Borehole nuclear reactor

Abstract

A nuclear power system includes a borehole nuclear reactor, including: a reactor core comprising nuclear fuel and positioned a borehole that is formed from a terranean surface to a subterranean formation; a neck that extends upward from the reactor; a heat exchanger positioned at the top of the neck and in thermal communication with the reactor core; and a pressurizer positioned in the borehole downhole of the reactor core or including a tube or pipe that rises near to the terranean surface. The pressurizer is configured to maintain a reactor pressure at a pressure of water at an operating depth in the subterranean formation. The reactor core, the neck, the heat exchanger, and the pressurizer can be connected to form a preassembled kit prior to installation in the borehole, or are configured to be assembled as parts are lowered into the borehole.

Description

Attorney Docket No.: 57302-0005WO1 BOREHOLE NUCLEAR REACTOR TECHNICAL FIELD

[0001] This disclosure relates to the field of nuclear power. BACKGROUND

[0002] Generating electrical power through the use of nuclear fuel may require infrastructure to house and secure the nuclear fuel for a fission reaction. For example, a pressure vessel can be used to contain high temperatures and pressures necessary to extract energy efficiently and to prevent the release of radioactive materials. A pressure vessel for a pressurized water reactor may have thick steel walls and weigh hundreds of tons. SUMMARY

[0003] One innovative aspect of the subject matter described in this specification is embodied in a borehole nuclear reactor that includes a reactor core including nuclear fuel and positioned in a first portion of a borehole that is formed from a terranean surface to a subterranean formation; a heat exchanger positioned in a second portion of the borehole and in thermal communication with the reactor core; and a pressurizer positioned in the borehole downhole of the reactor core. In an example instance, the reactor has a diameter equal to or less than that of the narrowest part of the borehole, and the reactor is fully assembled prior to its lowering to depth. In the example instance the “heat exchanger” uses heat from the primary (reactor) coolant loop to heat and in some instances boil water or other fluid in a secondary loop producing hot liquid or vapor which then moves to the surface through the secondary loop. In this instance any mechanical work or electric generation (for example by a steam turbine) takes place near the terrestrial surface. In this specification, the term “heat exchanger” is used to indicate a device that can exchange heat from one fluid to the other. If the second fluid boils, the heat exchanger can be called a “steam generator.” This specification, in some aspects, uses the term “heat exchanger” to include the instance in which the secondary fluid does not boil as well as the instance that the secondary fluid does boil.

[0004] Containment is provided primarily by the overbearing rock (rock, soil, and other geologic materials at shallower depth than the reactor). In an example instance, the borehole can be too small for a human to enter. No construction is done at depth, although in some instancesAttorney Docket No.: 57302-0005WO1 the reactor vessel can be remotely disconnected from the heat exchanger. Pressure, such as that required for a pressurized or boiling water reactor, and as required by some steam generators, can be obtained by columns of water above the reactor or above the steam generator, with no or minimal need of a special pressurizer.

[0005] One innovative aspect of the subject matter described in this specification is embodied in a borehole nuclear reactor that includes a reactor core including nuclear fuel and positioned in a first portion of a borehole that is formed from a terranean surface to a subterranean formation; a heat exchanger positioned in a second portion of the borehole and in thermal communication with the reactor core; a cold fluid flow path that extends from the terranean surface, through the borehole, and into the heat exchanger; and a hot fluid flow path that extends from the heat exchanger, up through the borehole, and into a mechanism at or near the terrestrial surface that can be used to produce electric power and useful heat.

[0006] The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. For instance, in some implementations, the pressurizer is configured to maintain a reactor pressure at a pressure of water at an operating depth in the subterranean formation.

[0007] In an aspect combinable with the example implementation, the reactor core, the heat exchanger, and the pressurizer are connected to form a preassembled kit prior to installation in the borehole. In some examples, the reactor core, the heat exchanger, and the pressurizer are configured to be easily assembled as parts are lowered into the borehole.

[0008] In another aspect combinable with one, some, or all of the previous aspects, the reactor core includes a first reactor core, the borehole nuclear reactor further including a second reactor core including nuclear fuel and positioned between the first reactor core and the heat exchanger, the heat exchanger in thermal communication with the first reactor core and the second reactor core.

[0009] In another aspect combinable with one, some, or all of the previous aspects, the borehole nuclear reactor includes one or more control rods housed in guide tubes. The guide tubes extend into the reactor core and into space above the nuclear reactor core to provide a location for the removal of the control rods out of the core.

[0010] In another aspect combinable with one, some, or all of the previous aspects, each of the one or more control rods are configured to operate by or with a stepping motor or anAttorney Docket No.: 57302-0005WO1 induction motor or with a hydraulic system or another method that would be evident to a person practiced in the art of remote control.

[0011] In another aspect combinable with one, some, or all of the previous aspects, the nuclear fuel includes a nuclear fuel assembly including a plurality of sectors, with a first portion of the sectors configured for an upflowing fluid and a second portion of the sectors configured for a downflowing fluid.

[0012] In another aspect combinable with one, some, or all of the previous aspects, the pressurizer includes a membrane that separates a fluid inside the reactor core from fluid outside the reactor core, the pressurizer configured to expanding and contract to control fluid pressure inside the reactor core. In another aspect combinable with one, some, or all of the previous aspects, pressure of the reactor core is maintained by a tube that goes towards the surface, with the pressure maintained by the weight of the water in that tube.

[0013] In another aspect combinable with one, some, or all of the previous aspects, the borehole nuclear reactor includes burnable neutron poison members in the reactor core, the neutron poison members including wires mounted on rings, wires mounted on or within disks, wires mounted on the fuel cladding, or otherwise placed in the reactor core.

[0014] In another aspect combinable with one, some, or all of the previous aspects, the wires are suspended across an annulus of a ring or on some other part of the reactor core which puts them in a region where they encounter neutrons. The wire geometry allows the poisoning to be approximately proportional to the amount of the fuel burn (typically measured in gigawatt-days per metric ton of nuclear fuel). The wire geometry is an example of a cylindrical geometry. The wire can be curved or bent and will still function to keep the poisoning approximately proportional to the amount of the fuel remaining in the reactor. Other geometries for the burnable poison can be used, including spheres and flat plates, and in some instances the burn rate of the burnable poison can be adjusted over time. The geometry can be configured to compensate for xenon instability of the borehole nuclear reactor.

[0015] In another aspect combinable with one, some, or all of the previous aspects, a burn of the nuclear fuel is controlled in part by use of the burnable neutron poisons members.

[0016] In another aspect combinable with one, some, or all of the previous aspects, the burnable neutron poisons members are positioned between portions of the nuclear fuel.Attorney Docket No.: 57302-0005WO1

[0017] In another aspect combinable with one, some, or all of the previous aspects, the first portion of the borehole is at least 500 meters in depth under the terranean surface.

[0018] In another aspect combinable with one, some, or all of the previous aspects, a diameter of the borehole is between 14 inches and 2 meters.

[0019] In another aspect combinable with one, some, or all of the previous aspects, the borehole is substantially vertical or slanted up to 45 degrees from vertical.

[0020] One innovative aspect of the subject matter described in this specification is embodied in a method for installing a borehole nuclear reactor, including: forming a preassembled kit by connecting: a reactor core; a heat exchanger; and a pressurizer; and inserting the preassembled kit into a borehole that extends from a terranean surface to a subterranean formation.

[0021] One innovative aspect of the subject matter described in this specification is embodied in a nuclear power system, including: a borehole nuclear reactor, further including a secondary coolant loop that extends between the heat exchanger and the terranean or near terranean surface; and power generation equipment positioned at or near the terranean surface. The secondary coolant loop of the borehole nuclear reactor is fluidly coupled to the power generation equipment.

[0022] In an aspect combinable with the example implementation, the nuclear power system includes a plurality of borehole nuclear reactors each including a respective secondary coolant loop. The secondary coolant loops of each borehole nuclear reactor of the plurality of borehole nuclear reactors is fluidly coupled to the power generation equipment.

[0023] One innovative aspect of the subject matter described in this specification is embodied in a method of operating a borehole nuclear reactor, including: obtaining a heated secondary coolant from the heat exchanger. The secondary coolant was heated by a primary coolant that was heated by the reactor core; and providing the secondary coolant to power generation equipment to generate electrical power. In some instances, the secondary loop can provide heat for other uses than electricity, such as heating a building, or heating for industrial purposes or to provide mechanical motion through a turbine or other heat engine.

[0024] In an aspect combinable with the example implementation, the method includes controlling reactivity in the reactor core by moving one or more control rods housed in guide tubes. The guide tubes extend into the reactor core and into a space above the reactor core. In some instances, that space extends into the heat exchanger.Attorney Docket No.: 57302-0005WO1

[0025] In an aspect combinable with any of the example implementations, the reactor core includes a pressurized water reactor (PWR) reactor core, a boiling water reactor (BWR) reactor core, a high-temperature gas-cooled reactor (HTGR) reactor core, a molten salt reactor (MSR), a liquid metal reactor core, a graphite reactor core, a fusion reactor, or any other type of nuclear reactor that allows a narrow arrangement that can be lowered into a borehole.

[0026] In another aspect combinable with one, some, or all of the previous aspects, the first portion of the borehole is at least 500 meters in depth under the terranean surface.

[0027] In another aspect combinable with one, some, or all of the previous aspects, the reactor core operates within a subterranean fluid at a pressure of about 160 atmospheres (atm) in the first portion of the subterranean formation.

[0028] In another aspect combinable with one, some, or all of the previous aspects, the subterranean fluid is water within the borehole.

[0029] In another aspect combinable with one, some, or all of the previous aspects, the water is fresh water, that is, water with less than 1 gram per liter of sodium chloride.

[0030] In another aspect combinable with one, some, or all of the previous aspects, the water is within a casing installed in the borehole.

[0031] In another aspect combinable with one, some, or all of the previous aspects, the subterranean fluid includes brine from the subterranean formation.

[0032] In another aspect combinable with one, some, or all of the previous aspects, the water contains dissolved material or particles that are added because of their property of having high neutron absorption.

[0033] In another aspect combinable with one, some, or all of the previous aspects, the water contains materials that reduce corrosion.

[0034] In another aspect combinable with one, some, or all of the previous aspects, a diameter of the borehole is between 14 inches and 2 meters.

[0035] In another aspect combinable with one, some, or all of the previous aspects, the nuclear fuel is supported by one or more nuclear fuel assemblies.

[0036] In another aspect combinable with one, some, or all of the previous aspects, the nuclear fuel assembly includes nuclear fuel assemblies, including the fuel rods, the upcoming coolant fluid, and the downcoming return fluid, with the combination adjusted to match the shape of the borehole.Attorney Docket No.: 57302-0005WO1

[0037] In another aspect combinable with one, some, or all of the previous aspects, the nuclear fuel assembly includes an adjusted fuel fraction (ratio of moderator to fuel) that is less than a fuel fraction of a standard nuclear fuel assembly.

[0038] In another aspect combinable with one, some, or all of the previous aspects, the adjusted fuel fraction is defined by a ratio of moderator to fuel that is greater than the ratio of moderator to fuel of a standard nuclear fuel assembly.

[0039] In another aspect combinable with one, some, or all of the previous aspects, the borehole is substantially vertical or slanted up to 45 degrees from vertical.

[0040] In another aspect combinable with one, some, or all of the previous aspects, the heat exchanger is configured to produce steam for transport to the terranean surface in the hot fluid flow path.

[0041] In another aspect combinable with one, some, or all of the previous aspects, a pressure of a working fluid in the fluid flow path in the secondary loop is maintained lower than a pressure of the working fluid in the fluid flow path in the primary loop.

[0042] Another aspect combinable with one, some, or all of the previous aspects includes insulation positioned adjacent the primary and secondary fluid flow paths.

[0043] In another aspect combinable with one, some, or all of the previous aspects, the insulation includes a vacuum, rock wool, aerogel, fiberglass, calcium-silicate, perlite, ceramic fiber, or a combination thereof, or any other insulator that would be evident to someone practiced in the field.

[0044] In another aspect combinable with one, some, or all of the previous aspects, a working fluid flows through the reactor core and the heat exchanger by natural convection.

[0045] In another aspect combinable with one, some, or all of the previous aspects, a burn of the nuclear fuel is controlled in part by use of gadolinium (Gd) or other burnable neutron poisons members.

[0046] In another aspect combinable with one, some, or all of the previous aspects, the burnable neutron poisons members include wires, threads, strings, or rods.

[0047] In another aspect combinable with one, some, or all of the previous aspects, a diameter of the burnable neutron poisons members is determined based on an expected neutron lifetime of the nuclear fuel.Attorney Docket No.: 57302-0005WO1

[0048] In another aspect combinable with one, some, or all of the previous aspects, the burnable neutron poisons members include wires mounted on a ring positionable between portions of the nuclear fuel.

[0049] In another aspect combinable with one, some, or all of the previous aspects, the portions of the nuclear fuel include fuel pellets.

[0050] Another aspect combinable with one, some, or all of the previous aspects includes a combination of gadolinium mixed with the nuclear fuel to modify a reactivity of the nuclear fuel over time.

[0051] In another aspect combinable with one, some, or all of the previous aspects, the combination resides in a presence of a nuclear reaction poison.

[0052] In another aspect combinable with one, some, or all of the previous aspects, the nuclear reaction poison includes boron.

[0053] In another aspect combinable with one, some, or all of the previous aspects, each of the cold fluid flow path and hot fluid flow path extends to the terranean surface and is configured to enable volume control, boron injection and dilution, injection and dilution of materials to adjust the acidity (pH) of the primary loop, removal of particles, and monitoring of a working fluid.

[0054] Another aspect combinable with one, some, or all of the previous aspects includes a casing installed in all or part of the borehole.

[0055] In another aspect combinable with one, some, or all of the previous aspects, the casing includes at least a portion that is uncemented or cemented with a flexible material to allow the casing to expand without undue stress or shape change when the reactor core increases a temperature.

[0056] In another aspect combinable with one, some, or all of the previous aspects, the flexible material includes flexible cement or sand.

[0057] In another aspect combinable with one, some, or all of the previous aspects, the reactor core is configured for decommissioning by plugging and sealing the borehole.

[0058] In another aspect combinable with one, some, or all of the previous aspects, the subterranean formation is suitable as a host site for the borehole nuclear reactor through radioisotope methods that show sufficient isolation of the subterranean formation from the terranean surface.Attorney Docket No.: 57302-0005WO1

[0059] In another aspect combinable with one, some, or all of the previous aspects, the nuclear fuel is reduced in fuel load relative to a standard nuclear fuel assembly to increase an effectiveness of a moderator.

[0060] In another aspect combinable with one, some, or all of the previous aspects, the reactor core includes a side-by-side arrangement of at least a portion of the primary cold fluid flow path and at least a portion of the primary hot fluid flow path to promote pre-heating of the coolant water as it descends to the bottom of the reactor.

[0061] In another aspect combinable with one, some, or all of the previous aspects, the nuclear fuel includes at least one rounded nuclear fuel assembly to substantially conform with a shape of the borehole or a casing, or both.

[0062] Another aspect combinable with one, some, or all of the previous aspects includes a pressurizer positioned in the borehole downhole of the reactor core.

[0063] In another aspect combinable with one, some, or all of the previous aspects, the pressurizer is configured to maintain a reactor pressure at a pressure of water at or near an operating depth in the subterranean formation.

[0064] Another aspect combinable with one, some, or all of the previous aspects includes one or more control rods.

[0065] In another aspect combinable with one, some, or all of the previous aspects, the one or more control rods are configured to operate by or with a stepping motor.

[0066] In another aspect combinable with one, some, or all of the previous aspects, the one or more control rods are configured to operate by or with one or more induction motors.

[0067] Another aspect combinable with one, some, or all of the previous aspects includes a filter configured to reduce iodine through natural convection. The filter can be proximate to the reactor or heat exchanger, in the space between them (referred to as the “neck”) or proximate to the terranean surface and connected to the reactor through a “sampling” pipe loop that brings a portion of the primary loop water to the proximate terranean surface where it can be sampled, and a separate length of pipe that returns to the primary loop of the reactor. The flow through this sampling pipe can be driven by natural convection or by a pump. In an example implementation, the pump is located proximate to the terrestrial surface.

[0068] Another aspect combinable with one, some, or all of the previous aspects includes a filter configured to reduce particulates through natural convection. The filter can be proximateAttorney Docket No.: 57302-0005WO1 to the reactor or heat exchanger, proximate to the neck, or proximate to the terranean surface and connected to the reactor through the sampling pipe loop.

[0069] Another aspect combinable with one, some, or all of the previous aspects includes a container of hydrogen gas configured to reduce oxygen ions in the primary fluid. The hydrogen can be proximate to the reactor or heat exchanger, in the neck, or proximate to the surface and connected to the reactor through the sampling pipe.

[0070] In another aspect combinable with one, some, or all of the previous aspects, the container is pre-compressed and configured to open at a depth of the subterranean formation.

[0071] In another aspect combinable with one, some, or all of the previous aspects, the nuclear fuel includes a nuclear fuel assembly including a plurality of sectors, with a first portion of the sectors configured for an upflowing fluid and a second portion of the sectors configured for a downflowing fluid.

[0072] In another aspect combinable with one, some, or all of the previous aspects, the cold fluid flow path is positioned in an annulus outside the nuclear fuel.

[0073] “Cold fluid” refers to the fluid that has been cooled by the heat exchanger and is cooler than the “hot fluid” that enters the heat exchanger. In one instance, the hot fluid is approximately 315°C and the cold fluid is approximately 275°C.

[0074] In another aspect combinable with one, some, or all of the previous aspects, the cold fluid flow path is positioned in a central region in, near, or at a center of the nuclear fuel.

[0075] In another aspect combinable with one, some, or all of the previous aspects, the cold fluid flow path is positioned in an outer region in, near, or at the perimeter (outside boundary) of the nuclear fuel.

[0076] In another aspect combinable with one, some, or all of the previous aspects, the hot fluid flow path includes a flexible conduit, and the cold fluid flow path includes a flexible conduit.

[0077] In another aspect combinable with one, some, or all of the previous aspects, the flexible conduits is configured to spool onto a spindle to facilitate installation and removal from the borehole.

[0078] An example innovative aspect of the subject matter described in this specification is embodied in a neutron poison member, comprising: a ring structure; and a plurality of wires mounted to the ring structure and suspended across an annulus of the ring structure, each wire comprising burnable neutron poison material.Attorney Docket No.: 57302-0005WO1

[0079] In an example aspect of the wire design, the wire is sufficiently thick that the outer portions of the wire shield the inner portions, so the burn is proportional to the area of the outer part of the wires. As the poison burns, the radius of the unburned poison decreases because of the cylindrical shape of the wires, and that reduces the surface area of the poison. Because the reduction of the surface area is proportional to the burn of the uranium (typically measured in gigawatt-days per metric ton of fuel) the amount of active poison (the amount of poison accessible to neutrons) reduces in proportion to the burn. In some instances, this proportionality will simplify the control of the nuclear reactor as the fuel burns.

[0080] In an aspect combinable with the example implementation, the burnable neutron poison material comprises gadolinium.

[0081] In another aspect combinable with one, some, or all of the previous aspects, the ring structure is formed from at least one of a ceramic or metal material.

[0082] In another aspect combinable with one, some, or all of the previous aspects, the plurality of wires each comprise a wire with a neutron poison core such as a gadolinium core.

[0083] In another aspect combinable with one, some, or all of the previous aspects, a diameter of the annulus is one centimeter or less.

[0084] The subject matter described in this specification can be implemented in various implementations and may result in one or more of the following advantages. The systems and methods described in this specification can enable reduced risk of radioactivity released to the environment. During the full reactor operation, from installation to decommissioning and disposal of the reactor underground, no highly radioactive material is expected to appear at surface or near- surface levels. The reactor itself is completely isolated from tornadoes, floods, airplane crashes, earthquakes, and human intrusion.

[0085] The secondary loop includes a pipe that carrying downcoming (downwards flowing) fluid to the heat exchanger, and a pipe that carries upcoming (upward flowing) fluid to the surface. In an example embodiment the downcoming fluid is liquid water, and the upcoming fluid is steam. In this specification, the pipe with the upcoming fluid will be referred to as the “steam” pipe even when the fluid is not steam.

[0086] An aspect of this arrangement is that it simplifies and enhances the resistance of the reactor to the “loss of coolant” accident that is of much greater concern to reactors located proximate to the terrestrial surface. The reactor is surrounded by water or brine, and except forAttorney Docket No.: 57302-0005WO1 the steam pipe, by 500 meters or more of water above it. The large amount of such water provides coolant for emergency core cooling in the case of a reactor failure. Moreover, since it is located above the reactor, no pump is needed to bring that water to the proximity of the reactor. This feature, of a large quantity of water in position to be fed by gravity to cool the core in an emergency, is a key feature of the reactor, and it provides significant public safety.

[0087] The placement of the reactor deep in a borehole also has the advantage of reducing the danger of radioactive material release to the environment due to an earthquake. If the reactor vessel is breached due to an earthquake, the depth of 500 meters or more provides a significant barrier to transport to the surface. Simulations show that earthquake faults, including large ones, will not transport radioactive material to the surface in amounts that threaten human health.

[0088] The borehole nuclear reactor has other features that reduce the danger to the public of a radioactivity release. As an example, the configuration mitigates the danger of a steam generator interface accident. These measures are made possible by the fact that the steam generator (also called the heat exchanger) can be long, such as 5 to 10 meters or longer. This length allows for a more gradual heat exchange from primary to secondary loop, and that in turn allows the use of a thicker and multiple-layer interface between primary and secondary loop. This thicker divider reduces the likelihood of breakage or corrosion. It also allows wider tubing, which in turn, reduces the danger of vibration induced by fluid flow. If the barrier is breached, then the risk is that radioactive water will enter the secondary loop and thence reach the surface. In the borehole nuclear reactor, the time for the steam to reach the surface is measured in minutes, and most of the Nitrogen – 16 decays prior to reaching the surface and the biosphere.

[0089] The disclosed implementations provide improved security by having limited amounts of radioactive material dispersed over numerous deep boreholes. For example, an individual borehole nuclear reactor may have between one and nine fuel assemblies, while an above-ground PWR typically has between 150 and 200 fuel assemblies. Security is strong when dangerous materials are buried at great depth.

[0090] Example implementations of the borehole nuclear reactor harness the geology of the Earth to provide safe, secure, and inexpensive nuclear power. Water pressure at a depth of 1600 meters (approximately one mile) is approximately 160 atmospheres, the same as artificially created in the thick steel-walled pressurizers in a gigawatt (1000 megawatt) Pressurize Water Reactor (PWR). Above the borehole nuclear reactor (in a 45 degree cone) is about 10 billion tonsAttorney Docket No.: 57302-0005WO1 of rock offering exceptional containment and security. Steel can be used to case the hole, but the steel can be thinner, compared to steel used in oil and gas wells, since there will be little pressure difference between the interior and the exterior of the casing.

[0091] A borehole nuclear reactor reduces most of the cost and complexity of a superstructure by using drillholes to harness the geology of the Earth for containment and security, and to provide 50 to 160 atmospheres or more of pressure. The cost of electric energy produced per kWh is therefore reduced because of the simplification of containment. The cost may be low enough to displace the use of coal. Low cost nuclear can reduce global warming by providing inexpensive low carbon energy around the world that will be economically advantageous.

[0092] 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, the drawings, and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] FIG. 1 is a schematic drawing of a geologic profile showing multiple implementations according to the present disclosure.

[0094] FIG. 2 is a schematic drawing of an example implementation of a deep borehole micro-modular pressurized water reactor (with a compressed vertical dimension) according to the present disclosure.

[0095] FIG.3 is a schematic diagram of a reactor core and steam generator for a borehole nuclear reactor.

[0096] FIG. 4 is a schematic drawing of an example implementation of a deep borehole micro-modular pressurized water reactor according to the present disclosure.

[0097] FIG. 5 is a schematic drawing of tandem reactors according to the present disclosure.

[0098] FIG. 6 shows an example schematic drawing of a cross-section of a secondary coolant pipe according to the present disclosure.

[0099] FIG. 7 is a schematic drawing of an example implementation of filter and surface / boron control tubes according to the present disclosure.Attorney Docket No.: 57302-0005WO1

[0100] FIG.8 is a schematic drawing of an example implementation of a pressurizer for a deep borehole micro-modular pressurized water reactor according to the present disclosure.

[0101] FIG. 9 is a schematic drawing of an example chemical, volume, and pressure control system.

[0102] FIGS. 10A and 10B are schematic drawings of two core flow cooling options according to the present disclosure.

[0103] FIG. 11 shows an example nuclear reactor fuel layout according to the present disclosure.

[0104] FIGS. 12A, 12B, and 12C are schematic drawings of example fuel rod layouts according to the present disclosure.

[0105] FIGS.13A and 13B are a perspective drawings of gadolinium wires integrated with fuel rods according to the present disclosure. DETAILED DESCRIPTION

[0106] Example implementations according to the present disclosure include configurations of a micro-reactor that is deployed in a deep narrow borehole (“borehole nuclear reactor”). The reactor is narrower than the borehole, so it does not require an enlarged space to at the operating depth. The example borehole nuclear reactor is a PWR; other options include a BWR (boiling water reactor), MSR (Molten Salt Reactor), LMR (liquid metal reactor), and HTGR (high temperature gas reactor). An example borehole reactor has a power rating of between 1 megawatt electric (MWe) and 20 MWe, is deployed in borehole having a diameter between eighteen and thirty inches and operates at an approximate depth of one mile. With a larger borehole, thirty to forty-five inches in diameter, the power can be increased to 50 MWe. The interior pressure of the reactor is approximately 1 atmosphere at the terranean surface but increases by approximately 1 additional atmosphere for every 10 meters depth; at 1 km, the pressure is approximately 100 atm, and at 1.6 km (about 1 mile) the pressure is approximately 160 atm A heat exchanger above the nuclear reactor produces steam in a secondary loop that carries the power to power generation equipment at the surface, such as a steam turbine. For example, the steam pipe can be insulated with rock wool, with only approximately 2% of the power lost as the steam flows to the surface. The depth provides pressure, and the geology is the primary source of containment, so surface structures are minimized.Attorney Docket No.: 57302-0005WO1

[0107] Example implementations of the borehole nuclear reactor use low enriched uranium (LEU) fuel pellets, identical to or similar to those used in gigawatt (1000 megawatt) light-water (PWR and BWR) reactors. Primary loop cooling can be achieved using convection, with no underground pumps needed. Reactor control can be performed using “black” control rods, which absorb essentially all incident neutrons, and “grey” control rods, which absorb a smaller number of neutrons. The control rods can be driven by linear stepping motors and / or other types of motors with no moving parts or using induction motors or hydraulic systems. The motors can be at depth, near the nuclear reactor, or they can be at the surface with cables or hydraulic tubes connecting them to the reactor control rods. Control can be maintained using the control rods and boron dissolved in primary coolant. Boron removal can be performed using tubes that extend to the surface. Gadolinium, a burnable poison, can be used in and near the fuel and in the coolant in the form of dissolved gadolinium salts, and in a solid form (e.g., pieces of metal or compound).

[0108] A strong negative temperature coefficient, in standard nuclear reactor terminology, refers to the fact that for some reactors, as the temperature in the reactor rises the reactivity decreases. A negative temperature coefficient provides safety against a constantly increasing reactivity, and also provides a mechanism for semi-automatic control of the reactor temperature as the fuel is depleted. In the borehole reactor, a negative temperature coefficient is derived from Doppler broadening in the fuel (fast response) and thermal expansion of primary coolant water and other reactor materials (larger negative temperature coefficient but slower response). If the reactor experiences a run-away chain reaction, then the temperature will rise, and the multiple negative temperature coefficients are expected to slow and limit this rise. These include the fast negative coefficients due to Doppler broadening in the fuel, and the slower negative coefficient due to thermal expansion of the water moderator. If these fail, and insertion of control rods (such an insertion is called, in the tradition of the nuclear industry, a SCRAM) also fails then its water in the reactor might boil, turning water to steam. Steam is too low density to continue the chain reaction, so the production of a significant amount of steam would terminate the chain reaction. And yet the steam at the high pressures found in a borehole are sufficiently good conductors of heat that the can provide cooling for the heat created by the radioactive decay of the remaining fuel, and serve to prevent a meltdown.

[0109] The borehole nuclear reactor has several properties that differentiate it from larger nuclear reactors. It can be off-grid. For decommissioning, the spent fuel can be lifted out forAttorney Docket No.: 57302-0005WO1 disposal using cables that are permanently attached to the reactor, or by using standard oil and gas retrieval methods (called “fishing” in the oil and gas industry) or left at depth and the hole sealed. New fuel can be placed in the borehole periodically, such as every 2 to 10 years. In some cases, a spent fuel assembly can be stored at depth rather than in surface dry casks, and a new borehole nuclear reactor can be placed on a platform above the previous nuclear reactor which then contains spent fuel. The waste heat can be dissipated in a cooling tower or in a river or other water system, and / or used for combined heat and power at a factory, campus, small town, or locally in a city. Monitoring of the borehole nuclear reactor can be performed in-place, but the borehole nuclear reactor can be brought to the surface if needed for inspection. Semi-autonomous load following can be employed.

[0110] The borehole nuclear reactor is modular, and FIG. 1 shows several vertical implementations, with one or many boreholes at one site. FIG.1 is not to scale. Each borehole is formed from a terranean surface 101 to a subterranean formation 111. At the depth of one mile from the terranean surface 101, a 50-foot tall borehole nuclear reactor is less than 1% of the depth. The one-mile depth is represented in FIG.1 by the arrow 105. This small vertical scale allows the simultaneous or sequential stacking of reactors at the bottom of the borehole.

[0111] More power can be generated by increasing the. power output of a single reactor or by adding more boreholes with multiple but separated reactors. This spacing would allow multiple reactors per acre in locations that have suitable geology. A conceptual geologic profile is shown in FIG. 1. FIG. 1 shows six different multiple reactor layout configurations; the ones shown are examples, and many other configurations are possible. On the left is a single borehole containing a 15 MWe reactor 102; in configuration 102, multiple reactors are depicted at the bottom of a single borehole that branches near the bottom. Next is a collection of a hundred or more borehole reactors 104 that deliver a total of 1500 MWe; next is a set of ten 10 MWe reactors 106 at one location (a campus, factory, military base, or small community); next is a single borehole 4 MWe reactor 108; next is a single borehole 1 MWe reactor 110; next is a single borehole 100 kWe reactor 112. In some examples, secondary coolant loops of multiple reactors are fluidly coupled to the same steam turbine or other piece of power generation equipment. In this way, power generation equipment can produce electrical power using the heated secondary coolant generated by the multiple reactors. The image in FIG 1. is not to scale; not all boreholes are shown, and multiple boreholes can fit in an area such as one acre. The depths of nearby boreholesAttorney Docket No.: 57302-0005WO1 can differ to provide increased separation that can facilitate licensing. The modular nature allows the reactors to be spread across a military base or a large city.

[0112] An example borehole nuclear reactor 200 is shown in FIG. 2. A nuclear reactor core 201 is positioned in a portion 121 of a borehole 130. The portion 121 of the borehole 130 can be at least 500 meters below the surface 101. In some examples, the portion 121 of the borehole 130 is approximately one mile below the surface 101. The borehole 130 can have a diameter 125 between approximately eighteen and thirty to forty-five inches.

[0113] The reactor core 201 can contain fuel rods 202. Only a few fuel rods are depicted; other fuel rod depiction are shown in FIGS.11 and 12. These can be standard fuel rods as currently used in the nuclear industry that are about 1 centimeter (cm) in diameter and 400 cm (14 feet) long, or they can contain fuel rods of custom size. The reactor core 201 can have a 17x17 PWR fuel assembly that is rounded to fit in the cylindrical reactor shape. The fuel assembly can have a square array pin (fuel rod) design. Other example fuel assemblies include 19x19 and hexagonal arrays. The number of fuel rods can be reduced in order to increase the ratio of moderator to fuel.

[0114] Primary coolant flow is convectively driven by the density difference between the hot and cold legs in the primary coolant loop. Primary water (e.g., water functioning as a primary coolant) flows up between the fuel rods 202 and into an upper reactor region 204 which holds control rods 206. A region containing the fuel rods 202 can be as wide as the reactor (as shown) or the region can be narrower than the reactor. The upper reactor region 204 can be referred to as the “neck”. In FIG 2, the reactor and the heat exchanger are shown as proximate, but the reactor and the heat exchanger can be separated for the purpose of increasing the length of the primary loop. A longer primary loop results in a larger pressure difference between the upcoming side and the downcoming side, and that can increase the rate of flow of the coolant in the primary loop. The separation is called the “neck” and has large pipes that have very low resistance to fluid flow. The neck can have an example length between zero and fifty meters. From the upper reactor region 204, the hot primary water (e.g., water at approximately 315°C) flows into the heat exchanger, or steam generator 210. The steam generator 210 is positioned in a portion 120 of the borehole 130 that is above the portion 121 of the borehole 130 in which the reactor Is positioned. The steam generator 210 is in thermal communication with the reactor core 201.

[0115] The hot primary water flows thorough a central insulated pipe 208 to the top of the heat exchanger 210 and down outside of the pipe 208, where the water comes into contact with theAttorney Docket No.: 57302-0005WO1 secondary loop. As heat transfers across a metal boundary 221 separating the primary and secondary loops, the hot primary water boils water in the secondary loop that is kept at a lower pressure. Convection can be enhanced by making the heat exchanger 210 longer (taller), or by increasing the height of the neck, the pipes that deliver the hot water upward and return the cooler water downward. The difference between temperature of the hotter water and cooler water is about 40°C.

[0116] FIG.2 uses a core design in which both the downflowing and upflowing water pass between fuel rods. This is done to enable more fuel to be placed in the limited space of the borehole 130. When the water reaches the bottom of the reactor core, the water flows up again through the fuel assembly. The flow through the reactor core is driven by the hotter and less dense water in the upward flowing section compared to the cooler water flowing downward. More fuel can also be introduced into the core by increasing the length (height) of the fuel rods, either by making the fuel rods longer, or by stacking the fuel rods.

[0117] Referring to FIG. 3, in some examples the downflowing water flows outside the core vessel 212 to the bottom of the fuel assembly 214, where the water flows up again through the fuel assembly 214. The downflowing water can flow in an annulus 215 that is outside the reactor core 201 in a region free of fuel rods.

[0118] Referring back to FIG.2, in the secondary loop cool water descends in a tube 216, then spreads to flow on the outside of the heat exchanger 210. When the water reaches the bottom 211 of the heat exchanger 210, the water turns inward and then flows up in a region 218 that is in thermal contact with the hot water from the primary loop. This section can be made very long (e.g., 40 feet or more), to allow robust metal to be used for the transfer, and to allow a greater pressure head to build in the primary loop to drive the convection. The secondary loop water boils, creating steam, and the steam rises to the surface 101 in the steam pipe 220. The flow through this long pipe serves to dry the steam.

[0119] The water level 244 in the cold-water feed tube 216 is below the surface 101 of the Earth. Maintaining the water level 244 below the surface 101 can be done to reduce the pressure in the secondary loop so that it will boil when it is heated by the hot section of the primary loop (which has a maximum temperature of about 315°C, although in some instances the maximum temperature of the primary loop can be lower or higher than this value). The water level 244 can be monitored by a depth gage 246 located near the surface of the secondary loop water. AcousticAttorney Docket No.: 57302-0005WO1 reflection can also be used or provide backup to the depth gage. The water can replenish without completely filling the upper part of the hole as needed. Alternatively, the height of the secondary water in the feed tube 246 can be adjusted by monitoring the steam pressure in the steam pipe 220; this monitoring can be done near or at the terrestrial surface.

[0120] Steam rises to the surface 101 in the insulated pipe 220, and at the surface 101 the steam can be used to drive a steam turbine (e.g., to produce electricity). The secondary coolant loop of the reactor is therefore fluidly coupled to the electric power generation equipment. The excess heat (about 2 / 3 of the thermal energy) can be used as a heat source, or it can be fed into cooling tower or other cooling system (such as a river or ocean) and the condensed water sent back underground to the reactor. In some instances, the hot water can be released to the environment and replace by cool water from a nearby water source, such as a river. The cooled and condensed water is returned to the secondary loop and flows down to the heat exchanger 210. Both the primary and secondary loops can be closed loops.

[0121] When a borehole is drilled, the hole can be filled with drilling mud. This mud cools the drill bit and provides support against rocks from the hole wall. Drilling mud can include fresh water, liquid hydrocarbon, brine, or actual mud (water with rock bits—used because of its high density). Once the hole is completed (or partially completed, in what is called staging), the drill bit and drill pipe are pulled up, and the hole is “cased”, that is, filled with a pipe (steel, fiberglass, or other material) that can support the expected pressure difference between the rock / brine outside of the pipe and the interior of the pipe.

[0122] In some examples, for a borehole nuclear reactor, the casing 226 can be filled with brine or fresh water 225. The brine or fresh water can contain material to reduce corrosion, and it can contain material to absorb neutrons. The water 225 within the casing (but outside the primary and secondary loops) provides a pressure reference at depth. Once the casing is filled with water, the pressure at the bottom of the borehole, in an example instance at about one mile depth below the terrestrial surface, will have a gravitational water pressure of approximately 160 atmospheres. A similar pressure will be provided in the sample tube 912. The pressure in the reactor can be kept at the desired value (160 atm in the example instance) by adjusting the pressure in the sample tube 912. The use of the sample tube as a pressurizer is described in greater detail with reference to FIG.9. Alternatively, the pressure in the primary loop can be maintained by using pressurizer 222 positioned below the reactor (e.g., downhole of the reactor core 201) and separated from the freshAttorney Docket No.: 57302-0005WO1 water 225 by a flexible membrane 224 to assure that the water inside the reactor is also at approximately 160 atmospheres. Example pressurizers are described in greater detail with reference to FIG.8.

[0123] An example reactor is about 30 feet in height 124 and about 26 inches in diameter 126. Its major components at depth are shown in the reactor layout diagrams in FIG.3 and FIG. 12C. The reactor core 201 can include four 17x17 fuel assemblies, sitting in a 2x2 square array. A plan view (horizontal cross section through the middle of the reactor) is shown in FIG 12C. The downcoming water flows in lune-shaped spaces 1226 between this square array 1228 and the circulator shape of the reactor vessel 236; this water provides additional moderation of the reactor.

[0124] The core sits under the steam generator and is connected to it by a region called the neck 232. In FIG.3, the neck 232 is shown as short relative to the height 124 of the reactor core 201, but the neck 232 can be made longer. In some instances, the neck can be as long as 50 meters. Such long necks may require very tall and expensive containment when proximate to the terrestrial surface, but in a borehole reactor the long necks have a natural containment by the surrounding rock. Control rods 206 when withdrawn from the reactor core 201 move into guide tubes 234 in the neck 232 which extend into the steam generator 210 if the neck is short. The guide tubes 234 can therefore extend into the reactor core 201 and into the heat exchanger 210.

[0125] The heat exchanger / steam generator 210 includes tubes carrying hot water from the core (the primary loop) past water in the secondary loop, which it heats and vaporizes. The primary loop can operate between 275°C and 315°C. Other low and high temperatures can be chosen for other instances. The secondary loop at depth is held at a lower pressure of about 65 to 100 atm. Water at 65 atm boils at 281 °C. The steam is brought to the surface 101 through an insulated pipe 620 within the water-filled casing.

[0126] Flow in the primary loop is driven by natural convection. Flow in the secondary loop can be controlled by pumps at the surface that feed water into the down-flowing steam generator supply pipe. In some examples, the only moving parts at depth are the control rods 206, the water in the primary and secondary loops, and optionally valves for boron control and water flow adjustment. In some examples, a reactor has no moving parts at depth other than the control rods 206, which are moved by electric pulses acting on permanent magnets on the rods (a linear stepping motor). If electric power fails, the rods will be drawn by gravity into the reactor core 201 where the rods will terminate the sustained chain reaction.Attorney Docket No.: 57302-0005WO1

[0127] A borehole nuclear reactor can have a cylindrical shape. An example reactor has a diameter 126 of between 14 and 30 inches (e.g., 16 inch diameter, 18-inch diameter, 24-inch diameter). An example reactor can have a height 124 of between 30 and 50 feet. The borehole nuclear reactor can operate between a temperature of 275 degrees Celsius (°C) and 315°C, although those temperatures can be adjusted to meet depth parameters and efficiency needs. If operated at a depth of one mile, the water pressure at this depth can be used to hold the pressure in the reactor to at about 160 atmospheres. For a BWR, a lower depth and a lower pressure can be used if desired. Pressure can be maintained using a steel pipe functioning as a pressurizer 222 that is in contact with the fresh water that fills the casing to the surface. If the steel pipe is flexible, then installation of the reactor at depth can make use of the technology of coiled tubing.

[0128] Reactor control can be maintained by control rods 206, boron (e.g., in the form of a boric acid) injection and removal from the primary loop water, gadolinium (e.g., in the form of a gadolinium salt) in or near the fuel elements, strong negative feedback mechanisms, or any combination thereof. The borehole nuclear reactor can be lowered into a fresh-water filled and cased borehole to the bottom of a mile-deep borehole using an overhead crane rather than a rig. A rig is the structure used to drill and case the hole; it is typically rented, and more expensive than an overhead crane. The ability to place the reactor with a crane helps reduce the cost of the reactor and is made possible by the small size and weight of the borehole reactor.

[0129] The control rods 206 generally fall into two categories, black and gray. The black rods can be used to control the reactor, but they are less absorbent for neutrons, and are primarily intended for emergency or other shutdown (SCRAM). Black rods are sometimes composed of an alloy of silver, indium, and cadmium, and sometimes boron. Gray rods, often used to help with load following, can be made of tungsten and stainless steel.

[0130] The control rods 206 can be lowered by using gravity to assist the downward motion. In some examples, the control rods 206 are moved by strong magnets (e.g., Alnico magnets, which operate up to 525°C, or samarium-cobalt magnets, special versions of which work up to 400°C) attached to the top of the rods. In some examples, a control rod 206 is moved by a series of electromagnet coils that form a linear motor 242. The linear motor 242 can be, for example, a linear stepping motor or a linear induction motor. The control rods 206 can have magnets interspersed along their lengths, in which case the stepping motors may have a shorter length. The linear motors 242 can be controlled by electric wires that rise to the surface, shown inAttorney Docket No.: 57302-0005WO1 FIG.6. When a control rod 206 needs to be moved, the coils can be phased to move the rod up or down one step (distance between coils) at a time. In some examples, no movement occurs in the linear motor 242 other than movement of the control rods 206. If the electric power from the surface fails, then the control rods 206 will lower by gravity into the reactor core 201. Thus, the control rod system is fail-safe in the instance of power loss.

[0131] In an example in which the stepping motor coils are inside the reactor, then the reactor wall can be penetrated by electric wires. In some examples, the control rods rise into pipes that protrude outside of the reactor and are closed at the tops of the pipes. Then the electric coils can be outside the reactor, since the pipes and the reactor vessel can be made of a material such as stainless steel that allows magnetic fields to penetrate. In this case, the only physical penetration of the reactor walls is by magnetic field.

[0132] The control rods 206 can be operated in groups or separately. The tubes 234 that hold the control rods 206 can be within the reactor or they can be tubes that extend out of a shorter reactor vessel, perhaps connected to the heat exchanger 210 above, as shown in FIG. 3. If the tubes 234 extend out of the reactor vessel 236, then there is no need to penetrate the reactor vessel with wires that operate the stepping motor; if the tubes 234 are entirely inside the reactor vessel 236, then such penetrating wires may need to be used. Since the pressure of the water in the casing is also set by the depth of fresh water, the water within the reactor vessel will have the same pressure as the water outside. Since radiation shielding is not needed, and since the pressure difference is close to zero, the primary need for strength of this vessel is to maintain integrity during surface handing and emplacement.

[0133] FIG. 4 shows an example reactor with different proportions compared to FIG. 3. FIG.4 shows the major bottomhole components of the 17-inch borehole nuclear reactor. At the very bottom is the pressurizer 222, which keeps the pressure inside the reactor at or near to the pressure in the casing water, which in an example implementation is about 160 atmospheres. The pressurizer 222 can have a height 402 of, for example, approximately four to eight meters. Next higher is the reactor core 201, which in this diagram uses the packed design of FIG. 10B. The reactor core 201 can have a height 404 of, for example, approximately two to eight meters. Above the reactor core 201 is space called the neck. In FIG.4, this space serves to provide greater pressure difference in the primary loop, and it also provides space to hold undeployed control rods 206. This space can also serve to provide additional pressure head for the convective circulation. AtAttorney Docket No.: 57302-0005WO1 the top is the heat exchanger 210, which here is depicted as a single pass with counter-flowing fluids having thermal contact in the primary and secondary loops. Single pass is possible because the long height of the drillhole, one mile, allows room for expansion.

[0134] In some examples, a borehole nuclear reactor can deliver approximately the same amount of power as delivered by a single fuel assembly in a gigawatt PWR, (e.g., about 5 MWe). In some cases, a higher power can be achieved, such as 15 to 50 MWe. If multiple boreholes are employed, then the entire collection (the nuclear “power plant”) up to or greater than 1,000 MWe.

[0135] In some examples, higher power can be achieved by using a taller reactor core. For example, the length (height) of the fuel assembly can be extended to eight meters or greater.

[0136] In some examples, higher power can be achieved by using multiple borehole reactors at a single site, as illustrated in FIG.1. The boreholes can be drilled close to each other, such as with a spacing of a few meters. With that spacing, 200 MWe of generating power can fit within one acre (4046 square meters), and a gigawatt plant can fit within a city block of approximately 2.5 acres.

[0137] In some examples, higher power can be achieved using a system of tandem reactors, as shown in FIG. 5. A tandem reactor system 500 includes two or more active nuclear reactors sitting one above the other with separate fuel assemblies and control systems, and with a shared heat exchanger. For example, a second reactor core 501b can be positioned between a first reactor core 501a and the heat exchanger 510. The heat exchanger 510 can be in thermal communication with both the first reactor core 501a and the second reactor core 501b.

[0138] An advantage of a tandem reactor system 500 over a single very tall reactor is that the tandem reactors can be assembled at the top of the borehole. The individual reactors 501a, 501b can be transported to the borehole and connected when the first reactor 501a has been partially lowered so that only the top of the first reactor 501a is at ground level. The first reactor 501a can be lowered into the borehole, and when only part of the first reactor 501a remains above the surface, the second reactor 501b can be attached to the first reactor 501a. Each reactor 501a, 501b has separate control rods 506a, 506b, respectively, so each reactor 501a, 501b can be operated separately. The tandem reactor system 500 can be operated for a longer period of time, or at higher power, compared to a single reactor.

[0139] The secondary loop begins at the steam generator 510 (immediately above the reactor at depth, or separated by a neck than can be up to 40 meters tall or greater) where coolAttorney Docket No.: 57302-0005WO1 water, returning from the turbine and entering the condenser at the surface, is converted to steam. The rising steam and the returning cool water travel up and down the cased borehole, respectively. This pathway also contains electric cables as well as the pressurizer water, which makes a continuous path from the reactor to the surface. The remaining space in the casing above the reactor, neck, and heat exchanger is filled with fresh water, water containing anti-corrosion chemicals, water containing chemical with high neutron absorption, chemicals to control the acidity (pH), water containing dissolved salt (brine), or water with a combination of such chemicals. This water provides pressure as well as a source for the Emergency Core Cooling System (ECCS).

[0140] FIG.6 shows a plan view section of the casing above the steam generator, from the steam generator to the surface 101. The casing 226 contains the insulated pipe 620 for conducting steam to the surface 101 and contains the return water pipe 602 of the secondary loop. The casing 226 contains boron and hydrogen control tubes 604, power cables 606, and reactor lift cables 608. The tubes 604 are used to control boric acid and other chemicals, as well as filtering for the removal of particulates. In some examples, the tubes 604 can be used for volume and pressure control for the primary loop. FIG.6 shows a support structure 621 for the two pipes, and a centralizer 622.

[0141] The pressure at the bottom of the secondary loop can be controlled by varying the water level in the supply pipe 602. In some instances, the hydrogen tube does not extend to the bottom but only to sufficient depth to assure ready dissolution of hydrogen gas. The casing 226 can be surrounded by cement 624 and a host rock 626. The casing 226 can have a sampling loop tube 912 for sampling the primary loop, described in greater detail with reference to FIG.9. The sampling tubes 912 extend down to and is connected to the primary loop.

[0142] The secondary loop consists of the cold water flowing downward through the cold water feed pipe 602, and the steam flowing upward through the steam pipe 620. The steam pipe 620 can be covered with up to several centimeters of insulator. The insulation can be vacuum, rock wool, aerogel, fiberglass, calcium-silicate, perlite, ceramic fiber, or a combination thereof. Approximately 2% of the energy in the steam will conduct and convect into the casing water as the steam rises to the surface. This heat will conduct and convect to surrounding rock and upward to the surface. Temperature rise of the surrounding rock is estimated to be no more than a few degrees Celsius.Attorney Docket No.: 57302-0005WO1

[0143] By the time the steam reaches the surface, the steam is expected to have been dried by the long passage through the steam pipe. If necessary, the steam will pass through an additional drier to remove droplets. Next, the steam will flow into and turn a conventional steam turbine to provides power for an electric generator. Alternative generators can also be used, including Stirling Engine, to provide higher efficiency or other advantages. At the output of the turbine, the low-pressure steam is condensed, mixed with water that condensed from steam within the turbine. Produced waste heat from the electric generator can be used for commercial, residential, or industrial purposes, or released into the air externally by a cooling system. A forced air cooler can be used. The condensed water can then be placed into the cold-water return pipe 602 to descend to the steam generator. In some instances, the cold-water return pipe 602 can also be insulated.

[0144] The level of water in the vertical cold-water return pipe 602 in the borehole can be adjusted to maintain the desired pressure in the steam and in the steam generator. For example, to maintain a pressure of 65 atm at the bottom of the secondary loop to facilitate steam generation, the liquid level of the cold-water pipe can be kept at 650 meters above the steam generator.

[0145]

[0146] For ease of installation, the steam pipe 620 and the water pipe 602 can be made of flexible material that can be rolled onto a coiled tubing feeder. Doing so would simplify the installation of the borehole nuclear reactor (which can be supported by the two pipes as the reactor is lowered into the borehole, or by cables 608). If cables are used, then three cables offer the ability to keep the reactor level as the reactor is lowered. In some examples, the insulated steam pipe 620 is lowered in 100-foot sections that each attach to its predecessor as the section is being lowered.

[0147] Boric acid can be injected into the primary loop coolant water at levels up to 4% to control the power production rate of the sustained chain reaction by the strong neutron-absorption property of boron. Natural boron has a neutron capture cross section of 760 bars for thermal neutrons. Injecting boron into the reactor primary coolant water can be performed by placing concentrated boric acid in a container at depth. The boron can be removed by a Chemical Purification System (CPS) or by dilution (replacement of primary loop borated water with non- borated water).

[0148] Referring to FIG.7, a boron removal system can bleed off a small amount of the primary water and run the water through a filter, or alternatively, replace the bled borated waterAttorney Docket No.: 57302-0005WO1 with fresh water. The system fits in the narrow confines of the hole. A chemical removal system for other substances, including I-135 and particulates, can be used because the quantities to be removed are small.

[0149] In some examples, two small tubes 702 and 704 of approximately 1 to 2 cm in diameter can extend from the primary loop to the terranean surface 101 or near terranean surface 101. Filtering of the primary water can be performed at this location. The tubes 702 and 704 can be used for injection of borated water, and to dilute the boron. The flow can be reversed to increase the boron concentration in the primary loop. Use of the sampling tube for boron control has many advantages over the alternative of putting a boron filter at depth near the reactor. Among other advantages, use of the sampling tube for boron control allows direct sampling of the primary loop water to do laboratory tests for early detection of corrosion and particulates created by fluid flow wear. The tubes 702, 704 are small, and normally filled with non-flowing water. These tubes can also be used to take up water when the coolant expands due to heating.

[0150] In some examples, a simple natural convective flow chemical purification system, or filter 706, can be included in the borehole nuclear reactor. In this example, a small amount of primary water is bled from the top of the primary loop and rises in a convection tube 708. The primary water then descends alongside the cool downflowing water of the secondary loop. The height of these flows and the temperature difference provide a pressure head sufficient to push the water thorough a filter which can remove small amounts of chemicals (such as iodine) and particulates.

[0151] In a 1000 MWe PWR, the pressure in the reactor is maintained at about 160 atmospheres. The high pressure allows the water in the primary loop to circulate at 315°C, high enough to give a good Carnot efficiency, and yet a sufficiently low that the water in the primary loop will not boil. For the borehole nuclear reactor, an approximate mile depth is chosen, in part, because it allows the use of a simple and robust means of pressurization.

[0152] Referring to FIG.8, in some examples, downhole of a reactor 810, a borehole 830 continues downward for an extended length 811. The extended length can be, for example, approximately ten meters or greater, approximately twenty meters or less. The lower section 840 of the borehole 830 that continues for the extended length 811 can be referred to as a “pressure reference cavity” or “sub-hole.” The lower section 840 can have a reduced diameter 836 compared to a diameter 831 of the portion 841 of the borehole 830 where the reactor 810 is positioned. TheAttorney Docket No.: 57302-0005WO1 lower section 840 of the borehole 830 can have a diameter 826 that is half the diameter 831, for example. This reduced size allows the reactor vessel to be supported by the bottom of the borehole rather than by cables that go to the terranean surface, or by a special platform or support structure configured to support the reactor vessel. In some instances, the lower hole can have the same diameter as the upper borehole, with the reactor supported by cables or other apparatus. In this second instance the wider hole below the reactor can be used as a storage or disposal space once the reactor fuel is spent; the reactor can be lowered into this space and a reactor with fresh fuel placed in the position that the reactor with spent fuel once occupied.

[0153] The pressurizer 822 sits in the lower section 840. The sub-hole fills with water at the local hydrostatic pressure, typically 160 atmospheres (1 atm for every 10 meters of depth). The bottom of the pressurizer 822 includes flexible (e.g., thin walled) pipe with a cross section that enhances the flexibility of the shape; in FIG. 8 this shape is elliptical. This flexible pipe descends into this region to function as a bellow, or, if it has a convoluted surface, as a bellows. In the instance of the elliptical bellow shape, because of its shape, the pipe responds quickly (getting more circular or more elliptical) to pressure differences between the outside casing water and the interior reactor water changes, thus equalizing the two pressures (except for the spring coefficient of the bellow). By making the pressurizer 822 longer, pressure can be equalized with lesser flexure and lesser danger of work-hardening.

[0154] A casing 816 can be filled with fresh water, and the casing water at depth can be approximately the same as the pressure desired in the reactor. If lower pressure is desired, for example to use a boiling water reactor instead of a PWR, then the borehole nuclear reactor can be operated at a shallower depth, or the casing 816 can be only partially filled with water.

[0155] The role of a pressurizer 822 is to equalize the two pressures (e.g., the pressure inside the reactor and the pressure outside the reactor and inside the casing). Equalizing the pressure can be done with bellows or other similar mechanisms, or with a piston or multiple of pistons. The pressurizer 822 can include a membrane 824 that separates fluid inside the reactor core from fluid outside the reactor core. The pressurizer 822 can expand and contract to control fluid pressure inside the reactor core. The example shown in FIG.8 includes an elliptically shaped flexible container that is extended below the reactor vessel 836 into the section 840 of the borehole 830 that is available for this use, but which can also be used for storage or disposal of a reactor containing spent fuel.Attorney Docket No.: 57302-0005WO1

[0156] The pressurizer 822 can play a major role when the reactor chain reaction is first stared. Initially, the water in the reactor will have temperature similar to that of the ambient rock at one mile depth (typically 40C to 60°C). When the chain reaction is in full operation, the water in the reactor is heated, and the density in the hot section drops and the water density in the “cold” down-flowing section has a higher density. As the water heats, its density decreases, and its volume expands. In a typical instance, this expansion can be 30% to 50% in volume. This volume increase can be accommodated by the pressurizer expansion, and it requires a long pressurizer, perhaps 10 meters long. If boron injection is used with dilution tubes that rise to the surface, then these tubes can provide for the initial reactor water expansion, and a smaller pressurizer can still be used to allow these tubes to be sealed except during the boron adjustment periods.

[0157] The filling of the casing with fresh water (water with low salt content compared to the brine in the adjacent rock) means that the pressure inside the casing is within a few atmospheres of the pressure of the brine outside the casing. In the oil and gas industry, in which the casing 816 is filled with a much lighter material (oil or gas), the pressure across the casing wall is much larger, and a thicker casing wall must be used than needed for the borehole nuclear reactor. Instead of a large pressurizer, the borehole nuclear reactor uses a simplified pressurizer to equalize the pressure inside the reactor to be equal to that in an underground reservoir that is at the 160-atmosphere hydrostatic pressure. Thus, there is no large pressure to contain; the pressure of the fresh light water inside the pressurizer is equal to the pressure of the ambient water at the bottom of the casing.

[0158] Other pressurizer designs can be used, such as an “accordion” bellows. Work- hardening of the pressurizer flexible surface can be minimized by keeping all flexures low and by minimizing cycling (multiple changes from low to high and back to low pressure). Corrosion resistance in the fresh reference water is effective for the lifetime of the reactor core, which expected to be 2 to 10 years.

[0159] The pressurizer 822 can have a height 834 of, for example, 8 meters long or greater, 16 meters long or greater, 20 meters long or greater. In some examples, the height 834 of the pressurizer 822 is longer than the height 124 of the reactor core 810. In some examples, the diameter 828 of the pressurizer 822 is less than the diameter 126 of the reactor core 810.

[0160] In some examples, the pressurizer 822 is be deployed as a flat collapsed ribbon when initially placed in the borehole and fit into the sub-hole at the bottom of the location where the reactor sits or is held in place. The sub-hole can have a height, or length 811, of approximatelyAttorney Docket No.: 57302-0005WO1 twenty meters. In some examples, the pressurizer is rolled off a spool, liked coiled tubing, and when the top end is just above the ground level, the pressurizer can be attached (e.g., screwed, welded) to the bottom of the reactor. When the reactor is at depth, and the water within the reactor is heated to an average of 305°C, the water in the reactor will expand, and the pressurizer will expand to accommodate this water. An alternative is to have the water expand into tubes that reach the surface. After the pressurizer 822 is fully expanded, the pressurizer 822 will undergo more modest changes in shape to maintain the pressure inside the reactor vessel 836 to within a few atmospheres of the pressure in the fresh water outside the reactor vessel 836.

[0161] In some examples, instead of being downhole from the reactor, the pressurizer can be uphole from the reactor. An example is shown in FIG.9, which shows an example chemical, volume, and pressure control system 900. A vault 902 can be located below the surface 101 (as shown) or at or above the surface 101. Radioactivity in the vault 902 is small since the radioactivity in the primary water is low; moreover, short-lived radioactivity in the primary loop is primarily nitrogen-16, and that decays with a half-life of 7 seconds, and will decay before reaching the surface. The primary radioactivity of this water is expected to be tritium. Tritium decays by emitting a low energy electron (beta decay) that will not penetrate the walls of the pipes and filtering system, so the expected radioactivity in the vault is expected to be the natural level (from nearby rock) approximately the same as if the reactor were not there. An access port 904 allows for opening of an upper section of the vault 902 in order to take samples of the primary loop coolant. A lid 906 would normally be lifted only during initial placement of the reactor; the cables that support the reactor are not shown. Most of the water in the pressure adjust and volume control tanks 922a, 922b comes from the initial expansion of the water in the reactor when the reactor is initially brought to criticality and the water within it expands by approximately 40% to 60%.

[0162] The pressure adjust and volume control tanks 922a, 922b can be placed underground, typically at a depth of 2 to 10 meters. The tanks 922a, 922b can be accessible by a portal 934 in the casing 916. The casing 916 is filled with water 936. The tanks 922a, 922b are isolated in this manner because the primary loop water, although expected to have extremely low radioactivity, could become radioactive if there is major damage in a fuel pellet and its casing. At the end of the reactor lifetime, the water can be left in the tanks 922a, 922b, or drained and movedAttorney Docket No.: 57302-0005WO1 to a permanent low-level waste disposal facility. Pressure in the tanks 922a, 922b is monitored by pressure sensors 938a, 938b.

[0163] A sampling loop tube 912 extends from the primary loop to a pressure adjust tank 922a near the surface 101 and also to filter 928. The tube 912 includes a supply line 912a and a return line 912b. The tube 912 can be seen near the top of FIG.3 and entering the pressure adjust and volume control tanks 922a, 922b in FIG.9. The tube 912 serves multiple purposes, including as a pathway for water from the initial expansion of the primary loop (when it is first heated) to enter the pressure tanks. The pressure of the air above the water in the tank 922a can be maintained at about 1 atm, or slightly above or below. The pressure in the primary loop will then be the hydrostatic depth of the water, about 160 atm. If the water expands or contracts, the excess (or deficit) water will move up or down the tube 912, but the pressure will remain constant. A pump 914 can move water from the tank 922a to the tank 922b to reduce the pressure, or from tank 922b to the tank 922a to increase the pressure. The pump 914 can be automated to keep pressure constant and can be overridden by an operator observing readings from the two pressure sensors.

[0164] The tube 912 provides a pressure at the bottom given by the hydrostatic height of the tubes, about 1 atmosphere of pressure for every 10 meters of depth. The pressure in the tube 912 at the surface is close to atmospheric pressure, although it can be held slightly above one atmosphere to allow better control of the pressure in the core. The pressurizing tube 912 also allows for continuous sampling of the reactor coolant for indications of primary loop damage, such as corrosion or wear.

[0165] The tubes 912a and 912b can provide a conduit system for a chemical and volume control system, including for boron injection and removal, for filtration of the primary coolant water, and for injection and dissolution of hydrogen gas into the primary loop. The system 900 includes a boron injection tank 924, a hydrogen injection tank 926, and a filter 928. The system 900 includes a fresh water tank 930 for boron dilution and a sample access 932. Although the water in the primary loop has a high temperature, the water in the sampling / pressure-control tube 912 is cooled by the casing water, which is, in turn, cooled by the surrounding rock.

[0166] FIGS.10A and 10B illustrate, in profile, two example primary loop configurations. FIG.10A shows down-going water 1004 from the heat exchanger returning to the bottom of the reactor through a path 1002 outside the core 1001. The path 1002 can be an annulus, or other shape that fits conveniently around the core. In FIG.12C, the return path are “lune” shapes, thatAttorney Docket No.: 57302-0005WO1 is, similar to the shape of a half moon or the letter “D”. FIG.10A shows the upflowing water 1005 passing past the fuel rods 1003 and into a space above. FIG.10B shows a profile for a fuel packed reactor configuration in which the down-going cool water 1014 passes through a group of fuel rods 1006 and is partially heated on the way down, and the upflowing water 1015 passing through another group of fuel rods 1007.

[0167] In some aspects, insulation 1010 can be positioned between the downflowing and up flowing water. If not used, then the thermal contact serves to pre-heat the water before it reaches the upward loop.

[0168] FIG.11 shows an example nuclear reactor layout for an 18-inch borehole. In place of a typical square cross-section fuel assembly, one with a rounded shape is used to allow more fuel rods to be added around the periphery. Some rods in the interior are removed to increase the ratio of moderator to fuel. In this layout, the cool water descends in an outer annulus 1102 surrounding the core 1104, enters the core on the bottom, and then flows up through the core 1104 and into the neck (if there is one) and into the heat exchanger.

[0169] Other example fuel configurations with larger numbers of fuel rods than shown in FIG.11 are shown in FIGS.12A, 12B, and 12C. In FIGS 12, the rods are shown as black dots. A nuclear fuel assembly can have multiple sectors, with a first portion of the sectors configured for an upflowing fluid and a second portion of the sectors configured for a downflowing fluid. In FIGS. 12A and 12B, the downflowing water, rather than configured as an annulus, is filled with fuel rods, and has a more symmetric upflow / downflow configuration.

[0170] FIG.12A shows a 17-inch reactor core 1200 that is divided into four sectors, half for downflow coolant and half for upflow. For example, sectors 1204a and 1204b are configured for downflowing water. Sectors 1206a and 1206b are configured for upflowing water.

[0171] FIG.12B shows a reactor 1250 that fits into a 24-inch casing, such as a casing in a borehole with a 26- to 30-inch diameter. For example, sectors 1254a and 1254b are configured for downflowing water. Sectors 1256a and 1256b are configured for upflowing water.

[0172] The number of sectors of a fuel assembly can be, for example, two, three, or greater. Each segment in these configurations is based on the design of a 17x17 gigawatt PWR fuel assembly. The outer shapes of the fuel assemblies are truncated to make a snug fit inside the cylindrical reactor vessel. The segments have physical dividers 1202, 1252 between the segments to divide the up flowing from the downflowing water. The dividers, or barriers, can be insulatedAttorney Docket No.: 57302-0005WO1 to enhance the temperature difference and, in that way, increase the forces for convective flow and the Carnot efficiency of the output. In some cases, the barriers are made of stainless steel. The reactor vessel 1210, 1260 can be made of steel or beryllium. The use of beryllium is to reflect neutrons, which increases the ability of the reactor to maintain a sustained change reaction with partially depleted fuel. As previously mentioned, the number of fuel rods can be reduced to increase the moderator-to-fuel ratio in the core.

[0173] FIG.12C shows a core of an example reactor in plan view. The core 1200 includes four 17x17 fuel assemblies 1240a, 1240b, 1240c, 1240d, surrounded by a round reactor vessel 236. The space 1226 between the fuel assemblies and the vessel 236 serves as the downcomer for return water from the steam generator to reach the bottom of the core 1200. The space 1226 between the reactor vessel and the casing is filled with fresh water. A feature of this arrangement is that the reactor can use standard PWR fuel assemblies.

[0174] The standard PWR fuel assemblies of the core 1200 can have 12-inch length diagonals and are 14-feet in height. Each assembly can have a standard 17 x 17 pin array with 264 fuel rods and 25 thimbles for control rods and instrumentation. Outside the fuel assemblies are four "lunes," lunar-shaped regions for the downcoming, returning cool water from the heat exchanger to the bottom of the reactor. The four assemblies and the lunes are contained in the cylindrical reactor vessel. This vessel is, in turn, contained by the casing that lines the borehole. In between the reactor vessel and the casing is fresh water that is continuous to the surface. The reactor can use low enriched uranium (LEU) at less than 5% U-235 enrichment in uranium-oxide ceramic fuel, gadolinium can be added to help level the burn. The reactor can also operate with more highly enriched uranium if that becomes available. An example of such uranium is HALEU (High Assay Low Enriched Uranium) which is enriched up to but not including 20% enrichment. (At 20% enrichment the U.S. government defines it as HEU, for Highly Enriched Uranium.) All of the reactor layouts shown in this report (FIGS.10, 11, 12) and those suggested by these layouts can use HALEU fuel. The benefit of using HALEU in place of LEU depends on the relative costs of HALEU compared to LEU fuel.

[0175] One aspect of the design shown in FIG. 12C is that each of the four sectors can include a standard 17x17 PWR fuel assembly with little or no modification. That aspect enhances the supply chain, that is, it improves the availability of the fuel assemblies. It can also improveAttorney Docket No.: 57302-0005WO1 the processing of licensing since licensing commissions have high familiarity with current fuel assemblies.

[0176] The borehole nuclear reactor can operate between a “cool” temperature of 275°C and a hot temperature of 315°C. These values can be changed depending on the depth of operation or desired Carnot efficiency; steam at higher temperature has a higher conversion efficiency to produce electricity. For that purpose, the reactor could be placed at a greater depth, since greater depth provides a higher pressure, which increases the boiling temperature of the water.

[0177] The effect on the reactivity of the core comes from the competition between the moderator and the U-238 in the uranium-dioxide, which is an absorber of fast neutrons. As the moderator density decreases, it takes a greater path length in the moderator to slow a neutron. The path length in the uranium-dioxide is virtually unchanged as a function of temperature, because of its relatively small temperature expansion coefficient. Thus, as the temperature rises, a larger fraction of neutrons will be absorbed on U-238 before they are fully moderated. When the temperature in the core rises, then the moderating power decreases, and the sustained chain reaction will stop. As the temperature drops, the chain reaction will begin again. This is a very strong negative feedback effect. Increased reactivity results in a higher convective flow rate in the primary loop, thus bringing the heat to the heat exchanger quicker.

[0178] A borehole nuclear reactor utilizes convective circulation in the primary loop to minimize the number of moving parts at depth. Convective circulation is facilitated by the density difference between the water in the hot up-flowing sector and the water in the “cold” downflowing sector in the primary loop.

[0179] Generally, the difference in temperature and density between the upward and downward flowing regions in the reactor is sufficient to use convective circulation instead of pumps for this purpose. The circulation can be increased or decreased by changing the heights of the neck and the height of the heat exchanger. In an example, a borehole nuclear reactor includes fuel assemblies having a length of four meters (14 feet). The neck is about 4 meters long and the heat exchanger is also 4 meters tall.

[0180] The coolant flow velocity can be increased by increasing the length of the neck and the of the heat exchanger (thus increasing the pressure head). The flow velocity can be decreased by incorporation of a at least one throttle valve in its path. One possible location for this throttle valve is at the bottom of the heat exchanger.Attorney Docket No.: 57302-0005WO1

[0181] There are several feedback mechanisms in the heat exchanger. If the flow in the primary loop is slow, then the heat transfer in the descending water occurs over a shorter distance, and this causes the flow velocity to increase. Similarly, if the flow is rapid, then hot water descends in the hot part of the heat exchanger, lowering the pressure difference, and reducing the primary flow velocity.

[0182] Lower flow reduces cooling in the reactor core, and that reduces the power production through the reactivity negative feedback mechanisms, primarily from thermal expansion of the coolant water, and the Doppler broadening in the fuel.

[0183] The steam pipe lies within the casing and must convey steam from the heat- exchanger / steam generator to the surface steam turbine. The velocity of flow depends on the diameter of the pipe. Increasing the diameter reduces pressure drop, and decreasing the diameter reduces heat loss.

[0184] When the steam reaches the surface, the steam is fed into a drier (if necessary) and then into steam turbine, Stirling engine, or other heat engine. In some examples, the borehole nuclear reactor can perform autonomous load following by following changing loads without need for human intervention. In some aspects, strong negative feedback that slows or quickens the chain reaction when the temperature rises or falls, respectively, can be used. A contributor to load following is the large negative temperature coefficient that arises from the high thermal expansion coefficient of the coolant.

[0185] In some examples, component of the borehole nuclear reactor can be connected to form a preassembled kit 140 prior to installation in the borehole. For example, components such as a fuel assembly, control rods, a heat exchanger, and a pressurizer can be assembled in a manufacturing facility and shipped to the borehole site on a flatbed truck. Fresh low-enriched uranium fuel has low radioactivity. With the fuel rods locked in place, no water in the reactor vessel, and the entire reactor carried in a shipping tube that has at least a portion with a high neutron absorption property (to prevent the assembly from undergoing a sustained or growing chain reaction if, for example, it is flooded with water by accident).

[0186] A feature of a borehole nuclear power plant is that the major components can be manufactured off site (including the nuclear reactor and the heat exchanger) and then assembled on site with relatively simple procedures, and then installed in the borehole. Some of the assembly can take place above the borehole as the following example shows. The reactor is lifted into aAttorney Docket No.: 57302-0005WO1 vertical orientation using an overhead crane and placed over the cased borehole. The borehole nuclear reactor can then be lowered into the borehole until only the top part is exposed. The neck and heat exchanger can then be attached. Next, this combination can be lowered and the steam pipe, cold water supply pipe, and other pipes can be attached. If these pipes are flexible, then the pipes can be delivered as coiled tubing.

[0187] In the borehole nuclear reactor, one purpose of the casing is to assure smooth guidance of the reactor to depth prior to the initiation of the reactor chain reaction, to assure easy removal of the reactor (if required by regulatory agencies). Another purpose is to prevent rockfalls from spallation and crumbling into the hole. There is a little pressure difference from inside to outside the casing since the interior space (excluding the cold water pipe and the steam pipe and electrical cables) is filled with fresh water at approximately the same pressure as the brine in the host rock. Oil and gas certified carbon steel casing would serve this purpose very well.

[0188] For the casing in the immediate vicinity of the reactor, in some instances the casing can be omitted; it is not needed for radiation protection since that is provided by the geology. The role of the casing at the bottom of the borehole is to reduce the risk of rock spallation against the reactor wall. For this purpose, it is anticipated that oil and gas certified casing would be adequate at the very bottom of the hole. The use of such standard casing provides a robust supply chain. The borehole nuclear reactor is configured to use LEU (low enriched uranium, ≤ 5% U-235) ceramic (uranium dioxide) fuel pellets. Use of standard LEU pellets enables the reactor to use existing supply chains to obtain fuel. The security and safety of the borehole nuclear reactor are ensured in by the 10 billion tons of rock in the 45 degree cone of rock above the reactor.

[0189] An example poison member 1300 for deploying the gadolinium in wires is shown in FIG.13.

[0190] In some instances, it is desirable for the gadolinium neutron poison to be completely consumed at about the same time when the sustainability of the chain reaction is no longer possible due to consumption of U-235. Complete consumption of the gadolinium can be accomplished by using the wire geometry for the gadolinium. Complete consumption will not occur if, for example, the gadolinium is distributed uniformly in the reactor moderator (as, for example, a dissolved gadolinium salt) or evenly within the fuel pellets. If the gadolinium is distributed evenly in the fuel, the when the U-235 is half gone, only half of the gadolinium could be gone, whereas it is desired that all gadolinium be gone at the stage when the chain reaction can no longer be sustained.Attorney Docket No.: 57302-0005WO1 The gadolinium is considered “gone” not when the metal gadolinium is completely absent, but when the neutron absorbing isotopes of gadolinium, Gd-155 and Gd-157, are depleted. Use of gadolinium to reach zero at the same time that the chain reaction is unsustainable is achieved by taking advantage of the self-shielding capability of gadolinium and by placing the gadolinium in a thin cylindrical geometer, that is, in wires. Because gadolinium has a very high cross-section for thermal neutron absorption, with a mean free path of only seven microns, its absorption is proportional to its area, not to its volume. To implement self-shielding, rather than introducing gadolinium uniformly in the fuel, the gadolinium can be arranged in a compact geometry such as a wire, a plate, or a sphere. The time of burn (actually, the integrated neutron flux to consume all the gadolinium) can be chosen to be any value desired by choice of the radius of the wire. The gadolinium, as it burns, does not disappear. Rather, the Gd-155 and Gd-157 are being converted to Gd-156 and Gd-158, which are not strong neutron absorbers. There is insufficient recoil when these isotopes absorb a thermal neutron to cause any disruption of the gadolinium metal. The use of gadolinium threads, or wires, is not confined to borehole PWRs, but can be valuable for large PWRs, BWRs, molten salt reactors, molten metal reactors, high temperature gas cooled reactors, and other types of reactors.

[0191] The mechanism that provides the gadolinium absorption to be proportional to the reactor burn is described here in greater detail. The U-235 burn rate can be considered constant to yield a constant power output. For a thick wire with initial radius r0 7 micron (μm), the radius of the poisoning isotopes of gadolinium decrease with time through surface burn out. Thus, the radius of the Gd-155 and Gd-157 components will decrease linearly with time. But the area of the cylinder (e.g., the thread, rod, or wire) is proportional to the radius, so the area also decreases linearly with time until the Gd-155 and Gd-157 are consumed. That means that the poisoning power decreases linearly with time until the center of the wire is reached. This linearity can be matched to the linear decrease (for constant power production) of the U-235. The linearity can also compensate for the linear build-up of long-lived neutron poisons that are created in the neutron-rich environment of the nuclear core. Gadolinium does not generally reduce the effect of these poisons, but it can make the total poison contribution constant with time, which eases the burden of control rods and boron adjustments.

[0192] As an example, if a nuclear reactor without gadolinium wires is operating with boric acid in the moderator and at a neutron multiplication factor of keff= 1.05, meaning that for everyAttorney Docket No.: 57302-0005WO1 fission the released neutrons will fission an additional 1.05 fissions, then the reactivity will grow exponentially. The desired value for a sustainable chain reaction is keff = 1.00. To reduce keff, more boric acid can be added to the moderator. An alternative is to add gadolinium wires into the core. To change keff = 1.05 to the desired keff = 1.00, the gadolinium must absorb 5% of the neutrons in the core. Since both the pellets and the gadolinium are strong absorbers of neutrons, the surface area of gadolinium is sufficient to absorb 5% of the neutrons can be accomplished by making the gadolinium wires have an area equal to 5% of the area of the fuel pellets. For fuel pellets with a diameter of 1 cm, the gadolinium thickness (all wires combined) in this example is approximately 5% of the diameter, or 0.05 cm. This estimate assumes the length of the gadolinium wires is equal to the length of stacked fuel pellets.

[0193] In the previous example, the total diameter of the gadolinium in the wires was estimated for the 5% neutron absorption example to be 5% of the 1-cm pellet diameter. This total width can be achieved with a single wire with diameter 0.05 cm, or with multiple wires. If five wires are used, then each wire has a diameter of 0.01 cm. The number of wires can be selected based on the time estimated to reach zero remaining absorption by the wire. That time can be chosen to match the lifetime of the fuel, that is, the time it takes for the fuel to be depleted to an enrichment at which a sustainable chain reaction in the reactor configuration is no longer possible.

[0194] FIGS. 13A and 13B show two possible installations of wires in or on a fuel rod. FIG.13A shows a perspective view of a segment of a fuel rod 1302, in which gadolinium wires 1304 extend across disks 1306 that are placed in between fuel pellets 1308. The fuel rod 1302 includes fuel pellets 1308a, 1308b, and 1308c stacked within a cladding 1310, which is a tube that holds the fuel pellets 1308. The fuel pellets 1308 each have a cylindrical shape.

[0195] Each disk 1306 can have a cylindrical shape with two circular surfaces that are congruent and located at each end of the cylinder. The wires 1304 can extend parallel to each other across a circular plane of the disk 1306. In some examples, a spacing between the wires 1304 is uniform across the circular plane of the disk 1306.

[0196] In the example of FIG. 13A, wires 1304a extend across a circular plane of disk 1306a, wires 1304b extend across a circular plane of disk 1306b, and wires 1304c extend across a circular plane of disk 1306c. Disk 1306a is positioned above fuel pellet 1308a. Disk 1306b is positioned between fuel pellet 1308a and fuel pellet 1308b. Disk 1306c is positioned between fuel pellet 1308b and fuel pellet 1308c.Attorney Docket No.: 57302-0005WO1

[0197] In some examples, wires 1304 nearer to the center of the disk 1306 have longer lengths compared to wires 1304 that are farther from the center of the disk. For example, wire 1304a-1, located nearer to the center of the disk 1306a, has a greater length than wire 1304a-2, located farther from the center of the disk 1306a.

[0198] In some examples, a disk 1306 can include gadolinium wires 1304 held by a ring, with the wires 1304 suspended across an annulus of the ring. A ring that suspends the wires can be a thin ring made of metal or ceramic. In some examples, the ring is a flat ring.

[0199] In some examples, the gadolinium wires 1304 are embedded in disks that are made of low neutron-adsorbent material such as zircaloy or aluminum. This layout has a feature that while the uranium is highly enriched, the low neutron-adsorbent material shields the gadolinium wires from a substantial fraction of the neutron flux, so the gadolinium is not depleted in the early stages of the reactor operation.

[0200] FIG. 13A shows the disks 1306 holding gadolinium wires 1304 between all fuel pellets 1308 in the section of the fuel rod 1302. In some examples, the disks can be placed only between certain fuel pellets, in order to adjust the burn within the reactor. For example, a disk 1304 can be placed between the fuel pellet 1308a and the fuel pellet 1308b, but not between the fuel pellet 1308b and the fuel pellet 1308c. As an example, if the neutron flux is larger in the central part of the reactor core, with keff = 1.05 when no gadolinium wires are present but keff = 1.01 near the outer regions of the core (further from the center) then the number of gadolinium- containing disks can be less in the outer regions. In some examples, the number of wires on each disk in outer regions of the core can be reduced compared to the number of wires on each disk in inner regions of the core.

[0201] The number of disks, or the number of wires on each disk, can also be varied with vertical position along the fuel rods. Such a variation can compensate for the reduced moderation of the coolant water as it rises through the reactor, is heated, and expands.

[0202] FIG. 13B shows a perspective view of a segment of a fuel rod 1312 in which gadolinium wires 1314 extend parallel to a cylindrical axis of the fuel rod 1312 and to cylindrical axes of fuel pellets 1308. The fuel rod 1312 includes fuel pellets 1318a, 1318b, and 1318c stacked within a cladding 1320. The gadolinium wires 1314 can be placed on the cladding 1320 and strung along the length of the fuel rod 1312. In some examples, the gadolinium wires 1314 are uniform in length. In some examples, a spacing between the wires 1314 is uniform around theAttorney Docket No.: 57302-0005WO1 circumference of the fuel rod 1312. The gadolinium wires 1314 can be placed on an outside surface of the cladding 1320 or on an inside surface of the cladding 1320. As with the disk configuration shown in FIG. 13A, the amount of gadolinium can be varied with location in the reactor.

[0203] In the examples of FIG. 13A and 13B, a gadolinium wire 1304, 1314, can be an alloy of gadolinium. In some examples, the gadolinium wire 1304, 1314 can be embedded in another material that provides additional strength. Throughout this specification, the term “wire” refers to the general case in which the gadolinium can be embedded in other materials.

[0204] Although this specification describes only two configurations for the placement of gadolinium wires, others are possible. For example, wires can be wrapped around the cladding. In some examples, the gadolinium wires can be embedded in the cladding, or embedded in the fuel pellets. The gadolinium is not mixed with the uranium dioxide but exists as short wires. In some examples, a length of a wire is at least three times greater than a diameter of the wire.

[0205] Although gadolinium is provided as an example herein, there are other burnable neutron absorbers that can be used in the wires. Examples of other burnable neutron absorbers are gadolinium oxide, boron-10, erbium, hafnium, and their oxides and other chemical forms. In some instances, it can prove advantageous to have combinations of these neutron-absorbing materials, and to vary the amount and the relative proportions in different parts of the reactor.

[0206] The use of neutron-absorbing wires is not restricted to small reactors. Such wires can be useful in large scale rectors, including PWRs, BWRs, and other types of nuclear reactors.

[0207] Gadolinium wires can also be used to homogenize the U-235 burn in the nuclear reactor. Because the neutron flux is not constant throughout the borehole nuclear reactor, the normal burn is uneven; there tend to be greater burn in the center of the core, for example, than elsewhere. If a neutron reflector is used, then there can also be greater burn around the rim. This non-homogeneity can lead to a premature drop of keff below 1, when the rapid burn-out regions are depleted, and the other regions are not sufficiently large to sustain a chain reaction without partial supply of neutrons from the burned-out parts.

[0208] To achieve homogeneity, more gadolinium wires can be placed in the regions of rapid burn. This assures a more uniform burn. Such uniformity is commonly achieved in a gigawatt reactor by using rods with differing U-235 enrichments in different locations. For the borehole nuclear reactor, it can be advantageous to put as much U-235 in the core as possible sinceAttorney Docket No.: 57302-0005WO1 it will not be replacing spent fuel, and that means 5% enrichment for all the fuel pellets. The gadolinium levels in the pellets will be determined by the necessary distribution determined to create a uniform burn.

[0209] The gadolinium poisoning can be tuned for other purposes. Flat plates of gadolinium have constant absorption, until they burn through. Wires have a linear decrease in poisoning, and spheres have a quadratic decrease in poisoning. A combination of these can be used, for example, to minimize the initial rise of Xe-135 buildup.

[0210] Iodine-135, a fission product of U-235, decays with a half-life of 6.6 hours to Xenon-135, which itself decays with a half-life of 9.2 hours. The net result is that when the borehole nuclear reactor is first turn on, the iodine initially builds up linearly, but xenon-135 builds up quadratically. This quadratic initial increase can be balanced by small spheres of gadolinium, since their area decreases quadratically with time. These small spheres can be built into the fuel or be placed externally (e.g., in thin sheets placed between fuel pellets), and their use can reduce the need for of control rods and boron adjustments.

[0211] In some aspects, gadolinium compounds in the primary loop can replace the use of dissolved boron hydroxide for the purpose of small changes in reactivity when following load or just compensating for U-235 depletion. Gadolinium has 66-times the thermal neutron absorption cross-section, so smaller amounts can be used. Therefore, removal of gadolinium can be easier to accomplish than removal of boron.

[0212] Gigawatt nuclear reactors often have a hydrogen gas bubble at the top of the pressurizer. The purpose is to keep enough hydrogen gas in the primary coolant to reduce oxygen dissolved in the water, particularly oxygen ions created by collision of neutrons with water. Such ions can enhance the corrosive power of water, but if hydrogen is dissolved in the water, the ions are neutralized.

[0213] For a borehole nuclear reactor, in the instance when a tube to the surface acts as a pressurizer, the top of the pressurizer is proximate to the surface, so the hydrogen bubble at the top of the reactor used in other reactors may not be appropriate. This issue can be addressed by placing the entry to the sample tube lower in the reactor / heat exchanger system, for example, in the neck. An alternative is to place a long inverted tube, closed at the top, either inside the reactor vessel, or just outside connecting near the bottom of the reactor. It can be placed in the same extension hole that contains the pressurizer. At the surface, before the borehole nuclear reactor is lowered intoAttorney Docket No.: 57302-0005WO1 the ground, this vessel can be filled with compressed hydrogen gas, or alternatively hydrogen gas at one atmosphere. As the borehole nuclear reactor is lowered, and the pressure in the reactor increases to its nominal 160 atmospheres, a valve separating the hydrogen from the primary loop water would relax, no longer held shut by the high pressure in the tank. When that happens, there is now high-pressure hydrogen in contact with the primary coolant water, hydrogen gas will dissolve in the primary loop, and oxygen ions will be suppressed in a similar manner to that in a surface reactor.

[0214] Another mode of burn for the borehole nuclear reactor is a “Cigar Burn.” In this, control rods that suppress a sustainable change reaction at the depth that they occupy, are moved together in the reactor and slowly raised; the nuclear fission burn is intense near the leading bottom edge of the control rods, and low everywhere else. The advantage of the cigar burn is in ease of reactor power control. The disadvantage is that xenon-135 buildup at the intense burn region absorbs neutrons and lowers keff. This will prevent a full burn of the U-235 at that level. This negative effect can be compensated by moving the control rods slowly up and down, to give the xenon (half-life of 9.2 hours) time to decay. The negative effect can also be reduced by use of gadolinium poisons.

[0215] Monitoring of the borehole nuclear reactor includes the usual PWR monitoring apparatus placed in an empty fuel rod or control rod location in the fuel assembly. Because the interior dimensions of the borehole nuclear reactor can be, e.g., identical to those in a large PWR, the same monitors can be used. These include monitors for temperature, pressure, neutron flux (several, for different energy sensitivities such as fast neutrons and thermal neutrons), and gamma ray measurements. A Nitrogen-16 detector can be placed at many possible locations in the core to monitor fast neutrons (which produce the short-lived 7 second half life N-16 from O-16).

[0216] Small cameras can be lowered within the casing (but outside the cold water and the steam pipes) to inspect the casing for corrosion or other changes. It is also possible to monitor the borehole nuclear reactor from the surface (by using, for example, acoustic and seismic imaging). The water in the casing can be sampled and can be analyzed for signs of corrosion.

[0217] The interior of a large PWR is inspected, typically, every 18 months when the reactor is shut down, the lid removed, and a third of the fuel assemblies are replace. The borehole nuclear reactor is configured to be recoverable, and if it is deemed necessary, it can be brought to the surface for inspection of the exterior of the reactor. The reactor can be lifted to the surface byAttorney Docket No.: 57302-0005WO1 the permanent cables attached to it; three such cables 608 are shown in FIG.6. An inspection can be performed using an X-ray of the reactor, or a 3D X-ray or gamma ray tomograph. Imaging can also be done using the existing Cs-137 within the reactor core. This might be best done in a room slightly underground, as shown in FIG. 9, so that the earth continues to provide shielding from gamma radiation from fission fragments in the nuclear reactor.

[0218] In addition, remote monitoring can be used, including passive and active seismic. Surface passive acoustic detection will alert to unanticipated sudden changes in the reactor, and seismic imaging offers assurance that no large-scale changes are taking place.

[0219] When the fuel has been depleted to the level that a sustained nuclear chain reaction is no longer possible, the fuel is defined as spent nuclear fuel or as nuclear waste. There are several options for the borehole nuclear reactor at this time. The decommissioning of the first reactor installed in a borehole can be performed as follows.

[0220] The decommissioned reactor can be brought to the surface for storage or disposal it in an approved facility. This can be storage in dry cask, or disposal at an underground facility. The nuclear reactor can be lifted to the surface using the permanent support cables 608 in FIG.6.

[0221] Alternatively, the decommissioned reactor can be stored, with its spent fuel, in place until a suitable disposal site is identified. The access hole can be continually inspected to certify that the hole will allow safe retrieval. If signs of corrosion or other potential impediments appear, then the hole can be cleaned and / or the reactor can be brought up for surface storage.

[0222] Alternatively, the decommissioned reactor can be shut down, and then the connection to the surface can be disconnected above the heat exchanger, leaving the reactor and the heat exchanger in place. The disconnected reactor can be considered stored (since it can be later retrieved) or disposed. Note that the primary loop is not breached in this approach. Doing this is underground “storage” is an alternative to above ground storage. A platform can be inserted above the decommissioned reactor and a new reactor can be lowered with fresh fuel to operate just above the decommissioned reactor. The borehole, casing, steam pipe, turbine, and condenser can all be reused. Full inspection of the borehole can be done before the new reactor is emplaced, and the parts above the reactor can be inspected after installation.

[0223] Alternatively, the decommissioned reactor can be shut down, and then the connection to the surface can be disconnected in or below the neck. The disconnected reactor can be considered stored (since it can be later retrieved) or disposed. Although the primary loop isAttorney Docket No.: 57302-0005WO1 breached in this approach, the radioactivity released to the casing water can be small enough to be acceptable. The borehole, casing, steam generator, steam pipe, turbine, and condenser can all be reused. Full inspection of the borehole can be done before the new reactor is emplaced, and the parts above the reactor can be inspected after installation. This option allows more space above the prior reactor to be used for the new reactor, since the heat exchanger and most of the neck can be removed.

[0224] Alternatively, the decommissioned reactor can be disposed of, with its spent fuel, at depth by sealing the borehole or the space just above the reactor. The steam generator, cold water pipe, and steam pipe can be removed, and so can most of the casing by cutting the casing at several depths and using a rig to bring the casing to the surface. This removed material should have low radioactive components from the reactor, so it will create little radiological risk to the public. If the waste is disposed in this manner, then no fission fragments or actinides are ever brought to the surface.

[0225] A “refill” may not be a replacement of the rods, but a placement of a new reactor above the prior one. This can include separating the steam pipe, water pipe, and electronics cable in the neck, or just above the heat exchanger. The separation can be done using oil and gas cutting tools and pulling the steam pipe, water pipe, and electronics cable to the surface, or by fabricating the steam pipe, water pipe, and electronics cable in such a manner (such as concentric tubes held in place by friction) that the steam pipe, water pipe, and electronics cable can be removed by pulling on the steam and cold water pipes). The primary reactor loop would not be breached, or breached in a minimal and controlled way so little radioactivity is released to the casing water. Then a platform is inserted to support the new reactor. This platform can be an expandable plug (e.g., a plug as used in oil and gas operations), or a sand fill. The old reactor is then considered to be in storage or disposal.

[0226] The next reactor is placed above the prior one. The entire length of the prior reactor is (as an example) 4 meters for the pressurizer, 4 meters for the core, 20 meters for the neck (if the neck is left below ground), and 4 meters for the heat exchanger (if the heat exchanger is left below ground), for a total, in this instance, of 12 to 32 meters. This can be longer or shorter, for example by factors of 2 or 1 / 2. For a reactor steam generator combination with a height of 32 meters, then 10 stacked reactors would take 320 meters, or 21% of the vertical or near-vertical length of the hole. If it is desirable for all of the reactors to operate at the same depth, then the original boreholeAttorney Docket No.: 57302-0005WO1 can be drilled deeper, 320 meters deeper in the instance discussed above with 10 reactors. The operating reactor can be placed at the chosen working depth, for example, 1 mile (1600 meters) and then lowered into the deeper hole when the fuel is spent, for storage or disposal. The next reactor can then be lowered into the borehole and placed at the same working depth of 1 mile. The permanent cables that support each reactor take little space in the borehole, so ten sets can be accommodated for ten sequential reactors.

[0227] An aspect of the borehole nuclear reactor is the high safety achieved by deep placement of the reactor and the spent fuel. Assurance of high safety is similar to the closely related case of disposing of spent nuclear fuel in deep boreholes. The achievement of low cost with the borehole nuclear reactor is by elimination of the expensive above ground components of other reactors. The borehole nuclear reactor harnesses natural geology to simplify the containment and security, and lets the mile depth provide the pressure needed for operation of the PWR. Examination of security and safety of nuclear waste disposal in boreholes is described in “Post- Closure Safety Analysis of Nuclear Waste Disposal in Deep Vertical Boreholes,” Energies 2021, 14(19), 6356, which is incorporated herein by reference.

[0228] A number of implementations have been described. Nevertheless, it will be understood that various modifications can be made without departing from the spirit and scope of this disclosure. For example, example operations, methods, or processes described herein can include more steps or fewer steps than those described. Further, the steps in such example operations, methods, or processes can be performed in different successions than that described or illustrated in the figures. Accordingly, other implementations are within the scope of the following claims.

Claims

Attorney Docket No.: 57302-0005WO1 WHAT IS CLAIMED IS:

1. A borehole nuclear reactor, comprising: at least one reactor core comprising nuclear fuel and positioned in a first portion of a borehole that is formed from a terranean surface to a subterranean formation; at least one heat exchanger positioned in a second portion of the borehole and in thermal communication with the at least one reactor core; and at least one pressurizer positioned in the borehole downhole of the at least one reactor core.

2. The borehole nuclear reactor of claim 1, wherein the at least one pressurizer is configured to maintain a pressure of the at least one reactor core and the at least one heat exchanger utilizing a gravitational pressure of water at an operating depth in the subterranean formation.

3. The borehole nuclear reactor of claim 1, wherein the at least one reactor core, the at least one heat exchanger, and the at least one pressurizer are connected to form a preassembled kit prior to installation in the borehole.

4. The borehole nuclear reactor of claim 1, wherein the at least one reactor core comprises a first reactor core, the borehole nuclear reactor further comprising a second reactor core comprising nuclear fuel and positioned between the first reactor core and the at least one heat exchanger, wherein the at least one heat exchanger is in thermal communication with the first reactor core and the second reactor core.

5. The borehole nuclear reactor of claim 1, comprising one or more control rods housed in guide tubes, wherein the guide tubes extend into the at least one reactor core and into the at least one heat exchanger.

6. The borehole nuclear reactor of claim 5, wherein each of the one or more control rods are configured to operate by or with a stepping motor or an induction motor or a hydraulic system.Attorney Docket No.: 57302-0005WO1 7. The borehole nuclear reactor of claim 1, wherein the nuclear fuel comprises a nuclear fuel assembly comprising a plurality of sectors, with a first portion of the sectors configured for an upflowing fluid and a second portion of the sectors configured for a downflowing fluid.

8. The borehole nuclear reactor of claim 1, wherein a pressure internal to the reactor is maintained at least in part by the at least one pressurizer, the at least one pressurizer including a membrane that separates a fluid inside the at least one reactor core from fluid outside the at least one reactor core, the at least one pressurizer configured to expanding and contract to control fluid pressure inside the at least one reactor core.

9. The borehole nuclear reactor of claim 1, wherein the pressure internal to the reactor core is maintained at least in part by a tube or pipe that rises vertically to the near terranean surface, with the pressure internal to the reactor core is determined by weight of water in the tube or pipe and any additional pressure applied at the surface of the water in the tube or pipe.

10. The borehole nuclear reactor of claim 1, comprising burnable neutron poison members in the at least one reactor core, the burnable neutron poison members each comprising a plurality of wires mounted on a ring structure or a disk structure or on or within a cladding.

11. The borehole nuclear reactor of claim 10, wherein the plurality of wires are suspended across an annulus of the ring structure.

12. The borehole nuclear reactor of claim 10, wherein the plurality of wires are embedded in the disk structure, the disk structure being located between fuel pellets of the reactor core.

13. The borehole nuclear reactor of claim 10, wherein the plurality of wires are made of at least one of gadolinium, boron, erbium, hafnium, dysprosium, or chemical compounds or alloys that contain of at least one of gadolinium, boron, erbium, hafnium, or dysprosium.

14. The borehole nuclear reactor of claim 10, wherein a burn of the nuclear fuel is controlled in part by use of the burnable neutron poisons members.Attorney Docket No.: 57302-0005WO1 15. The borehole nuclear reactor of claim 14, wherein the burnable neutron poison members are positioned between pellets of the nuclear fuel.

16. The borehole nuclear reactor of claim 14, wherein the burnable neutron poison members are positioned on the surface of the fuel cladding or within the fuel cladding.

17. The borehole nuclear reactor of claim 1, wherein the first portion of the borehole is at least 500 meters in depth under the terranean surface.

18. The borehole nuclear reactor of claim 1, wherein a diameter of the borehole is between 14 inches and 45 inches.

19. The borehole nuclear reactor of claim 1, wherein the borehole is substantially vertical or slanted up to 45 degrees from vertical.

20. The borehole nuclear reactor of claim 1, wherein emergency core cooling fluid includes water in the borehole above the at least one reactor core.

21. The borehole nuclear reactor of claim 1, wherein the borehole is drilled to a greater depth than the first portion of the borehole, with the greater depth configured for storage or disposal of the reactor core when the nuclear fuel is spent.

22. The borehole nuclear reactor of claim 1, comprising: a filter system configured to divert a portion of water in the primary loop of the at least one reactor core through a filter for removal of particles from the water, wherein a force provided by natural convection in the primary loop causes the portion of water to enter the filter system.

23. The borehole nuclear reactor of claim 1, comprising: a chemical and volume control system at or near the terranean surface, the chemical and volume control system configured to adjust dissolved boron by accessing a primary loop of the at least one reactor core with a pipe or tube that rises to the terranean surface or near the terranean surface from the primary loop, and descends to the primary loop.Attorney Docket No.: 57302-0005WO1 24. The borehole nuclear reactor of claim 1, comprising: a chemical and volume control system at or near the terranean surface, the chemical and volume control system configured to adjust dissolved hydrogen by accessing the primary loop of the at least one reactor core with a pipe or tube that rises to the terranean surface or near the terranean surface from the primary loop, and descends to the primary loop.

25. The borehole nuclear reactor of claim 1, comprising: a chemical and volume control system at or near the terranean surface, the chemical and volume control system comprising a tank configured to accept water from a primary loop of the at least one reactor core when volume of the water increases due to heating of the water, wherein the tank is configured to supply the water to the primary loop of the at least one reactor when the volume of the water decreases due to at least one of (i) thermal contraction of the water or (ii) leakage out of the primary loop.

26. The borehole nuclear reactor of claim 1, wherein the second portion of the borehole in which the at least one heat exchanger is positioned is proximate to the first portion of the borehole in which the at least one reactor core is positioned, the second portion of the borehole being at a depth of at least 500 meters from the terranean surface, the borehole nuclear reactor including an insulated pipe that carries steam from the at least one heat exchanger to the terranean surface or near the terranean surface.

27. The borehole nuclear reactor of claim 26, wherein a neck separates the at least one reactor core from the at least one heat exchanger, the neck configured to: accommodate control rods of the at least one reactor core; and provide buoyance to drive convective flow of a primary loop of the at least one reactor core by increasing heights of hot and cold parts of the primary loop.

28. The borehole nuclear reactor of claim 1, comprising one or more cables attached to the at least one reactor core, the one or more cables configured to: lower and raise the at least one reactor core, and hold the at least one reactor core at the first portion of the borehole, and lower the at least one reactor core to a portion of the borehole that is at a greater depth than the first portion of the borehole for temporary storage or long-term disposal.Attorney Docket No.: 57302-0005WO1 29. The borehole nuclear reactor of claim 28, wherein the one or more cables occupy a portion of the borehole, the one or more cables sized to enable multiple reactor cores to be lowered into the borehole and held in place above the at least one reactor core.

30. The borehole nuclear reactor of claim 1, wherein a configuration of the at least one reactor core is configured to use standard fuel assemblies.

31. The borehole nuclear reactor of claim 1, comprising: a steam pipe configured to carry steam from a depth of greater than 500 meters to proximate the terranean surface; and insulation that covers at least a portion of the steam pipe, the insulation comprising at least one of a vacuum, rock wool, aerogel, fiberglass, calcium-silicate, perlite, or ceramic fiber.

32. The borehole nuclear reactor of claim 1, wherein a neutron reflector is positioned around the at least one reactor core and configured to increase a fraction of neutrons emitted in nuclear fission that produce additional neutron fissions.

33. The borehole nuclear reactor of claim 32, wherein the neutron reflector is constructed from at least one of beryllium, carbon, steel, lead, or tungsten, or compounds of beryllium, carbon, steel, lead, or tungsten.

34. A method for installing a borehole nuclear reactor, comprising: forming a preassembled kit by connecting: a reactor core; and a heat exchanger; and inserting the preassembled kit into a borehole that is formed from a terranean surface to a subterranean formation.

35. The method of claim 34, comprising maintaining a pressure of an interior of the reactor core using a pressurizer, the pressurizer comprising a flexible membrane or surface that moves to cancel or reduce a pressure difference between the interior of the reactor core and fluid in the borehole outside of a reactor vessel that contains the reactor core.Attorney Docket No.: 57302-0005WO1 36. The method of claim 35, wherein the preassembled kit comprises the reactor core, the heat exchanger, a neck separating the reactor core from the heat exchanger, and the pressurizer.

37. The method of claim 35, wherein the pressure of the interior of the reactor core is maintained at least in part by a tube or pipe that extends from the reactor core towards the terranean surface.

38. A nuclear power system, comprising: the borehole nuclear reactor of claim 1, further comprising a secondary coolant loop that extends between the at least one heat exchanger and the terranean surface; and electric power generation equipment positioned at or near the terranean surface, wherein the secondary coolant loop of the borehole nuclear reactor is fluidly coupled to the power generation equipment.

39. The nuclear power system of claim 38, wherein the borehole nuclear reactor comprises a plurality of borehole nuclear reactors each comprising a respective secondary coolant loop, wherein the secondary coolant loops of each borehole nuclear reactor of the plurality of borehole nuclear reactors is fluidly coupled to the electric power generation equipment.

40. The nuclear power system of claim 39, comprising more than one borehole nuclear reactor at a same site to provide greater power from the electric power generation equipment than is available from a single borehole nuclear reactor.

41. The nuclear power system of claim 40, wherein two or more of the more than one borehole nuclear reactors output steam to the electric power generation equipment.

42. A method of operating the borehole nuclear reactor of claim 1, comprising: obtaining a heated secondary coolant from the at least one heat exchanger, wherein the secondary coolant was heated by a primary coolant that was heated by the at least one reactor core; and providing the secondary coolant to power generation equipment to generate electrical power.Attorney Docket No.: 57302-0005WO1 43. The method of claim 42, comprising controlling reactivity in part of at least one reactor core by moving one or more control rods housed in guide tubes, wherein the guide tubes extend into the at least one reactor core and into the at least one heat exchanger.

44. The method of claim 42, comprising controlling reactivity in part of at least one reactor core by controlling dissolved neutron poisons in the primary coolant.

45. The method of claim 42, comprising: controlling pressure in a secondary loop of the borehole nuclear reactor utilizing adjustment of a level of the secondary coolant in a feed pipe of the secondary loop.

46. A neutron poison member configured as a wire, cylinder, or thread, wherein a length of the wire, cylinder, or thread is at least three times greater than a diameter of the wire, cylinder, or thread.

47. A neutron poison member, comprising: a ring structure; and a plurality of wires mounted to the ring structure and suspended across an annulus of the ring structure, each wire comprising burnable neutron poison material.

48. The neutron poison member of claim 47, wherein the burnable neutron poison material comprises at least one of gadolinium, boron, erbium, hafnium, or dysprosium, or chemical compounds or alloys that contain at least one of gadolinium, boron, erbium, hafnium, or dysprosium.

49. The neutron poison member of claim 47, wherein the ring structure is formed from at least one of a ceramic or metal material.

50. The neutron poison member of claim 47, wherein each wire of the plurality of wires comprises a wire with a neutron poison core.

51. The neutron poison member of claim 47, wherein a diameter of the annulus is one centimeter or less.Attorney Docket No.: 57302-0005WO1 52. neutron poison system for a borehole nuclear reactor, comprising: a plurality of solid pieces of neutron poison, each solid piece having a geometry configured to compensate for xenon instability of the borehole nuclear reactor.

53. The neutron poison system of claim 52, wherein the geometry of a solid piece of neutron poison of the plurality of solid pieces of neutron poison comprises a sphere or a flat plate.

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