Borehole Nuclear Reactor

Abstract

The nuclear reactor system includes a borehole extending from the earth's surface through one or more subterranean formations, a reactor core positioned within the borehole, the reactor core comprising at least one nuclear fuel element, a primary coolant system configured to transfer a primary fluid coolant between the reactor core and a heat exchanger, and a secondary coolant system thermally coupled to the primary coolant system with the heat exchanger and configured to transfer a secondary fluid coolant between the heat exchanger and the earth's surface.

Description

[Technical Field]

[0001] The present disclosure relates to systems and methods for generating electrical power from a nuclear fission reactor formed by placing fissile material in a borehole. [Background technology]

[0002] Generally, highly radioactive materials, such as radioactive waste, chemical waste, biological waste, or other waste that is directly or indirectly harmful to living organisms, can be placed underground within (or outside of) a canister system. For example, radioactive waste (also referred to as nuclear waste) can be stored in deep, non-occupiable boreholes (suitable for storing such waste for years, decades, centuries, or longer) formed below the earth's surface into one or more underground formations. For example, non-occupiable boreholes (also referred to as boreholes or wellbores) are directional boreholes formed using conventional drilling equipment and can include vertical, curved, and horizontal sections (including multiple laterals in some cases). Alternatively, non-occupiable boreholes can be substantially vertical or inclined (e.g., formed offset from substantially vertical). The isolation provided by such boreholes suggests that they can be used not only for the disposal of hazardous materials but also as sites for the operation of nuclear power plants. Summary of the Invention [Means for solving the problem]

[0003] In an exemplary implementation, a nuclear reactor system includes a borehole extending from the earth's surface through one or more subterranean formations, a reactor core positioned within the borehole, the reactor core including at least one nuclear fuel element, a primary coolant system configured to transfer a primary fluid coolant between the reactor core and a heat exchanger, and a secondary coolant system thermally coupled to the primary coolant system with the heat exchanger and configured to transfer a secondary fluid coolant between the heat exchanger and the earth's surface.

[0004] In aspects combinable with example implementations, the at least one nuclear fuel element includes a fissile nuclear fuel element.

[0005] In another aspect that may be combined with any of the preceding aspects, the fissile fuel elements include at least one of enriched uranium, plutonium, uranium or plutonium oxide, or mixed oxide (MOX).

[0006] Another aspect that may be combined with any of the preceding aspects includes at least one canister that at least partially encloses at least one nuclear fuel element.

[0007] In another aspect that may be combined with any of the preceding aspects, the canister includes a first opening at an upper bore end of the canister and a second opening at a lower bore end of the canister.

[0008] In another aspect that may be combined with any of the preceding aspects, the primary coolant system is configured to transfer primary fluid coolant between the reactor core and the heat exchanger by at least one of natural circulation or pressure.

[0009] Another aspect that may be combined with any of the preceding aspects includes one or more pumps configured to provide force for transferring primary fluid coolant between the reactor core and the heat exchanger.

[0010] In another aspect that may be combined with any of the preceding aspects, the secondary coolant system is configured to transport the secondary fluid coolant between the heat exchanger and the surface by natural circulation.

[0011] Another aspect that may be combined with any of the preceding aspects includes one or more pumps configured to provide force for transporting the secondary fluid coolant between the heat exchanger and the surface.

[0012] In another aspect that may be combined with any of the preceding aspects, the primary fluid coolant includes water.

[0013] In another aspect that may be combined with any of the preceding aspects, the secondary fluid coolant includes water.

[0014] In another aspect that may be combined with any of the preceding aspects, the primary coolant system is fluidly isolated from the second coolant system.

[0015] In another aspect that may be combined with any of the preceding aspects, the heat exchanger is configured to transfer heat from a primary fluid coolant in the primary coolant system and heat a secondary fluid coolant in the second coolant system to a higher temperature liquid or vapor phase.

[0016] Another aspect that may be combined with any of the preceding aspects includes a power conversion system.

[0017] In another aspect that may be combined with any of the preceding aspects, the power conversion system is located at or near the earth's surface.

[0018] In another aspect that may be combined with any of the preceding aspects, the secondary fluid coolant includes a power conversion working fluid of a power conversion system.

[0019] In another aspect that may be combined with any of the preceding aspects, the at least one nuclear fuel element is positioned at a first depth in the borehole.

[0020] In another aspect that may be combined with any of the preceding aspects, the first depth is up to 0.5 km below the Earth's surface, or 1.0 to 1.5 km below the Earth's surface, or 1.5 km or more below the Earth's surface.

[0021] In another aspect that may be combined with any of the preceding aspects, the heat exchanger includes a barrier configured to fluidly isolate the primary coolant system from the second coolant system, the barrier being positioned at a second depth in the borehole, the second depth being less than the first depth.

[0022] Another aspect that may be combined with any of the preceding aspects includes one or more pipes extending from the surface to a third depth in the borehole, the third depth being greater than the second depth.

[0023] In another aspect that may be combined with any of the preceding aspects, the one or more pipes are configured to provide primary fluid coolant to a region of the borehole below the second depth.

[0024] In another aspect that may be combined with any of the preceding aspects, a primary coolant system includes a first pipe having a diameter smaller than a diameter of a borehole, the first pipe positioned within the borehole with an axis of the first pipe approximately parallel to an axis of the borehole, a reactor core positioned inside the first pipe, and the system includes an annulus defined between the first pipe and the borehole, the annulus including a flow circuit configured to transport primary fluid coolant downhole to the reactor core.

[0025] In another aspect that may be combined with any of the preceding aspects, the secondary coolant system includes a second pipe having a diameter smaller than a diameter of the borehole, the second pipe positioned within the borehole with an axis of the second pipe approximately parallel to an axis of the borehole, and the system includes an annulus defined between the second pipe and the borehole, the annulus including a flow circuit configured to transport the secondary fluid coolant downhole to the heat exchanger.

[0026] In another aspect that may be combined with any of the preceding aspects, the secondary coolant system includes a first pipe having a diameter smaller than the diameter of the borehole and configured to transport the secondary fluid coolant to the heat exchanger for heating, and a second pipe having a diameter smaller than the diameter of the borehole and configured to transport the heated secondary fluid coolant from the heat exchanger toward the surface.

[0027] In another aspect that may be combined with any of the preceding aspects, the borehole has a diameter of 36 inches or less.

[0028] In another aspect that may be combined with any of the preceding aspects, the borehole has a diameter of 4 inches or greater.

[0029] In another aspect that may be combined with any of the preceding aspects, the heat exchanger is positioned within a casing that is installed in the borehole.

[0030] In another aspect that may be combined with any of the preceding aspects, the primary coolant system includes at least one pipe installed outside the casing and the heat exchanger, and the secondary coolant system includes at least another pipe installed outside the casing.

[0031] In another aspect that may be combined with any of the preceding aspects, the casing is installed in a borehole with a cement formation and the heat exchanger is installed external to the cement formation.

[0032] In another aspect that may be combined with any of the preceding aspects, the reactor core is controllable between a low power output and a maximum power output.

[0033] In another aspect that may be combined with any of the preceding aspects, the reactor core is controllable using a fluid that includes a neutron absorber.

[0034] In another aspect that may be combined with any of the preceding aspects, the neutron absorber is controllably added to the primary fluid coolant.

[0035] In another aspect that may be combined with any of the preceding aspects, the reactor core is controllable with at least one control rod configured to move near or adjacent to at least one nuclear fuel element.

[0036] In another aspect that may be combined with any of the preceding aspects, the reactor core is a first reactor core, the nuclear fuel elements include a first nuclear fuel element, and the system includes a second reactor core positioned in the borehole, the second reactor core including at least one second nuclear fuel element.

[0037] In another aspect that may be combined with any of the preceding aspects, the first and second reactor cores are individually controllable between low power output and maximum power output.

[0038] In another aspect that may be combined with any of the preceding aspects, each of the first and second reactor cores is individually controllable using a fluid that includes a neutron absorber.

[0039] In another aspect that may be combined with any of the preceding aspects, the fluid is transported individually to each of the first and second nuclear reactor cores through one or more pipes extending from the Earth's surface to the first and second nuclear reactor cores.

[0040] In another aspect that may be combined with any of the preceding aspects, the neutron absorber is controllably added to the fluid.

[0041] In another aspect that may be combined with any of the preceding aspects, each of the first and second reactor cores is individually controllable using a respective first and second control rod that is movable within or near the respective first and second nuclear reactor cores.

[0042] Another aspect that may be combined with any of the preceding aspects includes one or more inverted cups positioned within the primary coolant system and within the primary fluid coolant flow path.

[0043] In another aspect that may be combined with any of the preceding aspects, the one or more inverted cups are coupled to the cladding of at least one nuclear fuel element.

[0044] Another aspect that may be combined with any of the preceding aspects includes a reflector configured to reflect neutrons generated in the reactor core without absorbing the neutrons.

[0045] In another aspect that may be combined with any of the preceding aspects, the reflector includes at least one of beryllium, carbon, a beryllium alloy, or a carbon alloy.

[0046] In another aspect that may be combined with any of the preceding aspects, certain subterranean formations of the one or more subterranean formations adjacent to the reactor core are configured to act as reflectors to reflect neutrons generated in the reactor core.

[0047] In another aspect that may be combined with any of the preceding aspects, the casing is of a material sufficient to act as a reflector to reflect neutrons generated within the reactor core.

[0048] In another aspect that may be combined with any of the preceding aspects, the material of the casing includes at least one of carbon steel, stainless steel, ceramic, a plastic material, or fiberglass.

[0049] In another exemplary implementation, a method of constructing a nuclear reactor system includes forming a borehole from the earth's surface through one or more subterranean formations, moving at least one nuclear fuel element into the borehole, positioning the at least one nuclear fuel element at a first depth in the borehole, moving a heat exchanger into the borehole, the heat exchanger including a fluid barrier, and positioning the heat exchanger within the borehole with the fluid barrier positioned at a second depth in the borehole, the second depth being less than the first depth.

[0050] In aspects combinable with example implementations, the at least one nuclear fuel element includes a fissile nuclear fuel element.

[0051] Another aspect that may be combined with any of the preceding aspects includes at least one canister that at least partially encloses at least one nuclear fuel element.

[0052] In another aspect that may be combined with any of the preceding aspects, the canister includes an opening at one or both ends, and the method includes positioning the canister in the borehole with the opening facing downhole.

[0053] Another aspect that may be combined with any of the preceding aspects includes mounting the power conversion system at or near the ground surface.

[0054] Another aspect that may be combined with any of the preceding aspects includes inserting one or more pipes extending from the surface of the earth to a third depth in the borehole, the third depth being greater than the second depth.

[0055] In another exemplary implementation, a method includes identifying a nuclear reactor system, the nuclear reactor system including: a borehole extending from the earth's surface through one or more subterranean formations; a reactor core positioned within the borehole, the reactor core including at least one nuclear fuel element; a primary coolant system; and a secondary coolant system thermally coupled to the primary coolant system with a heat exchanger and configured to transfer a secondary fluid coolant between the heat exchanger and the earth's surface. The method includes transferring a primary fluid coolant in the primary coolant system between the reactor core and the heat exchanger, transferring a secondary fluid coolant heated in the heat exchanger by the primary fluid coolant in the secondary coolant system between the heat exchanger and the earth's surface, and generating electrical power using the heated secondary fluid coolant.

[0056] Aspects combinable with example implementations include transporting a primary fluid coolant into a region of the borehole below a depth of a fluid barrier of the heat exchanger and transporting a secondary fluid coolant into a region of the borehole above a depth of the fluid barrier, the fluid barrier fluidically isolating the primary fluid coolant from the second fluid coolant.

[0057] Another aspect that may be combined with any of the preceding aspects includes transporting the primary fluid coolant into the borehole through one or more pipes extending from the earth's surface to a depth in the borehole that is deeper than the depth of the fluid barrier.

[0058] In another aspect that may be combined with any of the preceding aspects, the primary and secondary fluid coolants include water.

[0059] In another exemplary implementation, a nuclear reactor system includes: a first borehole extending from the earth's surface through one or more subsurface formations; a reactor core positioned within the first borehole, the reactor core comprising at least one nuclear fuel element; a second borehole extending from the earth's surface through one or more subsurface formations and separated from the first borehole by a portion of the rock formation; a heat exchanger positioned within the second borehole in thermal communication with the reactor core through the portion of the rock formation; and a coolant system thermally coupled to the heat exchanger and configured to transport a fluid coolant between the heat exchanger and the earth's surface.

[0060] In aspects combinable with example implementations, the at least one nuclear fuel element includes a fissile nuclear fuel element.

[0061] Another aspect that may be combined with any of the preceding aspects includes at least one canister that at least partially encloses at least one nuclear fuel element.

[0062] In another aspect that may be combined with any of the preceding aspects, the canister includes an opening at an axial end of the canister.

[0063] In another aspect that may be combined with any of the preceding aspects, the coolant system is configured to transport the fluid coolant between the heat exchanger and the surface by natural circulation.

[0064] Another aspect that may be combined with any of the preceding aspects includes one or more pumps configured to provide force for transporting the fluid coolant between the heat exchanger and the surface.

[0065] In another aspect that may be combined with any of the preceding aspects, the fluid coolant includes water.

[0066] In another aspect that may be combined with any of the preceding aspects, the heat exchanger is configured to transfer heat from a primary coolant system to a fluid coolant in a secondary coolant system.

[0067] Another aspect that may be combined with any of the preceding aspects includes a power conversion system.

[0068] In another aspect that may be combined with any of the preceding aspects, the power conversion system is located at the earth's surface.

[0069] In another aspect that may be combined with any of the preceding aspects, the fluid coolant includes a power conversion working fluid of a power conversion system.

[0070] In another aspect that may be combined with any of the preceding aspects, the at least one nuclear fuel element is positioned at a first depth in a first borehole.

[0071] In another aspect that may be combined with any of the preceding aspects, the first depth is less than 0.5 km from the Earth's surface, 1.0 to 1.5 km from the Earth's surface, or 1.5 km or more from the Earth's surface.

[0072] In another aspect that may be combined with any of the preceding aspects, the first borehole has a diameter of 36 inches or less.

[0073] In another aspect that may be combined with any of the preceding aspects, the first borehole has a diameter of 4 inches or greater.

[0074] In another aspect that may be combined with any of the preceding aspects, the reactor core is a first reactor core, the nuclear fuel elements include a first nuclear fuel element, and the system includes a second reactor core positioned in the first borehole, the second reactor core including at least one second nuclear fuel element.

[0075] In another aspect that may be combined with any of the preceding aspects, the first and second reactor cores are individually controllable between low power output and maximum power output.

[0076] In another aspect that may be combined with any of the preceding aspects, each of the first and second reactor cores is individually controllable using a fluid that includes a neutron absorber.

[0077] In another aspect that may be combined with any of the preceding aspects, the fluid is transported individually to each of the first and second nuclear reactor cores through one or more sets of pipes extending from the Earth's surface to the first and second nuclear reactor cores.

[0078] In another aspect that may be combined with any of the preceding aspects, the neutron absorber is controllably added to the fluid.

[0079] In another aspect that may be combined with any of the preceding aspects, each of the first and second reactor cores is individually controllable using a control rod assembly that is movable within or near the respective first and second nuclear reactor cores.

[0080] The details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, drawings, and claims. [Brief explanation of the drawings]

[0081] [Figure 1] 1-3 are schematic diagrams of exemplary implementations of a downhole (or borehole) nuclear reactor system according to the present disclosure. [Figure 2] 1-3 are schematic diagrams of exemplary implementations of a downhole (or borehole) nuclear reactor system according to the present disclosure. [Figure 3] 1-3 are schematic diagrams of exemplary implementations of a downhole (or borehole) nuclear reactor system according to the present disclosure.

[0082] [Figure 4] FIG. 4 is a schematic diagram of another exemplary implementation of a downhole (or borehole) nuclear reactor system according to the present disclosure.

[0083] [Figure 5A] 5A and 5B are schematic diagrams of another exemplary implementation of a downhole (or borehole) nuclear reactor system according to the present disclosure. [Figure 5B] 5A and 5B are schematic diagrams of another exemplary implementation of a downhole (or borehole) nuclear reactor system according to the present disclosure.

[0084] [Figure 6] FIG. 6 is a schematic diagram of a nuclear fuel element for a borehole nuclear reactor system according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0085] A nuclear fission reactor is described. Typically, nuclear fuel stored in a canister in a vertical, inclined, or directional borehole generates an underground nuclear fission reactor, and a power system may be fluidly (e.g., gas, liquid, mixed-phase fluid) coupled to the underground nuclear fission reactor to remove thermal energy from the borehole to or near the surface to produce useful power. The reactor fits conveniently into a borehole (or borehole) buried deep underground and unoccupied by humans. An exemplary nuclear fission reactor according to the present disclosure can take advantage of the fact that the hydrostatic pressure of saltwater in rock is typically 1 atmosphere per 10 meters. Thus, due to the depth to which the nuclear fuel is stored in the borehole, a nuclear reactor in a borehole at 1 kilometer (km) would have a pressure of 100 atmospheres, and at 1.5 km, the pressure would be 150 atmospheres, approximately equal to that of a pressurized water reactor (PWR).

[0086] At a pressure of 150 atmospheres, water boils at approximately 325°C. Light water (H2O), heavy water (D2O), other materials containing hydrogen, or combinations can be used for the moderator (e.g., liquid and gas, and reactor coolant). A moderator in a nuclear reactor is a substance that slows down neutrons. In the example of a light water moderator, collisions between fast neutrons and hydrogen atoms in water slow the fast neutrons to a lower velocity. At lower velocities, neutrons are more likely to propagate a nuclear fission chain reaction.

[0087] Control of the rate of the chain reaction can be accomplished physically (such as by adding or removing boron from the reactor core region) or automatically (such as by using hydrogen-bearing materials in the moderator to reduce moderation as the temperature increases). Steam accumulation from boiling can also be used to automatically reduce moderator density and thus slow or stop the fission chain reaction if the reactor becomes too hot. Thus, the reactor can self-regulate without external intervention (e.g., control rods) to keep the reactor temperature below the boiling point. A heat exchanger within the borehole serves to heat a separate reservoir of water that circulates to the surface, thus bringing heat uncontaminated by radioactive isotopes to the surface. The sides of the borehole (or, in some aspects, the borehole casing) can function to bring low-pressure water to the heat exchanger, where it boils, to deliver power to the surface in the form of steam. Because the reactor is less than a few million to several billion tons of rock, the reactor can be installed in a borehole without a separately installed (or conventional) containment structure. Furthermore, because the reactor is in a geological formation where the ambient water or brine pressure is equivalent to that in a pressurized water reactor (PWR) or boiling water reactor (BWR), no thick-walled pressure vessel is required.

[0088] For example, this disclosure describes one or more fuel assemblies similar in configuration and size to those of a PWR or BWR fuel assembly installed near the bottom of a vertical borehole (e.g., a 1.5 km deep borehole). The assembly is surrounded by "water" (e.g., water or water with an additive such as a soluble neutron poison) to cause the fissile fuel (e.g., uranium with more than 0.7% uranium-235 (U-235), or plutonium-239 (Pu-239), or both uranium and plutonium) to undergo a nuclear fission chain reaction. In other embodiments, other materials, such as graphite or uranium zirconium hydride, can be used as moderators. In some embodiments, a combination of moderators, such as graphite and water, can be used.

[0089] In some aspects, the fuel assembly is placed inside a canister with an open bottom and top. In an exemplary embodiment, if the water in the fuel assembly boils (i.e., if the fuel rises above 325°C for a reactor at a depth of 1.5 km), gas will accumulate in the void collector and the fission chain reaction will decrease until the temperature drops. Thus, the reactor temperature can self-regulate to 325°C or about 325°C. The temperature can be regulated to lower temperatures at shallower depths and higher temperatures at deeper depths.

[0090] A heat exchanger that removes heat from the reactor is located above the reactor within the borehole. In this embodiment, the heat exchanger also isolates the water (e.g., as the primary coolant) from the water flow (e.g., as the secondary coolant) that is brought to the surface to transport the heat upward. Heat can be transported from the fuel to the heat exchanger using a pump or using natural circulation from heat-driven convection. The light water that brings the heat to the surface can do so using natural circulation or by being actively pumped. The secondary water can be kept pressurized so that the water reaches the surface as hot water, or the water can be allowed to boil in the heat exchanger and reach the surface as steam.

[0091] Fresh water (non-saline) can be used as the fluid that transports heat to or near the surface, although other fluids, such as saltwater, hydrocarbons, or gases such as helium or nitrogen, can also be used. If a gas is used, the gas-carrying pipe can be surrounded by a fluid-filled pipe (e.g., containing fresh water), which provides emergency core cooling for the reactor. Alternatively, if water is used as the secondary fluid, this water can also serve as the source for the emergency core cooling system (ECCS).

[0092] The upward-flowing hot fluid can be partially isolated from the surrounding rock and the downward-flowing cold water by an insulator. The rock formation surrounding the hot, upward-flowing pipe can also act as an insulator. As the temperature of this rock increases, the heat flow from the upward-flowing hot pipe into the surrounding rock decreases, and therefore the rock acts as an insulator when heated.

[0093] Schematic diagrams of an exemplary reactor system 100 are shown in FIGS. 1-3. FIG. 1 is an overview of a borehole nuclear reactor system for a vertical borehole embodiment. In this embodiment, only one borehole is depicted, containing only one fuel assembly. In other embodiments, multiple fuel assemblies can be installed within the borehole, either vertically positioned or distributed through branches within the borehole. Furthermore, multiple boreholes can be used to increase the total power delivered to the surface or near the surface. Additionally, once the fuel in the borehole reactor is depleted, it can be removed, or in a preferred implementation, it can be left in place, covered with sand or other support, and a second nuclear reactor installed above it. FIG. 2 shows the reactor section of a borehole nuclear reactor system. FIG. 3 shows an exemplary heat exchange section of a borehole nuclear reactor system. Dashed line 110 represents the same level, i.e., depth, in FIGS. 1-3. All materials within the reactor can be designed for low corrosion. One way to do this is to include corrosion-inhibiting materials in the fluid. Another is to make pipes and other parts from low-corrosion or non-corrosive materials, including plastics. A third is to coat metal surfaces with corrosion-inhibiting materials, including quartz and diamond. Other methods may be apparent to engineers practicing in the corrosion prevention field.

[0094] Eventually, once the reactor fuel is depleted, it can be removed from the borehole or left at depth, and the borehole can be sealed (e.g., with a borehole plug or otherwise). The casing against the surface can be left in place or cut and extracted. The fuel may not be removed from the borehole 115. In some cases, the fuel can be disposed of in the borehole 115 by sealing it above the reactor once the fuel is depleted. Such borehole disposal provides a very high level of safety. The reactor system 100 may be less expensive to build and operate compared to a standard BWR or PWR reactor. Thus, in some examples, the nuclear fission chain reaction can be designed to burn more slowly. The reactor system 100 can therefore operate for long periods, e.g., 30 to 60 years or more, with little or no need for nuclear fuel replacement. Additionally, more highly enriched fuel (e.g., about 4%) than is typically used in commercial nuclear reactors can be used to extend the operational life of the borehole reactor, i.e., to extend the time it can deliver useful power without requiring the use of additional fuel. One example of such fuel is highly assayed low-enriched uranium (HALEU), typically enriched to approximately 20% U-235. Another exemplary embodiment is that when the fuel is becoming sufficiently depleted, i.e., when the amount of fissile material such as U-235 has been significantly reduced, instead of removing the fuel assembly, it can be left in place and a new fuel assembly can be placed in place closer to the access hole. Doing this allows the highly radioactive depleted fuel to remain safely at depth, which will eventually be disposed of by the action of sealing the borehole above the new fuel assembly (i.e., at a location within the borehole closer to the surface than the new fuel assembly). The reactor system 100 can be used to provide heat in the form of hot water, steam 106, and / or by using an electrical power conversion system (eg, generator 102).The generator 102 can be, for example, a turbine or a thermocouple stack or other heat-driven generator. The heat can also be used for commercial purposes (such as heating buildings or heat-intensive industries) without conversion to electricity.

[0095] FIG. 1 is an overview of a borehole nuclear reactor system 100. FIG. 1 includes reference arrows 105 that define the uphole and downhole directions. While this illustration shows a vertical borehole, the borehole can be directionally drilled in an inclined, substantially horizontal, or other configuration. A generator 102 is located at the Earth's surface 104. A vertical borehole 115 descends to a depth of approximately one-half kilometer (km) or greater (e.g., 1.0 km or greater, 1.5 km or greater, 2.0 km or greater, or other predetermined depth). The borehole 115 includes a casing 216, which can be fabricated to adhere to the borehole wall, for example, by being cemented to the borehole wall with cement 214.

[0096] In some aspects, a gap can be left between the casing and the borehole wall so that the water that typically fills this gap can boil and turn to steam, which can help provide insulation to reduce subsequent heat flow into the rock. The borehole 115 can be narrow, for example, 4 to 36 inches in diameter.

[0097] At the bottom of the borehole 115, nuclear fuel is held within a reactor 130, which includes one or more fuel assemblies (i.e., rods) 218. Details of the reactor 130 are described with reference to FIG. 2. In some examples, the reactor 130 includes individual fuel elements, i.e., fuel rods 218. The reactor 130 is surrounded by a moderator. In some examples, the moderator is water. The reactor section is isolated from the surface 104 by a heat exchanger 120 and a pressure-equalizing barrier 212 (shown in FIG. 3). The water serves as both a coolant and a moderator. The water flows by natural circulation through hot water pipes 205 up the reactor 130 to the heat exchanger 120.

[0098] An insulated pipe 208 is positioned within the borehole 115. During operation, water 108 flows downwardly on the outside of the pipe 208. The secondary water 108 is heated by the primary water as the secondary water 108 passes through the heat exchanger 120. The heated water, i.e., steam 106, then flows upwardly inside the pipe 208. The pipe 208 thus transports the heat generated by the reactor 130 to the surface 104.

[0099] Many methods can be used to cause the fuel to go critical, i.e., undergo a sustained chain reaction. One way to accomplish this can be by using a "neutron reflector," which is a material with low neutron absorption properties that can scatter neutrons leaving the reactor region back into the reactor region. In some cases, the neutron reflector is made of graphite (carbon) or beryllium. These materials can serve to help slow down the neutrons.

[0100] 3, the heat exchanger 120 is depicted in a simplified schematic diagram. Any suitable heat exchanger system can be used to remove heat from the reactor 130. The primary water is fluidly isolated from the secondary water by a barrier, for example, a pressure equalization barrier 212. The pressure equalizer can be omitted if it is desired to maintain the secondary water at a lower pressure to promote boiling. The barrier is positioned above the region of the system 100 through which the heavy water flows and where the reactor 130 is located.

[0101] The system 100 includes a primary water supply and drain pipes 201 and 202, respectively. The primary fluid supply and drain pipes 201, 202 can be used to exchange the primary fluid (or portions thereof) and also provide pressure control for the reactor 130. In some implementations, two small pipes, such as the water supply and drain pipes 201, 202, penetrate the pressure-equalizing barrier 212. The pipes 201, 202 can be used to partially exchange or remove the primary water near the reactor. For example, primary water, which may initially contain dissolved boron salts, can be exchanged with fresh water, salt water, or water with additional boron salts (materials that quench a nuclear chain reaction).

[0102] The use of boiling (gas generation) to reduce reactivity (and thus control chain reactions) is called a "negative void coefficient." Other methods for controlling reactivity are also well known in the reactor design community. These include the use of neutron reflectors and absorbers with temperature-dependent efficiencies. In addition to these passive (non-moving) methods, control rods can be inserted into or near the reactor. These can be controlled from the surface, or they can automatically move into the reactor region as the temperature increases. Such movement can be controlled by using a material with a temperature expansion coefficient large enough to allow a mechanism to move the control rod. Such mechanisms are found, for example, in bimetallic thermometers, where a relatively small expansion can be used to drive the readout needle over a large distance.

[0103] Pipes 201, 202 can provide control of the fluid content within the reactor (e.g., pumping boron concentrate), and they can also provide pressure stability. In another configuration, there can be a second pipe leading from the reactor region to the surface to provide pressure stability. For example, when gas bubbles form, the space occupied by the bubbles pushes water up the pipe instead of increasing the reactor pressure. Such a pipe does not require water circulation; the pipe allows sufficient upward flow to prevent pressure buildup within the reactor region. The fission chain reaction can be turned off, if desired, by, for example, pumping pure primary water out and replacing it with water containing a neutron poison (i.e., a material that absorbs neutrons) or a high-pressure gas such as argon or nitrogen.

[0104] A pressure-equalizing barrier 212 between the primary and secondary waters provides an alternative or redundant method (to the piping described in the previous paragraph) for controlling pressure within the reactor. A barrier 212 can also be installed between the primary water and brine from the local rock formation to maintain the pressure in the primary system equal or nearly equal to that of the host rock formation. The barrier 212 can be formed from a flexible material that bends, for example, in a bellows configuration, when the pressures are unequal. In another case, the barrier 212 can be a pipe with a noncircular cross-section that contracts (or becomes more circular) in response to the pressure difference inside and outside the pipe. In some examples, the pressure-equalizing barrier 212 can be composed of a permeable material, such as sandstone or sand, that allows flow whenever pressures are unequal across the barrier. A permeable plug can allow some mixing between the moderator material (such as heavy water) and the brine or heat extraction fluid (secondary water) (although the amount of mixing would be small under normal operation). The mixed water can be exchanged within the reactor by using the primary drain and feed pipes 201, 202. The primary drain and feed pipes 201, 202 can be filled with water near the bottom and contain a movable plug in the middle, with other water filling above the movable plug.

[0105] The hydrostatic weight of the water in the pipes 201, 202 can provide a pressure of approximately 100 atmospheres. The pressure can be controlled in several ways. The tops of the water pipes 201, 202 can have gas at the top, which allows the water to flow up the pipes without significantly increasing the pressure in the reactor 130 (if gas bubbles are produced in the reactor). The water pressure is equal to the hydrostatic pressure of the water at the bottom of the pipes 201, 202, which can be 50 atmospheres to 150 atmospheres or more (if they lead to the surface 104).

[0106] The water pipes 201, 202 can be used for emergency moderator replacement when there is a desire to turn off the nuclear reactor 130, or for replacement with neutron-poisoned water. Thus, the water pipes 201, 202 can function as a reactivity control mechanism for the reactor. In some examples, control rods can be included within the reactor 130. Any suitable method of controlling the reactivity of the reactor 130 can be implemented.

[0107] In FIG. 2 , fuel, e.g., fuel rods 218, are held inside canister 220. The bottom and top of canister 220 remain open to provide a passage for primary coolant loop 213. If the fuel becomes overheated, i.e., if the temperature rises sufficiently to boil water in the primary loop (which occurs at PWR pressures at temperatures between 250 and 325°C), gas bubbles, i.e., steam 206, will reduce the density of the moderator near fuel rods 218 and reduce the rate of the nuclear chain reaction. Thus, the formation of gas bubbles provides a “negative feedback” for controlling and stopping excess power production. The bubble trapping mechanism thus provides a “negative feedback” for controlling and stopping excess power production.

[0108] To enhance the negative temperature coefficient from the voids, inverted cups or tubes can be installed within the reactor to capture the gas bubbles. These inverted cups can also be elongated in shape, like a traditional "test tube," to provide more gas accumulation capacity. The cups can be made of a thermally conductive metal, such as Zircaloy, to ensure they can transfer heat from the fuel rods to the liquid coolant portion.

[0109] The flow of the secondary coolant fluid can be provided by a pump or by natural circulation. If the pump fails or is turned off, the circulation of the secondary water will be reduced and more heat will be transferred to the secondary water and then to the surrounding rock 222. In some examples, the secondary water can be the power conversion working fluid of a power conversion system. For example, when the secondary water is heated to steam 106 by the heat exchanger 120, it can be used as the power conversion working fluid for a steam turbine generator, e.g., generator 102.

[0110] The design in FIG. 3 shows the pipe 208 bringing the hot water 106 (or steam 106) to the surface 104 as insulated with insulation 204. Insulation can be added to various parts of the system 100 to improve efficiency. In another embodiment, the hot HO pipe 208 can be used for the cold water supply, and the cold water pipe 201 is for the hot water return. In this alternative embodiment, as well as in the embodiment shown in the figure, the insulation between the outer pipe 201 and the casing and rock can be reduced (or omitted). Doing so can cause the rock surrounding the casing to warm or cool, which takes longer to occur. Similarly, if insulation is used between the pipe and the casing, the rock temperature can still change over time because the insulator is not perfect. In these embodiments, the rock temperature eventually approaches that of the pipe, and when this happens, the rock acts as an efficient insulator itself, countering future loss of energy through the rock.

[0111] Various embodiments illustrated in Figures 1-3 can be implemented in accordance with the present disclosure. In some examples, carbon (e.g., low-boron graphite) can be added to the design to provide a moderator component that will not boil and will also act as a neutron reflector to increase reactivity. Hydrogen-bearing compounds such as uranium beryllium hydride can be added because they provide a very strong negative temperature coefficient. Fuel enrichment can be as low as natural uranium (0.7%), approximately at levels typically used in PWRs (4.5%), as high as enrichment levels used in many "fourth generation" nuclear reactors (19.9%), or higher.

[0112] Any suitable type of pressure equalizer can be used. Any suitable type of heat exchanger can be used. In some examples, the concrete, i.e., cement 214, between the casing 216 and the borehole wall can be omitted. A casing centralizer can be used to provide pipe stability.

[0113] The secondary water brought to the surface 104 can be kept under pressure so that it reaches the surface 104 as a liquid, or the secondary water can be allowed to boil in a heat exchanger so that the secondary water arrives at the surface 104 as steam 106.

[0114] Other forms of fuel and moderator can be used, including TRISO fuel and pebble fuel (typically including TRISO in larger pellets) and molten salt or molten metal-containing fissile material. The reactor system 100 can use heat pipes in the vertical section to bring heat to the surface 104. This can also allow water to boil at depth. In implementations that include heat pipes, pressure for the secondary water can be supplied by secondary water pipes.

[0115] Other aspects of the exemplary embodiments according to the present disclosure can include one or more other features. For example, a turbine for generating electricity can be installed inside the borehole 115 to extract energy while keeping the majority of the borehole pressurized. The borehole 115 can be inclined rather than vertical, or otherwise directionally drilled. Many additional variations exist that will be apparent to those skilled in the art of hydrology, heat transfer, and nuclear power.

[0116] The reactor system 100 can be designed to deliver low levels of power (e.g., heat, electricity) for short periods of time, or for long periods (30-60 years or more) by using control rods and neutron poisons to reduce the fission reactor's rate while keeping the reactor critical. The delivered power can also be extended by using more highly enriched fuel, by using multiple fuel assemblies, or by inserting a new reactor into the same borehole as the fissile fuel in the previous reactor is depleted. At the end of that period, the depleted fuel can be recovered. However, this design offers the option of leaving the depleted fuel at depth and sealing the reactor. A borehole 115 in the appropriate geology (e.g., low-permeability rock, no deep aquifers, little vertical flow of saltwater within the rock) can provide a high level of protection to humans at the surface 104 for periods of one million years or more.

[0117] The hot water (or steam 106) that reaches the surface 104 can be used directly for heat or to produce electricity using a steam turbine, thermopile, or other power generator (e.g., generator 102). The steam 106 can also be used to start engines for other uses, such as manufacturing. With its local or modular capabilities, the heat of a borehole nuclear reactor can be effectively utilized, for example, to heat a large building, campus, or to provide energy for a factory. In this aspect, the system 100 has an advantage over larger plants that must be located remotely from where the power is used.

[0118] System 100 can also be used for military operations, where an appropriately sized borehole is drilled in a few days and a prefabricated reactor is lowered into the borehole. For military use, the reactor's depth provides a very high level of military "hardness," i.e., impregnable against attack by missiles, drones, bombs, or terrorist attacks. If it becomes necessary to abandon the reactor, the vertical access hole can be easily destroyed by inserting explosives. Furthermore, at the end of the life of system 100 (e.g., when the fissile material is no longer capable of sustaining a nuclear reaction as desired), the reactor portion and heat exchanger of system 100 can be removed or allowed to remain within borehole 115, which can be appropriately sealed to store fissile material (permanently or temporarily) in borehole 115.

[0119] The pressure equalization barrier (212 in FIG. 3) or any direct interface between the primary and secondary circulation systems can also serve as a safety mechanism. It can be constructed in such a way that it will open if the pressure difference becomes greater than a critical and undesirable level (e.g., the reactor fluid boils too forcefully to be equalized by the plug or by pipes 201 and 202). If that occurs, the hot reactor fluid will rise over this plug, and cold water from above the plug will flow downward. This process provides an "emergency core cooling system" (ECCS) as required for PWR reactors under current standards. This ECC is provided passively; that is, unlike the ECCs in many PWRs, it does not require external power to be supplied to surround the reactor with cold water and is gravity-driven.

[0120] Many variants exist regarding the idea of ​​placing a nuclear reactor within a borehole. For example, two holes can be drilled that converge directly above the heat exchanger. In this embodiment, hot water can come to the surface from the second hole, and the first hole can be used to supply cold water, provide a path for the exchange water supply, and provide a passage for the control of control rods (which can be operated electrically using an underground motor or by a cable rising to or near the surface). The separation of the two holes allows the rock to act as an insulator between the cold and hot water. Joining two holes together at depth is a technique that is within the state of the art of current drilling practice. In another embodiment, three holes are drilled: a relatively large diameter hole for placing the fuel assembly and heat exchanger, a second hole to provide a path for the hot water to come to the surface, and a third hole to provide pressure for the passages for the injection and removal of fluids into and from the reactor region. These fluids may include water or fluids that transport neutron-absorbing poisons. The auxiliary holes (all but those allowing for the installation of fuel and heat exchangers) do not need to be wide in diameter, but can be between 2 inches and 10 inches in diameter, allowing them to be constructed at relatively low cost. Additional boreholes intersecting the main large diameter borehole are also possible and can serve to improve the efficiency or safety of the borehole reactor.

[0121] 4 illustrates another exemplary implementation of a borehole (or borehole) nuclear reactor system 400 according to the present disclosure. The borehole nuclear reactor system 400 is a variation of system 100 in which the same or substantially similar components as those shown in FIG. 1 are included in FIG. 4, but in which heat-producing nuclear fuel is disposed of or otherwise located in a disposal borehole 401, which is separated from a heat exchanger borehole 415 by a rock formation 406. Thus, the heat exchanger 120 (as previously described) is located in a separate borehole from the reactor 130, which includes, for example, one or more disposal canisters 402 (which together form the reactor 130), which contain nuclear waste 404. The nuclear waste 404 in the canister 402 is positioned in a disposal borehole 401, which is formed from the Earth's surface 104 into one or more subsurface formations, including rock formations 406. The disposal borehole 401 is shown as a vertical borehole (e.g., a well bore or borehole), but can be directional, inclined, horizontal, or a combination thereof. Details of the reactor 130 are described with reference to FIG. 2.

[0122] FIG. 4 includes reference arrows 105 that define the uphole and downhole directions. While this illustration shows a vertical heat exchanger (HX) borehole 415, the borehole 415 can be directionally drilled in an inclined configuration, a substantially horizontal configuration, or other configurations. A generator 102 is located at the Earth's surface 104. The HX borehole 415 can descend to a depth of approximately one-half kilometer (km) or greater (e.g., 1.0 km or greater, 1.5 km or greater, 2.0 km or greater, or other predetermined depth). The HX borehole 415 can include a casing that can be fabricated to adhere to the borehole wall, for example, by being cemented to the borehole wall with cement. The HX borehole 415 can be narrow (e.g., 4 to 36 inches in diameter).

[0123] At the bottom of the HX borehole 415, nuclear fuel is held within the reactor 130, which contains one or more fuel assemblies. A pipe 208 (which may or may not be insulated) is positioned within the HX borehole 415. During operation, secondary water 108 flows downwardly on the outside of the pipe 208. The water 108 is heated (by heat transferred from the nuclear waste 404 in the reactor 130 through the rock formation 406 to the primary water) as the secondary water 108 passes through the heat exchanger 120. The heated secondary water, i.e., steam 106, then flows upward inside the pipe 208. The pipe 208 thus transports heat generated by the reactor 130 (in the separate disposal borehole 401) to the surface 104.

[0124] Other variations on the disclosed implementations are also contemplated by the present disclosure. For example, a heat exchanger of a borehole nuclear reactor system, such as heat exchanger 120, can be located outside the casing in the borehole (such as between the casing and the rock formation). In some aspects, in such implementations, cold water can be transported through piping that is outside the casing (e.g., within the rock formation, within a cement layer that attaches the casing to the rock formation, or between the rock formation and the casing). The cold water can be heated to hot water or steam by an external heat exchanger and brought to or near the surface 104, where the heat can be used in the generator 102.

[0125] As another variation, a reactor for a borehole nuclear reactor system (such as reactor 130) can be divided into or include multiple independently controllable reactors. For example, multiple reactors 130 within a reactor region can be operated simultaneously to provide maximum power, or individual reactors 130 can be periodically shut down or reduced in thermal power output when little or no power is needed or desired. Control of each reactor 130 (of the multiple reactors 130) can be accomplished, for example, by placing strong neutron absorbers near or within one or more of the multiple reactors 130. This can be done, for example, by having tubing or pipes extending from one or more reactors 130 to the surface 104. If the tubes are filled with water, a particular reactor 130 will produce maximum power. If filled with a fluid containing a strong neutron absorber, such as boron or cadmium or their salts, the reactivity of a particular reactor 130 can be reduced. A set of pipes (supply, return fluid) may be installed and provided for each reactor 130 of the plurality of reactors 130 .

[0126] In some aspects, control of one or more of the plurality of reactors 130 can be accomplished using a control rod (e.g., a solid control rod comprising boron or cadmium or salts thereof) that is controllably movable in and out of the individual reactors 130. The controllable movement of the control rod can be accomplished mechanically or hydraulically, such as by using a fluid in a controlled pipe at or near the surface 104.

[0127] 5A and 5B are schematic diagrams of another exemplary implementation of a borehole (or borehole) nuclear reactor system 500 according to the present disclosure. System 500 is similar in some or most aspects to borehole nuclear reactor system 100 and is shown with additional detail. FIG. 5A illustrates a heat exchanger portion 501 of system 500, while FIG. 5B illustrates a nuclear reactor portion 503 (which is the downhole of heat exchanger portion 501) of system 500. Heat exchanger portion 501 is positioned or installed within a borehole 505 formed from the earth's surface into a subsurface formation 502. In some aspects, subsurface formation 502 can serve to slow down and reflect neutrons generated in the fission reaction back into nuclear reactor zone 503 during the nuclear fission reaction.

[0128] In this example, borehole 505 includes casing 504 (including optional perforations 508), which is cemented to borehole 505 using cement 506. In an exemplary implementation, when heat exchanger section 501 is installed in borehole 505 that is approximately 18 inches in diameter, section 501 is approximately 40 feet in length.

[0129] As shown in FIG. 5A, the heat exchanger portion 501 includes a primary fluid flow path 524, which includes an ascending section 526 (within the ascending sections 595) and a descending section 528 (between the ascending sections 595). The riser section 526 turns into a faller section 528 at a closed end of the primary fluid flow path 524 at or near the upper bore end of the heat exchanger section 501. In some aspects, a thermal insulation layer 534 is installed between the riser section 526 and the faller section 528. In combination, the riser section 526 and the faller section 528 (i.e., the primary fluid flow path 524) form at least a portion of a closed circuit for circulating the primary fluid coolant therein (e.g., naturally, by convection, or both) between the nuclear reactor section 503 and the heat exchanger section 501. The primary fluid coolant can be water (e.g., heavy water, light water, or other water-based liquid with additives).

[0130] 5A , heat exchanger portion 501 includes a secondary fluid flow path 523 that includes a downflow section 512 and an upflow section 518. Generally, downflow section 512 extends downhole within borehole 505 from or near the generator and condenser at the surface until they turn at a closed end into upflow section 518, which also extends to the generator at the surface. In some aspects, insulation layer 522 is installed between downflow section 512 and upflow section 518. In combination, downward flow section 512 and upward flow section 518 (i.e., secondary fluid flow path 523) form at least a portion of a closed circuit for circulating the secondary fluid coolant therein (e.g., forcedly, naturally, by convection, or a combination thereof) between heat exchanger section 501 and a generator at the surface. As shown, secondary fluid flow path 523 and primary fluid flow path 524 are in thermal communication so that heat can be transferred from the primary fluid coolant to the secondary fluid coolant (as described in more detail below), but the fluid flow paths are fluidly disconnected so that mixing of the primary and secondary fluid coolant does not occur within heat exchanger section 501. The secondary fluid coolant can be light water or other water-based liquid with additives.

[0131] Turning now to FIG. 5B, this figure illustrates the nuclear reactor portion 503 of the system 500. As shown, a reactor vessel 590 is installed within the borehole 505 (possibly within the brine 580 that fills the borehole 505). The reactor vessel 590 encloses a core 560 (defined by a core reflector 562), which includes, at least in part, nuclear fuel elements 570 and a control rod system 550 (which may be controlled from the surface, for example). The descender sections 528 extend downhole into the core 560 at the periphery of the core vessel 562 until they turn at the closed downhole end of the reactor vessel 590 to meet the ascender sections 526 within the core 560 at the nuclear fuel elements 570. In some aspects, the core vessel 562 can include thermal insulation material that provides thermal isolation between the downcomer section 528 and the core 560, as well as neutron reflector surfaces that face the nuclear fuel elements 570. The nuclear reactor section 503 can be, for example, about 14 to 28 feet in length.

[0132] In some aspects, core reflector 562 can be comprised of a material sufficient to reflect neutrons without absorbing them, such as, for example, carbon, beryllium (or any of their alloys). In some aspects, subsurface formation 502 can act as a neutron reflector or moderator based on its geological properties. Thus, in combination, core reflector 562 (if provided) and formation 502 can act in combination to contribute to the reactivity of nuclear reactor portion 503. Additionally or alternatively, casing 504 can be comprised of a material sufficient to reflect neutrons (or otherwise act to increase reactivity) without absorbing them, such as, for example, carbon steel or stainless steel. Casing 504 can also be comprised of materials such as ceramic, plastic material, or fiberglass.

[0133] In some aspects, there may be one, several, or many nuclear fuel elements 570. For example, the nuclear fuel element may be a nuclear fuel assembly rod (e.g., with cladding that holds the nuclear fuel pellets, such as that shown in FIG. 6). In some aspects, the nuclear fuel element may be nuclear fuel (e.g., one or more nuclear fuel assemblies) enclosed within a canister. In some aspects, the nuclear fuel element may be another form of fissile material, such as TRISO fuel, metallic uranium or plutonium, oxides of uranium or plutonium, or a mixture of uranium oxide and plutonium (MOX).

[0134] 5A and 5B, a pressure control tube 510 extends within borehole 505 through heat exchanger section 501 and into nuclear reactor section 503. In some aspects, pressure control tube 510 allows for pressure control of fluids within reactor system 500 and the insertion and removal of primary reactor coolant fluid. For pressure control, tube 510 allows an operator to set the pressure in the primary reactor loop to approximate the pressure of brine 580 in subterranean formation 502, thereby allowing for the use of thinner, less expensive metal for reactor vessel 595.

[0135] In an exemplary operation of borehole nuclear reactor system 500, nuclear reactor section 503 operates (e.g., naturally, conventionally, or both) to heat (and, when cooled, reheat) low temperature primary fluid coolant 532 that circulates from heat exchanger section 501 through downcomer section 528 and into core 560. As low temperature primary fluid coolant 532 rises through core 560 and through nuclear fuel elements 570, nuclear fuel elements 570 heat low temperature primary fluid coolant 532 into high temperature primary fluid coolant 530 that enters upcomer 595 and upcomer section 526.

[0136] Hot primary fluid coolant 530 flows through upcomer 595 and upcomer section 526 of system 500 and into heat exchanger portion 501. At the closed end of primary fluid coolant passage 524, hot primary fluid coolant 530 turns into downcomer section 528 and begins to transfer heat to ascending secondary fluid coolant 516 circulating in upflow section 518 of secondary fluid coolant passage 523. As heat is transferred, ascending secondary fluid coolant 516 can remain in liquid form or can change phase (at least partially) to gaseous form to form steam 520, which ultimately rises to a generator at the surface.

[0137] Steam 520 used in the generator generally changes phase back to a liquid and circulates through downflow section 512 as low-temperature secondary fluid coolant 514. Low-temperature secondary fluid coolant 514 circulates through downflow section 512 and into upflow section 518 in secondary fluid coolant flow passages 523 where it is heated by rising secondary fluid coolant 516.

[0138] This process is repeated when the nuclear reactor section 503 (and / or the generator) is in operation. For example, in the event of an uncontrolled nuclear fission reaction occurring within the nuclear reactor section 503, or to dampen a controlled nuclear fission reaction, the control rod assembly 550 can be operated (e.g., inserted into the core 560) to stop or reduce the nuclear fission reaction of the nuclear fuel elements 570.

[0139] Although a particular type of nuclear reactor (enriched uranium, plutonium, their oxides, or mixed oxides) is shown in nuclear reactor portion 503, the present disclosure contemplates that many forms of nuclear reactor may be used with fissile material forming nuclear fuel elements 570. For example, a molten salt reactor, a molten metal reactor, a TRIGA reactor, a TRISO fuel reactor, a boiling water reactor, a high temperature gas-cooled reactor, or another type of Gen IV reactor that may be installed in a borehole that is not occupyable by humans (such as borehole 505) are all contemplated by the present disclosure.

[0140] Additionally, although a single nuclear reactor section 503 is shown in Figure 5B, there can be multiple nuclear reactor sections 503 (and multiple heat exchanger sections 501) mounted within the borehole 505, either in a single vertical borehole from the borehole 505 or in multiple inclined or horizontal boreholes. Additionally, additional systems found in standard reactors, such as filtration systems, can also be easily added to the reactors described herein.

[0141] FIG. 6 is a schematic diagram of a nuclear fuel element 600 (fuel "rod" or "pin") for a borehole nuclear reactor system according to the present disclosure. This diagram explicitly illustrates one implementation of the previously described void collector. As shown in this example, the nuclear fuel element 600 includes a plurality of fissionable nuclear fuel pellets 604 (or other discrete forms of fissionable nuclear fuel) within cladding 602 (e.g., metal or other material in the form of a tube, such as Zircaloy or aluminum alloy). In some aspects, the nuclear fuel element 600 represents a nuclear fuel rod (many of which make up a nuclear fuel assembly within a support structure).

[0142] As shown in this example, a plurality of cups 606 are installed or attached to (or integral with) the cladding 602. Each cup 606 is inverted such that the cup 606 includes a closed end 608 and an open end 610. When installed within the nuclear reactor portion of a borehole nuclear reactor system, the closed end 608 faces toward the uphole, while the open end 610 faces toward the downhole. Thus, the open end 610 faces (and receives) the flow of primary fluid coolant toward the uphole, through the reactor core, and within the upwelling section of the reactor.

[0143] In some aspects, cup 606 acts as a bubble trap during operation of the nuclear reactor zone. For example, cup 606 can operate to accumulate primary fluid coolant in vapor form if the primary fluid coolant boils. In some aspects, cups 606 can be formed such that they have a low aspect ratio (e.g., the cup diameter is shorter, perhaps much shorter, than the cup length). It is expected that many of the voids "captured" by cup 606 will be generated inside cup 606 but will remain there instead of rising upward.

[0144] As the primary fluid coolant temperature drops, the vapor within cup 606 will drop and convert back to a liquid phase. Thus, cup 606 can act to provide or enhance a negative temperature coefficient for reactivity of the nuclear reactor portion of the borehole nuclear reactor system.

[0145] Although the cups 606 are shown as cylinders, other shapes with similar functionality can also be used. The cups 606 can have good thermal contact with the cladding 602 so that they do not reduce the cooling of the cladding 602 by the primary fluid coolant. In some aspects, they will enhance this cooling by providing additional surface area for the cladding.

[0146] While the specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or what may be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features described herein in the context of separate implementations may also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation may also be implemented separately in multiple implementations or in any suitable subcombination. Furthermore, while features may be described above as operative in a certain combination and may even be initially claimed as such, one or more features from a claimed combination may, in some cases, be deleted from that combination, and the claimed combination may be directed to subcombinations or variations of subcombinations.

[0147] Similarly, although operations are depicted in the figures in a particular order, this should not be understood as requiring such operations to be performed in the particular order shown, or in a sequential order, or that all of the illustrated operations be performed, to achieve desirable results. In some situations, multitasking or parallel processing may be advantageous. Also, it should be understood that the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and that the program components and systems described may generally be integrated together in a single software product or packaged in multiple software products.

[0148] Several implementations have been described. Nevertheless, it should be understood that various modifications may be made without departing from the spirit and scope of the present disclosure. For example, the example operations, methods, or processes described herein may include more or fewer steps than those described. Furthermore, the steps of such example operations, methods, or processes may be performed in a different sequence than that described or illustrated in the figures. Accordingly, other implementations are within the scope of the following claims.

Claims

1. 1. A nuclear reactor system, comprising: a borehole extending from the earth's surface through one or more subsurface formations; a reactor core positioned within the borehole, the reactor core comprising at least one nuclear fuel element; and a primary coolant system configured to transfer a primary fluid coolant between the reactor core and a heat exchanger; a secondary coolant system thermally coupled to the primary coolant system using the heat exchanger; Equipped with 10. A nuclear reactor system, wherein the secondary coolant system is configured to transfer a secondary fluid coolant between the heat exchanger and the surface.

2. 10. The nuclear reactor system of claim 1, wherein said at least one nuclear fuel element comprises a fissile nuclear fuel element.

3. 3. The nuclear reactor system of claim 2, wherein said fissile fuel elements comprise at least one of enriched uranium, plutonium, uranium or plutonium oxide, or mixed oxide (MOX).

4. 10. The nuclear reactor system of claim 1, comprising at least one canister at least partially enclosing said at least one nuclear fuel element.

5. 5. The nuclear reactor system of claim 4, wherein said canister includes a first opening at an upper bore end of said canister and a second opening at a lower bore end of said canister.

6. 10. The nuclear reactor system of claim 1, wherein said primary coolant system is configured to transport said primary fluid coolant between said reactor core and said heat exchanger by at least one of natural circulation or pressure.

7. 10. The nuclear reactor system of claim 1, comprising one or more pumps configured to provide force to transfer said primary fluid coolant between said reactor core and said heat exchanger.

8. 10. The nuclear reactor system of claim 1, wherein said secondary coolant system is configured to transfer said secondary fluid coolant between said heat exchanger and said surface by natural circulation.

9. 10. The nuclear reactor system of claim 1, comprising one or more pumps configured to provide force for transporting said secondary fluid coolant between said heat exchanger and said surface.

10. 10. The nuclear reactor system of claim 1, wherein said primary fluid coolant comprises water.

11. 10. The nuclear reactor system of claim 1, wherein said secondary fluid coolant comprises water.

12. 10. The nuclear reactor system of claim 1, wherein said primary coolant system is fluidly isolated from said second coolant system.

13. 10. The nuclear reactor system of claim 1, wherein the heat exchanger is configured to transfer heat from the primary fluid coolant in the primary coolant system and heat the secondary fluid coolant in the second coolant system to a higher temperature liquid or vapor phase.

14. 10. The nuclear reactor system of claim 1, comprising a power conversion system.

15. 15. The nuclear reactor system of claim 14, wherein said power conversion system is located at or near the Earth's surface.

16. 15. The nuclear reactor system of claim 14, wherein said secondary fluid coolant comprises a power conversion working fluid of said power conversion system.

17. 10. The nuclear reactor system of claim 1, wherein said at least one nuclear fuel element is located at a first depth in said borehole.

18. The first depth is Is it at most 0.5 km from the ground surface? 1.0 to 1.5 km from the ground surface, or 18. The nuclear reactor system of claim 17, wherein the reactor system is at least 1.5 km above the Earth's surface.

19. 17. The nuclear reactor system of claim 16, wherein said heat exchanger includes a barrier configured to fluidly isolate said primary coolant system from said second coolant system, said barrier being positioned at a second depth in said borehole, said second depth being less than said first depth.

20. 20. The nuclear reactor system of claim 19, comprising one or more pipes extending from the surface to a third depth in the borehole, the third depth being greater than the second depth.

21. 21. The nuclear reactor system of claim 20, wherein the one or more pipes are configured to provide the primary fluid coolant to a region of the borehole below the second depth.

22. 10. The nuclear reactor system of claim 1, wherein said primary coolant system comprises a first pipe having a diameter smaller than a diameter of said borehole, said first pipe positioned within said borehole with an axis of said first pipe substantially parallel to an axis of said borehole, said reactor core positioned inside said first pipe, said system comprising an annulus defined between said first pipe and said borehole, said annulus comprising a flow circuit configured to transport said primary fluid coolant downhole to said reactor core.

23. 23. The nuclear reactor system of claim 22, wherein said secondary coolant system comprises a second pipe having a diameter smaller than the diameter of said borehole, said second pipe positioned within said borehole with an axis of said second pipe substantially parallel to the axis of said borehole, said system comprising an annulus defined between said second pipe and said borehole, said annulus comprising a flow circuit configured to transport said secondary fluid coolant downhole to said heat exchanger.

24. the secondary coolant system a first pipe having a diameter smaller than the diameter of the borehole and configured to transport the secondary fluid coolant to the heat exchanger for heating; a second pipe having a diameter smaller than the diameter of the borehole and configured to transport heated secondary fluid coolant from the heat exchanger toward the surface; 14. The nuclear reactor system of claim 13, comprising:

25. 10. The nuclear reactor system of claim 1, wherein said borehole has a diameter of 36 inches or less.

26. 10. The nuclear reactor system of claim 1, wherein said borehole has a diameter of 4 inches or greater.

27. 10. The nuclear reactor system of claim 1, wherein said heat exchanger is positioned within a casing installed within said borehole.

28. 30. The nuclear reactor system of claim 27, wherein said primary coolant system comprises at least one pipe mounted external to said casing and said heat exchanger, and said secondary coolant system comprises at least another pipe mounted external to said casing.

29. 28. The nuclear reactor system of claim 27, wherein said casing is installed in said borehole with a cement layer and said heat exchanger is installed external to said cement layer.

30. 10. The nuclear reactor system of claim 1, wherein said reactor core is controllable between a low power output and a maximum power output.

31. 31. The nuclear reactor system of claim 30, wherein said reactor core is controllable using a fluid comprising a neutron absorber.

32. 32. The nuclear reactor system of claim 31, wherein said neutron absorber is controllably added to said primary fluid coolant.

33. 31. The nuclear reactor system of claim 30, wherein said reactor core is controllable with at least one control rod configured to move near or adjacent said at least one nuclear fuel element.

34. 10. The nuclear reactor system of claim 1, wherein said reactor core is a first reactor core, said nuclear fuel elements comprising first nuclear fuel elements, and said system comprises a second reactor core positioned in said borehole, said second reactor core comprising at least one second nuclear fuel element.

35. 35. The nuclear reactor system of claim 34, wherein said first and second reactor cores are individually controllable between low power output and maximum power output.

36. 35. The nuclear reactor system of claim 34, wherein each of said first and second reactor cores is individually controllable using a fluid comprising a neutron absorber.

37. 37. The nuclear reactor system of claim 36, wherein said fluid is transported individually to each of said first and second nuclear reactor cores through one or more tubes extending from said surface to said first and second nuclear reactor cores.

38. 38. The nuclear reactor system of claim 37, wherein said neutron absorber is controllably added to said fluid.

39. 35. The nuclear reactor system of claim 34, wherein each of said first and second reactor cores is individually controllable using respective first and second control rods movable within or near said respective first and second nuclear reactor cores.

40. 10. The nuclear reactor system of claim 1, comprising one or more inverted cups positioned within said primary coolant system and within said primary fluid coolant flow path.

41. 41. The nuclear reactor system of claim 40, wherein said one or more inverted cups are bonded to a cladding of said at least one nuclear fuel element.

42. 10. The nuclear reactor system of claim 1, comprising a reflector configured to reflect the neutrons generated in the reactor core without absorbing the neutrons.

43. 43. The nuclear reactor system of claim 42, wherein said reflector comprises at least one of beryllium, carbon, a beryllium alloy, or a carbon alloy.

44. 10. The nuclear reactor system of claim 1, wherein certain subterranean formations of said one or more subterranean formations adjacent said reactor core are configured to act as reflectors for reflecting neutrons generated within said reactor core.

45. 30. The nuclear reactor system of claim 27, wherein said casing is comprised of a material sufficient to act as a reflector to reflect neutrons generated within said reactor core.

46. 30. The nuclear reactor system of claim 27, wherein said casing material comprises at least one of carbon steel, stainless steel, ceramic, plastic material, or fiberglass.

47. 1. A method of constructing a nuclear reactor system, said method comprising: forming a borehole from the earth's surface through one or more subsurface formations; moving at least one nuclear fuel element into the borehole; positioning the at least one nuclear fuel element at a first depth in the borehole; moving a heat exchanger into the borehole, the heat exchanger comprising a fluid barrier; and positioning the heat exchanger in the borehole; Including, The method, wherein the fluid barrier is positioned at a second depth in the borehole, the second depth being less than the first depth.

48. 48. The method of claim 47, wherein the at least one nuclear fuel element comprises a fissile nuclear fuel element.

49. 48. The method of claim 47, comprising at least one canister at least partially enclosing said at least one nuclear fuel element.

50. 50. The method of claim 49, wherein the canister includes an opening at one or both ends, the method including positioning the canister in the borehole, the opening facing downhole.

51. 48. The method of claim 47, comprising installing a power conversion system at or near the ground surface.

52. 48. The method of claim 47, comprising inserting one or more pipes extending from the surface to a third depth in the borehole, the third depth being greater than the second depth.

53. 1. A method, comprising:

1. Identifying a nuclear reactor system, the nuclear reactor system comprising: a borehole extending from the earth's surface through one or more subsurface formations; a reactor core positioned within the borehole, the reactor core comprising at least one nuclear fuel element; and a primary coolant system; a secondary coolant system thermally coupled to the primary coolant system using a heat exchanger; the secondary coolant system configured to transfer a secondary fluid coolant between the heat exchanger and the surface; transferring a primary fluid coolant in the primary coolant system between the reactor core and the heat exchanger; transporting a secondary fluid coolant heated in the heat exchanger by the primary fluid coolant in the secondary coolant system between the heat exchanger and the surface; generating electrical power using the heated secondary fluid coolant; and A method comprising:

54. transporting the primary fluid coolant into a region of the borehole below a depth of the heat exchanger fluid barrier; transferring the secondary fluid coolant into a region of the borehole above the depth of the fluid barrier; and 54. The method of claim 53, comprising: wherein the fluid barrier fluidly isolates the primary fluid coolant from the second fluid coolant.

55. 55. The method of claim 54, comprising transporting the primary fluid coolant into the borehole through one or more pipes extending from the earth's surface to a depth in the borehole that is deeper than the depth of the fluid barrier.

56. 54. The method of claim 53, wherein the primary and secondary fluid coolants comprise water.

57. 1. A nuclear reactor system, comprising: a first borehole extending from the earth's surface through one or more subsurface formations; a reactor core positioned within the first borehole, the reactor core comprising at least one nuclear fuel element; and a second borehole extending from the earth's surface through the one or more subsurface formations and separated from the first borehole by a portion of the rock formation; a heat exchanger positioned within the second borehole in thermal communication with the reactor core through the portion of the rock formation; a coolant system thermally coupled to the heat exchanger and configured to transfer a fluid coolant between the heat exchanger and the surface; 1. A nuclear reactor system comprising:

58. 58. The nuclear reactor system of claim 57, wherein said at least one nuclear fuel element comprises a fissile nuclear fuel element.

59. 58. The nuclear reactor system of claim 57, comprising at least one canister at least partially enclosing said at least one nuclear fuel element.

60. 60. The nuclear reactor system of claim 59, wherein said canister comprises an opening at an axial end of said canister.

61. 58. The nuclear reactor system of claim 57, wherein said coolant system is configured to transport said fluid coolant between said heat exchanger and said surface by natural circulation.

62. 58. The nuclear reactor system of claim 57, comprising one or more pumps configured to provide force to transfer said fluid coolant between said heat exchanger and said surface.

63. 58. The nuclear reactor system of claim 57, wherein said fluid coolant comprises water.

64. 58. The nuclear reactor system of claim 57, wherein said heat exchanger is configured to transfer heat from said primary coolant system to said fluid coolant in said secondary coolant system.

65. 58. The nuclear reactor system of claim 57, comprising a power conversion system.

66. 60. The nuclear reactor system of claim 59, wherein said power conversion system is located at the surface of the earth.

67. 66. The nuclear reactor system of claim 65, wherein said fluid coolant comprises a power conversion working fluid of said power conversion system.

68. 58. The nuclear reactor system of claim 57, wherein said at least one nuclear fuel element is located at a first depth in said first borehole.

69. 69. The nuclear reactor system of claim 68, wherein said first depth is less than 0.5 km from said Earth's surface, between 1.0 and 1.5 km from said Earth's surface, or greater than 1.5 km from said Earth's surface.

70. 58. The nuclear reactor system of claim 57, wherein said first borehole has a diameter of 36 inches or less.

71. 58. The nuclear reactor system of claim 57, wherein said first borehole has a diameter of 4 inches or greater.

72. 58. The nuclear reactor system of claim 57, wherein said reactor core is a first reactor core, said nuclear fuel elements comprising first nuclear fuel elements, and said system comprises a second reactor core positioned in said first borehole, said second reactor core comprising at least one second nuclear fuel element.

73. 73. The nuclear reactor system of claim 72, wherein said first and second reactor cores are individually controllable between low power output and maximum power output.

74. 74. The nuclear reactor system of claim 73, wherein each of said first and second reactor cores is individually controllable using a fluid comprising a neutron absorber.

75. 74. The nuclear reactor system of claim 73, wherein said fluid is transported individually to each of said first and second nuclear reactor cores through one or more sets of tubes extending from said Earth's surface to said first and second nuclear reactor cores.

76. 74. The nuclear reactor system of claim 73, wherein said neutron absorber is controllably added to said fluid.

77. 74. The nuclear reactor system of claim 73, wherein each of said first and second reactor cores is individually controllable using a control rod assembly movable into or near said respective first and second nuclear reactor cores.

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