WO2025059308A1

WO2025059308A1 - Borehole nuclear power plant system

Info

Publication number: WO2025059308A1
Application number: PCT/US2024/046394
Authority: WO
Inventors: Richard A. Muller
Original assignee: Deep Fission Inc
Current assignee: Deep Fission Inc
Priority date: 2023-09-12
Filing date: 2024-09-12
Publication date: 2025-03-20
Anticipated expiration: 2026-03-12

Abstract

A method of operating a nuclear reactor system includes: operating a first nuclear reactor to produce steam, the first nuclear reactor positioned in a borehole that extends from a terranean surface through one or more subterranean formations; shutting down the first nuclear reactor; placing a second nuclear reactor into the borehole; and operating the second nuclear reactor to produce steam. A method for controlling a nuclear reactor system includes boron injection and removal, and adjusting a pressure in at least one pressurizer vessel of a pair of pressurizer vessels. A first pressurizer vessel is in fluid communication with primary coolant at a first depth of a reactor vessel. A second pressurizer vessel is in fluid communication with the primary coolant at a second depth of the reactor vessel. The reactor vessel is positioned in a borehole that extends from a terranean surface through one or more subterranean formations.

Description

BOREHOLE NUCLEAR POWER PLANT SYSTEM

TECHNICAL BACKGROUND

[0001] This disclosure relates to systems and methods for generating power from a power plant formed by placing a nuclear reactor in a borehole.

BACKGROUND

[0002] Highly radioactive material, such as radioactive waste, chemical waste, biologic waste, or other waste that is generally harmful to living creatures whether directly or indirectly, can be placed underground within (or outside of) canister systems. As an example, radioactive waste (also referred to as nuclear waste) can be stored in deep, human-unoccupiable boreholes that are formed from a terranean surface into one or more subterranean formations that are suitable to store such waste for years, decades, centuries, or longer. For instance, the human- unoccupiable boreholes (also called wellbores) can be directional boreholes formed with conventional drilling equipment and include vertical, curved, and horizontal portions (including multilaterals in some cases). Alternatively, the human-unoccupiable boreholes can be substantially vertical or slanted (e.g., formed offset from substantially vertical). The isolation afforded by such boreholes suggests that they can be used, not only for the disposal of dangerous material, but as a site for the operation of a nuclear power plant.

SUMMARY

[0003] In an example implementation, a method of operating a nuclear reactor system, includes: operating a first nuclear reactor to produce steam, the first nuclear reactor positioned in a borehole that extends from a terranean surface through one or more subterranean formations; shutting down the first nuclear reactor; placing a second nuclear reactor into the borehole; and operating the second nuclear reactor to produce steam.

[0004] In an aspect combinable with the example implementation, the method includes: before placing the second nuclear reactor into the borehole, removing the first nuclear reactor from the borehole; and placing the second nuclear reactor into the borehole at a position that is the same or similar distance from the terranean surface as the first nuclear reactor.

[0005] In another aspect combinable with any of the previous aspects, the method includes: before placing the second nuclear reactor into the borehole, moving the first nuclear reactor from an initial position in the borehole to a position that is farther from the terranean surface than the initial position; and placing the second nuclear reactor into the borehole at a position that is the same or similar distance from the terranean surface as the initial position. [0006] In another aspect combinable with any of the previous aspects, the method includes: before placing the second nuclear reactor into the borehole, placing a platform above the first nuclear reactor in the borehole; and placing the second nuclear reactor into the borehole atop the platform at a position that is nearer to the terranean surface than the first nuclear reactor. [0007] In another aspect combinable with any of the previous aspects, placing the platform above the first nuclear reactor comprising placing at least one of sand or cement into the borehole.

[0008] In another aspect combinable with any of the previous aspects, operating the first nuclear reactor to produce steam includes transporting a secondary fluid coolant through a secondary coolant system that is thermally coupled to a primary coolant system that is thermally coupled to a reactor core of the first nuclear reactor.

[0009] In another aspect combinable with any of the previous aspects, the secondary coolant system is thermally coupled to the primary coolant system by a heat exchanger; and the method includes transporting steam through a steam pipe from the heat exchanger to the terranean surface.

[0010] In another aspect combinable with any of the previous aspects, the method includes: before placing the second nuclear reactor into the borehole, removing the heat exchanger and the steam pipe from the borehole; and after placing the second nuclear reactor into the borehole, replacing the heat exchanger and the steam pipe into the borehole.

[0011] In another aspect combinable with any of the previous aspects, the method includes: before placing the second nuclear reactor into the borehole, removing the steam pipe from the borehole, and leaving the heat exchanger in place in the borehole; and after placing the second nuclear reactor into the borehole, replacing the steam pipe into the borehole.

[0012] In another aspect combinable with any of the previous aspects, operating the second nuclear reactor to produce steam includes transporting a secondary fluid coolant through a secondary coolant system that is thermally coupled to a primary coolant system that is thermally coupled to a reactor core of the second nuclear reactor. [0013] In another aspect combinable with any of the previous aspects, the secondary coolant system is thermally coupled to the primary coolant system by a heat exchanger; and the method includes transporting steam through a steam pipe from the heat exchanger to the terranean surface.

[0014] In another aspect combinable with any of the previous aspects, the secondary coolant system includes: a heat exchanger; a central tube, the method including transporting the secondary fluid coolant through the central tube from the heat exchanger to the terranean surface; and an outer concentric tube, the method including transporting the secondary fluid coolant through the outer concentric tube from the terranean surface to the heat exchanger.

[0015] In another aspect combinable with any of the previous aspects, the method includes: shutting down the second nuclear reactor; placing a third nuclear reactor into the borehole; and operating the third nuclear reactor to produce steam.

[0016] In an example implementation, a method includes adjusting a pressure in at least one pressurizer vessel of a pair of pressurizer vessels. A first pressurizer vessel of the pair of pressurizer vessels is in fluid communication with primary coolant at a first depth of a reactor vessel, a second pressurizer vessel of the pair of pressurizer vessels is in fluid communication with the primary coolant at a second depth of the reactor vessel, the second depth being different from the first depth, the reactor vessel is positioned in a borehole that extends from a terranean surface through one or more subterranean formations, the reactor vessel contains a reactor core including at least one nuclear fuel element, and the primary coolant is in thermal communication with the reactor core, and causing a change to a pressure of the primary coolant in response to adjusting the pressure in the at least one pressurizer vessel.

[0017] In an aspect combinable with the example implementation, the first pressurizer vessel is in fluid communication with the primary coolant through a first pressurizer tube that extends from the first pressurizer vessel to the first depth of the reactor vessel, andthe second pressurizer vessel is in fluid communication with the primary coolant through a second pressurizer tube that extends from the second pressurizer vessel to the second depth of the reactor vessel.

[0018] In an aspect combinable with any of the previous aspects, the first depth is nearer to a bottom of the reactor vessel than the second depth. [0019] In another aspect combinable with any of the previous aspects, the first pressurizer vessel and the first pressurizer tube contain boronated fluid.

[0020] In another aspect combinable with any of the previous aspects, the method includes causing a change to a boron concentration of the primary coolant in response to adjusting the pressure in the at least one pressurizer vessel.

[0021] In another aspect combinable with any of the previous aspects, the pair of pressurizer vessels are located above the terranean surface.

[0022] In another aspect combinable with any of the previous aspects, the method includes injecting a boronated fluid into the reactor vessel through an injector tube by inserting a plunger into the injector tube.

[0023] In another aspect combinable with any of the previous aspects, the method includes inserting the plunger into the injector tube at a first end of the injector tube that is at or near the terranean surface to cause the boronated fluid to enter the reactor vessel at a second end of the injector tube that is inside the reactor vessel.

[0024] In another aspect combinable with any of the previous aspects, a portion of the injector tube inside the reactor vessel is curved, and the second end of the injector tube is oriented towards the terranean surface.

[0025] In another aspect combinable with any of the previous aspects, a one-way valve is positioned at the second end of the injector tube, the one-way valve permitting flow of the boronated fluid into the reactor vessel.

[0026] In an example implementation, a nuclear reactor system includes a reactor vessel positioned in a borehole that extends from a terranean surface through one or more subterranean formations; a reactor core contained within the reactor vessel and including at least one nuclear fuel element; a primary coolant in thermal communication with the reactor core; and a pair of pressurizer vessels, including: a first pressurizer vessel in fluid communication with the primary coolant at a first depth of the reactor vessel, and a second pressurizer vessel in fluid communication with the primary coolant at a second depth of the reactor vessel, the second depth being different from the first depth, the pair of pressurizer vessels configured such that adjusting pressure in at least one pressurizer vessel of the pair of pressurizer vessels causes a change to a pressure of the primary coolant. [0027] In an aspect combinable with the example implementation, the first pressurizer vessel is in fluid communication with the primary coolant through a first pressurizer tube that extends from the first pressurizer vessel to the first depth of the reactor vessel, andthe second pressurizer vessel is in fluid communication with the primary coolant through a second pressurizer tube that extends from the second pressurizer vessel to the second depth of the reactor vessel.

[0028] In an aspect combinable with any of the previous aspects, the first depth is nearer to a bottom of the reactor vessel than the second depth.

[0029] In another aspect combinable with any of the previous aspects, the first pressurizer vessel and the first pressurizer tube contain boronated fluid.

[0030] In another aspect combinable with any of the previous aspects, adjusting the pressure in the at least one pressurizer vessel causes a change to a boron concentration of the primary coolant.

[0031] In another aspect combinable with any of the previous aspects, the pair of pressurizer vessels are located above the terranean surface.

[0032] In another aspect combinable with any of the previous aspects, the system includes: an injector tube; and a plunger. The injector tube is configured to inject boronated fluid into the reactor vessel when the plunger is inserted into the injector tube.

[0033] In another aspect combinable with any of the previous aspects, a first end of the injector tube is at or near the terranean surface, the plunger is at the first end of the injector tube, and a second end of the injector tube is inside the reactor vessel.

[0034] In another aspect combinable with any of the previous aspects, a portion of the injector tube inside the reactor vessel is curved, and the second end of the injector tube is oriented towards the terranean surface.

[0035] In another aspect combinable with any of the previous aspects, the system includes a one-way valve positioned at the second end of the injector tube, the one-way valve permitting flow of the boronated fluid into the reactor vessel.

[0036] 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

[0037] FIGS. 1 to 3 are schematic diagrams of an example implementation of a borehole nuclear reactor power plant system according to the present disclosure.

[0038] FIG. 4 is a schematic diagram of an example pressure control system for a borehole nuclear reactor power plant system according to the present disclosure.

[0039] FIGS. 5 A and 5B are schematic diagrams of another example implementation of a borehole nuclear reactor system according to the present disclosure.

[0040] FIGS. 6A to 6G show example boreholes with different configurations for downward flowing coolant water and the upward flowing steam for a borehole nuclear reactor system according to the present disclosure.

[0041] FIG. 7 is a schematic diagram of an example implementation of a borehole nuclear reactor power plant system with multiple reactors according to the present disclosure. [0042] FIG. 8 shows a cross-section of an example reactor core for a borehole reactor according to the present disclosure.

DETAILED DESCRIPTION

[0043] A nuclear fission reactor is described. In general, nuclear fuel stored, e.g., in canisters in a vertical, slanted, or directional borehole creates an underground fission reactor to which a power system can be fluidly (e.g., gas, liquid, mixed-phase fluid) coupled to remove thermal energy from the borehole to bring useful power to the surface or near surface. The reactor fits in a human-unoccupiable borehole buried deeply in the ground. The example nuclear fission reactors according to the present disclosure can take advantage of the fact that the hydrostatic pressure of the brine in the rock is typically one atmosphere per ten meters. Thus, due to the depth at which nuclear fuel is stored in the borehole, the nuclear reactor in a borehole at one kilometer (km) can have a pressure of one hundred atmospheres; at 1.5 km the pressure can be one hundred-fifty atmospheres, approximately equal to that of a pressurized water reactor (PWR).

[0044] At 150 atmospheres of pressure, water boils at about 325°C. Light water (H2O), heavy water (D2O), other materials containing hydrogen, or a combination can be used for a moderator (e.g., liquids and gases and reactor coolant). The moderator of a nuclear reactor is a substance that slows neutrons. In the example of a light water moderator, collisions between fast neutrons and hydrogen atoms of the water cause the fast neutrons to slow to lower speeds. At lower speeds, the neutrons are more susceptible to propagate a nuclear fission chain reaction. [0045] Control of the rate of the chain reaction can be accomplished physically (such as adding or subtracting boron to the reactor core region) or automatically (such as inclusion of hydrogen-bearing materials in the moderator that reduce the moderation when the temperature rises). The accumulation of vapor from boiling can also be used to automatically reduce the moderator density and thus slow or stop the fission chain reaction if the reactor grows too hot. Thus, in some aspects, the reactor can self-control, without external intervention (e.g., control rods), to keep reactor temperature at or below the boiling point. A heat exchanger in the borehole serves to heat a separate reservoir of water that circulates to the surface, thus bringing heat, uncontaminated by radioisotopes, to the surface. Sides of the borehole (or in some aspects, a borehole casing) can function to bring lower pressure water to the heat exchanger, where it can boil, to deliver power to the surface in the form of steam. Because the reactor is below millions to billions tons of rock, the reactor can be installed in the borehole without a separately installed or conventional containment building. Further, because the reactor is in a geologic formation in which the ambient water or brine pressure is comparable to that in a PWR or boiling water reactor (BWR), no thick-walled pressure vessel is needed.

[0046] For example, the present disclosure describes one or more fuel assemblies similar in configuration and size to those of a PWR or BWR fuel assembly placed near the bottom of a vertical borehole (e.g., 1.5 km deep borehole). The assembly is surrounded by “water” (e.g., water or water with additives such as soluble neutron poisons), to make the fissile fuel (e.g., uranium with greater than 0.7% uranium-235 (U-235), or which contains plutonium-239 (Pu- 239), or which contains both uranium and plutonium) undergo a fission chain reaction. In other embodiments, other materials such as graphite or uranium-zirconium hydride, can be used as a moderator. In some embodiments, a combination of moderators can be used, such as graphite and water.

[0047] In some aspects, the fuel assemblies are placed inside canisters that have open bottoms and tops. In example embodiments, if the water in the fuel assembly boils (that is, if the fuel rises above 325°C for a reactor at a depth of 1.5 km) then gas can accumulate in void collectors, and the fission chain reaction can reduce, until the temperature lowers. Thus, the temperature of the reactor can be self-regulated at or about 325°C. The temperature can be regulated at lower temperatures at lesser depths, and at higher temperatures at greater depths. [0048] Above the reactor in the borehole is a heat exchanger that removes heat from the reactor. In this embodiment, the heat exchanger also isolates the water (e.g., as a primary coolant) from a flow of water (e.g., as a secondary coolant) that is brought to the surface to carry the heat upward. The heat can be carried from the fuel to the heat exchanger using a pump or using natural circulation from heat-driven convection. The light water that brings the heat to the surface can do so using natural circulation, or by being actively pumped. The secondary water can be kept pressurized so that the water reaches the surface as hot water, or the water can be allowed to boil in the heat exchanger and come to the surface as steam.

[0049] Fresh (non-saline) water can be used as the fluid that carries the heat to the surface or near surface, but other fluids can also be used, such as brine, hydrocarbons, or gas such as helium or nitrogen. If a gas is used, then the gas conveyance pipe can be surrounded by a fluid filled pipe (containing, for example, fresh water) that provides emergency core cooling for the reactor. Otherwise, if water is used as the secondary fluid, then this water can also serve as a source for an emergency core cooling system (ECCS).

[0050] The upward flowing hot fluid can be partially isolated from the surrounding rock, and from the downward flowing cool water, by insulators. The rock formations surrounding the hot upward flowing pipe can also serve as an insulator. As the temperature of this rock rises, the rate of heat flow from the upward flown hot pipe into the surrounding rock can diminish, and thus the rock, once heated, serves as an insulator.

[0051] A diagram of an example reactor system 100 is shown in FIGS. 1 to 3. FIG. 1 is an overview of a borehole nuclear reactor system for the embodiment of a vertical borehole. In this embodiment, only one hole is depicted, and it contains only one fuel assembly. In other embodiments, multiple fuel assemblies can be placed in a borehole, either vertically arranged, or spread though branches in the borehole. Further, multiple boreholes can be used to increase the total power delivered to the surface or near surface. In addition, once the fuel in a borehole reactor is depleted, it can be removed, or, in the preferred implementation, left it place, covered with sand or other support, and a second nuclear reactor placed above it. FIG. 2 shows the reactor section of the borehole nuclear reactor system. FIG. 3 shows an example heat exchange section of the borehole nuclear reactor system. The dashed line 110 represents the same level, or depth, in FIGS. 1 to 3. All materials in the reactor can be designed for low corrosion. One method of doing this is to include anti-corrosion materials in the fluids. Another is to make the pipes and other parts from low or non-corrosion materials including plastics. A third is to coat the metal surfaces with anti-corrosive materials, including quartz and diamond. Other methods can be evident to an engineer practiced in the anti-corrosion field.

[0052] Ultimately, when the reactor fuel is spent, the fuel can be removed from the borehole, or left at depth, with the borehole sealed (e.g., with a plug or otherwise). The casing to the surface can be left in place or cut and pulled out. The fuel might never be removed from the borehole 115. In some cases, the fuel can be disposed in the borehole 115 by sealing the borehole 115 above the reactor when the fuel is spent. Such borehole disposal offers very high levels of safety. The reactor system 100 can be less expensive to build and operate compared to a standard BWR or PWR reactor. Thus, in some examples, the fission chain reaction can be designed to burn more slowly. The reactor system 100 can thus operate for long periods of time, e.g., thirty to sixty years or longer, with need for no or few replacements of the nuclear fuel. In addition, fuel that is more highly enriched than typically used in a commercial nuclear reactor (e.g., about 4%) can be used to extend the working lifetime of the borehole reactor, that is, the time that it can deliver useful power without requiring the use of additional fuel. One example of such fuel is high assay low enriched uranium (HALEU) which is typically enriched to almost 20% U-235. Another example embodiment is that when the fuel has become sufficiently spent, that is the amount of fissionable material such as U-235 has been significantly reduced, that instead of removing the fuel assembly, it can be left in place, and a new fuel assembly put in position closer to the access hole. Doing this allows the highly radioactive spent fuel to remain safely at depth, where it can eventually be disposed by the action of sealing the borehole above the new fuel assembly (i.e., at a location in the borehole that is closer to the surface than the new fuel assembly). The reactor system 100 can be used to provide heat in the form of hot water, steam 106, and/or by using a power conversion system, e.g., generator 102. The generator 102 can be, for example, a turbine or thermocouple stack or other heat-driven generator. The heat can also be used for commercial purposes (such as heating buildings or for heat-intensive industry) without conversion to electricity.

[0053] FIG. 1 is an overview of a borehole nuclear reactor system 100. FIG. 1 includes a reference arrow 105 defining uphole and downhole directions. This figure shows a vertical borehole, but the borehole can be directionally drilled into a slanted or near-horizontal or other configuration. A generator 102 is located at a ground surface 104. A vertical borehole 115 descends to a depth of approximately one-half kilometer (km) or greater (e.g., 1.0 km or greater, 1.5 km or greater, 2.0 km or greater, or other predetermined depth). The borehole 115 includes a casing 216 that can be made to adhere to the borehole wall, e.g., by being cemented with cement 214 to the borehole wall.

[0054] In some aspects, a gap can also be left between the casing and the borehole wall, so that water that typically fills this gap can boil and become steam, and that steam can help to provide insulation reducing subsequent heat flow into the rock. The borehole 115 can be narrow, e.g., four to thirty-six inches in diameter.

[0055] At or near the bottom of the borehole 115, nuclear fuel is held in a reactor 130 including one or more fuel assemblies (or rods) 218. Details of the reactor 130 are described with reference to FIG. 2. In some examples, the reactor 130 includes an individual fuel element, or fuel rod 218. The reactor 130 is surrounded by a moderator. In some examples, the moderator is water. The reactor section is isolated from the surface 104 by a heat exchanger 120 and a pressure equalizer barrier 212 (shown in FIG. 3). The water serves as both coolant and moderator. The water flows by natural circulation to the heat exchanger 120 above the reactor 130 through hot water pipe 205.

[0056] An insulated pipe 208 is positioned in the borehole 115. During operation, water 108 flows downward outside of the pipe 208. The secondary water 108 is heated by the primary water as the secondary water 108 passes through the heat exchanger 120. The heated water, or steam 106, then flows upward inside the pipe 208. The pipe 208 thus carries heat generated by the reactor 130 to the surface 104.

[0057] Many methods can be used to cause the fuel to become critical, that is, to undergo a sustained chain reaction. One method to accomplish this can be to use a “neutron reflector”, a material that has low neutron absorption properties, and which can scatter the neutrons that are leaving the reactor region back into it. In some instances, the neutron reflector is made of graphite (carbon) or beryllium. These materials can also serve to help moderate the neutrons. [0058] Referring to FIG. 3, the heat exchanger 120 is depicted in a simplified schematic. Any appropriate heat exchanger system can be used for removing heat from the reactor 130. The primary water is fluidly isolated from the secondary water by a barrier, e.g., pressure equalizer barrier 212. The pressure equalizer can be omited if it is desired to maintain the secondary water at a lower pressure to facilitate boiling. The barrier is positioned above the region of the system 100 in which heavy water flows, and in which the reactor 130 is located.

[0059] The system 100 includes primary water supply and drain pipes 201 and 202, respectively. The primary fluid supply and drain pipes 201, 202 can be used to replace the primary fluid (or part of it) and also to provide pressure control for the reactor 130. In some implementations, two small pipes, e.g., water supply and drain pipes 201, 202, penetrate the pressure equalizer barrier 212. The pipes 201, 202 can be used to partially replace or remove the primary water near the reactor. For example, the primary water, which might initially contain dissolved boron salts, can be replaced with fresh water, brine, or water with additional boron salts (material that quenches the fission chain reaction).

[0060] The use of boiling (creation of gas) to reduce reactivity and thus control the chain reaction is called a “negative void coefficient.” Other methods to control the reactivity are well- known in the reactor design community. They include use of neutron reflectors and absorbers with efficiency that is temperature dependent. In addition to these passive (not moving) methods, control rods can be inserted into or near the reactor. These can be controlled from the surface, or they can automatically move into the reactor region when the temperature rises. Such movement can be controlled by using materials with a temperature coefficient of expansion large enough to allow a mechanism to move the control rods. Such mechanisms are found, for example, in bimetallic thermometers in which relatively small expansion can be used to drive a readout needle over a large distance.

[0061] The pipes 201, 202 can provide control of the content of the fluid in the reactor (for example, channeling the boron concentration), and they can also provide pressure stability. In another configuration, there can be a second pipe that goes from the reactor region to the surface to provide pressure stability. For example, when bubbles form, the space taken by the bubbles pushes water up the pipe instead of causing reactor pressure to increase. Such a pipe need not circulate water; the pipe allows enough flow upward to prevent an increase in pressure in the reactor section. The fission chain reaction can be turned off, if desired, by pumping out, e.g., pure primary water and replacing that water with water containing a neutron poison (that is, a material that absorbs neutrons), or high pressure gas such as argon or nitrogen. [0062] The pressure equalizer barrier 212 between the primary water and secondary water provides an alternative (to the pipe described in the prior paragraph) or redundant method to control the pressure in the reactor. The barrier 212 can also be placed between the primary water and the brine from the local rock formation, to maintain the pressure of the primary system as equal to or approximately equal to that of the host rock formation. The barrier 212 can be formed from a flexible material that bends when the pressures are unequal, for example, in the form of a bellows. In another instance, the barrier 212 can be a tube with a non-circulator cross section that compresses or becomes more circular depending on the pressure difference inside and outside of the tube. In some examples, the pressure equalizer barrier 212 can be composed of a permeable material, such as sandstone or sand that allows flow whenever the pressures are unequal across the barrier. Although a permeable plug could allow some mixing between the moderating material (such as heavy water) and either the rock brine or the heat extraction fluid (secondary water), under normal operations the amount of mixing can be small. The mixed water can be replaced in the reactor by using the primary drain and supply pipes 201, 202. The primary drain and supply pipes 201, 202 can be filled with water near the bottom, and include a moveable plug part way up, with other water filling above the movable plug.

[0063] The hydrostatic weight of the water in the pipes 201, 202 can supply about one hundred atmospheres of pressure. The pressure can be controlled in several ways. The top of the water pipes 201, 202 can have gas at the top that allows the water to flow up the pipe (if bubbles are produced in the reactor) without significantly increasing the pressure in the reactor 130. The pressure of the water can be equal to the hydrostatic pressure of water at the bottom of the pipes 201, 202 which (if they go to the surface 104) can be fifty to one hundred-fifty atmospheres or more.

[0064] The water pipes 201 , 202 can be used for an emergency replacement of the moderator, if there is a desire to turn off the nuclear reactor 130, or for replacement by neutron- poisoned water. Thus, the water pipes 201, 202 can function as reactivity control mechanisms for the reactor. In some examples, control rods can be included in the reactor 130. Any appropriate method of controlling reactivity of the reactor 130 can be implemented.

[0065] In FIG. 2, the fuel, e.g., fuel rods 218, is held inside a canister 220. The bottom and the top of the canister 220 are left open to provide a pathway for the primary coolant loop 213. If the fuel overheats, that is, rises in temperature sufficiently to boil the water in the primary loop (which, at PWR pressures, occurs at a temperature of 250 to 325°C) then the bubbles, or vapor 206, can reduce the density of the moderator near the fuel rods 218, and reducing the rate of the fission chain reaction. Thus, the formation of bubbles provides “negative feedback” to control and stop a power production overrun. Thus, the bubble capture mechanism provides “negative feedback” to control and stop a power production overrun.

[0066] To enhance the negative temperature coefficient from voids, inverted cups or tubes can be placed in the reactor to trap bubbles. These inverted cups can also be elongated in shape, like conventional “test tubes,” to provide more gas accumulation capability. The cups can be made of a thermally- conductive metal such as zircoloy to assure that they can transfer heat from the fuel rods to the part of the liquid coolant.

[0067] The flow of the secondary coolant fluid can be provided by a pump or by natural circulation. If the pump fails, or is turned off, then the circulation of the secondary water can be reduced, and more of the heat can be transferred to the secondary water and then to the surrounding rock 222. In some examples, the secondary water can be a power conversion working fluid of a power conversion system. For example, the secondary water, when heated to steam 106 by the heat exchanger 120, can be used as a power conversion working fluid for a steam turbine generator, e.g., generator 102.

[0068] The design in FIG. 3 shows the pipe 208 that brings the hot water (or steam 106) to the surface 104 as being insulated with an insulator 204. Insulation can be added to various parts of the system 100 to improve efficiency. In another embodiment, the steam pipe 208 can be used for cool water supply, and the cool water pipe 201 can be for hot water return. In this alternate embodiment, as well as in the embodiment shown in the figures, insulation between the outer pipe 201 and the casing and rock might be reduced or omitted. Doing this can cause the rock surrounding the casing to either warm or cool, respectively, although that can take longer to take place. Similarly, if insulation is used between that pipe and the casing, the temperature of the rock can still change with time since the insulator is not perfect. In these embodiments, the temperature of the rock can eventually be close to that of the pipe, and when this happens, the rock itself serves as an effective insulator against future loss of energy through the rock.

[0069] Various embodiments that are exemplified in FIGS. 1-3 can be implemented in accordance with the present disclosure. In some examples, carbon (e.g., low boron graphite) can be added to the design to provide a component of the moderator that will not boil and also operate as a neutron reflector to increase the reactivity. A hydrogen-bearing compound such as uranium-beryllium-hydride can be added because it gives a very strong negative temperature coefficient. The enrichment of the fuel can be as low as natural uranium (0.7%), at approximately the level typically used in a PWR (4.5%), as high as enrichment levels used in many “fourth generation” nuclear reactors (19.9%), or higher.

[0070] Any appropriate type of pressure equalizer can be used. Any appropriate type of heat exchanger can be used. In some examples, the concrete, or cement 214, between the casing 216 and the borehole wall can be omitted. Casing centralizers can be used to provide stability of the pipes.

[0071] The secondary water brought to the surface 104 can be kept pressurized, so it reaches the surface 104 as a liquid, or the secondary water can be allowed to boil in the heat exchanger, so the secondary water arrives at the surface 104 as steam 106.

[0072] Other forms of fuel and moderators can be used, including Tri-structural Isotropic (TRISO) fuel and pebble fuel (typically containing TRISO in larger pellets), and molten salt or molten metal containing fissile material. The reactor system 100 can use a heat pipe in the vertical section to bring the heat to the surface 104. This can similarly allow the water to boil at depth. In an implementation including a heat pipe, pressure for the secondary water can be supplied by the secondary water pipes.

[0073] Other aspects of example embodiments according to the present disclosure can include one or more other features. For example, a turbine to generate electricity can be placed inside the borehole 115 to extract energy while keeping most of the borehole pressurized. The borehole 115 can be slanted or otherwise directionally drilled, rather than vertical. There are many more variations that are evident to someone practiced in the field of hydrology, heat transfer, and nuclear power.

[0074] The reactor system 100 can be designed to deliver a low level of power (e.g., heat, electrical power) for a short period or for an extended period (thirty to sixty years or more) by using control rods and neutron poisons to reduce the rate of fission reactors while keeping the reactor critical. The power delivered can also be extended by use of more highly enriched fuel, or by using multiple fuel assemblies or inserting new reactors in the same hole once the fissile fuel in the previous reactor has become depleted. At the end of that period, the spent fuel can be recovered. However, this design offers the option of leaving the spent fuel at depth and sealing the reactor. The borehole 115, in appropriate geologies (e.g., low permeability rock, no deep aquifers, little vertical flow of brine within the rock) can offer high levels of protection to humans at the surface 104 for a million years or longer.

[0075] The hot water (or steam 106) that reaches the surface 104 can be used directly for heat, or to produce electricity using a steam turbine, thermopile, or other electric generator, e.g., generator 102. The steam 106 can also be used to run engines for other uses such as manufacturing. The ability to be local, or modular, the borehole nuclear reactor heat can be usefully used, for example, to heat a large building, campus, or to provide energy for a factory. In this aspect, the system 100 has an advantage over larger plants that must be located far from where the power is used.

[0076] The system 100 can also be used for military operations, in which a borehole of suitable size is dug in a few days, and a pre-fabricated reactor is lowered into the borehole. For military use, the depth of the reactor offers a very high level of military “hardness,” that is, invulnerability to attach by missile, drone, bomb, or terrorist attack. If it is necessary to abandon the reactor, the vertical access hole can easily be destroyed by inserting explosives. Further, at an end of life of system 100 (e.g., when the fissile material is no longer capable of sustaining a nuclear reaction as desired), the reactor portion and heat exchanger of the system 100 can be removed or can be allowed to remain in the borehole 115, which can be appropriately sealed to store the fissile material therein (permanently or temporarily).

[0077] The pressure equalizer barrier (212 in FIG. 3) or any direct interface between primary and secondary circulation systems, can also serve as a safety mechanism. It can be made in such a way that it can be opened if the pressure difference grows larger than a critical and undesired level, for example, if the reactor fluid boils too vigorously to be equalized by the plug or by the pipes 201 and 202. If that happens, then the hot reactor fluid can rise past this plug, and cool water from above the plug can flow down. This process provides “emergency core cooling system” (ECCS) as is required for PWR reactors under present code. This ECCS is provided passively, that is, unlike the ECCS at many PWRs, it does not require outside power to be supplied to surround the reactor with cool water, but is gravity driven.

[0078] To enable hot water from the primary loop to create steam in the secondary loop, the relative pressure can be adjusted. This can be accomplished by having one or more pressurizer tubes that goes from the reactor water to the surface. FIG. 5B shows a nuclear reactor system 500 with two pressurizer tubes 512, 510. In this configuration the tube 512 opens near the bottom of the reactor vessel 590 and the tube 510 opens near the top of the reactor vessel 590. Other configurations are possible. The pressurizer tubes 512, 510 can also be used to lower the reactor vessel 590 into the borehole, or to lift the reactor vessel 590 out of the borehole. The water in the pressurizer tubes 512, 510 is held at a pressure that is above the ambient pressure such that the water in the primary loop will not boil, but the water in the secondary loop can boil.

[0079] An example system for pressurizing the water in the pressurizer tubes is shown in FIG. 4. The tubes go from the reactor to the surface through the borehole. At the top, the tubes 512, 510 are attached to pressurizer vessels 412, 410 respectively. In some examples, the pressurizer vessels 412, 410 are located above the terranean surface.

[0080] The pressure at the bottom of the pressure tube 512 is equal to the sum of pressure at the top of the pressure tube 512 (in the pressurizer vessel 412) and the pressure from the weight of the water in the tube 512. The pressure at the bottom of the pressure tube 510 is equal to the sum of pressure at the top of the pressure tube 510 (in the pressurizer vessel 410) and the pressure from the weight of the water in the tube 510. In some examples, pressure in the pressurizer vessels 410, 412 can be controlled by activating and deactivating heaters. For example, heaters can be turned on to raise the pressure inside a pressurizer vessel, and heaters can be turned off to lower the pressure inside the pressurizer vessel. In some examples, pressure in the pressurizer vessels 410, 412 can be controlled by pumping fluid into and/or out of the pressurizer vessels 410, 412. In some examples, pressure in the pressurizer vessels 410, 412 can be controlled by spraying cool fluid into the pressurizer vessels 410, 412.

[0081] For the secondary loop, the pressure is the pressure from the weight of the water. The pressure in the secondary loop is controlled by adjusting the level of water in the pipe that supplies water to the steam generator (the heat exchanger). The pressure in the primary loop can be maintained higher than the pressure in the secondary loop.

[0082] In a pressurized water reactor, it is useful to introduce of a dissolved neutron poison, typically a boron salt such as BH3O3) along with an acid neutralizer such as LiOH, to reduce the criticality coefficient and to keep it near a reactivity coefficient of one. A neutron poison is a material that has a high neutron absorption probability, and so when the neutron poison is injected into the coolant, it reduces the reactivity coefficient. As the U-235 is depleted, the amount of poison introduced is reduced.

[0083] The pressure control tubes can be used to introduce the poison into the reactor. In some examples, one tube 512 can be filled with a boronated fluid, such as water that contains dissolved boron salt, and the other tube 510 can be filled with unsalted water. The tube 512 with the dissolved salts can contain a heavier liquid, which can be compensated by lowering the pressure in the steam pressurizer at the surface. When it is desired to remove some boron, then the pressure in the unsalted side (e.g., the pressurizer vessel 410, the tube 510) can be increased. This causes salt-free water to enter the reactor, and the boron-containing water can flow upwards in the other tube. Explained in a different way, by making the pressure of the bottom of the unsalted pressurizer tube 510 higher than that of the salted tube 512, unsalted water can flow into the reactor and salted water can flow out of the reactor. In some examples, a desired change is one part per million per day, and so only a few liters or less of flow can be used.

[0084] In some examples, a separate tube can be used to introduce poison into the reactor in addition to or instead of introducing poison through the pressurizer tubes. For example, referring to FIG. 4, an injector tube 555 has a plunger 554 at a first end. The first end of the injector tube 555 can be located at or near the terranean surface such that the plunger 554 is human accessible. The first end of the injector tube 555 can be, for example, within ten meters of the terranean surface or within twenty meters of the terranean surface. In some examples, the first end of the injector tube is above the terranean surface. In some examples, the first end of the injector tube is below the terranean surface. Below the plunger and inside the injector tube 555 is a fluid. The fluid can be, for example, a boronated fluid such as BH3O3. The boron in BH3O3 acts as a strong neutron poison. FIG. 5B shows the injector tube 555 entering into the reactor vessel 590.

[0085] The injector tube 555 allows the fluid to flow into the reactor vessel 590 but not out through the injector tube 555. In FIG. 5B this is accomplished by having the injector tube undergo a "J" shaped change in direction, so the end of the tube 555 is pointing upward. A weight 556 can be placed over the injector tube opening, to prevent flow back into the injector tube. In some examples, the weight is a weighted ball. When a sufficient force is placed on the plunger 554, the ball can be lifted off the end of the injector tube 555 and the liquid containing the neutron poison can flow into the reactor vessel 590. In some examples, there is a cage 557 surrounding the weight 556 to make sure that the weight 556 can reseat when the force from the plunger 554 is reduced. The weight 556 operates as a one-way valve. There are alternative ways to place a one-way valve at the end of the injector tube 555. As with the tubes 510, 512, the injector tube 555 can serve an additional purpose by being used to lower the reactor into its position at or near the bottom of the borehole and/or to lift the reactor out of the borehole.

[0086] The water reaching the surface can contain some radioactive material, so a containment region around that pressurizer should be maintained and the radioactive material disposed of in an appropriate way. One way to dispose of the contaminated water is to put it into the brine 402 that is outside the primary and secondary loops.

[0087] FIGS. 5A and 5B are schematic diagrams of components of a borehole nuclear reactor system 500 according to the present disclosure. System 500 is similar in some or most aspects to the borehole nuclear reactor system 100 and shown with more detail. FIG. 5 A shows a heat exchanger portion 501 of the system 500, while FIG. 5B shows a nuclear reactor portion 503 of the system 500 (that is downhole of the heat exchanger portion 501). The heat exchanger portion 501 is positioned or installed in a borehole 505 that is formed from a terranean surface into a subterranean formation 502. In some aspects, the subterranean formation 502 can serve to moderate and reflect neutrons back into the nuclear reactor section 503 during a fission reaction generated therein.

[0088] In this example, the borehole 505 includes a casing 504 (that includes optional perforations 508) that is cemented into the borehole 505 with cement 506. In example implementations, the heat exchanger portion 501 if installed in a borehole 505 that is approximately 18 inches in diameter, and the portion 501 is approximately 40 feet in length. In some examples, the portion 501 is less than 40 feet in length (e.g., 35 feet or less, 25 feet or less). In some examples, the portion 501 is greater than 40 feet in length (e.g., 45 feet or more, 50 feet or more). A tall heat exchanger having a length of approximately 40 feet enables tubing of the heat exchanger to be thicker, since the heat exchange does not have to occur over a short section, and the piping can be kept straight except for the curves at the end of the heat exchanger. These advantages are possible because of the configuration of the borehole nuclear reactor, which provides very long vertical distance in which the heat exchanger can be placed.

[0089] As shown in FIG. 5A, the heat exchanger portion 501 includes a primary fluid flowpath 524 that includes a riser section 526 (within riser 595) and downcomer sections 528 (between the riser 595. The riser section 526 turns to the downcomer sections 528 at a closed end of the primary fluid flowpath 524 at or near an uphole end of the heat exchanger portion 501. In some aspects, an insulating layer 534 is installed between the riser section 526 and the downcomer sections 528. In combination, the riser section 526 and the downcomer sections 528 (i.e., the primary fluid flowpath 524) form at least a part of a closed-circuit for circulating (e.g., naturally or by convection or both) a primary fluid coolant therewithin between the nuclear reactor portion 503 and the heat exchanger portion 501. The primary fluid coolant can be water (e.g., heavy water, light water, or other water-based liquid with additives).

[0090] As further shown in FIG. 5 A, the heat exchanger portion 501 includes a secondary fluid flowpath 523 that includes downflow sections 513 and upflow sections 518. Generally, the downflow sections 513 extend downhole in the borehole 505 from at or near a generator and condenser at the terranean surface until they turn at closed ends into the upflow sections 518, which also extend to the generator at the terranean surface. In some aspects, an insulating layer 522 is installed between the downflow sections 513 and the upflow sections 518. In combination, the downflow sections 513 and the upflow sections 518 (i.e., the secondary fluid flowpath 523) form at least a part of a closed-circuit for circulating (e.g., forcibly, naturally or by convection or a combination thereof) a secondary fluid coolant therewithin between the heat exchanger portion 501 and the generator at the terranean surface. As shown, the secondary fluid flowpath 523 and the primary fluid flowpath 524 are in thermal communication such that heat can be transferred from the primary fluid coolant to the secondary fluid coolant (as explained in more detail later), but the fluid flowpaths are fluidly decoupled so that mixing of the primary and secondary fluid coolants does not occur within the heat exchanger portion 501. The secondary fluid coolant can be light water other water-based liquid with additives.

[0091] Referring to FIG. 5B, this figure shows the nuclear reactor portion 503 of the system 500. As shown, a reactor vessel 590 is installed in the borehole 505 (and likely within a brine 580 that fills the borehole 505). The reactor vessel 590 at least partially encloses a core 560 (defined by a core vessel 562) that includes nuclear fuel elements 570 and a control rod assembly 550 (that can be controlled from, e.g., the terranean surface). The downcomer sections 528 extend downhole into the core 560 at an outer circumference of the core reflector 562 until, at a closed, downhole end of the reactor vessel 590, they turn to meet the riser section 526 within the core 560 at the nuclear fuel elements 570. In some aspects, the core vessel 562 can include a neutron reflector surface that faces the nuclear fuel elements 570, as well as an insulating material that provides thermal separation between the downcomer sections 528 and the core 560. The nuclear reactor portion 503 can be, e.g., approximately 14-28 feet in length.

[0092] In some aspects, the core vessel 562 can be made of a material sufficient to reflect neutrons without absorbing the neutrons, such as, for example, carbon, beryllium (or any of their alloys). In some aspects, the subterranean formation 502 can act as a reflector or moderator of neutrons based on its geological properties. Thus, in combination, the reflective core vessel 562 (if provided) and the formation 502 can operate in combination to contribute to the reactivity of the nuclear reactor portion 503. Additionally, or alternatively, the casing 504 can be made of a material sufficient to reflect neutrons without absorbing the neutrons (or otherwise act to increase reactivity), such as, for example, carbon steel or stainless steel. The casing 504 can also be made of a material such as ceramic, a plastic material, or fiberglass.

[0093] In some aspects, there can be one, some, or many nuclear fuel elements 570. For example, a nuclear fuel element can be a nuclear fuel assembly rod (e.g., with a cladding that holds nuclear fuel pellets). In some aspects, a nuclear fuel element can nuclear fuel (e.g., one or more nuclear fuel assemblies) enclosed in a canister. In some aspects, a nuclear fuel element can be another form of fissile material, such as TRISO fuel, metallic uranium or plutonium, oxides of uranium or plutonium, or mixtures of uranium and plutonium oxides (MOX).

[0094] As shown in FIGS. 5A and 5B, pressure control tubes 510, 512 extend within the borehole 505 through the heat exchanger portion 501 and into the nuclear reactor portion 503. In some aspects, the pressure control tubes 510, 512 allow pressure control of the fluid in the reactor system 500 and the insertion and removal of a primary reactor coolant fluid. In the case of pressure control, the tubes 510, 512 allow the operator to set the pressure in the primary reactor loop to be close to the pressure of the brine 580 in the subterranean formation 502, thereby allowing the use of thinner and less expensive metal for the reactor vessel 590.

[0095] In an example operation of the borehole nuclear reactor system 500, the nuclear reactor portion 503 operates to heat (and re-heat, once cooled) a cool primary fluid coolant 532 that circulates (e.g., naturally, by convention, or both) from the heat exchanger portion 501 and into the core 560 through the downcomer sections 528. As the cool primary fluid coolant 532 rises through the core 560 and through nuclear fuel elements 570, the nuclear fuel elements 570 heat the cool primary fluid coolant 532 into hot primary fluid coolant 530 that enters the riser 595 and into the riser section 526.

[0096] The hot primary fluid coolant 530 flows through the riser 595 and the riser section 526 and into the heat exchanger portion 501 of the system 500. The hot primary fluid coolant 530 turns into the downcomer sections 528 at the closed end of the primary fluid coolant flowpath 524 and begins to transfer heat to a rising secondary fluid coolant 516 that circulates in the upflow sections 518 of the secondary fluid coolant flowpath 523. As heat is transferred, the rising secondary fluid coolant 516 can remain in liquid form or phase change (at least partially) to gas form in order to form steam 520 that, eventually, rises to the generator at the terranean surface.

[0097] The steam 520 that is used in the generator, generally, changes phase back to liquid and circulates through the downflow sections 513 as cold secondary fluid coolant 514. The cold secondary fluid coolant 514 circulates through the downflow sections 513 and into the upflow sections 518 within the secondary fluid coolant flowpath 523, where it is heated to the rising secondary fluid coolant 516.

[0098] This process repeats as the nuclear reactor portion 503 (and/or the generator) is in operation. In the case of, e.g., an uncontrolled fission reaction that occurs in the nuclear reactor section 503, or in order to abate a controlled fission reaction, the control rod assembly 550 can be operated (e.g., inserted into the core 560) to stop or reduce the nuclear fission reaction of the nuclear fuel elements 570.

[0099] Although a particular type of nuclear reactor is shown in the nuclear reactor portion 503, (enriched uranium, plutonium, their oxides, or mixed oxides) the present disclosure contemplates that many forms of nuclear reactors can be used with fissile materials that forms the nuclear fuel elements 570. For example, molten salt reactors, molten metal reactors, Training, Research, Isotopes, General Atomics (TRIGA) reactors, TRISO fuel reactors, boiling water reactors, high temperature gas cooled reactors, or another kind of 4th generation reactors that can be installed in a human-unoccupiable borehole (such as borehole 505) are all contemplated by the present disclosure.

[0100] Furthermore, although a single nuclear reactor portion 503 is shown in FIG. 5B, there can be multiple nuclear reactor portions 503 (as well as multiple heat exchanger portions 501) installed in the borehole 505, whether within a single vertical borehole or multiple, slant or lateral boreholes from the borehole 505. Moreover, additional systems found in standard reactors, such as filtration systems, can be readily added to the reactors described here.

[0101] FIGS. 6A to 6F show example cross-sections (plan views) of the borehole with several different configurations for the downward flowing coolant water (labeled "W") and the upward flowing steam (labeled "S"). In some examples, the upflowing steam and the downflowing coolant water are conveyed in a common borehole. In some examples, the upflowing steam and the downflowing coolant water are conveyed in separate boreholes that link at the heat exchanger.

[0102] One or more of the pipes can be insulated. The insulation can be achieved by aerogel or vacuum insulation. If the cold water pipe is insulated and the steam pipe is uninsulated, then the steam pipe can heat the casing, cement (if used), and the surrounding rock. This heating can cause some loss of power carried by the steam pipe during the initial operation, but as the rock heats, the rate of heat transfer can slow because of the decreased temperature difference between the surrounding rock. Thus, the surrounding rock formation serves as an insulator. The insulation value of the rock increases as the water within it turns to vapor, since vapor is less conductive than water.

[0103] In some examples, cement between the casing and the formation is not used. In another, the casing is perforated to allow flow of rock brine within the casing (but not penetrating the cooling water or the steam pipes). In both of these cases, the temperature of the brine can rise above its boiling point and turn to water vapor. When this happens, for the non- concentric configurations shown in FIG. 6A to 6F, the water vapor can serve as another heat insulator, or steam insulator, that reduces the flow of heat from the steam pipe and the flow of heat into the coolant pipe.

[0104] Referring to FIG. 6G, in some examples, the piping includes a transition region 650 between two of the configurations shown in FIG. 6A to 6F. For example, in an example configuration 606, the upflowing steam is in the central pipe, surrounded by the downflowing coolant water. In another example configuration 602, the downflowing coolant water is in the central pipe, surrounded by the upflowing steam in the surrounding pipe, enabling the use of steam insulation. The transition region 650 connects a section of the pipe with the configuration 606 to a section of the pipe with the configuration 602. The transition region 650 includes a section with configuration 604, with the steam pipe and water pipe being side by side. 1 [0105] Referring to FIG. 7, a reactor for a borehole nuclear reactor system (such as reactor 130) can be split into or include multiple, independently-controllable reactors 130a, 130b. For example, multiple reactors 130 within a reactor region can be operated simultaneously to provide maximum power, or individual reactors 130 can be periodically shut down or reduced in heat power output when less generated power is needed or desired. The control of each reactor 130 (of multiple reactors 130) can be accomplished by, for example, putting a strong neutron absorber in a vicinity or one or more of the multiple reactors 130. This can be done, for example, by having tubing or pipes that extend from the one or more reactors 130 to the surface 104. If the tubes are filled with water, then a particular reactor 130 can produce maximum power. If filled with a fluid containing a strong neutron absorber, such as boron or cadmium or their salts, then the reactivity of the particular reactor 130 can be reduced. A set of tubes (supply and return fluids) could be installed and provided for each reactor 130 of multiple reactors 130. [0106] In some aspects, control of one or more of the multiple reactors 130 can be accomplished with control rods (e.g., solid control containing boron or cadmium or their salts) that are controllably moveable into and out of individual reactors 130. The controllable movement of the control rods can be performed mechanically or hydraulically, such as by using fluids in pipes that are controlled at or near the surface 104.

[0107] In some examples, stacked reactors are operated sequentially. A first reactor 130a can be placed in the borehole 115 and operated for a period of time. When the first reactor is no longer to be used, the spent fuel can be disposed by leaving it the first reactor place. The reactor core can be sealed or can be placed unsealed beneath a second reactor 130b. The second reactor 130b can be placed on top of the first reactor 130a. The second reactor 130b is then operated, while the first reactor 130a is no longer in operation.

[0108] The deep borehole reactor has several elements that attach to tubes or cables that extend from the reactor to the surface. In some examples, the cables support the reactor to hold the reactor in place in the borehole.

[0109] When the fuel in the first reactor 130a is spent, the steam pipe can be removed and the first reactor 130a left in place. In some examples, the connection between the remaining tubes (e.g., 510, 512, 555) and control rod cables can be left in place and the hole filled with a sealing material. In some examples, the tubes and cables can be severed near the bottom of the cables. One way to sever the tubes and cables is to lower a severing gun or a cutter that detaches the tubes and cables; then the upper parts of the tubes and cables can be removed. In some examples, the tubing is cut above the steam generator. The steam generator can be left in the borehole 115 along with the fuel assembly.

[0110] In some examples, the second reactor 130b is placed above the first reactor 130a after the tubes and cables have been removed. The tubes and cables can be inserted into the borehole 115 and attached to the second reactor 130b. The second reactor 130b can then operate in the same manner that the first reactor 130a operated. The second reactor 130b can sit directly on top of the first reactor 130a. In some examples, the second reactor 130b can be operated using the same control system as the first reactor 130a.

[0111] In some examples, the first reactor 130a can be disconnected from the surface by, for example, a cutting tool. The first reactor 130a can then be covered with sand, cement, or other material. In some examples, the sand serves as a platform on which the second reactor 130b can sit. In some examples, an additional platform can be placed above the first reactor 130a in order to support the second reactor 130b. In some examples, the heat exchanger is removed prior to installing the second reactor 130b, and the platform is placed on top of the first reactor 130a. In some examples, the heat exchanger remains in place prior to installing the second reactor 130b, and the platform is placed on top of the heat exchanger. The second reactor 130b, and a new heat exchanger, can then be placed on top of the platform.

[0112] The process of stacking reactors can be repeated multiple times for additional reactors. For example, a third reactor (not shown) can be placed above the second reactor 130b when operation of the second reactor 130b is completed. Any number of reactors can be stacked upon one another and operated sequentially.

[0113] In some examples, during operation of the first reactor 130a, the first reactor 130a can be positioned at an initial position, or operating depth, within the borehole. After operation of the reactor 130a completes, the first reactor 130a can be lowered from the operating depth to a storage depth. The storage depth can be a position 140 that is deeper in the borehole 115 than the operating depth. The second reactor 130b can then be placed into the borehole 115 and lowered to the operating depth.

[0114] By moving the first reactor 130a deeper into the borehole 115 after operation is completed, the second reactor 130b can operate from the same or similar operating depth as the first reactor 130a. For example, the operating depths of the first reactor 130a and the second reactor 130b can be similar such that a portion of the borehole occupied by the second reactor 130b during operation of the second reactor 130b overlaps with a portion of the borehole occupied by the first reactor 130a during operation of the first reactor 130a. In some examples, the operating depths of the first reactor 130a and the second reactor 130b are similar such that a difference between operating depths is less than a specified distance (e.g., five meters or less, ten meters or less, twenty meters or less).

[0115] In some examples, the borehole 115 includes storage space for multiple nuclear reactors. After operation of the second reactor 130b completes, the second reactor 130b can also be moved deeper into the borehole 115 for storage and disposal. In this example, the first reactor 130a and the second reactor 130b can each be moved to the position 140 that is deeper than the operating depth, and a third reactor (not shown) can be placed at the operating depth.

[0116] In some examples, the heat exchanger 120 is not permanently attached to the first reactor 130a, and the steam pipe 208 and heat exchanger 120 can be removed from the borehole 115, leaving the first reactor 130a with its spent fuel at the bottom of the hole. Then the second reactor 130b can be lowered to sit above the first reactor 130a. The same heat exchanger 120 and steam pipe 208 can be placed above the second reactor 130b, or a new heat exchanger 120 and/or steam pipe 208 can be used.

[0117] The reuse of the borehole, the heat exchanger 120, and/or the steam pipe 208 can significantly reduce the cost of the second reactor. Additionally, having a plan to put a second reactor in place allows the first reactor to be operated at a higher power rate with shorter lifetime, since it can be anticipated that the reactor can be replaced at the end of its lifetime.

[0118] FIG. 8 shows a cross-section of an example reactor core 800 for a borehole reactor. The reactor core 800 includes a single fuel assembly 802 with a circular cross-section similar to that of the borehole and casing.

[0119] The fuel assembly 805 includes fuel rods 804. The fuel rods 804 can be conventional light water reactor (LWR) hollow fuel rods made of a zirconium or aluminum alloy, with uranium dioxide fuel pellets stacked in the interior. The reactor core 800 is narrow enough to fit into a borehole, and yet has a sufficiently high efficiency in the use of neutrons that it can achieve criticality over a range of enrichments.

[0120] The reactor core 800 includes four hundred thirty-one fuel rods 804 and twenty- one control rods 806 in the inserted position. This configuration enables a sufficient ratio of water to fuel. When the control rods 806 are removed, the spaces that the control rods 806 occupied fill with coolant water from the primary loop.

[0121] In some examples, the reactivity of any section of the reactor core 800 can be adjusted by changing the ratio of moderator to fuel. For example, some fuel rod positions can remain empty, forming gaps 812 that can fill with the water moderator. Other ways to change the moderator-to-fuel ratio include changing the diameter of the fuel rods 804, changing the diameter of the control rods 806, and leaving gaps between fuel pellets that can fill with the water moderator. The process of changing the moderator-to-fuel ratio can be implemented to different degrees in different parts of the reactor core 800 to achieve the desired burn pattern.

[0122] Surrounding the fuel rods 804 is a core vessel 808. In some examples, the core vessel 808 is made of beryllium, a metal having a low neutron absorption and high neutron reflectivity. Surrounding the core vessel 808 is the primary loop return water, or cool primary fluid coolant 532, which also has low neutron absorber and high reflectivity. Because of their light atomic weight, both the beryllium, and the hydrogen in the water, serve as moderators for the fission, which improves the reactivity (criticality) coefficient.

[0123] Surrounding the primary loop return water is a reactor vessel 810. In some examples, the reactor vessel 810 is made of beryllium, steel, aluminum, zirconium, or other metal chosen for its low neutron absorption and ability to resist corrosion.

[0124] The reactor can be placed in a host rock 502 that is chosen for the criteria of having low neutron absorption, high neutron reflection, and moderating capability. The depth of the reactor as well as the location can be adjusted to find the most suitable rock.

[0125] For a borehole reactor, there is room in the long borehole to allow the use of much longer fuel rods, compared to standard PWR fuel rods. The use of longer rods has many potential benefits, including higher power output and longer lifetime before the fissile fuel is spent. In the deep borehole, the rods could be between four and twenty meters in length, or more. In some examples, after the hole is drilled and optionally cased, then the reactor can be assembled on the surface in a horizontal position, and one end raised by a crane. The crane can lower the reactor into the hole, and when the top of the reactor is near the surface, the heat exchanger 120 and steam pipe 208 can be attached.

[0126] In some examples, shorter fuel rods of between two and four meters can be used. Individual reactors can be made that can attach to each other, with the top of the first reactor attached to the bottom of the second reactor. The control rods from one reactor can be attached to the corresponding control rods of the second reactor, or they could be controlled separately. [0127] In some examples, a depth of the borehole reactor is approximately one mile. In some instances, greater or lesser depths can be used. Greater depths allow for a higher pressure, and therefore a higher boiling point of water, which allows the reactor to operate at a higher temperature. A higher temperature can result in a higher Carnot efficient for the reactor system. A smaller depth can also be used. At a depth of a half mile, the pressure is lower, but the cost of construction is reduced, and particularly for small power systems, there can be a cost-benefit advantage to using the reduced depth.

[0128] The reactor systems described herein can deliver low power (e.g., 1 MWe) for longer periods of time (e.g., three decades or more), or high power (e.g., 10 MWe) for shorter periods of time (e.g., several years). In some examples, when the fuel is spent, then the reactor can be removed. The pressurizer tubes 510, 512, 555 can be used to lift the reactor to the surface. If the heat exchanger 120 is attached to the reactor, then the heat exchanger 120 can be lifted up using other lifting methods, bringing everything to the surface.

[0129] Flexible tubing is typically a pipe or tube that is sufficiently thin that it can be wrapped around a large spool at the surface. The use of this flexible tubing allows it to be placed in the hole with a relatively small, inexpensive, and easily transportable workover rig, rather than requiring an expensive drilling rig. Flexible tubing can be used for piping for the borehole reactor. Any of the following can be made from flexible tubing: the steam pipe 208, the tubes 510, 512, the tube 555. The flexible tubing can be sufficiently flexible to fit around the flexible tubing spool, thereby allowing the drilling rig to be removed after the hole is drilled, or if it is a cased hole, after the casing has been put in place.

[0130] In some examples, the casing is cemented between its outer surface and the host rock. This can be done for many reasons: to provide a seal so that oil and gas will not flow outside the pipe, and to provide stability for the casing against shaking from the transport of gas and oil.

[0131] In one embodiment of the deep borehole reactor, the casing is left uncemented. The hot rising steam in the borehole can be used to heat the casing, and this can cause the gap between the borehole and the casing to become void of liquid and to fill with steam. This steam serves as an additional insulator for the hot pipe and increases the efficiency of transport of the reactor steam to the surface. In some examples, the cold water pipe, with downward flowing water, is insulated to keep it cool.

[0132] In some examples, the casing is perforated, to allow the flow of liquids in and out of the casing. This can allow the space within the casing also to be filled with steam, and thereby provide additional insulation for the rising steam.

[0133] The use of burnable poisons, also known as burnable absorbers, such as gadolinium, is well known for use in controlling the reactivity of nuclear reactors. When fresh fuel is in a nuclear reactor, neutrons are abundant, these poisons absorb neutrons, and help keep the reactivity under control. For example, if the reactivity coefficient is 1.2, the presence of these poisons and their ability to remove neutrons without causing the release of additional neutrons can help bring the reactivity coefficient down to the desired value of 1.

[0134] In an example configuration a chain reaction can occur with a fuel enrichment (U- 235/U-238) as low as 2.75%. However, if instead of a uniform distribution the reactor has the distribution of enrichment that results from an extended chain reaction, then the criticality ends when the average enrichment is 3.5%. The chain reaction goes below criticality, that is, reactivity coefficient less than one at 3.5% rather than 2.75%. This shortening of the range of criticality has negative implications for the cost of delivered energy.

[0135] To overcome this waste, the burnable poison can be distributed in the fuel assembly in a nonuniform manner. In some examples, a burnable poison can be placed poison in such a manner that it can assure improved total fission of the U-235 fuel and its fissionable byproducts such as Plutonium-239. In some examples, the burnable poison is placed in the outer part of the core. The poison protects the outer U-235 until the inner parts have undergone extensive fissioning, and then, once the burnable poison was sufficiently depleted, the outer parts of the core participated more fully in the chain reaction.

[0136] In various instances, the neutron poison can be gadolinium, erbium, UO2-M2O3, ZrB2, AI2O3-B4C, and other materials known to those practiced in the field. The poisons can be attached to the fuel pellets, to the cladding of the control rods, or in special holders (including rods and wires and tubes containing moderator). The poisons can be discrete, or they can be mixed with the fuel pellets. They can be uniformly mixed, or non-uniformly mixed. In some examples, the neutron poison includes rod-shaped pieces that are place in the center cores of the individual pellets. [0137] While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what can be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation.

Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features can be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination can be directed to a subcombination or variation of a subcombination.

[0138] Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

[0139] A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, example operations, methods, or processes described herein may include more steps or fewer steps than those described. Further, the steps in such example operations, methods, or processes may 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

WHAT IS CLAIMED IS:

1. A method of operating a nuclear reactor system, comprising: operating a first nuclear reactor to produce steam, the first nuclear reactor positioned in a borehole that extends from a terranean surface through one or more subterranean formations; shutting down the first nuclear reactor; placing a second nuclear reactor into the borehole; and operating the second nuclear reactor to produce steam.

2. The method of claim 1, comprising: before placing the second nuclear reactor into the borehole, removing the first nuclear reactor from the borehole; and placing the second nuclear reactor into the borehole at a position that is the same or similar distance from the terranean surface as the first nuclear reactor.

3. The method of claim 1, comprising: before placing the second nuclear reactor into the borehole, moving the first nuclear reactor from an initial position in the borehole to a position that is farther from the terranean surface than the initial position; and placing the second nuclear reactor into the borehole at a position that is the same or similar distance from the terranean surface as the initial position.

4. The method of claim 1, comprising: before placing the second nuclear reactor into the borehole, placing a platform above the first nuclear reactor in the borehole; and placing the second nuclear reactor into the borehole atop the platform at a position that is nearer to the terranean surface than the first nuclear reactor.

5. The method of claim 4, wherein placing the platform above the first nuclear reactor comprising placing at least one of sand or cement into the borehole.

6. The method of claim 1 , wherein operating the first nuclear reactor to produce steam comprises transporting a secondary fluid coolant through a secondary coolant system that is thermally coupled to a primary coolant system by a heat exchanger, wherein the primary coolant system is thermally coupled to a reactor core of the first nuclear reactor, the method comprising transporting steam through a steam pipe from the heat exchanger to the terranean surface.

7. The method of claim 6, comprising: before placing the second nuclear reactor into the borehole, removing the heat exchanger and the steam pipe from the borehole; and after placing the second nuclear reactor into the borehole, replacing the heat exchanger and the steam pipe into the borehole.

8. The method of claim 6, comprising: before placing the second nuclear reactor into the borehole, removing the steam pipe from the borehole, and leaving the heat exchanger in place in the borehole; and after placing the second nuclear reactor into the borehole, replacing the steam pipe into the borehole.

9. The method of claim 6, wherein the secondary coolant system comprises: a central tube, the method comprising transporting the secondary fluid coolant through the central tube from the heat exchanger to the terranean surface; and an outer concentric tube, the method comprising transporting the secondary fluid coolant through the outer concentric tube from the terranean surface to the heat exchanger.

10. The method of claim 1, wherein operating the second nuclear reactor to produce steam comprises transporting a secondary fluid coolant through a secondary coolant system that is thermally coupled to a primary coolant system by a heat exchanger, wherein the primary coolant system is thermally coupled to a reactor core of the second nuclear reactor, the method comprising transporting steam through a steam pipe from the heat exchanger to the terranean surface.

11. The method of claim 1 , comprising: shutting down the second nuclear reactor; placing a third nuclear reactor into the borehole; and operating the third nuclear reactor to produce steam.

12. A method for controlling a nuclear reactor system, comprising: adjusting a pressure in at least one pressurizer vessel of a pair of pressurizer vessels, wherein: a first pressurizer vessel of the pair of pressurizer vessels is in fluid communication with primary coolant at a first depth of a reactor vessel, a second pressurizer vessel of the pair of pressurizer vessels is in fluid communication with the primary coolant at a second depth of the reactor vessel, the second depth being different from the first depth, the reactor vessel is positioned in a borehole that extends from a terranean surface through one or more subterranean formations, the reactor vessel contains a reactor core comprising at least one nuclear fuel element, and the primary coolant is in thermal communication with the reactor core, and causing a change to a pressure of the primary coolant in response to adjusting the pressure in the at least one pressurizer vessel.

13. The method of claim 12, wherein: the first pressurizer vessel is in fluid communication with the primary coolant through a first pressurizer tube that extends from the first pressurizer vessel to the first depth of the reactor vessel, and the second pressurizer vessel is in fluid communication with the primary coolant through a second pressurizer tube that extends from the second pressurizer vessel to the second depth of the reactor vessel.

14. The method of claim 13, wherein the first depth is nearer to a bottom of the reactor vessel than the second depth.

15. The method of claim 13, wherein the first pressurizer vessel and the first pressurizer tube contain boronated fluid.

16. The method of claim 15, comprising causing a change to a boron concentration of the primary coolant in response to adjusting the pressure in the at least one pressurizer vessel.

17. The method of claim 12, wherein the pair of pressurizer vessels are located above the terranean surface.

18. The method of claim 12, comprising: injecting a boronated fluid into the reactor vessel through an injector tube by inserting a plunger into the injector tube.

19. The method of claim 18, comprising inserting the plunger into the injector tube at a first end of the injector tube that is at or near the terranean surface to cause the boronated fluid to enter the reactor vessel at a second end of the injector tube that is inside the reactor vessel.

20. The method of claim 19, wherein a portion of the injector tube inside the reactor vessel is curved, and the second end of the injector tube is oriented towards the terranean surface.

21. The method of claim 20, wherein a one-way valve is positioned at the second end of the injector tube, the one-way valve permitting flow of the boronated fluid into the reactor vessel.

22. A nuclear reactor system, comprising: a reactor vessel positioned in a borehole that extends from a terranean surface through one or more subterranean formations; a reactor core contained within the reactor vessel and comprising at least one nuclear fuel element; a primary coolant in thermal communication with the reactor core; and a pair of pressurizer vessels, comprising: a first pressurizer vessel in fluid communication with the primary coolant at a first depth of the reactor vessel, and a second pressurizer vessel in fluid communication with the primary coolant at a second depth of the reactor vessel, the second depth being different from the first depth, the pair of pressurizer vessels configured such that adjusting pressure in at least one pressurizer vessel of the pair of pressurizer vessels causes a change to a pressure of the primary coolant.

23. The nuclear reactor system of claim 22, wherein: the first pressurizer vessel is in fluid communication with the primary coolant through a first pressurizer tube that extends from the first pressurizer vessel to the first depth of the reactor vessel, and the second pressurizer vessel is in fluid communication with the primary coolant through a second pressurizer tube that extends from the second pressurizer vessel to the second depth of the reactor vessel.

24. The nuclear reactor system of claim 23, wherein the first depth is nearer to a bottom of the reactor vessel than the second depth.

25. The nuclear reactor system of claim 23, wherein the first pressurizer vessel and the first pressurizer tube contain boronated fluid.

26. The nuclear reactor system of claim 25, wherein adjusting the pressure in the at least one pressurizer vessel causes a change to a boron concentration of the primary coolant.

27. The nuclear reactor system of claim 22, wherein the pair of pressurizer vessels are located above the terranean surface.

28. The nuclear reactor system of claim 22, comprising: an injector tube; and a plunger, wherein the injector tube is configured to inject boronated fluid into the reactor vessel when the plunger is inserted into the injector tube.

29. The nuclear reactor system of claim 28, wherein: a first end of the injector tube is above the terranean surface, the plunger is at the first end of the injector tube, and a second end of the injector tube is inside the reactor vessel.

30. The nuclear reactor system of claim 29, wherein a portion of the injector tube inside the reactor vessel is curved, and the second end of the injector tube is oriented towards the terranean surface.

31. The nuclear reactor system of claim 30, comprising a one-way valve positioned at the second end of the injector tube, the one-way valve permitting flow of the boronated fluid into the reactor vessel.