement component through a downcomer heat exchange duct defined by the exterior of the displacement component and the inner surface of the reactor vessel. This design reduces the overall size of the reactor vessel for a given volume of salt compared to designs with a radial or bottom reflector inside.

Description

Detailed Description of the Invention

[0001] [Related Applications] This application was filed as a PCT international patent application on October 6, 2021, and claims the benefit of and priority to U.S. Non-provisional Application No. 17 / 388,824, filed July 29, 2021, which is incorporated herein by reference.

[0002] [Introduction] The use of molten nuclear fuel (or simply molten fuel) in nuclear power-producing reactors offers significant advantages over solid fuel reactors. For example, molten nuclear reactors generally offer greater power density compared to solid fuel reactors, as well as lower fuel costs due to the relatively high production costs of solid fuel.

[0003] Molten fluoride fuel salts suitable for use in nuclear reactors have been developed using uranium tetrafluoride (UF4) mixed with other fluoride salts. Molten fluoride salt reactors operate at average temperatures between 600°C and 860°C. Binary, ternary, and quaternary chloride fuel salts of uranium and other fissile elements are described in co-assigned U.S. patent application Ser. No. 14 / 981,512, entitled "MOLTEN NUCLEAR FUEL SALTS AND RELATED SYSTEMS AND METHODS," which is incorporated herein by reference. In addition to chloride fuel salts including one or more of UCl4, UCl3F, UCl3, UCl2F2, and UClF3, the application also describes: 37 Further disclosed are fuel salts with modified amounts of Cl, bromide fuel salts such as UBr3 or UBr4, thorium chloride fuel salts, and methods and systems for using the fuel salts in molten fuel reactors. The average operating temperature of a chloride salt reactor is expected to be between 300°C and 800°C, but could be much higher (e.g., >1000°C).

[0004] Low-power experimental reactors are useful for investigating various aspects of reactor design and operation. Because producing a lot of power is not a goal in itself, novel designs can be pursued for low-power reactors that would not be feasible in a normal commercial environment.

[0005] [Modified Low-Power Fast-Spectrum Molten-Fuel Reactor Design with Improved Neutronics] This document describes alternative designs for low-power, fast-spectrum molten fuel salt reactors that can be used to advance the understanding of molten salt reactors, their design, and operation. Furthermore, as described herein, the described designs may be adapted for extraterrestrial use, such as for use as power generators for low-gravity, lunar, Mars, or space bases. These low-power reactors include a core space defined by axial and radial neutron reflectors housed within a reactor vessel. In the reactor vessel, heated fuel salt flows from the core, through ducts between the radial neutron reflectors and the reactor vessel, and back into the core. Heat generated by nuclear fission within the core is transferred from the molten fuel through the reactor vessel to the coolant in experimental designs or directly to the extraterrestrial environment in extraterrestrial designs. The molten fuel may be actively pumped and / or the flow of molten fuel may be driven by natural circulation caused by density differences between the high-temperature and low-temperature molten fuel.

[0006] When adapted for experimental use, these low-power reactors include reactor systems designed to investigate, for example, the following phenomena: low effective delayed neutron fraction due to delayed neutron precursor advection and the presence of plutonium in the fuel salt; negative fuel density (expansivity) reactivity coefficient; reactivity effects associated with asymmetric flow and thermal distribution (velocity and temperature) of fuel salt entering the core; K-effective stability (reactivity fluctuations) due to flow instabilities and / or recirculations; and criticality (startup), reactivity control, and shutdown techniques.

[0007] When adapted for extraterrestrial use, the design takes advantage of reduced radiation exposure requirements and the natural heat sinks provided by the extraterrestrial environment: for example, heat may be dissipated directly into the cold of space through thermoelectric generators mounted on the exterior of the reactor vessel.

[0008] These and various other features and advantages which characterize the systems and methods described herein will become apparent upon reading the following detailed description and review of the associated drawings. Additional features will be set forth in the following description and in part will be obvious from the description, or may be learned by practice of the technology. The advantages and features of the technology will be realized and attained by the configurations particularly pointed out in this description and claims, as well as the accompanying drawings.

[0009] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

[0010] BRIEF DESCRIPTION OF THE DRAWINGS The following figures, which form part of this application, are illustrative of the described technology and are not intended in any way to limit the scope of the claimed invention, which scope is based on the claims appended hereto.

[0011] Figure 1 shows a functional block diagram of a pool-type reactor designed for use with a fuel salt.

[0012] FIG. 2 shows a diagram of one possible physical implementation of the nuclear reactor shown in FIG.

[0013] 3A-3D illustrate one embodiment of the reactor system of FIG.

[0014] FIG. 4 shows the fuel salt and flow paths within the reactor of FIG.

[0015] 5A and 5B illustrate one embodiment of a reflector assembly that may be used in the reactor system of FIG.

[0016] 6A-6D show various embodiments of the control drum.

[0017] FIG. 7 illustrates one embodiment of a vessel head assembly.

[0018] Figure 8 shows the main components of a nuclear reactor (excluding the shielding vessel).

[0019] FIG. 9 illustrates one embodiment of a fuel pump assembly.

[0020] FIG. 10 shows a reactor vessel with dimples on the exterior surface rather than fins for improved heat transfer.

[0021] 11A-11F show various views of an alternative embodiment of a low power nuclear reactor system.

[0022] 12A-12C illustrate an embodiment of a nuclear reactor facility having alternative primary and secondary cooling systems instead of a heat rejection system.

[0023] FIG. 13 shows a functional block diagram of a pool-type reactor system designed for use with molten nuclear fuel in an extraterrestrial environment or other suitably cryogenic environment.

[0024] 14A-14B illustrate yet another embodiment of a pool-type reactor system in which all flow paths of the molten fuel, except for the flow of molten fuel through the core and pumping chamber, are in contact with and defined by the inner surface of the reactor vessel.

[0025] FIG. 15 shows two alternative embodiments of upper molten fuel outlet channel and pump layouts that could be used in any of the reactor system embodiments described herein.

[0026] FIG. 16 illustrates yet another embodiment of the upper molten fuel exit channel and the surface elements of the radial reflector that define the channel.

[0027] FIG. 17 shows an alternative embodiment of a nuclear reactor system.

[0028] FIG. 18 shows an alternative embodiment of the reactor in which the reflector is outside the reactor vessel.

[0029] 19A-19E illustrate various options available for reactivity control using external radial reflectors.

[0030] FIG. 20 illustrates an embodiment of a low power reactor design adapted to reduce the reactivity change associated with flowing delayed neutron precursors.

[0031] 21A and 21B show an embodiment of a nuclear reactor in which a transverse swirl flow is induced to flow the fuel salt along the inner surface of the lateral surface of the reactor vessel.

[0032] 22A and 22B show an alternative embodiment of a reactor design in which the fuel salt flows in a swirling manner around the interior surface of the reactor vessel.

[0033] Detailed Description While the technology introduced above and discussed in detail below can be implemented with a variety of molten nuclear fuels, the designs herein are described as using molten fuel salts, more specifically, molten chloride salts of sodium chloride and plutonium. However, it will be understood that any type of fuel salt now known or later developed may be used, and that the technology described herein may be equally applicable regardless of the type of fuel used (e.g., salts having one or more of U, Pu, Th, or any other actinide). It should be noted that the minimum and maximum operating temperatures of the fuel within the reactor may vary depending on the fuel salt used so that the salt is maintained in a liquid phase throughout the reactor. The minimum temperature may be as low as 300°C to 350°C, and the maximum temperature may be as high as 1400°C or higher.

[0034] Before disclosing and describing the design and operating concepts of a low-power, fast-spectrum nuclear reactor, it is understood that the present disclosure is not limited to the specific structures, process steps, or materials disclosed herein and encompasses equivalents thereof, as would be recognized by one of ordinary skill in the art. It is also understood that the terminology used herein is used solely for the purpose of describing particular embodiments of the reactor and is not intended to be limiting. As used herein, it should be noted that the singular forms "a," "an," and "the" include plural referents unless otherwise specified. Thus, for example, a reference to "lithium hydroxide" should not be construed as quantitative or source-limiting, a reference to "a step" may include multiple steps, a reference to "producing" or the "products" of a reaction should not be construed as all of the products of the reaction, and a reference to "reacting" may include a reference to one or more of such multiple reaction steps. Thus, the process of reacting may involve multiple reactions or repeated reactions of similar materials to produce a specified reaction product.

[0035] As used herein, two components may be said to be in "thermal communication" if energy in the form of heat can be transferred directly or indirectly between the two components. For example, the walls of a container may be said to be in thermal communication with the material in contact with the wall. Similarly, two components may be said to be in "fluid communication" if a fluid is transferred between them. For example, in a circuit in which a liquid flows from a compressor to an expander, the compressor and the expander are in fluid communication. Thus, given a sealed container of heated liquid, although the liquid may be considered to be in thermal communication with the environment outside the container (through the walls of the container), the liquid is not in fluid communication with the environment because it does not flow freely into the environment.

[0036] (Experimental reactor design) FIG. 1 shows a functional block diagram of a pool-type nuclear reactor 100 designed for use with molten nuclear fuel. In the illustrated embodiment, the reactor 100 includes a reactor system 110, a primary coolant system 112, and a heat rejection system 114. The reactor system 110 generates heat through the fission of molten salt fuel. The heat is removed from the reactor system 110 via the primary coolant system 112. This removed heat is then rejected to the atmosphere by the heat rejection system 114. While the illustrated embodiment 100 is designed for use with chloride fuel salts, such as uranium, plutonium, or thorium, or a combination of chloride fuel salts, alternative embodiments of the reactor may be designed for use with any fuel salt, such as fluoride fuel salts and fluoride-chloride fuel salts. Examples of nuclear fuel salts include mixtures of one or more fissile fuel salts (e.g., PuCl, UCl, UClF, UCl, UClF, ThCl, and UClF, etc.) with one or more non-fissile salts (e.g., NaCl, MgCl, CaCl, BaCl, KCl, SrCl, VCl, CrCl, TiCl, ZrCl, ThCl, AcCl, NpCl, AmCl, LaCl, CeCl, PrCl, and NdCl, etc. For example, PuCl-NaCl, UCl-NaCl, and UCl-MgCl salts are contemplated.

[0037] The nuclear reactor system 110 includes a core 102. The core 102 is a central open channel that, during operation, contains a volume of molten fuel sufficient to produce a critical density of fast neutrons (neutrons having energies of 0.5 MeV or greater). The size and shape of the channel are defined by a neutron reflector assembly within the reactor vessel. The reflector assembly surrounds the core 102 and acts to reflect (reflect) fast neutrons generated within the core 102 back into the core 102, thereby increasing the fast neutron density. The reflector assembly is discussed in more detail with reference to subsequent figures.

[0038] The size of the core 102 is selected based on the type of fuel used; that is, its volume is sufficient to hold the amount of molten fuel necessary to achieve critical mass within the core 102. In one embodiment, the core 102 is not moderated during operation; that is, the core does not include moderator rods or other moderating elements so that the energy of fast neutrons within the core is not reduced. In one embodiment, the core 102 includes only molten fuel. The ability of the core 102 to achieve criticality from molten fuel within the core itself is one aspect that separates the fast reactor designs herein from thermal neutron reactors and fast reactors that use collections of individual fuel pins, each of which contains a small amount of molten fuel that is insufficient to achieve criticality during operation, but which can form a critical mass when collected in sufficient numbers into a fuel assembly.

[0039] The reactor core 102 and reflector assembly are surrounded by a reactor vessel 104, which, in the illustrated embodiment, is itself within a shielded vessel 116. The reactor 100 is referred to as pool-type to indicate that the molten fuel is contained within the reactor vessel 104. The reactor vessel 104 forms a pool that is filled with liquid molten fuel during operation. Solid components, such as elements of the reflector assembly, may be within the pool formed by the reactor vessel 104 or may occupy a portion of the space within the reactor vessel 104. Such components are referred to herein as displacement elements because they displace fuel from the space it occupies within the reactor vessel. Some displacement elements may perform no function other than occupying space within the reactor vessel. Other displacement elements, such as the reflector assembly, may perform functions in addition to displacing molten fuel within the reactor vessel 104, such as directing the circulation of molten fuel to affect the neutronics of the core.

[0040] In one embodiment, the shielding vessel 116 provides additional neutron shielding around the reactor core as an added level of safety and may also serve as secondary containment in the event of a reactor vessel breach. In one embodiment, the reactor vessel 104 and the shielding vessel 116 are fabricated from solid steel. Depending on the operating conditions, any suitable high-temperature corrosion-resistant steel (e.g., 316H stainless steel), HT-9, molybdenum alloys, zirconium alloys (e.g., ZIRCALOY™), SiC, graphite, niobium alloys, nickel or nickel alloys (e.g., HASTELLOY™ N, INCONEL™ 617, or INCONEL™ 625), or high-temperature ferritic, martensitic, or stainless steels may be used. The operating conditions will be dictated, at least in part, by the fuel selection. Materials suitable for use as shielding include steel, boron-added steel, nickel alloys, MgO, and graphite. For example, in one embodiment, all molten fuel contacting (salt wet) components may be made from or coated with INCONEL™ 625 (UNS Designation No. 6625) to reduce corrosion of these components.

[0041] In the illustrated embodiment, one or more pumps 118 are provided to circulate the molten fuel. In an alternative embodiment, the reactor system 110 is designed to operate under natural circulation and no pumps are provided. During operation, heated fuel is circulated between the reactor core 102, where the heat of fission is generated, and the interior surface of the reactor vessel 104, where the fuel is cooled and the heat of fission is removed.

[0042] The reactor vessel 104 is cooled by a primary coolant system 112. During steady-state operation, the temperature within the core 102 remains stable, and excess heat generated by nuclear fission is removed by the primary coolant system 112. In one embodiment, the primary coolant system 112 is composed of one or more cooling circuits (only one circuit is shown in FIG. 1 ), each of which includes a heat exchanger 106 and a coolant blower 108. Alternatively, a liquid coolant could be used with a liquid-to-air heat exchanger and pump. The coolant blower 108 forces the cooled primary coolant gas past the exterior of the reactor vessel 104 by flowing the coolant through a space provided for the primary coolant between the reactor vessel 104 and the shield vessel 116. Passing the primary coolant along the exterior of the reactor vessel removes heat from the reactor vessel 104. Although some heat may be lost through parasitic losses, under steady-state conditions, most, if not all, of the heat generated in the core 102 is removed by the primary coolant system 112. To aid in the transfer of heat, as discussed in more detail below, fins, pins, dimples, or other heat transfer elements may be provided on the exterior surface of the vessel 104 to increase the surface area of the exterior surface exposed to the primary coolant.

[0043] The heated primary coolant then flows to heat exchanger 106. The heated primary coolant gas passes through heat exchanger 106 where it is cooled and air is heated. The cooled primary coolant is then recirculated to reactor system 110, completing the primary coolant flow circuit.

[0044] In one embodiment, an inert gas (e.g., nitrogen or argon) is used as the primary coolant gas. However, any gas may be used. In an alternative embodiment, the reactor 100 may be designed to use any fluid, either gas or liquid, as the primary coolant.

[0045] The heat rejection system 114 uses air as a working fluid. The heat rejection system 114 takes in ambient air at ambient temperature and pressure. Using an air blower 128, the ambient air is forced through the heat exchanger 106. In the heat exchanger 106, the ambient air receives heat from a thermal coolant. The heated air from the heat exchanger 106 is then exhausted to the environment. Similar to the primary cooling system 112, the heat rejection system 114 may include multiple independent heat rejection circuits (again, only one circuit is shown in FIG. 1 ). Each heat rejection circuit may include its own independently controllable blower 128, air intake 120, heated air exhaust vent 122, and associated piping / ducting.

[0046] In one embodiment, multiple independent cooling circuits and heat rejection circuits may be used. For example, in one embodiment, four separate independent cooling circuits are used. In addition, an independent heat rejection circuit may be provided for each cooling circuit. In other embodiments, instead of four independent pairs of primary cooling circuits / heat rejection circuits, there are two, three, five, six, seven, eight, nine, ten, or more independent pairs of primary cooling systems 112 and heat rejection systems 114. However, a one-to-one correspondence between primary cooling circuits and heat rejection circuits is not required. For example, in one embodiment, reactor 100 may have four primary cooling circuits but only two heat rejection circuits, with each heat rejection circuit serving two primary cooling circuits. Other configurations are possible.

[0047] One aspect of this design is that the low power output of the reactor makes it feasible to reject excess heat from nuclear fission to the environment. In the illustrated embodiment, the primary coolant system 112 is provided as a safety system to contain the primary coolant in case of any release of nuclear fuel or fission products from the reactor system 110 into the primary coolant circuit. In an alternative design, heat may be rejected directly to the environment by venting the primary coolant directly to the environment. In this embodiment, the primary coolant system 112 is essentially eliminated so that heat is removed by the heat rejection system 114, although such a design may require additional safety devices, such as an emergency shutdown system, to ensure safety requirements are met. In such an embodiment, air may be used as the primary coolant. In an alternative embodiment, water may be used as the primary coolant, and the blower 128 may be replaced with a pump 128 that vents heated water to the environment.

[0048] Alternatively, heat removed from the reactor could be advantageously used to provide thermal energy to other systems. For example, in one embodiment, primary coolant could be passed to a thermal energy system to be reused as thermal energy within the reactor facility.

[0049] Figure 2 shows a diagram of one possible physical implementation of the reactor shown in Figure 1. In Figure 2, the physical components of the system are shown, such as the coolant gas blower 208, the air blower 228, the fuel salt pump assembly 218 and the shield vessel 216, as well as some of the piping / ducting between the systems.

[0050] In the illustrated physical implementation, the reactor system 210 is provided with four cooling circuits 212 and four heat rejection circuits 214, only one of each circuit is shown. The reactor system 210 is provided within a central room, and each of the primary cooling circuits 212 and heat rejection circuits 214 is separated from the reactor system 210 and the other circuits by walls for containment purposes.

[0051] Each cooling circuit 212 includes a gas-to-air heat exchanger 230 and a coolant gas blower 208. The coolant gas blower 208 drives the flow of coolant gas around the circuit 212. As described above, in a circuit, the coolant gas passes over the exterior of the reactor vessel, where it is heated, and then passes to the gas-to-air heat exchanger 230. In the gas-to-air heat exchanger 230, heat is transferred to the air in the associated heat rejection circuit 214. The circuit then returns the cooled coolant gas to the reactor, where it is reheated. In the illustrated embodiment, the coolant gas blower 208 is shown in the cooled coolant section of the circuit 212. In an alternative embodiment, the coolant gas blower 208 may be in the heated coolant section of the circuit 212.

[0052] Each heat rejection circuit 214 includes an air blower 228. The air blower 228 brings in ambient air from the environment and passes the air through a gas-to-air heat exchanger 230. The heated air is then exhausted to the environment. In the illustrated embodiment, the air blower 228 is shown in the ambient air section of the circuit 214. In an alternative embodiment, the air blower 228 may be in the heated air section of the circuit 214.

[0053] 3A-3D illustrate one embodiment of the nuclear reactor system of FIG. 1. FIG. 3A is a cutaway view along section AA shown in FIG. 3B. The cutaway view shows a reactor vessel 304 and some of the reactor vessel's internal components (the shielding vessel 305 is not shown in FIG. 3A). In the illustrated embodiment, the nuclear reactor system 300 uses molten chloride fuel salt as nuclear fuel. The nuclear reactor system 300 has a single molten salt pump assembly 318 to circulate fuel salt through a central reactor core 302 and into four individual fuel salt flow circuits. Although four individual flow circuits are shown, any number of fuel salt flow circuits may be used. For example, fuel salt exiting the reactor core may be distributed to two, three, four, five, six, eight, or twelve individual circuits, as desired by the reactor designer.

[0054] The pump assembly 318 includes a pump motor 320. The pump motor 320 rotates a shaft 322 having an impeller 324 attached to the distal end of the shaft. In one embodiment, rotation of the impeller 324 drives a flow of fuel salt upward through the central core and downward through four heat exchange ducts along the inner surface of the reactor vessel 304 in the heat transfer section, although in an alternative embodiment, the flow may be reversed. The pump assembly 318 is discussed in more detail below.

[0055] The reactor vessel 304 is provided with fins 326 on its exterior surface as shown. The fins 326 assist in heat transfer from the reactor vessel 304 to the coolant. Alternatively, any high surface area configuration, such as a dimpled jacket (as shown in FIG. 10 ) or alternating pins, may be used instead of or in addition to the fins. In the illustrated embodiment, the fins 326 are located on four sections of the exterior of the reactor vessel 304 sidewall. These sections are the only sections (heat transfer areas) that remove radiative heat from the reactor vessel 304. The fins 326 are located opposite the downward flow path of the fuel salt (heat exchange duct 306) and are located on the portion of the reactor vessel 304 sidewall that is not in contact with the fuel salt and is therefore fin-free. However, in an alternative embodiment, the fins 326 are located on the entire exterior surface of the reactor vessel vertical wall, regardless of the location of the heat transfer area on the reactor vessel 304. In yet another embodiment, fins or other heat transfer elements are provided around the entire periphery of the side and bottom surfaces of the reactor vessel. In yet another embodiment, heat may be transferred between the fuel salt and the primary coolant via a heat exchanger.

[0056] Surrounding the core on the sides and bottom is a neutron reflector assembly 330. The reflector assembly 330 includes a radial reflector 332 that defines the lateral extension of the core 302 and a lower axial reflector 334 that defines the bottom of the core 302. In one embodiment, the neutron reflector assembly 330 is comprised of a compressed powder or solid brick of reflector material contained within a reflector structure that acts as a container for the reflector material. In one aspect, the neutron reflector assembly 330 can be considered a large container that acts as a displacement space. That is, the neutron reflector assembly 330 displaces salt within the reactor vessel, thereby defining where fuel salt may reside within the reactor vessel. The neutron reflector assembly 330 is discussed in more detail below.

[0057] In the illustrated embodiment, vessel head 340 provides some additional neutron reflection. In an alternative embodiment, additional reflector material may be incorporated within vessel head 340 or between the vessel head and radial reflector 332. For example, in one embodiment, reflector assembly 330 includes an upper axial reflector 336 between vessel head 340 and radial reflector 332. Similarly, external shielding (not shown in FIG. 3A ) may be provided around the reactor for added safety.

[0058] In the illustrated embodiment, the vessel head 340 includes a hollow upcomer 342 terminating in a flange 344, and a main deck 346. The flange 344 mounts the pump assembly 318. The main head deck 346 sealingly covers the reactor vessel 304 and, in the illustrated embodiment, includes a control drumwell (see FIG. 7). The motor-to-impeller shaft 322 is housed within the upcomer 342. The upcomer 342 defines a chamber above the impeller that is in fluid communication with the fuel salt within the reactor. The chamber is referred to as an expansion chamber 348. Within this chamber is a free surface level 349 of the fuel salt within the reactor system 300. During operation, the headspace within the expansion chamber 348 above the fuel salt is filled with an inert cover gas. A cover gas management system (not shown) is provided which controls the pressure of the gas in the expansion chamber 348 and also cleans the cover gas as needed. The pressure on the cover gas can also be used to force fuel salt out of the reactor vessel 304 through access / removal ports (not shown in FIGS. 3A-3D) which are provided to deliver and remove liquid from the reactor vessel 304.

[0059] The level 349 of the fuel salt in the expansion chamber 348 will change as the fuel salt expands and contracts (e.g., during startup and shutdown). This level 349 may be used as an indicator of the current operating state or condition of the reactor system. A monitoring device may be provided to indicate the height of the fuel salt free surface level 349 during operation. Control decisions, such as opening or closing one or more flow restriction devices 360 (described below), rotating the control drum 350, or increasing or decreasing the flow rate and / or temperature of coolant to the reactor system 300, may be made partially or completely based on the output of the level monitoring device. For example, in one embodiment, a range of the free surface level 349 may be set as a target to indicate normal operation, and one or more of the control decisions described above may be automatically made by a controller to maintain the fuel salt level within the target range.

[0060] An overflow port 347 may be provided in the upcomer 342 to remove excess fuel salt to a fuel salt overflow tank (not shown).

[0061] During subcritical, non-fission heating operation, the fuel salt within reactor system 300 may be maintained above the fuel salt melting point. In one embodiment, this may be achieved by using multiple electric heaters 351 mounted on the vessel head 340 and / or the exterior of reactor vessel 304. For example, in one embodiment, heater 352 is located between fins 326 in the space between reactor vessel 304 and shield vessel 305. Alternatively, heater 351 may be included within the primary coolant system, e.g., within each cooling circuit, and used to heat the primary coolant (gas / liquid), which in turn heats reactor system 300 to maintain the fuel salt at the desired temperature. In other words, the primary coolant system may be used as the primary heating system to heat and / or maintain reactor system 300 at the appropriate temperature when the reactor is subcritical.

[0062] Reactivity control of the reactor system 300 is achieved via one or more independently rotated control drums 350. In the illustrated embodiment, four control drums are used, although any number and configuration of control drums may be used. The control drums 350 are cylindrical bodies of reflector material 352 and are provided with localized surfaces 354 made of neutron absorbing material. The reflector assembly 330 defines a receiving space for each control drum 350, as shown, so that the control drums 350 can be inserted into the reactor vessel 304 laterally adjacent to the core 302. The control drums 350 can be independently rotated within the reflector assembly 330 so that the neutron absorbing surfaces 354 are closer to or farther from the core 302, thereby controlling the amount of fast neutrons that can bounce back into the core 302 and be available for nuclear fission. When the absorber surface 354 rotates closer to the core 302, fast neutrons are absorbed rather than reflected, reducing the reactivity of the reactor system 300. Through the rotation of the control drum, the reactor can be maintained in a critical, subcritical, or supercritical state.

[0063] Although a control drum 350 is shown, in an alternative embodiment, insertable control rods or control sleeves of absorbent material or neutron reflectors may be used instead of or in addition to the control drum 350. Additionally, additional control elements may be provided for emergency use, including, for example, one or more control rods of absorbent material that may be inserted / dropped into the reactor core 302 itself in an emergency.

[0064] Additionally, although the control drum 350 is illustrated as a cylinder that substantially fills the drum chamber or drum well 356 (see also FIG. 7), the control drum 350 could be any shape. Furthermore, the control drum 350 need not completely fill the drum well 356. For example, in one embodiment, the drum has a crescent-shaped horizontal cross section, which allows for easier insertion and removal around the pump flange of the vessel head.

[0065] In yet another embodiment, the control drum 350 may include a space for inserting and removing liquid absorbent material instead of the absorbent surface 354. In this embodiment, the control drum 350 or drum well 356 may include one or more empty spaces that may be filled with liquid absorbent material to control the reactivity of the nuclear reactor system 300. For example, the control drum 350 shown in FIG. 6B may be static, and during operation, the absorbent surface 354 may be empty of absorbent material, and during shutdown, the absorbent surface 354 may be filled with liquid absorbent material to reduce reactivity to subcriticality.

[0066] An optional flow restriction device 360 for controlling the flow of fuel salt within one of the fuel salt circuits is shown in FIGS. 3 and 4. The flow restriction device 360 is located at the top of one of four fuel salt upper flow passages 361 between the core 302 and the inner surface of the reactor vessel 304. While only one flow restriction device 360 in one of the four flow circuits is shown, in alternative embodiments, such a device may be provided in some or all of the other fuel salt flow circuits. The molten salt flow restriction device 360 (which may be any one of a valve, gate valve, sluice gate, pinch valve, etc., but a gate valve is shown) controls the flow rate of fuel salt through the circuit. The flow restriction device 360 may be used to induce asymmetry in the flow into the core 302 and to change the effective delayed neutron ratio by varying the amount of delayed neutron precursors flowing (advected) outside the core. This allows the operation of reactor 300 to be varied to explore different operating scenarios and reactor conditions.

[0067] Another custom feature of reactor system 300 is the design of the pump suction region below impeller 324. Rather than direct flow from the center of core 302 into impeller 324, a contoured plug 362 is provided directly below impeller 324 between impeller 324 and core 302. In one embodiment, plug 362 is supported by one or more vertical and / or horizontal members. Plug 362 may be integrated into reflector assembly 330 or may be part of vessel head 340 or pump assembly 318 (as shown in Figures 3A, 3D, and 7, the plug and pump chamber are integrated into vessel head 340). In one embodiment, plug 362 is made from a shield material such as INCONEL™ 625. In an alternative embodiment, plug 362 is made from a reflective material, such as those described with respect to the radial reflector. The flow of molten fuel ascending through the core 302 is channeled around this plug 362, through one or more annular inlet regions, and then ascends into the pump impeller 324. This design serves several purposes. First, the plug 362 acts as a virtual top reflector or top shield for the core 302 (and can be thought of as defining the top of the core 302), providing radiation shielding between the high flux regions of the core 302 and the pump impeller 324. Second, the support members supporting this pump suction plug 362 can be adjusted, as needed, to provide optimal inlet conditions for the pump, which can reduce or enhance vortices.

[0068] 3B shows a plan view of the top of the reactor system 300. In the illustrated embodiment, the pump and vessel head flanges slightly overlap the location of the control drum 350. Additionally, as shown, the fins 326 on the exterior of the reactor vessel 304 do not extend all the way to the shield vessel 305; the space between the two vessels 304, 305 is a continuous gas space filled with primary coolant. This is just one possible embodiment. In an alternative embodiment, the fins 326 are in contact with the shield vessel 305. In another embodiment, the four finned regions are separate coolant flow paths, and the annular spaces between the fin locations are either static spaces (filled with a solid material, such as a neutron absorber material, or an inert gas) or may house heating elements.

[0069] Figure 3C shows a horizontal cross section of the reactor through the center of the core 302 and details the fins 326 on the reactor vessel 304. Figure 3C also shows the fuel salt path on the interior surface of the reactor vessel opposite the fins in the heat transfer area. Again, the control drum 350 is shown in its minimum reactivity configuration.

[0070] 3C further illustrates further details of one embodiment of the radial reflector 332. In the illustrated embodiment, the radial reflector 332 is made from five separate pieces. The five separate pieces include a central annular reflector 332a with a cutout on the outside of the annulus to accommodate the control drum 350, and four outer arc-shaped reflectors 332b positioned around the outside of the central annular reflector 332a. In the illustrated embodiment, the outer structure 309 holds the reflector material of the arc-shaped reflectors 332b. In one design, the arc-shaped reflectors 332b are solid, while in other embodiments, the reflectors 332b are solid.

[0071] 3C shows further details of one embodiment of the heat exchange duct 306. In the illustrated embodiment, cladding 308 is provided between the heating fuel salt duct 306 and the radial reflector 332a, which in the illustrated embodiment is shown on the exterior of the reflector structure 309. The cladding 308 is made from a material that is resistant to corrosion by the nuclear fuel.

[0072] 3D illustrates a cutaway view of one embodiment of reactor system 300, showing shield vessel 305, reactor vessel 304, and portions of several internal components of the reactor system. In the illustrated embodiment, reactor vessel 304 is supported by a support skirt 370. Additionally, the primary coolant piping / ducting within and outside the space between shield vessel 305 and reactor vessel 304 is illustrated, and the direction of coolant gas flow is indicated. In the illustrated embodiment, cooled coolant flows through lower coolant inlet duct 372, upward through the region between shield vessel 305 and reactor vessel 304, and over fins 326. Heated coolant then exits through coolant outlet duct 374. A separate coolant circuit is provided for each set of fins 326, with outlet duct 374 and inlet duct 372 located immediately above and below the fins, respectively.

[0073] 3D shows that the space above the control drum 350 is empty. In an alternative embodiment, this space may be filled with a suitably shaped reflector to provide additional reflection to the core. This reflector is removable and does not interfere with drum rotation.

[0074] Figure 4 shows the fuel salt and flow circuits within the reactor 300 of Figure 3. Figure 4 shows the overall salt volume 400 contained within the reactor system 300. In addition to the flow paths, Figure 4 shows a schematic of a pump stator (in the form of a directing vane 412), a flow restrictor 360 (in the form of a gate valve) within one flow path, and a flow regulator 420 (in the form of an orifice ring plate).

[0075] During operation, heated fuel salt flows upward through the core 302 and into the impeller chamber 410. The rotating impeller 324 (not shown in FIG. 4 ) drives the fuel salt (indicated by arrows) through the pump stator's directed vanes 412. Here, the fuel salt flow splits into one of four upper heating fuel salt outlet channels 414. The outlet channel 414 carries the fuel salt over the radial reflector 332 and into the heat exchange duct 416. In the illustrated embodiment, the upper heating fuel salt outlet channel 414 is narrower nearest the pump impeller 324 and widens as it approaches the reactor vessel 304.

[0076] Heat exchange duct 416 is a channel between radial reflector 332 and the inner surface of reactor vessel 304, extending from near the top of radial reflector 332 to approximately the bottom of radial reflector 332. In one embodiment, one wall of heat exchange duct 416 is formed by reactor vessel 304 so that fuel flowing downward through heat exchange duct 416 is in direct contact with reactor vessel 304 and, therefore, in thermal communication with the coolant on the other side of reactor vessel 304.

[0077] The fuel salt exits the heat exchange duct 416 via a lower cooled fuel salt delivery channel 418. The lower cooled fuel salt delivery channel 418 is a channel through the reflector assembly 330 between the lower axial reflector 334 and the radial reflector 332. The lower cooled fuel salt delivery channel 418 delivers the cooled fuel salt from the heat exchange duct 416 into the bottom of the core 302.

[0078] The flow conditioner 420 may be located at or near the location where the cooled fuel salt enters the core 302 from the lower cooled fuel salt delivery channel 418. The flow conditioner 420 ensures that the flow entering the core is well dispersed without jet-like behavior or large vortices or recirculations as the flow turns inside the lower edge of the radial reflector 332. In the illustrated embodiment, the flow conditioner 420 is an orifice plate designed to optimize the flow of cooled fuel salt. In an alternative embodiment, the flow conditioner 420 may take alternative forms, such as directional baffles, tube bundles, honeycombs, porous materials, etc.

[0079] 4 more clearly shows the fuel salt in the expansion chamber 348 within the upcomer 342 and the free surface level 349 of the fuel salt. The expansion chamber 348 allows the heated fuel salt to expand in volume during operation.

[0080] Figures 5A and 5B illustrate one embodiment of a reflector assembly that could be used in the reactor system of Figure 3. The neutron reflector assembly 500 is provided in two sections, a lower axial reflector 502 and a radial reflector 504, which, when combined, function as an integrated component that performs several functions, including defining the shape and size of the reactor core 302; reflecting fast neutrons from the core back into the core; and, when installed within the reactor vessel, defining the flow circuit of molten fuel within the reactor vessel (see arrows in Figure 5A).

[0081] In one embodiment, the individual components of the reflector assembly include a reflector structure or container. The reflector structure or container forms the exterior surface and, consequently, the shape of that portion of the reflector assembly. The interior space of the reflector structure is filled, in whole or in part, with a reflector material. For example, in one embodiment, a compacted powder and / or bricks of the reflector material are contained within the reflector structure. The reflector structure may be made of steel or any other suitable sturdy, heat-resistant, and corrosion-resistant material, as described above with respect to the reactor vessel. The reflector material within the reflector structure may be Pb, Pb-Bi alloy, zirconium, steel, iron, graphite, beryllium, tungsten carbide, SiC, BeO, MgO, ZrSiO4, PbO, Zr3Si2, and Al2O3, or any combination thereof.

[0082] 5A, the radial reflector 504 may be a unitary structure consisting of an outer shell of steel (as described above) filled with a reflector material. In one embodiment, MgO is used as the reflector material in the form of bricks (e.g., sintered bricks), pressed powder, or a combination of the two, with the reflector structure itself made from 316H stainless steel with the fuel-exposed surface coated with INCONEL™ 625.

[0083] The reflector assembly components are designed to accommodate thermal expansion mismatch and swelling due to neutron radiation and temperature changes. For reflector materials such as MgO, the neutron reflector filler material may be processed as a powder. The powder typically has a density between 66 and 85% of the theoretical density limit. Higher densities can be achieved by secondary operations such as area reduction by drawing and annealing, and vibratory compaction.

[0084] There are several strategies for assembling reflector assembly components into the reactor vessel. In one strategy, the reflector structure is sized for a desired fit with the reactor vessel at operating temperatures. The reactor vessel is preheated using the heater(s) described above, and then the reflector assembly components are inserted into the vessel. During insertion, the components may be at the same temperature as the vessel or a lower temperature than the vessel. The reactor vessel may then be allowed to cool. This results in a permanent shrink fit between the reactor vessel and the reflector assembly and a proper fit at operating temperatures. In a second strategy, the reflector structure is sized for a slip fit with the reactor vessel at a given temperature, such as room temperature. This creates a gentle transition in fit at operating temperatures.

[0085] FIG. 5B shows a cross-sectional view of the reflector assembly 500 showing the shape of the core 510, the heating fuel salt outlet channel 512, the heat exchange duct 514, and the cooling fuel salt delivery channel 516 defined by the shapes of the radial reflectors 504 and the axial reflectors 502.

[0086] 6A, 6B, and 6C illustrate one embodiment of a control drum and its use as a reactivity control device. As shown in FIGS. 6A and 6C, each control drum 600 includes a retractable rotating arm 602. By manipulating the arm 602, the drum 600 may be lowered and raised within a drum space provided within the reflector assembly. In one embodiment, the drum 600 may be removed entirely. Furthermore, in one embodiment, the arm 602 allows the drum to be rotated any amount in either direction.

[0087] In the illustrated embodiment, the drum is made from the reflector material 610 described above and includes a surface 612 of absorbing material. In one embodiment, the absorbing material is B4C, although any suitable neutron absorbing material may be used. Other neutron absorbing materials include cadmium, hafnium, gadolinium, cobalt, samarium, titanium, dysprosium, erbium, europium, molybdenum, and ytterbium, as well as alloys thereof. Some other neutron absorbing materials include combinations such as Mo2B5, hafnium diboride, titanium diboride, dysprosium titanate, and gadolinium titanate.

[0088] In one embodiment, similar to the construction of a neutron reflector, the drum is constructed by constructing an outer structure or container, such as steel, and then filling the appropriate portions with the appropriate materials. For example, in one embodiment, the drum structure has two spaces, one space filled with one or more neutron absorbing materials and the other space filled with one or more neutron reflecting materials.

[0089] As described above, rotation of the control drum changes the distance between the absorber surface and the core, and also changes the amount of reflective material between the absorber and the core. Figures 6A and 6B show four control drums 600 in their least reactive configuration, with the absorber surface 612 of each drum as close as possible to the core. Figure 6A shows four drums. Figure 6B, on the other hand, is a top view of the reactor system 300, showing the four drums 600 within the vessel head. This helps maximize neutron density reduction within the core. In this reactor design, the relative size, amount, and distance of the absorber material from the core in this configuration are sufficient to maintain the reactor in a subcritical state. In one embodiment, the control drum is sized so that, when rotated to the position shown in Figure 6B, it can maintain a subcritical state under all possible shutdown conditions and conditions.

[0090] 6D shows two views of an alternative embodiment of a control drum having a different design for the absorbent surface 612. In this embodiment, the absorbent surface 612 is a layer of uniform thickness that extends approximately halfway around the circumference of the drum 600 inside the drum structure, which is otherwise filled with a reflector material.

[0091] FIG. 7 illustrates one embodiment of a vessel head. In the illustrated embodiment, vessel head 700 is either a single piece as shown, or an assembly including a head plate 702, a well 704 for receiving a control drum inserted into the reflector assembly, one or more openings 706 for accessing the interior of the reactor vessel (e.g., an opening for a flow restrictor is shown), an upcomer 708 providing an annular space for the fuel salt expansion volume described above, and a flange 710 providing connection to a pump assembly. Additionally, in this embodiment, a pump chamber including a shield plug protecting the impeller is incorporated into vessel head 700 such that pump chamber component 712 fits into the top of the central open channel formed by the radial reflectors upon installation of the vessel head. Vessel head 700 may be fabricated as a single element, for example, by 3D printing or milling from a single piece of material, or may be assembled from various elements and attached by welding or other methods. As mentioned above, a reflector material may be incorporated into the vessel head 700, or a separate upper axial reflector (not shown) could be provided that would be positioned between the head plate 702 and the reflector assembly shown in Figures 5A and 5B.

[0092] 8 is an exploded view showing the major components of one embodiment of a nuclear reactor system. In the illustrated embodiment, reactor system 800 includes a reactor vessel 804, a reflector assembly 802 (two parts: a lower axial reflector 802a and a radial reflector 802b), a vessel head 806, one or more flow restrictor(s) 808, a control drum 810, and a pump assembly 812. Each component can be independently manufactured off-site and then transported and easily assembled at the desired location. Because reactor system 800 is designed as a low-power reactor, these major components can be kept relatively small (for a nuclear reactor) for ease of manufacture, transportation, assembly, maintenance, and replacement.

[0093] FIG. 9 shows a fuel pump assembly 900. As described above, the pump assembly 900 includes a motor 904, a shaft 908, and an impeller 910. The motor is separated from the reactor core by a motor support structure 906, through which the shaft 908 passes. The fuel salt pump 900 is attached to the vessel head via a flange 902. In the illustrated embodiment, the pump assembly 900 includes a fluid column 912 between the flange 902 and the impeller 910. During installation, the fluid column 912 is inserted into the upcomer of the vessel head, and the fluid column 912 includes an expansion chamber. In an alternative design, the housing is replaced with a support structure that provides the upper portion of the pump stator.

[0094] As shown, the pump is a vertical cantilever pump (without salt-wetted bearings) with an integrated fluid column 912 with controlled cover gas pressure and double mechanical seals. In the illustrated embodiment of the pump assembly, the impeller 910 faces downward in a so-called "end suction" configuration. This orientation supports the reactor system design, in which the pump draws flow from above the center of the core and pushes it radially out the four channels. This impeller orientation is made possible by providing fluid column 912 in fluid communication with the suction side of the pump, so that cover gas pressure on the liquid in the column and hydrostatic pressure from the fuel salt above impeller 910 can be used to provide the required net positive suction head (NPSH) for the pump. In one embodiment, the system may be operated under positive cover gas pressure (i.e., greater than 1 atmosphere) to ensure proper operation of the pump.

[0095] Given the need to direct the pump discharge from the volute and diffuse it into one or more high aspect ratio channels (i.e., the four upper heated fuel salt outlet channels 414), the pump incorporates a stator region with curved vanes to smoothly redirect the flow (see FIG. 4), which increases efficiency and impeller 910 stability compared to a single volute / single outlet configuration.

[0096] 10 shows a reactor vessel 1004 with dimples 1006 on its exterior surface rather than fins for improved heat transfer. As discussed above, any heat transfer element may be used to improve heat transfer between the reactor vessel 1004 and the coolant anywhere that coolant flows over the exterior of the reactor vessel. While not shown, the same is true for the fuel salt, and any form of heat transfer element may be provided on the interior surface of the reactor vessel to improve heat transfer between the molten fuel and the reactor vessel.

[0097] The reactor vessel may also vary in thickness, being thicker in locations where heat transfer between the reactor vessel interior and the coolant is not desired and thinner in the heat transfer areas. For example, referring to FIG. 3C , the reactor vessel 304 where the fins 326 are attached may be thinner than the thickness at any other location on the vessel 304. Note that the reactor vessel 304 and / or the shield vessel 305 may be a single, unitary construction of one material (e.g., steel) or may be multi-layered. For example, the reactor vessel may include a structural steel layer with an inner cladding of a different material selected based on its resistance to corrosion by fuel salts.

[0098] 11A-11G show various views of an alternative embodiment of a low-power nuclear reactor system 1100. Similar to the systems described above, the reactor system 1100 includes a reactor vessel 1104 containing a reflector assembly 1120 that defines a reactor core 1102 within the reactor vessel 1104. Again, the reflector assembly 1120 includes a lower axial reflector 1122, an upper axial reflector 1144, and a radial reflector 1124.

[0099] Figure 11A shows an isometric view of reactor system 1100 showing external details of vessel head 1106. Figure 11B shows a plan view of reactor system 1100. Figure 11C shows a cutaway view of reactor system 1100 along section AA identified in Figure 11B. Not all items are referenced in any of the figures.

[0100] The vessel head 1106 is similar to that described above and includes a flange 1108 for connecting to a pump assembly and an upcomer 1113 including an expansion chamber 1114. A control drum opening 1110 is shown in the vessel head 1106, providing access to a control drum well 1111 for the control drum, along with a fuel port access opening 1112. In the illustrated embodiment, the fuel port access opening 1112 allows the reactor vessel 1104 to be filled and drained with fuel. The fuel port access opening provides access to a dip tube 1116, which extends from the vessel head 1106 to a lower axial reflector 1122. In the illustrated embodiment, the lower end of the dip tube 1116 terminates in a collection channel 1126 defined by the lower axial reflector 1122. The collection channel 1126 is the lowest point in the reactor vessel 1104 that is not filled with a displacement element. By connecting the dip tube 1116 to the collection channel 1126, liquid can be easily drained from the reactor system by pressurizing the cover gas of the reactor system 1100. The free surface level 1125 of the molten fuel drops due to gravity and collects at the lowest point of the reactor system 1100 where the molten fuel is accessible.

[0101] In one embodiment, the free surface level 1125 of fuel salt in the reactor system 1100 may be monitored by monitoring the level in the dip tube 1116. This eliminates the need to incorporate monitoring equipment into the upcomer 1113. Measurements may be made using a laser level monitor, a conductance monitor, or any other device known in the art.

[0102] Access via dip tube 1116 also allows for reactivity control through the insertion of liquid absorbents, which are known in the art and may be added to the molten fuel through dip tube 1116 in situations where reduced reactivity is desired. For example, lithium is an absorbent material, and certain lithium salts are liquids in the operating temperature range envisioned for reactor system 1100.

[0103] The reactor system 1100 differs from the systems shown above in that, in the illustrated embodiment, it has larger heat exchange ducts 1136 so that almost the entire inner surface of the reactor vessel is in direct contact with the fuel salt and serves as a heat transfer area. As shown in the plan view of FIG. 11B , the fins 1130 on the exterior of the reactor vessel 1104 extend around the entire periphery of the vertical wall of the reactor vessel 1104. Furthermore, heated fuel salt flows over almost the entire inner surface of the reactor vessel 1104 opposite the fins 1130. In the illustrated embodiment, four standoff ridges 1134 are shown on the exterior of the radial reflector 1124. The standoff ridges 1134 contact the reactor vessel, keep the radial reflector centered on the reactor vessel, and form lateral boundaries for the four heat exchange ducts 1136. The standoff ridges 1134 may be solid and continuous, thereby separating the fuel salt flow between adjacent heat exchange ducts 1136. In an alternative embodiment, the spacing ridges 1134 may be discontinuous, for example, a series of individual contact points, allowing fuel to flow between what would otherwise be considered adjacent fuel salt ducts 1136. In yet another embodiment, instead of four spacing ridges 1134, the radial reflector 1124 may be provided with several individual spacing elements spaced around the exterior of the radial reflector so that fuel salt flows over substantially all of the outer surface of the radial reflector 1124.

[0104] 11D is a cross-sectional view through the center of the reactor system 1100, showing some of the enclosure components in more detail. In the illustrated embodiment, the finned areas on the vertical sides of the reactor vessel 1104 are surrounded by a jacket 1140 through which coolant flows. In one embodiment, the vertical outer wall of the jacket 1140 is provided with a layer 1142 of either a reflective or absorptive material for added safety. An overflow port 1184 is provided in the upcomer 1113 in case the reactor system 1100 is overfilled.

[0105] Figure 11F shows a top isometric view of the lower axial reflector 1122 and radial reflectors 1124, and a bottom isometric view of the upper axial reflector 1144, so that the resulting channels defined by the reflector assembly 1120 can be easily seen. The surfaces facing the fuel salt are contoured to define heating fuel salt outlet channels 1180 on top of the radial reflectors 1124 and cooling fuel salt delivery channels 1182 that return the cooling salt to the core 1102 from where it contacts the reactor vessel 1104. Figure 11E shows the shape of the fuel salt within the reactor vessel as a result of the displacement elements shown in Figures 11C and 11F.

[0106] 11C provides further details on an embodiment of the reflector assembly components. For example, the radial reflector 1124 is shown as a radial reflector shell 1124a enclosing a reflector material 1124b. In one embodiment, the reflector shell 1124a is made from INCONEL™ 625, and the reflector material 1124b includes magnesium oxide. Similarly, the lower axial reflector 1122 is shown as a shell 1122a and an interior filled with reflector material 1122b.

[0107] Other aspects of reactor system 1100 are similar to those described for the systems above. For example, four control drums 1150, which function similarly to those described above, are provided for reactivity control. Backfill reflector plugs 1152 on the control drums 1150 are further shown in FIG. 11C.

[0108] The overall pump design is also similar to that described above, including the use of a protective plug 1146 between the impeller and the core. In the embodiment shown in Figure 11C, the plug 1146 is made from a shielding material and is integrated into the radial reflector 1124. A lower skirt 1156 is provided to support the bottom of the reactor vessel 1104.

[0109] 12A-12C illustrate an embodiment of a nuclear reactor facility 1200 having alternative primary and secondary cooling systems in place of a heat rejection system. In the illustrated embodiment, a reactor system 1202 is housed by a shield assembly 1204. The shield assembly 1204 includes a removable top plug 1206 through which the reactor system 1202 may be accessed. In the illustrated embodiment, the shield assembly 1204 includes a base 1208, a rectangular sidewall component 1210, and a top 1212 having the removable plug 1206. In the illustrated embodiment, coolant ducts 1221 of a cooling circuit 1222, molten salt piping, and other piping and electrical elements penetrate the shield assembly 1204 at various locations.

[0110] 12A-12C show an alternative layout for a primary cooling system 1220. Again, the primary cooling system 1220 is shown as having four independent cooling circuits 1222. In the illustrated embodiment, nitrogen is the primary coolant, and each cooling circuit 1222 includes a heat exchanger 1224 and a blower 1226. In the illustrated embodiment, the heat exchanger 1224 transfers heat from the primary coolant to a facility heating system (not shown). Alternatively, the reactor system heat could be rejected to the environment, as described above.

[0111] A cover gas management system 1228 is shown near the shield assembly 1204. As described above, the cover gas management system 1228 maintains the pressure of the cover gas in the headspace above the fuel salt in the vessel head and also cleans the cover gas. The system 1228 may include a pump or blower 1229 for pressure control and any number of vessels for source gas storage, contaminant removal, and contaminant storage. Cover gas management systems are known in the art, and any suitable configuration or type may be used.

[0112] A reactor system controller 1230 is also shown near the shield assembly 1204. The controller 1230 monitors and controls the operation of the reactor system 1202.

[0113] A flush salt drain tank 1240 and a fuel salt overflow / drain tank 1242 are shown. Flush salt (e.g., a non-nuclear salt compatible with the fuel salt) may be used to prepare the reactor system for receiving the fuel salt. Flush salt may also be used to flush the reactor system 1202 after the fuel salt has been removed. Additionally, flush salt may be used to dilute the fuel salt, reducing its fissile density and, consequently, its reactivity.

[0114] 12B, the reactor facility includes a reactor building, again with removable access panels at the top of the building as shown, for accessing the reactor system 1202, the shield assembly 1204, and the components comprising the reactor room.

[0115] 14A-14B illustrate yet another embodiment of a pool-type nuclear reactor system 1400. FIG. 14A shows a molten fuel volume within a reactor vessel 1404. Similar to the systems described above, a central cylindrical core 1402 is defined within the reactor vessel 1404 by an internal radial reflector 1406 (shown in phantom as the open space between the fuel salt and the reactor vessel) that is spaced apart from the reactor vessel 1404. A pump chamber 1408 is located within the reactor vessel 1404 and includes an impeller and stator that are rotated by an external motor.

[0116] However, in the reactor system 1400 in Figures 14A-14C, there is no upper or lower axial reflector inside the reactor vessel 1404. Instead, when not in the core 1402 or the pump chamber 1408, the flow of molten fuel follows the inner surface of the reactor vessel 1404 in one or more channels 1418 defined by the space between the radial reflectors 1406 and the reactor vessel 1404. In the illustrated embodiment, the molten fuel flows upward through the reactor 1402 and into the pump chamber 1408. Rotation of the impeller projects the molten fuel upward and radially toward the reactor vessel 1404, forcing it to flow along the top of the interior of the reactor vessel 1404. The molten fuel flow then follows the inner surface of the reactor vessel 1404 radially outward and then downward along the heat transfer region of the vertical portion of the reactor vessel 1404. At the bottom of the reactor vessel 1404, the vessel 1404 is shaped to provide a collection channel 1410 near the outer diameter of the vessel 1404, and also provides a flow control cone that directs the molten fuel into the bottom of the core 1402. In this manner, the shape of the inner bottom surface of the reactor vessel 1404 provides a return flow path for the molten fuel.

[0117] Internal supports and flow control elements may be provided, for example, as shown in FIG. 14B. FIG. 14B shows internal vanes 1412 for directing the molten fuel flow out of the pump chamber 1408 and along the inner surface of the reactor vessel 1404. Other flow control elements, such as baffles, orifice plates, or vanes, may be provided to direct and control the molten fuel flow as needed. Additionally, as described above, internal supports may be provided at any location to center the radial reflector 1406 within the reactor vessel 1404. Such supports may also be used to control the flow of molten fuel.

[0118] Additional external reflectors may be provided outside the reactor vessel to improve the neutronics of the reactor system 1400. For example, an external lower axial reflector may be provided below the reactor vessel 1404. Additionally, an external upper axial reflector may be provided above the reactor vessel 1404.

[0119] FIG. 15 illustrates two alternative embodiments of upper molten fuel outlet channel and pump layouts that could be used in any of the reactor system embodiments described herein. FIG. 15 is a cross-sectional view of a nuclear reactor system 1500 showing the upper portion of a radial reflector 1501 surrounding a reactor core 1502 within a reactor vessel 1504. Molten fuel flows upward and out of the core 1502, around a protective plug 1506, and into a pump chamber 1508. A rotating impeller 1510 within the pump chamber drives the molten fuel upward and radially out of the pump chamber 1508 toward the inner surface of the top of the reactor vessel 1504. The molten fuel then flows into a heated molten fuel outlet channel 1512 that follows the contour of the inner surface of the top of the reactor vessel 1504. As noted above, while shown as a single channel that flows along the entire inner surface of the top of the reactor vessel 1504, the channel could be split into separate, independent channels, if desired.

[0120] In the illustrated embodiment, an expansion space 1514 is provided within the heated molten fuel outlet channel 1512 of the nuclear reactor system 1500. The expansion space 1514 is where the distance between the inner surface of the reactor vessel 1504 and the exterior of the radial reflector 1401 increases, thereby slowing the flow of molten fuel through that portion of the heated molten fuel outlet channel 1512 and thereby slowing the flow of molten fuel through the entire fuel circuit. The expansion space 1514 allows for better mixing of the flow exiting the pump chamber and better dispersion of the molten fuel, resulting in more uniform flow and temperature in the molten fuel as it enters the heat exchange duct 1516.

[0121] Figure 16 illustrates yet another embodiment of an upper molten fuel outlet channel and the surface elements of a radial reflector that define the channel. Figure 16 is a cross-sectional view of a nuclear reactor system 1600 showing the upper portion of a radial reflector 1601 that surrounds a reactor core 1602 within a reactor vessel (not shown). Molten fuel flows upward and out of the core 1602, around a protective plug 1606, and into a pump chamber 1608. A rotating impeller (not shown) within the pump chamber drives the molten fuel upward and radially out of the pump chamber 1608 toward the inner surface of the top of the reactor vessel. The molten fuel then flows into a heated molten fuel outlet channel 1612 that follows the contour of the inner surface of the top of the radial reflector 1601.

[0122] Reactor system 1600 is shown as having four separate heated molten fuel outlet channels 1612. These channels join together into a single manifold channel 1614. The single manifold channel 1614 then distributes the molten fuel into a single heat exchange duct 1616 that extends around the inner surface of the reactor vessel and the outer lateral surface of the radial reflector 1601. The manifold channel 1614 provides better mixing of the flow exiting the pump chamber and better dispersion of the molten fuel, resulting in more uniform flow and temperature in the molten fuel as it enters the heat exchange duct 1616.

[0123] Figure 17 illustrates an alternative embodiment of a nuclear reactor system. The embodiment illustrated in Figure 17 is similar to the embodiment of Figures 14A-14B in that, with the exception of molten fuel flow through a reactor core 1702 and a pump chamber 1708, the flow path of the molten fuel is in contact with and defined by the inner surface of a reactor vessel 1704.

[0124] 17 shows a volume of molten fuel within a reactor vessel 1704, where a central cylindrical core 1702 is defined by an internal radial reflector 1706 within the reactor vessel 1704, spaced apart from the reactor vessel 1704. A pump chamber 1708, protected from the core 1702 by a reflective plug 1705, is located inside the reactor vessel 1704 and contains an impeller 1709 rotated by an external motor. Similar to the previous designs, a control drum 1750 is located within the reflector 1706 for reactivity control.

[0125] However, in reactor system 1700, radial reflector 1706 can be said to comprise an upper axial component above the top of core 1702, while the lower axial reflector is not internal to reactor vessel 1704. Instead, as shown, an external lower axial reflector 1754 is provided. In the illustrated embodiment, molten fuel flows upward through core 1702, around reflective plug 1705, and into pump chamber 1708. Rotation of impeller 1709 projects the molten fuel upward and radially toward reactor vessel 1704, forcing it to flow along the top of the interior of reactor vessel 1704. The molten fuel then flows radially outward along the inner surface of reactor vessel 1704 and then downward along the heat transfer area of the vertical portion of reactor vessel 1704 in heat exchange duct 1712.

[0126] Figure 17 shows that the wall thickness of the reactor vessel 1704 is thinner in the heat transfer region than in other portions of the reactor vessel 1704. In Figure 17, the wall thickness at the top of the reactor vessel 1704 is substantially greater than the sides of the heat transfer region.

[0127] At the bottom of the reactor vessel 1704, the vessel 1704 is shaped to provide a collection channel 1710 near the outer diameter of the vessel 1704. The collection channel 1710 is in fluid communication with an access port 1752 at the top of the reactor vessel 1704 via a dip tube (not shown). The bottom of the reactor vessel 1704 further includes a flow control cone 1720 and a flow control orifice plate 1722 that delivers molten fuel into the bottom of the core 1702. The shape of the bottom inner surface of the reactor vessel 1704 thus provides a return flow path for the molten fuel. The reactor vessel 1704 further includes an integral skirt for supporting the reactor system 1700 on the floor of a nuclear reactor facility.

[0128] (Extraterrestrial reactor design) It would be desirable to have a power system capable of operating in cryogenic or extraterrestrial environments, for example to power satellites, spacecraft, or extraterrestrial installations (e.g., manned or unmanned lunar or Mars bases, etc.).

[0129] 13 shows a functional block diagram of a pool-type nuclear reactor system 1300. The pool-type nuclear reactor system 1300 is designed for use with molten nuclear fuel in an extraterrestrial environment or other suitably cryogenic environment. The reactor system 1300 is generally the same design as the reactor systems described above, except that rather than removing heat using coolant from the exterior of the reactor vessel, heat is dissipated to the external environment through a solid-state thermal-to-electricity conversion system mounted on the exterior of the reactor vessel. This converts heat directly into electricity, which can be used to operate devices.

[0130] In the illustrated embodiment, the nuclear reactor system 1300 includes a reactor core 1302 defined by a reflector assembly 1303 housed in a reactor vessel 1304. In the simplified cross-sectional view shown, the reflector assembly 1303 includes a radial reflector 1310, an upper axial reflector 1312, and a lower axial reflector 1314. One or more heating fuel salt outlet channels 1316 at the top of the core 1302 are defined between the radial reflectors 1310 and the upper axial reflector 1312. One or more cooling fuel salt return channels 1318 are defined between the radial reflectors 1310 and the lower axial reflector 1314. One or more heating fuel salt ducts 1320 connect the heating fuel salt outlet channels 1316 with the cooling fuel salt return channels 1318 to complete a fuel salt circuit within the nuclear reactor system.

[0131] The fuel salt circuit routes heated fuel salt along the interior surface of the reactor vessel 1304. At the interior surface of the reactor vessel 1304, heat is transferred through the vessel wall to a solid-state thermoelectric generator (TEG) (e.g., a thermionic or thermoelectric system, etc.). TEGs are known in the art, and any suitable design or type may be used. The TEG generates a current in an external circuit by applying a temperature difference (ΔT). The magnitude of ΔT determines the magnitude of the voltage difference (ΔV), and the direction of heat flow determines the voltage polarity. International Patent Application WO2014 / 114950 provides a further description of the operation of a TEG.

[0132] In one embodiment, the TEG consists of a collection of individual thermoelectric (TE) modules arranged in a fault-tolerant configuration wrapped around the exterior of the outer reactor vessel. The exterior surfaces of the TE modules are exposed to the surrounding environment (e.g., the Martian or Lunar atmosphere, or directly into space when deployed in orbit or deep space) and can passively reject waste heat by radiating it to the environment. In one embodiment, fuel salt within the reactor core maintains a temperature of 500-600°C. Assuming the Martian surface is approximately -65°C and deep space is -270°C, the ΔT available to the TEG in an extraterrestrial environment could be 550-800°C or higher.

[0133] In one embodiment, the reactor system uses natural circulation to drive the flow of fuel salt around the circuit. Calculations show that even in lunar gravity, natural circulation drives flow through the core at a rate of several centimeters per second. Alternatively, for zero-gravity embodiments, one or more electric pumps may be provided anywhere in the fuel salt circuit to drive the flow of fuel salt. The pump or pumps may be powered by the TEG.

[0134] In one embodiment, the fuel is a molten salt fuel mixture including a combination of NaCl, PuCl3, and / or UCl3 (e.g., the eutectic 64NaCl-36PuCl3, which melts at about 450°C). Several options are possible to avoid the use of Pu. However, these options always lead to larger and more massive cores, thereby increasing the cost of extraterrestrial deployment. KCl and MgCl2 are alternative carrier salts that may also be suitable for use in the reactor system 1300.

[0135] Beryllium and beryllium oxide may be used as reflector materials in extraterrestrial deployments, although as noted above, others are possible.

[0136] Unlike the above designs, in addition to the reflector, the reactor system 1300 includes an intravessel radiation shield 1322 that reduces radiation dose to external equipment (especially the TEG) and personnel. An enriched B4C structure is one practical option that reduces external radiation dose by several orders of magnitude with acceptable weight. In the illustrated embodiment, the intravessel shield 1322 is located on the exterior of the radial reflector 1310, between the radial reflector 1310 and the heating fuel salt duct 1320. Additional intravessel or extravessel shields may be provided, for example, above the upper axial reflector 1312 or below the lower axial reflector 1314.

[0137] In the illustrated embodiment, an inner vessel 1304a and an outer vessel 1304b are provided on portions of the top and side walls of the reactor vessel 1304, with fuel salt flowing between them in a heating fuel salt duct 1320. The inner vessel 1304a separates the shield 1322 from contact with the fuel salt, thereby protecting the shield 1322 from corrosion. In an alternative embodiment similar to the embodiment described above, the inner vessel 1304a is omitted. For example, the material for the shield 1322 and the reflector material of the radial reflector 1310 may be contained within a single structure, the outer surface of which is in contact with the molten fuel and defines the heat exchange duct 1320.

[0138] To prevent heat loss to the ambient environment around reactor system 1300, surfaces of the reactor vessel not in contact with the TEG may be insulated by external insulation. In one embodiment, greater than 90% of the heat generated by the core during steady-state operation is dissipated through the TEG and used to generate electricity. In another embodiment, greater than 99% of the heat generated is dissipated through the TEG. In an alternative embodiment, all or substantially all (e.g., greater than 90%) of the entire exterior surface of reactor system 1300 could be covered by the TEG.

[0139] The design calculation is 50 to 100kW. thA natural circulation system operating at 1 / 6 the gravity of Earth coupled with a thermoelectric device produces 10-15kW of 120VDC power. e For minimum mass systems, fueling with PuCl3 is preferred, although UCl3 (or a ternary mixture of NaCl, PuCl3 and UCl3) is also an option.

[0140] Notwithstanding the appended claims, the present disclosure is also defined by the following clauses. 1. a core in the form of an open channel that can achieve criticality when containing molten nuclear fuel; a heat exchange duct in fluid communication with said reactor core; a reactor vessel containing the reactor core and the heat exchange duct, the reactor vessel having an inner surface in thermal communication with the heat exchange duct and an outer surface in thermal communication with a coolant duct, whereby during criticality, heat from molten nuclear fuel in the heat exchange duct is transferred through the reactor vessel from the inner surface to the outer surface of the reactor vessel and further to the coolant in the coolant duct; and radial reflectors within the reactor vessel between the heat exchange ducts and the core, the radial reflectors defining the lateral boundaries of the core; Including, molten fuel reactors. 2. 10. The nuclear reactor of claim 1, further comprising a lower axial reflector defining a bottom of the core. 3. 3. The nuclear reactor of clause 1 or 2, further comprising an upper axial reflector defining the top of the core. 4. 4. The nuclear reactor of any one of clauses 1 to 3, wherein the heat exchange duct is fluidly connected to the reactor core to receive heated molten fuel from a first location in the reactor core and to discharge cooled molten fuel to a second location in the reactor core that is different from the first location. 5. 5. The nuclear reactor of any one of clauses 1-4, further comprising one or more heat transfer elements on the exterior surface of the reactor vessel. 6. 6. The nuclear reactor of any one of clauses 1-5, further comprising one or more fins, pins, or dimples on the exterior surface of the reactor vessel adapted to increase the heat transfer surface area of the exterior surface. 7. further comprising a shielding vessel containing the reactor vessel; 7. The reactor of any one of clauses 1 to 6, wherein the coolant duct is located between the shielding vessel and the reactor vessel. 8. 8. The nuclear reactor of any one of clauses 1 to 7, further comprising at least one flow restriction device capable of controlling the flow of molten nuclear fuel through said heat exchange duct. 9. 9. The nuclear reactor of any one of clauses 1-8, further comprising a vessel head assembly adapted to seal the top of the reactor vessel. 10. The vessel head assembly includes: drum well for receiving the control drum; penetrations for accepting flow restricting devices; a pump flange for connection to the pump assembly; and an upcomer including an expansion space within said vessel head assembly in fluid communication with said reactor core; 9. A nuclear reactor as described in clause 9, further comprising: 11. a control drum including a body of neutron reflecting material at least partially covered with neutron absorbing material, the control drum rotatably disposed within the drum well in the vessel head assembly; further comprising 11. The nuclear reactor of claim 10, wherein rotation of the control drum within the drumwell changes the reactivity of the reactor. 12. a pump assembly attached to the pump flange of the vessel head assembly, the pump assembly including an impeller for drawing molten nuclear fuel from the reactor core to the impeller and for driving the molten nuclear fuel into the heat exchange duct; 11. The reactor of claim 10, further comprising: 13. 13. The nuclear reactor of clause 12, further comprising a shield plug between the impeller and the core. 14. 14. The nuclear reactor of clause 13, wherein the shield plug comprises a reflector material and / or a shield material. 15. 10. The nuclear reactor of clause 9, further comprising an access port in said vessel head assembly in fluid communication with said reactor core. 16. 3. The nuclear reactor of clause 2, wherein the lower axial reflector defines a collection channel that is the lowest point in the reactor vessel in fluid communication with the core. 17. 17. The nuclear reactor of clause 16, further comprising at least one dip tube fluidly connecting the collection channel with an access port. 18. 18. The nuclear reactor of any one of clauses 1 to 17, further comprising at least one flow restriction device capable of controlling the flow of molten nuclear fuel through said heat exchange duct. 19. 19. The nuclear reactor of any one of clauses 1-18, further comprising an impeller that draws molten nuclear fuel from the reactor core to the impeller and drives the molten nuclear fuel into the heat exchange duct. 20. 20. The nuclear reactor of clause 19, further comprising a shield plug between the impeller and the core. twenty one. 21. The nuclear reactor of any one of clauses 1-20, wherein the heat exchange duct is fluidly connected to the core to receive heated molten fuel from a first location in the open channel and to discharge cooled molten fuel to a second location in the open channel. twenty two. the first location is near the top of the core; 22. The nuclear reactor of clause 21, wherein the second location is near the bottom of the core. twenty three. 23. The nuclear reactor of any one of clauses 1-22, further comprising a cooling system capable of transferring heat received by the coolant from the molten nuclear fuel through the reactor vessel to the ambient atmosphere. twenty four. The cooling system comprises: a primary cooling circuit including the coolant duct, a heat exchanger, and a coolant blower configured to circulate the coolant through the primary cooling circuit, whereby heat from the heated coolant from the coolant duct is transferred to air via the heat exchanger; and a heat rejection system including an air blower that directs air through the heat exchanger and to a vent to ambient atmosphere; 24. The molten fuel reactor of claim 23, further comprising: twenty five. 25. The nuclear reactor of any one of clauses 1-24, further comprising a sensor configured to monitor the height of a free surface of molten nuclear fuel within the reactor. 26. 10. The nuclear reactor of claim 1, wherein the molten nuclear fuel includes one or more fissile fuel salts selected from PuCl3, UCl4, UCl3F, UCl3, UCl2F2, ThCl4, and UClF3, and one or more non-fissile salts selected from NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, AmCl3, LaCl3, CeCl3, PrCl3, and NdCl3. 27. a core in the form of an open channel, which, if it contains molten nuclear fuel, can achieve criticality from the mass of the molten nuclear fuel; a heat exchange duct in fluid communication with said reactor core; a reactor vessel containing the reactor core and the heat exchange duct, the reactor vessel having an inner surface and an outer surface, the inner surface contacting the heat exchange duct such that the heat exchange duct is in thermal communication with the outer surface; and a thermoelectric generator having a first surface and a second surface, the thermoelectric generator producing electricity from a temperature differential between the first surface and the second surface, the first surface of the thermoelectric generator in thermal communication with the exterior surface of the reactor vessel, and the second surface of the thermoelectric generator exposed to the ambient environment; Including, nuclear reactors. 28. 28. The nuclear reactor of clause 27, further comprising radial reflectors within the reactor vessel between the heat exchange ducts and the core, the radial reflectors defining lateral boundaries of the core. 29. 29. The nuclear reactor of claim 27 or 28, further comprising a lower axial reflector defining a bottom of the core. 30. 30. The nuclear reactor of any one of clauses 27 to 29, further comprising an upper axial reflector defining the top of the core. 31. 29. The nuclear reactor of clause 28, further comprising shielding within the reactor vessel between the radial reflector and the heat exchange duct. 32. 32. The nuclear reactor of any one of clauses 27 to 31, further comprising a pump powered by electricity generated by the thermoelectric generator, the pump including an impeller in the reactor vessel capable of circulating molten nuclear fuel between the reactor core and the heat exchange duct. 33. 29. The nuclear reactor of claim 28, wherein the radial reflector is a steel vessel filled with a reflective material. 34. 34. The nuclear reactor of any one of clauses 27-33, wherein the molten nuclear fuel comprises one or more fissile fuel salts selected from PuCl3, UCl4, UCl3F, UCl3, UCl2F2, ThCl4, and UClF3, and one or more non-fissile salts selected from NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, AmCl3, LaCl3, CeCl3, PrCl3, and NdCl3. 35. 35. The nuclear reactor of any one of clauses 27 to 34, wherein greater than 90% of the thermal energy generated in the reactor core is dissipated through the thermoelectric generator. 36. 36. The nuclear reactor of any one of clauses 27 to 35, further comprising one or more insulating plates on the exterior surface of the reactor vessel. 37. a core space that, if it contains molten nuclear fuel, can achieve criticality from the mass of the molten nuclear fuel within the core space; a reactor vessel including the core space, the reactor vessel being in thermal communication with the core; and a thermoelectric generator having a first surface and a second surface, the thermoelectric generator producing electricity from a temperature differential between the first surface and the second surface, the first surface of the thermoelectric generator in thermal communication with the reactor vessel, and the second surface of the thermoelectric generator exposed to the ambient environment; Including, molten fuel reactors. 38. a radial reflector within the reactor vessel between the reactor vessel and the core, the radial reflector defining a lateral boundary of the core space; and a heat exchange duct within the reactor vessel, the heat exchange duct being between the radial reflector and the reactor vessel and in fluid communication with the core space; 38. The reactor of claim 37, further comprising: 39. 39. The nuclear reactor of claim 38, wherein at least one surface of the heat exchange duct is formed by the reactor vessel. 40. 40. The nuclear reactor of any one of clauses 37 to 39, further comprising a lower axial reflector defining a bottom of said core volume. 41. 41. The nuclear reactor of any one of clauses 37 to 40, further comprising an upper axial reflector defining a top of said core volume. 42. 42. The nuclear reactor of any one of clauses 37 to 41, further comprising shielding within the reactor vessel between the radial reflector and the heat exchange duct. 43. 43. The nuclear reactor of any one of clauses 37-42, wherein the molten nuclear fuel comprises one or more fissile fuel salts selected from PuCl3, UCl4, UCl3F, UCl3, UCl2F2, ThCl4, and UClF3, and one or more non-fissile salts selected from NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, AmCl3, LaCl3, CeCl3, PrCl3, and NdCl3. 44. reactor vessel; a radial reflector within the reactor vessel defining a reactor core in the form of an open channel capable of achieving criticality when containing molten nuclear fuel; and a heat exchange duct between the radial reflector and the reactor vessel, the heat exchange duct being in fluid communication with the reactor core; 1. A molten fuel nuclear reactor comprising: A molten fuel reactor, wherein the reactor vessel has an inner surface in thermal communication with the heat exchange duct and an outer surface in thermal communication with a coolant duct, such that during criticality, heat from molten nuclear fuel in the heat exchange duct is transferred through the reactor vessel from the inner surface to the outer surface of the reactor vessel and further to the coolant in the coolant duct. 45. 45. The nuclear reactor of clause 44, further comprising a lower axial reflector defining a bottom of the core. 46. 46. The nuclear reactor of claim 44 or 45, further comprising an upper axial reflector defining the top of the core. 47. 47. The nuclear reactor of any one of clauses 44-46, wherein the heat exchange duct is fluidly connected to the reactor core to receive heated molten fuel from a first location in the reactor core and to discharge cooled molten fuel to a second location in the reactor core that is different from the first location. 48. 48. The nuclear reactor of any one of clauses 44-47, further comprising one or more heat transfer elements on the exterior surface of the reactor vessel. 49. 49. The nuclear reactor of any one of clauses 44-48, further comprising one or more fins, pins, or dimples on the exterior surface of the reactor vessel adapted to increase the heat transfer surface area of the exterior surface. 50. further comprising a shielding vessel containing the reactor vessel; 50. The reactor of any one of clauses 44 to 49, wherein the coolant duct is between the shielding vessel and the reactor vessel. 51. 51. The nuclear reactor of any one of clauses 44 to 50, further comprising at least one flow restriction device capable of controlling the flow of molten nuclear fuel through said heat exchange duct. 52. 52. The nuclear reactor of any one of clauses 44-51, further comprising a vessel head assembly adapted to seal the top of the reactor vessel. 53. The vessel head assembly includes: drum well for receiving the control drum; penetrations for accepting flow restricting devices; a pump flange for connection to the pump assembly; and an upcomer including an expansion space within said vessel head assembly in fluid communication with said reactor core; 53. The reactor of claim 52, further comprising: 54. a control drum including a body of neutron reflecting material at least partially covered with neutron absorbing material, the control drum rotatably disposed within the drum well in the vessel head assembly; further comprising 54. The nuclear reactor of clause 53, wherein rotation of the control drum within the drumwell changes the reactivity of the reactor. 55. a pump assembly attached to the pump flange of the vessel head assembly, the pump assembly including an impeller for drawing molten nuclear fuel from the reactor core to the impeller and for driving the molten nuclear fuel into the heat exchange duct; 54. The reactor of claim 53, further comprising: 56. 56. The nuclear reactor of clause 55, further comprising a shield plug between the impeller and the core. 57. 57. The nuclear reactor of clause 56, wherein the shield plug comprises a reflector material and / or a shield material. 58. 53. The nuclear reactor of clause 52, further comprising an access port in the vessel head assembly in fluid communication with the reactor core. 59. 46. The nuclear reactor of clause 45, wherein the lower axial reflector defines a collection channel that is the lowest point in the reactor vessel in fluid communication with the core. 60. 60. The nuclear reactor of clause 59, further comprising at least one dip tube fluidly connecting the collection channel with an access port. 61. 61. The nuclear reactor of any one of clauses 44 to 60, further comprising at least one flow restrictor capable of controlling the flow of molten nuclear fuel through said heat exchange duct. 62. 62. The nuclear reactor of any one of clauses 44 to 61, further comprising an impeller that draws molten nuclear fuel from the reactor core to the impeller and drives the molten nuclear fuel into the heat exchange duct. 63. 63. The nuclear reactor of clause 62, further comprising a shield plug between the impeller and the core. 64. 64. The nuclear reactor of any one of clauses 44-63, wherein the heat exchange duct is fluidly connected to the core to receive heated molten fuel from a first location in the open channel and to discharge cooled molten fuel to a second location in the open channel. 65. the first location is near the top of the core; 65. The nuclear reactor of clause 64, wherein the second location is near the bottom of the core. 66. 66. The nuclear reactor of any one of clauses 44 to 65, further comprising a cooling system capable of transferring heat received by the coolant from the molten nuclear fuel through the reactor vessel to the ambient atmosphere. 67. The cooling system comprises: a primary cooling circuit including the coolant duct, a heat exchanger, and a coolant blower configured to circulate the coolant through the primary cooling circuit, whereby heat from the heated coolant from the coolant duct is transferred to air via the heat exchanger; and a heat rejection system including an air blower that directs air through the heat exchanger and to a vent to ambient atmosphere; 66. A nuclear reactor as described in clause 66, further comprising: 68. 68. The nuclear reactor of any one of clauses 44 to 67, further comprising a sensor configured to monitor the height of a free surface of molten nuclear fuel within the reactor. 69. 69. The nuclear reactor of any one of clauses 44-68, wherein the molten nuclear fuel includes one or more fissile fuel salts selected from PuCl3, UCl4, UCl3F, UCl3, UCl2F2, ThCl4, and UClF3, and one or more non-fissile salts selected from NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, AmCl3, LaCl3, CeCl3, PrCl3, and NdCl3. 70. reactor vessel; a radial reflector within said reactor vessel defining a reactor core in the form of an open channel capable of achieving criticality when containing molten nuclear fuel; a heat exchange duct between the radial reflector and the reactor vessel, the heat exchange duct being in fluid communication with the reactor core; and a thermoelectric generator having a first surface and a second surface configured to generate electricity from a temperature differential between the first surface and the second surface, the first surface of the thermoelectric generator being in thermal communication with the exterior surface of the reactor vessel and the second surface of the thermoelectric generator being exposed to an ambient environment; A nuclear reactor comprising: The reactor vessel has an inner surface and an outer surface, the inner surface contacting the heat exchange ducts such that the heat exchange ducts are in thermal communication with the outer surface. 71. 71. The nuclear reactor of clause 70, further comprising radial reflectors within the reactor vessel between the heat exchange ducts and the core, the radial reflectors defining lateral boundaries of the core. 72. 72. The nuclear reactor of claim 70 or 71, further comprising a lower axial reflector defining a bottom of the core. 73. 73. The nuclear reactor of any one of clauses 70 to 72, further comprising an upper axial reflector defining the top of said core. 74. 72. The nuclear reactor of clause 71, further comprising shielding within the reactor vessel between the radial reflector and the heat exchange duct. 75. 75. The nuclear reactor of any one of clauses 70 to 74, further comprising a pump powered by electricity generated by the thermoelectric generator, the pump including an impeller in the reactor vessel capable of circulating molten nuclear fuel between the reactor core and the heat exchange duct. 76. 75. The nuclear reactor of claim 71 or 74, wherein the radial reflector is a steel vessel filled with a reflective material. 77. 77. The nuclear reactor of any one of clauses 70-76, wherein the molten nuclear fuel includes one or more fissile fuel salts selected from PuCl3, UCl4, UCl3F, UCl3, UCl2F2, ThCl4, and UClF3, and one or more non-fissile salts selected from NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, AmCl3, LaCl3, CeCl3, PrCl3, and NdCl3. 78. 78. The nuclear reactor of any one of clauses 70-77, wherein greater than 90% of the thermal energy generated in the reactor core is dissipated through the thermoelectric generator. 79. 79. The nuclear reactor of any one of clauses 70-78, further comprising one or more insulating plates on the exterior surface of the reactor vessel.

[0141] FIG. 18 illustrates an alternative embodiment of a nuclear reactor 1800 in which the majority of the reflector material is outside the reactor vessel 1804. In the illustrated embodiment, the reactor vessel 1804 is a cylinder that houses all of the salt and a displacement component 1806 (which may be a reflector) in the upper section of the reactor vessel 1804. In the illustrated embodiment, unlike the displacement component 1806, the reflector elements, including the radial reflector 1802 and bottom reflector 1803, are located outside the vessel 1804. Similar to the above design, the salt flows around the exterior of the displacement component 1806 through a downcomer heat exchange duct 1808 defined by the exterior of the displacement component 1806 and the interior surface of the reactor vessel 1804. This design reduces the overall size of the reactor vessel 1804 for a given volume of salt compared to the previously described designs with a radial reflector or bottom reflector inside.

[0142] An unmoderated pool of fuel salt at the bottom of the reactor vessel serves as the reactor core 1810. The displacement component 1806 includes a draft tube section 1818. The draft tube section 1818 extends nearly to the bottom of the reactor vessel 1804, forcing the fuel salt to flow along most of the inner surface of the reactor vessel 1804 before being redirected into the reactor core 1810. The fuel salt heated by the nuclear fission occurring in the core 1810 rises in the center of the reactor vessel 1804 through an upcomer duct 1812 located in the center of the displacement component 1806, as shown. In the illustrated embodiment, an impeller 1814 is located at the top of the upcomer duct 1812 to help drive the flow of fuel salt. As described above, the impeller 1814 is driven by a motor 1816 external to the reactor vessel 1804. A casing containing the impeller 1814 is formed by the displacement component 1806 and the reactor vessel 1804. In an alternative embodiment, the reactor 1800 is designed to operate in natural circulation, and the pump is omitted.

[0143] Again, cooling of the reactor 1800 is achieved by flowing a coolant gas or fluid along the exterior of the reactor vessel 1804. In the illustrated embodiment, the coolant duct 1820 is formed in an annular region between the exterior of the reactor vessel 1804 and the interior surface of the radial reflector 1802. In the illustrated embodiment, no fins are provided within the coolant duct 1820; that is, the coolant duct 1820 is an open channel through which the coolant flows. In this embodiment, removing the fins increases the reactivity of the reactor because it has been determined that the fins impede neutrons from bouncing back into the reactor core.

[0144] In one embodiment, the coolant is flowed co-currently with the fuel salt, i.e., both the coolant and the fuel salt flow downwardly over opposing surfaces of the sidewall of the reactor vessel 1804. Co-current flow, with or without the use of fins, is equally applicable to all embodiments of the reactor described herein.

[0145] In this embodiment, the reactor vessel 1804 is fabricated from a material with sufficient strength and properties to withstand the high neutron flux incident near the region of the reactor core 1810. By placing a reflector on the outside of the reactor vessel, the diameter of the reactor vessel can be reduced. Assuming a downcomer duct 1808 of the same diameter, the cross-sectional flow area is reduced, and for the same mass flow rate, the velocity of the fuel salt moving through the duct 1808 is increased for this design. The increased velocity is expected to increase the heat transfer coefficient. A smaller vessel diameter requires less structural strength, which may result in a smaller wall thickness. A thinner reactor vessel wall improves the heat transfer characteristics between the downcomer heat exchange duct 1808 and the coolant duct 1820.

[0146] Other aspects of the design include a sufficiently high riser 1822 between the top of the reactor vessel 1804 and a pump connection flange 1824. This riser 1822 defines an expansion space 1826 for the fuel salt. The heat exchange characteristics through the reactor vessel wall can be modified by increasing or decreasing the lateral height of the reactor vessel to increase the heat transfer area.

[0147] The reactor shown in FIG. 18 may include any and all of the reactor components of the above-described embodiments, although some of the elements described above are not shown. For example, a shielding plug may be provided in the upcomer duct 1812 to protect the impeller from neutrons generated in the reactor core 1810. A conical lower axial reflector may be provided at the bottom of the vessel 1804. This may be incorporated into the displacement component 1806 or may be a separate component. As described above, a removable vessel head may be provided at the top of the reactor vessel 1804, or, as shown, the vessel may be a continuous body including the riser 1822.

[0148] 19A-19E illustrate various options available for reactivity control when a radial reflector 1902 is located outside the reactor vessel 1904 in a design such as that shown in FIG. 18. By moving all or a portion of the radial reflector 1902, the reactivity of the reactor 1900 may be controlled. FIGS. 19A-19C illustrate cross-sectional views of the reactor, each showing a different possible radial reflector configuration. In one embodiment, as shown, a radial absorber 1908 or neutron shield outside the reflector 1902 may be provided to confine neutrons not blocked by the reflector 1902.

[0149] In Figure 19A, the external radial reflector 1902 is shown in its most reactive configuration, in which the reflector completely surrounds the reactor vessel 1904. In this configuration, side-traveling neutrons generated within the reactor core 1906 are reflected back into the core by the radial reflector 1902.

[0150] Figure 19B shows a reduced reactivity configuration in which the radial reflector 1902 is lowered (or the upper portion of the reflector is removed) so that it no longer completely surrounds the core 1906 as shown in Figure 19A. In this configuration, some of the neutrons generated within the core 1906 escape rather than bouncing back into the core, thereby reducing the reactivity of the reactor. In this embodiment, a cooling jacket 1930 may be provided to insulate the coolant duct 1910 from the movement of the reflector 1902 to ensure that the coolant flows along the exterior of the reactor vessel 1904.

[0151] Figure 19C shows yet another embodiment in which a portion of the radial reflector 1902 is movable for reactivity control, while maintaining the size and length of the coolant duct 1910. In Figure 19C, a portion 1902a of the reflector is raised, reducing the overall thickness of the reflector material around the core 1906, thereby reducing the reactivity of the reactor 1900.

[0152] FIG. 19D is a plan view of the reactor 1900, illustrating yet another alternative for reactivity control using this design. In the illustrated embodiment, control elements 1920 may be inserted into the coolant duct 1910 formed between the reflector 1902 and the exterior surface of the reactor vessel 1904. The control elements 1920 may be neutron reflectors or neutron absorbers. Similar to the control rods, these control elements 1920 are shown as four separate arc-shaped plates that may be raised or lowered within the coolant duct 1910. If the elements 1920 are made of an absorbing material, their insertion results in a reduction in the reactivity of the reactor 1900. If the elements are reflectors or made of a reflective material, their insertion into the coolant duct 1910 may increase the reactivity of the reactor 1900, and their removal may decrease the reactivity of the reactor. Although shown as four arcuate plates, any number or shape of elements 1920 may be used, including, for example, cylindrical rods or flat plates sized to fit into the coolant ducts.

[0153] Figure 19E illustrates yet another embodiment of reactor control. Figure 19E is a plan view of reactor 1900 illustrating the use of a control drum 1922 in the reflector. Similar to the control drums described above, control drum 1922 can rotate within a control drum recess in reflector 1902 such that either an absorbent surface 1924 or a reflective surface 1926 on the control drum is exposed to the reactor core.

[0154] The various forms of reactor control in Figures 19A-19E could be used separately or together in any combination. For example, the arcuate control element of Figure 19D could be used in conjunction with a separable reflector 1902 that could change from the configuration shown in Figure 19A to the configuration shown in Figure 19B or Figure 19C. As another example, the reflector of Figure 19A could include one or more control drums as shown in Figure 19E and could be further lowered to the position shown in Figure 19B. All combinations are contemplated.

[0155] 19A-19C illustrate a further aspect of this design related to a reactor vessel 1904. In one embodiment, the reactor vessel 1904 is designed to freely change size and shape in response to thermal expansion. In the embodiment shown, the reactor vessel 1904 is supported from below by a support structure 1932 or stand. In the embodiment shown in FIG. 19A, the support structure 1932 includes a lower axial reflector 1912. The side walls of the reactor vessel 1904 are free to move and are allowed to change diameter by providing ducts on either side of the reactor vessel wall.

[0156] In the illustrated embodiment, the base of the reactor vessel 1904 has a generally convex, conical, or frusto-conical shape to help direct the flow of salt from the downcomer duct into the center of the core 1906. This shape has other advantages, including providing more strength than a flat surface and accommodating thermal expansion better than a flat bottom. In an alternative embodiment (not shown), a second displacement component may be provided at the bottom of the vessel as a lower axial reflector, which may also provide a convex shape for directing the flow of fuel salt.

[0157] As mentioned above, rather than being rigidly mounted, the reactor vessel 1904 may simply rest on the support structure 1932 to allow for free thermal expansion of the vessel 1904. In an alternative embodiment, the vessel 1904 may be suspended from above via a pump flange. The displacement component 1914 may be suspended from the top of the vessel 1904, from the vessel head if one is provided, or from the pump assembly. In an alternative embodiment, the displacement component 1914 may be loosely contained within the vessel 1904 and rest on the bottom vessel 1904 via a downcomer wall, one or more supports, or other elements that maintain the displacement component 1914 in place within the vessel 1904 without being rigidly mounted to the vessel.

[0158] Figure 20 shows an embodiment of a low-power reactor design adapted to reduce the reactivity change associated with flowing delayed neutron precursors. Delayed neutrons are neutrons emitted by excited fission product nuclei during beta decay following the fission that created the product nuclei. Typically, delayed neutrons are emitted within 10 seconds after fission. -14 Neutrons emitted after 1000 s are considered delayed neutrons. Delayed neutrons are not typically a significant design criterion in molten salt reactors designed to produce power. Typically, in power-producing designs, there is always a significant amount of fuel salt outside the core, traveling through the fuel salt cooling circuits and heat exchangers. In these designs, delayed neutrons have little effect on the reactor reactivity because most of the delayed neutrons are released before the fuel salt completes its circuit through the heat exchangers and returns to the core. In fact, while minimizing the amount of fuel salt outside the core (due to the high cost of fuel salt) is typically a design criterion, in power-producing molten salt reactors that circulate fuel salt through shell-and-tube heat exchangers, the effect of delayed neutrons on reactivity is ignored because so much of the salt is required to be outside the core for heat transfer purposes.

[0159] However, in the test reactor design proposed herein, delayed neutrons could have a significant effect on the reactor reactivity. While one criterion for reactor design is typically to minimize the amount of fuel salt outside the core due to the high cost of fuel salt, it has been determined that these low-power test reactor designs may require increasing the fuel salt volume outside the core beyond what may be needed for heat transfer purposes. The fuel salt reservoir outside the core but within the fuel salt flow circuit, which essentially serves no heat transfer purpose, is provided solely for the purpose of increasing the volume of fuel salt in the fuel salt circuit outside the core. One perspective on this reservoir is that it artificially increases the residence time of fuel salt in the fuel salt circuit outside the core without any heat transfer benefit.

[0160] Figure 20 illustrates an embodiment in which a delayed neutron reservoir 2002 is provided in the fuel salt circuit outside of the reactor core 2004. The reactor 2000 is similar to the reactor shown in Figure 19, with a reactor vessel 2008 surrounding a displacement component 2006 and a free space filled with fuel salt containing the reactor core 2004. The fuel salt delayed neutron reservoir 2002 is created outside of the reactor core 2004 by sizing the displacement component 2006 to manage the reactivity associated with delayed neutrons.

[0161] In the illustrated embodiment, reservoir 2002 is above displacement component 2006. However, reservoir 2002 could be located anywhere within the fuel salt flow path outside of core 2004. Increasing the volume of fuel salt outside of core 2004 prevents a large portion of delayed neutrons from affecting the fission reactivity within core 2004.

[0162] In one embodiment, the delayed neutron reservoir 2002 is a volume V of salt in the reactor core. core the total volume of salt in the reactor vessel 2008, V tot In this embodiment, the volume of the reservoir 2002 is determined based on V core / V tot The ratio is increased until the desired ratio of 75 to 99% of V is achieved. core / V tot (i.e., V core / V tot Target ratios of 0.75-0.99 were beneficial, and V of 85-95%, 88-92%, and 89-91% were beneficial. core / V tot It has been confirmed that the ratio of the total volume of salt in the reactor vessel 2008, V tot is the volume of the core V core and the volume of the reservoir V res and the volume of salt within the fuel salt circuit but outside the core and reservoir, V cir (Note: V cirincludes the salt volume in the heat transfer downcomer duct 2010 and upcomer duct 2012, but, depending on the design, does not include the expansion space in the riser (because the expansion space is typically not part of the flow circuit and does not change the residence time of the fuel salt outside the reactor core 2004). In an alternative embodiment, the delayed neutron reservoir 2002 is core / V tot is less than 95%, less than 91%, less than 90%, about 90%, less than 89%, less than 85%, or less than 75%. core / V tot The minimum ratio is 50%.

[0163] 21 and 22 show alternative designs for manipulating the flow of fuel salt as it circulates through the interior of the reactor vessel. The flow of fuel salt has been generally described above as having an upward vertical flow through the upcomer duct and a downward vertical flow in the downcomer duct. This is the simplest flow scheme and results in the shortest residence time of the fuel salt near the surface in the downcomer heat exchange duct and within the surface of the reactor vessel. However, other flow schemes are possible that alter the heat transfer behavior of the reactor.

[0164] Figures 21A and 21B show two views of a nuclear reactor 2100 according to one embodiment. In this embodiment, a lateral swirl flow (indicated by the dashed lines) is induced to flow the fuel salt along the inner surface of the lateral face of the reactor vessel 2102. In the illustrated embodiment, vanes 2104 are provided on the surface of the displacement component 2106 in the downcomer duct 2108 so that the flow of fuel salt is directed tangentially downward along the inner surface of the reactor vessel, rather than straight down. Figure 21A is a cross-sectional view of the reactor 2100, and Figure 21B is a cutaway view showing the vanes 2104 on the displacement component 2106.

[0165] In the illustrated embodiment, a series of vanes 2104, resembling threads on a screw, are provided in the downcomer duct 2108 between the displacement component 2106 and the inner surface of the reactor vessel 2102. The vanes 2104 could be attached to the displacement component 2106, attached to the inner surface of the reactor vessel 2102, or a combination of both. The vanes 2104 could extend the full width of the downcomer duct 2108, connecting the reactor vessel 2104 with the displacement component 2106, or the vanes 2104 could extend only partially into the downcomer duct 2108. The resulting swirling flow increases the time the salt has to travel around the inner surface of the reactor vessel 2102 before reaching the bottom of the vessel and flowing upward through the core. Modeling indicates that the swirling motion continues within the core as the fuel salt heats up. This improves the uniformity of heating of the fuel salt exiting the core.

[0166] Figures 22A and 22B show an alternative embodiment of a reactor design in which the fuel salt flows in a swirling manner around the inner surface of the reactor vessel. In this embodiment, the fuel salt is removed from the reactor vessel 2202 through a central outlet port 2204 and re-injected through injection ports 2206 that are tangential to the side of the reactor vessel 2202. Figure 22A is a cross-sectional view of the reactor 2200 showing the induced salt flow in dashed lines, and Figure 22B is a perspective view showing the outlet port 2204 and injection port 2206. A swirling flow of the fuel salt can be induced by directing the fuel salt flow tangentially along the inner surface of the reactor vessel.

[0167] In an alternative embodiment, more than one injection port 2206 may be used. The injection ports 2206 may be angled slightly downward or may be horizontal as shown.

[0168] Figures 21A, 21B, 22A, and 22B are just two examples of how swirling flow of fuel salt along the inner surface of the reactor vessel may be achieved. Other methods of creating a swirling motion in the salt flow are also contemplated, such as providing vanes along the inner surface of the reactor vessel or one or more directional nozzles or jets in the outlet of the pump. Any suitable method may be used herein.

[0169] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the present technology are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. However, any numerical value inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0170] Notwithstanding the appended claims, the present disclosure is also defined by the following clauses. 1. 1. A molten fuel nuclear reactor, comprising: a reactor vessel having an interior surface and an exterior surface; a displacement component within the reactor vessel that cooperates with the interior surface of the reactor vessel to define a reactor core capable of achieving criticality when containing molten nuclear fuel, a central upcomer duct, and a downcomer duct in fluid communication with the reactor core and the central upcomer duct; a radial reflector around the reactor vessel; a coolant duct between the reactor vessel and the radial reflector; Equipped with the inner surface of the reactor vessel is in thermal communication with the downcomer duct and the outer surface of the reactor vessel is in thermal communication with the coolant duct; This allows heat from the molten nuclear fuel in the downcomer duct to be transferred through the reactor vessel from the inner surface to the outer surface of the reactor vessel, and then to the coolant in the coolant duct. 2. 10. The nuclear reactor of claim 1, further comprising a lower axial reflector below the reactor vessel. 3. 3. The nuclear reactor of clauses 1 and 2, wherein the displacement component incorporates neutron reflective material to bounce neutrons from the core back into the core. 4. 4. The nuclear reactor of any one of clauses 1-3, wherein the downcomer duct is fluidly connected to the reactor core to receive heated molten fuel from a first location in the reactor core and to discharge cooled molten fuel to a second location in the reactor core that is different from the first location. 5. 5. The nuclear reactor of any one of clauses 1 to 4, wherein the displacement component includes a central penetration therethrough defining the central upcomer duct, and a draft tube. 6. 6. The nuclear reactor of any one of clauses 1-5, further comprising at least one vane attached to the displacement component that directs molten nuclear fuel obliquely along the inner surface of the reactor vessel. 7. 7. The nuclear reactor of any one of clauses 1-6, further comprising a vessel head assembly sealing a top of the reactor vessel. 8. The radial reflector is a drum well for receiving a control drum; a control drum including a body of neutron reflecting material at least partially covered with neutron absorbing material, the control drum being rotatably disposed within the drum; Furthermore, 8. The nuclear reactor of any one of clauses 1 to 7, wherein rotation of the control drum within the drumwell changes the reactivity of the reactor. 9. 8. The nuclear reactor of clause 7, further comprising an access port in the vessel head assembly in fluid communication with the reactor core. 10. the radial reflector is movable relative to the reactor vessel; 10. The reactor according to any one of clauses 1 to 9, wherein the reactivity of the reactor can be changed by moving the radial reflectors. 11. the radial reflector is a plurality of reflector elements; 11. The nuclear reactor of claim 10, wherein moving the radial reflector includes moving a first reflector element of a plurality of the reflector elements. 12. 12. The nuclear reactor of any one of clauses 1 to 11, further comprising an impeller that draws molten nuclear fuel from the reactor core to the impeller and drives the molten nuclear fuel into the downcomer duct. 13. 13. The nuclear reactor of clause 12, further comprising a shield plug between the impeller and the core. 14. 14. The nuclear reactor of any one of clauses 1-13, wherein the downcomer duct is fluidly connected to the reactor core to receive heated molten fuel from a first location in the central upcomer duct and to discharge cooled molten fuel to a second location in the reactor core. 15. 15. The reactor of any one of clauses 1 to 14, further comprising a control element in said coolant duct that can be moved to control the reactivity of said reactor. 16. the control element includes one or both of a neutron reflecting material and a neutron absorbing material; 16. The nuclear reactor of claim 15, wherein the control element is selected from an arc plate, a flat plate, or a rod. 17. The cooling system comprises: a primary cooling circuit including the coolant duct, a heat exchanger, and a coolant blower, the coolant blower configured to circulate the coolant through the primary cooling circuit, whereby heat from the heated coolant from the coolant duct is transferred to air via the heat exchanger; a heat rejection system including an air blower directing air through the heat exchanger to a vent to ambient atmosphere; 17. The reactor of any one of clauses 1 to 16, further comprising: 18. 18. The nuclear reactor of any one of clauses 1-17, wherein the molten nuclear fuel comprises one or more fissile fuel salts selected from PuCl3, UCl4, UCl3F, UCl3, UCl2F2, ThCl4, and UClF3, and one or more non-fissile salts selected from NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, AmCl3, LaCl3, CeCl3, PrCl3, and NdCl3. 19. The total volume V of molten nuclear fuel in the reactor vessel tot The volume V of molten nuclear fuel in the core relative to cor 19. The reactor according to any one of clauses 1 to 18, wherein the ratio is 75 to 99%. 20. The total volume V of molten nuclear fuel in the reactor vessel tot The volume V of molten nuclear fuel in the core relative to cor 19. The reactor of any one of clauses 1 to 18, wherein the ratio is 85 to 95%. twenty one. The total volume V of molten nuclear fuel in the reactor vessel tot The volume V of molten nuclear fuel in the core relative to cor 19. The reactor of any one of clauses 1 to 18, wherein the ratio is 88 to 92%. twenty two. The total volume V of molten nuclear fuel in the reactor vessel tot The volume V of molten nuclear fuel in the core relative to cor 19. The reactor of any one of clauses 1 to 18, wherein the ratio is 89 to 91%. twenty three. The total volume V of molten nuclear fuel in the reactor vessel tot The volume V of molten nuclear fuel in the core relative to cor 19. The reactor of any one of clauses 1 to 18, wherein the ratio is less than 95%. twenty four. The total volume V of molten nuclear fuel in the reactor vessel tot The volume V of molten nuclear fuel in the core relative to cor 19. The reactor of any one of clauses 1 to 18, wherein the ratio is less than 91%. twenty five. The total volume V of molten nuclear fuel in the reactor vessel tot The volume V of molten nuclear fuel in the core relative to cor 19. The reactor of any one of clauses 1 to 18, wherein the ratio is about 90%. 26. The total volume V of molten nuclear fuel in the reactor vessel tot The volume V of molten nuclear fuel in the core relative to cor 19. The reactor of any one of clauses 1 to 18, wherein the ratio is less than 90%. 27. A nuclear reactor, a reactor vessel having a reactor core in the form of an open space at the bottom of the reactor vessel that can achieve criticality when containing molten nuclear fuel; a radial reflector on the exterior of the reactor vessel; a displacement component within the reactor vessel above the reactor core, the displacement component defining an upcomer duct in the form of an open channel extending therethrough and in fluid communication with the reactor core; a downcomer heat exchange duct between the displacement component and the reactor vessel, the downcomer heat exchange duct in fluid communication with the upcomer duct and the reactor core; the reactor vessel having an inner surface and an outer surface, the inner surface contacting the downcomer heat exchange duct such that the downcomer heat exchange duct is in thermal communication with the outer surface; a thermoelectric generator having a first surface and a second surface, the thermoelectric generator configured to generate electricity due to a temperature differential between the first surface and the second surface; Equipped with the first surface of the thermoelectric generator is in thermal communication with the exterior surface of the reactor vessel; The nuclear reactor, wherein the second surface of the thermoelectric generator is exposed to a coolant duct between the radial reflector and the reactor vessel. 28. 1. A molten fuel nuclear reactor, comprising: a core space that, if it contains molten nuclear fuel, can achieve criticality from the mass of the molten nuclear fuel; a reactor vessel including the core space and in thermal communication with the core; a radial reflector positioned around the reactor vessel and spaced from the reactor vessel; a coolant duct between the radial reflector and the reactor vessel, the coolant duct being in thermal communication with the reactor core; A nuclear reactor comprising:

[0171] It will be apparent that the systems and methods described herein are well adapted to achieve the stated objects and advantages, as well as those inherent therein. Those skilled in the art will recognize that the methods and systems herein may be implemented in numerous ways and are therefore not limited by the exemplary embodiments and examples described above. For example, while the reactor system described above is illustrated with the core, radial reflector, and reactor vessel generally cylindrical in cross section with a circular or annular design, the cross-sectional shape may be any shape, including circular, square, hexagonal, pentagonal, octagonal, or any polygonal shape. Additionally, the cross-sectional shape or diameter may vary at various locations in the reactor system. For example, the core may be frustoconical, such as that described in U.S. Patent Application Publication No. 2017 / 0216840, which is incorporated herein by reference. In this regard, any number of features of the various embodiments described herein may be combined into a single embodiment, and alternative embodiments having fewer than all of the features described herein or having more than all of the features described herein are contemplated.

[0172] While various embodiments have been described for purposes of this disclosure, various changes and modifications may be made which are fully within the scope contemplated by this disclosure. Numerous such changes may be made which would be readily suggested to one skilled in the art and which are encompassed by the spirit of this disclosure. [Brief explanation of the drawings]

[0173] [Figure 1] 1 shows a functional block diagram of a pool-type reactor designed for use with fuel salt. [Figure 2] 2 shows a diagram of one possible physical implementation of the nuclear reactor shown in FIG. 1. [Figure 3A] 2 illustrates an embodiment of the reactor system of FIG. 1. [Figure 3B] 2 illustrates an embodiment of the reactor system of FIG. 1. [Figure 3C] 2 illustrates an embodiment of the reactor system of FIG. 1. [Figure 3D] 2 illustrates an embodiment of the reactor system of FIG. 1. [Figure 4] FIG. 4 shows the fuel salt and flow paths within the reactor. [Figure 5A] 4 illustrates one embodiment of a reflector assembly that may be used in the nuclear reactor system of FIG. 3. [Figure 5B] 4 illustrates one embodiment of a reflector assembly that may be used in the nuclear reactor system of FIG. 3. [Figure 6A] 1 shows various embodiments of a control drum. [Figure 6B] 1 shows various embodiments of a control drum. [Figure 6C] 1 shows various embodiments of a control drum. [Figure 6D] 1 shows various embodiments of a control drum. [Figure 7] 1 illustrates one embodiment of a vessel head assembly. [Figure 8] The main components of the reactor are shown (excluding the shielding vessel). [Figure 9]1 illustrates an embodiment of a fuel pump assembly. [Figure 10] 1 shows a reactor vessel with dimples on the exterior surface rather than fins for improved heat transfer. [Figure 11A] 1A-1C are various views of an alternative embodiment of a low power nuclear reactor system. [Figure 11B] 1A-1C are various views of an alternative embodiment of a low power nuclear reactor system. [Figure 11C] 1A-1C are various views of an alternative embodiment of a low power nuclear reactor system. [Figure 11D] 1A-1C are various views of an alternative embodiment of a low power nuclear reactor system. [Figure 11E] 1A-1C are various views of an alternative embodiment of a low power nuclear reactor system. [Figure 11F] 1A-1C are various views of an alternative embodiment of a low power nuclear reactor system. [Figure 12A] 1 illustrates an embodiment of a nuclear reactor facility having alternative primary and secondary cooling systems instead of a heat rejection system. [Figure 12B] 1 illustrates an embodiment of a nuclear reactor facility having alternative primary and secondary cooling systems instead of a heat rejection system. [Figure 12C] 1 illustrates an embodiment of a nuclear reactor facility having alternative primary and secondary cooling systems instead of a heat rejection system. [Figure 13] 1 shows a functional block diagram of a pool-type nuclear reactor system designed for use with molten nuclear fuel in an extraterrestrial environment or other suitably cryogenic environment. [Figure 14A] 1 illustrates yet another embodiment of a pool-type reactor system in which all flow paths of the molten fuel, except for the flow of molten fuel through the core and pumping chamber, are in contact with and defined by the inner surface of the reactor vessel. [Figure 14B] 1 illustrates yet another embodiment of a pool-type reactor system in which all flow paths of the molten fuel, except for the flow of molten fuel through the core and pumping chamber, are in contact with and defined by the inner surface of the reactor vessel. [Figure 15] 10A-10C illustrate two alternative embodiments of upper molten fuel outlet channel and pump layouts that could be used in any of the reactor system embodiments described herein. [Figure 16] 10 illustrates yet another embodiment of an upper molten fuel exit channel and surface elements of a radial reflector that define the channel. [Figure 17] 1 illustrates an alternative embodiment of a nuclear reactor system. [Figure 18] 1 illustrates an alternative embodiment of a nuclear reactor in which the reflector is outside the reactor vessel. [Figure 19A] 1 illustrates the various options available for reactivity control using external radial reflectors. [Figure 19B] 1 illustrates the various options available for reactivity control using external radial reflectors. [Figure 19C] 1 illustrates the various options available for reactivity control using external radial reflectors. [Figure 19D] 1 illustrates the various options available for reactivity control using external radial reflectors. [Figure 19E] 1 illustrates the various options available for reactivity control using external radial reflectors. [Figure 20] 1 illustrates an embodiment of a low-power reactor design adapted to reduce reactivity changes associated with flowing delayed neutron precursors. [Figure 21A] 1 illustrates an embodiment of a nuclear reactor in which a transverse swirl flow is induced to cause fuel salt to flow along the inner surface of the lateral surface of the reactor vessel. [Figure 21B] 1 illustrates an embodiment of a nuclear reactor in which a transverse swirl flow is induced to cause fuel salt to flow along the inner surface of the lateral surface of the reactor vessel. [Figure 22A] 1 illustrates an alternative embodiment of a reactor design in which the fuel salt flows in a swirling manner around the interior surface of the reactor vessel. [Figure 22B] 1 illustrates an alternative embodiment of a reactor design in which the fuel salt flows in a swirling manner around the interior surface of the reactor vessel.

Claims

1. 1. A molten fuel nuclear reactor, comprising: a reactor vessel having an interior surface and an exterior surface; a displacement component within the reactor vessel that cooperates with the interior surface of the reactor vessel to define a reactor core capable of achieving criticality when containing molten nuclear fuel, a central upcomer duct, and a downcomer duct in fluid communication with the reactor core and the central upcomer duct; a radial reflector around the reactor vessel; a coolant duct between the reactor vessel and the radial reflector; Equipped with the inner surface of the reactor vessel is in thermal communication with the downcomer duct and the outer surface of the reactor vessel is in thermal communication with the coolant duct; This allows heat from the molten nuclear fuel in the downcomer duct to be transferred through the reactor vessel from the inner surface to the outer surface of the reactor vessel, and then to the coolant in the coolant duct.

2. 10. The nuclear reactor of claim 1 further comprising a lower axial reflector below said reactor vessel.

3. 10. The nuclear reactor of claim 1, wherein the displacement component incorporates neutron reflective material to bounce neutrons from the core back into the core.

4. 4. The nuclear reactor of claim 1, wherein the downcomer duct is fluidly connected to the reactor core to receive heated molten fuel from a first location in the reactor core and to discharge cooled molten fuel to a second location in the reactor core that is different from the first location.

5. 5. The nuclear reactor of claim 1, wherein the displacement component includes a central penetration therethrough that defines the central upcomer duct, and a draft tube.

6. The nuclear reactor of any one of claims 1 to 5, further comprising at least one vane attached to the displacement component that directs molten nuclear fuel obliquely along the inner surface of the reactor vessel.

7. The nuclear reactor of any one of claims 1 to 6, further comprising a vessel head assembly sealing the top of the reactor vessel.

8. The radial reflector is a drum well for receiving a control drum; a control drum including a body of neutron reflecting material at least partially covered with neutron absorbing material, the control drum being rotatably disposed within the drum; Furthermore, The nuclear reactor of any one of claims 1 to 7, wherein rotation of the control drum within the drumwell changes the reactivity of the reactor.

9. 8. The nuclear reactor of claim 7, further comprising an access port in said vessel head assembly in fluid communication with said reactor core.

10. the radial reflector is movable relative to the reactor vessel; A nuclear reactor according to any one of claims 1 to 9, whereby the reactivity of the reactor can be changed by moving the radial reflectors.

11. the radial reflector is a plurality of reflector elements; 11. The nuclear reactor of claim 10, wherein moving the radial reflector comprises moving a first reflector element of a plurality of the reflector elements.

12. 12. The nuclear reactor of claim 1, further comprising an impeller that draws molten nuclear fuel from the reactor core to the impeller and drives the molten nuclear fuel into the downcomer duct.

13. 13. The nuclear reactor of claim 12 further comprising a shield plug between the impeller and the core.

14. 14. The nuclear reactor of any one of claims 1 to 13, wherein the downcomer duct is fluidly connected to the reactor core to receive heated molten fuel from a first location in the central upcomer duct and to discharge cooled molten fuel to a second location in the reactor core.

15. A nuclear reactor according to any preceding claim, further comprising a control element in the coolant duct that can be moved to control the reactivity of the reactor.

16. the control element includes one or both of a neutron reflecting material and a neutron absorbing material; 16. The nuclear reactor of claim 15, wherein the control elements are selected from arc plates, flat plates, or rods.

17. The cooling system comprises: a primary cooling circuit including the coolant duct, a heat exchanger, and a coolant blower, the coolant blower configured to circulate the coolant through the primary cooling circuit, whereby heat from the heated coolant from the coolant duct is transferred to air via the heat exchanger; a heat rejection system including an air blower directing air through the heat exchanger to a vent to ambient atmosphere; The nuclear reactor of any one of claims 1 to 16, further comprising:

18. The molten nuclear fuel is PuCl ３ , UCl ４ , UCl ３ F, UCl ３ , UCl ２ F ２ , ThCl ４ , and UC1F ３ and one or more fissionable fuel salts selected from NaCl, MgCl ２ , CaCl ２ , BaCl ２ , KCl, SrCl ２ , VCl ３ , CrCl ３ , TiCl ４ , ZrCl ４ , ThCl ４ , AcCl ３ , NpCl ４ , AmCl ３ , LaCl ３ , CeCl ３ , PrCl ３ , and NdCl ３ 18. A nuclear reactor according to any preceding claim, comprising one or more non-fissile salts selected from:

19. The total volume V of molten nuclear fuel in the reactor vessel ｔｏｔ The volume V of the molten nuclear fuel in the core relative to ｃｏｒ A nuclear reactor according to any one of claims 1 to 18, wherein the ratio is between 75 and 99%.

20. The total volume V of molten nuclear fuel in the reactor vessel ｔｏｔ The volume V of the molten nuclear fuel in the core relative to ｃｏｒ A nuclear reactor according to any one of claims 1 to 18, wherein the ratio is between 85 and 95%.

21. The total volume V of molten nuclear fuel in the reactor vessel ｔｏｔ The volume V of the molten nuclear fuel in the core relative to ｃｏｒ A nuclear reactor according to any one of claims 1 to 18, wherein the ratio is between 88 and 92%.

22. The total volume V of molten nuclear fuel in the reactor vessel ｔｏｔ The volume V of the molten nuclear fuel in the core relative to ｃｏｒ The nuclear reactor according to any one of claims 1 to 18, wherein the ratio is between 89 and 91%.

23. The total volume V of molten nuclear fuel in the reactor vessel ｔｏｔ The volume V of the molten nuclear fuel in the core relative to ｃｏｒ A nuclear reactor according to any one of claims 1 to 18, wherein the ratio of

24. The total volume V of molten nuclear fuel in the reactor vessel ｔｏｔ The volume V of the molten nuclear fuel in the core relative to ｃｏｒ A nuclear reactor according to any one of claims 1 to 18, wherein the ratio of

25. The total volume V of molten nuclear fuel in the reactor vessel ｔｏｔ The volume V of the molten nuclear fuel in the core relative to ｃｏｒ A nuclear reactor according to any one of claims 1 to 18, wherein the ratio of is about 90%.

26. The total volume V of molten nuclear fuel in the reactor vessel ｔｏｔ The volume V of the molten nuclear fuel in the core relative to ｃｏｒ A nuclear reactor according to any one of claims 1 to 18, wherein the ratio of

27. A nuclear reactor, a reactor vessel having a reactor core in the form of an open space at the bottom of the reactor vessel that can achieve criticality when containing molten nuclear fuel; a radial reflector on the exterior of the reactor vessel; a displacement component within the reactor vessel above the reactor core, the displacement component defining an upcomer duct in the form of an open channel extending therethrough and in fluid communication with the reactor core; a downcomer heat exchange duct between the displacement component and the reactor vessel, the downcomer heat exchange duct in fluid communication with the upcomer duct and the reactor core; the reactor vessel having an inner surface and an outer surface, the inner surface contacting the downcomer heat exchange duct such that the downcomer heat exchange duct is in thermal communication with the outer surface; a thermoelectric generator having a first surface and a second surface, the thermoelectric generator configured to generate electricity due to a temperature differential between the first surface and the second surface; Equipped with the first surface of the thermoelectric generator is in thermal communication with the exterior surface of the reactor vessel; The nuclear reactor, wherein the second surface of the thermoelectric generator is exposed to a coolant duct between the radial reflector and the reactor vessel.

28. 1. A molten fuel nuclear reactor, comprising: a core space that, if it contains molten nuclear fuel, can achieve criticality from the mass of the molten nuclear fuel; a reactor vessel including the core space and in thermal communication with the core; a radial reflector positioned around the reactor vessel and spaced from the reactor vessel; a coolant duct between the radial reflector and the reactor vessel, the coolant duct being in thermal communication with the reactor core; A nuclear reactor comprising:

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