GB2629056A

GB2629056A - Nuclear fission power plant

Info

Publication number: GB2629056A
Application number: GB2403377.1A
Authority: GB
Inventors: D Lee Joseph; G Bauld James; A Brown Sam; J Wakeling Megan; Pepper Samuel
Original assignee: Rolls Royce Submarines Ltd
Current assignee: Rolls Royce Submarines Ltd
Priority date: 2024-03-08
Filing date: 2024-03-08
Publication date: 2024-10-16
Also published as: GB202403377D0

Abstract

An extra-terrestrial nuclear fission reactor 10 comprises a plurality of thermoelectric generators (TEGs) 80 coupled to each of a plurality of heat pipes 40 that extend from an unmoderated reactor core. Heat generated by the unmoderated fuel 30 in the fast-fission core is transferred to the thermoelectric generators via the heat pipes and is ultimately radiated away therefrom, electricity being generated by virtue of a temperature gradient between the hot and cold ends of each thermoelectric element. In addition to a plurality of primary neutronic control elements in the form of rotatable control drums 50, a neutron reflector 70 is also disposed around the periphery of the reactor core. At least one linearly movable control rod 54 may also be provided for selective insertion into the core. The unmoderated fuel of the reactor may comprise High Assay Low Enriched Uranium (HALEU) and more preferably fuel enriched to 19.75% uranium-235.

Description

NUCLEAR FISSION POWER PLANT

TECHNICAL FIELD

[0001] This disclosure relates to a nuclear fission power plant configured for extra-terrestrial use and a kit of parts and method for a nuclear fission power plant configured for extra-terrestrial use.

BACKGROUND

[0002] In space applications, it is desirable to have a reliable and sustainable power source. Solar panels are often used for satellites. However, their power density is low and they do not generate electricity when in shadow. This is a particular problem for moon or planetary expeditions where a facility could be in shadow for prolonged periods of time. Solar panels could be supplemented with battery storage, but batteries add significant weight, which is not viable for a rocket launch. In any event, the low power density of solar panels limits their application. In particular, a moon or planet based facility may have a high power requirement.

[0003] The high-power density of a nuclear fission power plant and ability to continue generation without sunlight make nuclear fission power plants in extra-terrestrial applications an attractive option. However, an extra-terrestrial nuclear fission power plant would need to withstand the harsh environment of space (or moon/planet), require minimum maintenance and survive the large vibrations associated with a rocket launch. The low or zero gravity force and lack of a readily available heat sink present additional challenges. Weight and size are also issues as any power plant would likely need to fit within the confines of a rocket.

[0004] The present disclosure seeks to address these issues.

SUMMARY

[0005] According to a first aspect there is provided a nuclear fission power plant configured for extra-terrestrial use, the nuclear fission power plant comprising: an unmoderated nuclear reactor core, the unmoderated nuclear reactor core comprising an unmoderated fuel; a neutron reflector disposed around a periphery of the unmoderated nuclear reactor core; a plurality of primary neutronic control elements; a plurality of heat pipes, each heat pipe comprising a first portion extending within the unmoderated nuclear reactor core and a second portion extending beyond an end of the unmoderated nuclear reactor core; and a plurality of thermoelectric generators, wherein a plurality of the thermoelectric generators are coupled to each heat pipe, each thermoelectric generator having a first hot end coupled to the second portion of a corresponding heat pipe and a second cold end configured to radiate heat.

[0006] The unmoderated fuel may comprise High Assay Low Enriched Uranium (HALEU). The unmoderated fuel may be enriched to substantially 19.75% Uranium-235. The unmoderated fuel may comprise uranium dioxide, UO2. The unmoderated fuel may comprise pellets of fuel stacked within a metal tube. The unmoderated nuclear reactor core may comprise a fast neutron reactor.

[0007] The heat pipes may comprise a heat pipe working fluid comprising an alkali metal, such as sodium.

[0008] The plurality of primary neutronic control elements may comprise rotatable control drums disposed around the periphery of the unmoderated nuclear reactor core. Additionally or alternatively, the nuclear fission power plant may comprise a linearly movable control rod. The primary neutronic control elements and the linearly moveable control rod may comprise boron carbide, B4C.

[0009] The heat pipe second portions may fan out with respect to one another. The heat pipe second portions may be deployable from a stowed configuration to a deployed configuration in which the heat pipe second portions fan out with respect to one another. The heat pipes may be configured such that the thermoelectric generators are spaced apart from the unmoderated nuclear reactor core, e.g. by 5 metres, by 10 metres or more.

[0010] According to a second aspect there is provided a kit of parts for a nuclear fission power plant configured for extra-terrestrial use, the kit of parts being configured at least partially for extra-terrestrial assembly and comprising: an unmoderated nuclear reactor core, the unmoderated nuclear reactor core comprising an unmoderated fuel; a neutron reflector configured to be disposed around a periphery of the unmoderated nuclear reactor core; a plurality of primary neutronic control elements; a plurality of heat pipes, each heat pipe, when the nuclear fission power plant is assembled, comprising a first portion extending within the unmoderated nuclear reactor core and a second portion extending beyond an end of the unmoderated nuclear reactor core; and a plurality of thermoelectric generators, wherein a plurality of the thermoelectric generators are couplable to each heat pipe, each thermoelectric generator having a first hot end couplable to the second portion of a corresponding heat pipe and a second cold end configurable to radiate heat.

[0011] The kit of parts may comprise a linearly movable control rod configured to be insertable into a cavity within the unmoderated nuclear reactor core.

[0012] According to a third aspect there is provided a method for a nuclear fission power plant configured for extra-terrestrial use, the method comprising controlling the nuclear fission power plant to: generate heat with an unmoderated nuclear reactor core comprising an unmoderated fuel, wherein a neutron reflector is disposed around a periphery of the unmoderated nuclear reactor core, and the nuclear fission power plant comprises a plurality of primary neutronic control elements; transfer heat from the unmoderated nuclear reactor core using a plurality of heat pipes, each heat pipe comprising a first portion extending within the unmoderated nuclear reactor core and a second portion extending beyond an end of the unmoderated nuclear reactor core; transfer heat from the second portion of the heat pipes to a plurality of thermoelectric generators; wherein a plurality of the thermoelectric generators are coupled to each heat pipe, each thermoelectric generator having a first hot end coupled to the second portion of a corresponding heat pipe and a second cold end; radiate heat from the second cold end of each of the thermoelectric generators; and generate electricity by virtue of a temperature gradient across the thermoelectric generators.

[0013] The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive any feature described herein may be applied to any aspect and/or combined with any other feature described herein.

BREIF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and not to scale, and in which: [0015] Figure 1 is a schematic diagram showing an example of a nuclear fission power plant configured for extra-terrestrial use; [0016] Figure 2 is a schematic diagram showing another example of a nuclear fission power plant configured for extra-terrestrial use; [0017] Figure 3 is a schematic diagram showing an arrangement of thermoelectric generators for a nuclear fission power plant; and [0018] Figure 4 is a flowchart depicting a method for a nuclear fission power plant configured for extra-terrestrial use.

DETAILED DESCRIPTION

[0019] Wth reference to Figure 1, the present disclosure relates to a nuclear fission power plant 10 specifically configured for extra-terrestrial use, i.e., away from the Earth's surface. The nuclear fission power plant 10 may be exclusively configured for extra-terrestrial use. As such, the nuclear fission power plant 10 may be referred to as an extra-terrestrial nuclear fission power plant 10. However, the nuclear fission power plant 10 may be at least partially assembled on Earth and may be launched, e.g. from a rocket, into space. In a particular example, the nuclear fission power plant 10 may be intended for use on the lunar surface of the Moon orbiting Earth. However, the nuclear fission power plant 10 may also be used in space or on other planets and moons.

[0020] The nuclear fission power plant 10 may be a micro-reactor. As such, the nuclear fission power plant 10 may be readily transportable, in particular on a rocket.

[0021] As depicted, the nuclear fission power plant 10 comprises an unmoderated nuclear reactor core 20. The unmoderated nuclear reactor core 20 comprises a fuel 30. The fuel 30 may comprise High Assay Low Enriched Uranium (HALEU), e.g., with the concentration of the fissile isotope uranium-235 (U-235) being between 5% and 20% of the mass of uranium. In particular, the fuel 30 may be enriched to substantially 19.75% Uranium-235. This level of enrichment allows for better energy density whilst maintaining a safe level of enrichment.

[0022] The fuel 30 may comprise uranium dioxide, UO2. Such fuels may undergo fission from fast, high-energy neutrons, that have not undergone any moderation (slowing down). The fuel 30 may be in pellet form and may be clad, e.g. in plated metal or a tubular cladding. The fuel pellets may be stacked within a metal tube. For example, pellets of fuel may be stacked within the metal tube, before being welded closed with a gap at the top to trap fission gases.

[0023] The unmoderated nuclear reactor core 20 comprises an unmoderated fuel.

The use of a "fast reactor" (i.e. a reactor based on fission from fast neutrons) removes the need to include moderator material, which reduces the space and mass requirements for the nuclear fission power plant. It also makes the design more resilient to the rigours of launch, as moderator materials tend to be more fragile, and therefore need careful designing in order to prevent them from breaking during launch. Furthermore, fast reactors can also operate at higher temperatures, and therefore can have higher efficiencies compared with moderated systems.

[0024] Heat energy may be extracted from the unmoderated nuclear reactor core 20 using two-phase passive convection. For example, the nuclear fission power plant 10 may further comprise a plurality of heat pipes 40. The heat pipes 40 may be substantially elongate with each heat pipe 40 having a first portion 41 extending within (e.g. across a substantial length of) the unmoderated nuclear reactor core 20 and a second portion 42 that extends beyond an end of the unmoderated nuclear reactor core 20. The heat pipes 40 may be distributed within the unmoderated nuclear reactor core 20, e.g., to ensure efficient heat transfer and heat distribution within the unmoderated nuclear reactor core 20.

[0025] The heat pipes 40 are heat-transfer devices that use phase transition to transfer heat between two parts of the heat pipe. At the hot part of the heat pipe 40 (i.e., the first portion 41 in the unmoderated nuclear reactor core 20), a volatile liquid within the heat pipe 40 turns into a vapour by absorbing heat from around the heat pipe. The vapour then travels along the heat pipe 40 to a cold part of the heat pipe and condenses back into a liquid, releasing the latent heat (i.e., at the second portion 42). The liquid then returns to the hot part of the heat pipe 40 and the cycle repeats.

[0026] Heat pipes 40 have been selected as they have excellent reliability, lifetime, and redundancy (for example, when a plurality of heat pipes are used, the nuclear power plant may continue to operate even if a particular heat pipe has failed). The heat pipes 40 may use an alkali metal as their working fluid (for example sodium) as this provides excellent heat transportation when using the passive two-phase convection mechanism and operates within the 400-1000°C temperature range intended for the unmoderated nuclear reactor core 20.

[0027] The nuclear fission power plant 10 further comprises a plurality of primary neutronic control elements 50. The primary neutronic control elements 50 may be controlled to vary whether the primary neutronic control elements 50 absorb or reflect neutrons from the nuclear reactor core 20, and thereby control reactivity levels in the nuclear reactor core 20. The primary neutronic control elements 50 may comprise rotatable control drums disposed around the periphery of the nuclear reactor core 20. Rotation of the drums by an actuator 52 may vary whether the primary neutronic control elements 50 absorb or reflect neutrons from the nuclear reactor core 20. The actuator(s) 52 may be controlled by a suitable controller, which may receive data from one or more sensors. For example, the actuators 52 may be controlled by a first controller 55. The first controller 55 may be in communication with multiple systems (not shown), including sensor system(s) of the nuclear fission power plant 10. The sensor systems may be configured to determine a rate of fission in the nuclear reactor core 20. Each drum may comprise a neutron-reflecting material 51 (e.g. graphite or beryllium oxide) and may further comprise a neutron-absorbing material 53 (e.g. boron carbide, 134C). Beryllium oxide has very good neutron reflecting properties for its mass, has a high thermal conductivity, and a high temperature stability. Using beryllium oxide as the neutron-reflecting material of the drum may therefore increase a performance of the drum, which may consequently permit a mass of the nuclear fission power plant 10 to be reduced. The neutron-absorbing material 53 may be disposed over at least a portion of an outer circumference of the drum. To promote reactivity, the control drums may be positioned such that more of the reflecting material 51 is facing toward the core, thereby directing more neutrons back into the nuclear reactor core. To slow down reactivity, each control drum cylinder may be rotated so that more of the neutron-absorbing material 53 is facing toward the core, thereby absorbing more neutrons to slow down the rate of fission in the nuclear reactor core. In this way, reactivity levels of the nuclear reactor core 20 may be controlled and the rotatable drums may provide the primary form of control. The primary neutronic control elements 50 may be used for fine control, such as during a normal operating mode of the nuclear fission power plant 10. The primary neutronic control elements 50 may also be used for coarse control, such as in an emergency or shut-down mode of the nuclear fission power plant 10. The rotatable drums may be rotated quickly to rapidly increase their neutron absorbing properties, e.g., in the event of an emergency. The rotatable control drums may be the sole form of control of the unmoderated nuclear reactor core 20. The rotatable drums of the primary neutronic control elements 50 are advantageously compact and sufficiently robust to withstand the vibrations of a rocket launch.

[0028] Two or more independent actuators 52 may be provided for redundancy. For example, an actuator 52 may be provided at each end of a rotatable drum. In another arrangement, the rotatable drums may be arranged in two or more independent sets of rotatable drums with each set having its own actuator. The rotatable drums within a particular set may alternate with rotatable drums from another set. For example, there may be two independent sets of six rotatable drums interspersed with one another about the circumference of the unmoderated nuclear reactor core 20. Likewise, two or more independent control systems for the actuator(s) may be provided for redundancy, for example an independent control system may be provided for each set of rotatable drums.

[0029] The rotatable drums are advantageously compact and very reliable compared with other control methods, such as linear control rods, which require more space. The rotatable drums also are less impacted by environmental conditions such as vibration and are therefore more robust. In contrast to linear control rods, the rotatable drums are also highly reliable since the space they occupy does not change and they do not rely on linear movement into a void.

[0030] However, with reference to Figure 2, it is also contemplated that in addition to the plurality of primary neutronic control elements 50 the nuclear fission power plant 10 may additionally comprise at least one linearly movable control rod 54. The linearly movable control rod 54 may be selectively inserted into a corresponding cavity 56 in the unmoderated nuclear reactor core 20. Movement of the control rod 54 by a control rod actuator 57 may vary the number of neutrons absorbed by the control rod in the nuclear reactor core.

[0031] The nuclear fission power plant 10 may further comprise a neutron reflector 70 disposed around a periphery of the unmoderated nuclear reactor core 20. The neutron reflector 70 may reflect neutrons back towards the unmoderated nuclear reactor core 20. The neutron reflector 70 may be formed from aluminium oxide or graphite. Aluminium oxide (A1203) may be used as a reflection material, as it provides acceptable neutron reflection properties for its mass. Aluminium oxide also has excellent machinability and is good for shaping into specific reactor geometries. Nuclear-grade graphite exhibits similar properties and could therefore also be used as the reflection material.

[0032] The nuclear fission power plant 10 may further comprise a plurality of thermoelectric generators (TEGs) 80. A plurality of the thermoelectric generators 80 may be coupled to each of the heat pipes 40, in particular to the second portions 42 of the heat pipes 40. Each thermoelectric generator 80 may have a hot first end coupled to the second portion 42 of a corresponding heat pipe 40 and a cold second end configured to radiate heat. The thermoelectric generators 80 may be distributed along the length of the heat pipe second portions 42. The thermoelectric generators 80 may also be distributed around the heat pipe, e.g., with thermoelectric generators 80 on opposing sides of the heat pipe 40.

[0033] The thermoelectric generators 80 may generate an electrical current by virtue of a temperature gradient across each thermoelectric generator. The thermoelectric generators 80 may generate electricity from the diffusion of electrons across the temperature gradient. The thermoelectric generators 80 may be electrically coupled together and may supply an electrical current to one or more loads (not shown).

[0034] Heat may be removed directly from the cold second end of the thermoelectric generators 80 via direct radiation out to space. However, with reference to Figure 3, a radiator panel 82 may be provided at the cold second end of the thermoelectric generators 80. The radiator panel 82 may extend across multiple thermoelectric generators 80. The radiator panel 82 may present a larger surface area for the heat to radiate from. With the inclusion of a radiator panel 82, heat from the second portions 42 of the heat pipes 40 will heat the hot first end of each thermoelectric generator 80, and heat energy will be radiated away from the cold second ends of each thermoelectric generator 80 via the radiator panel 82, creating a temperature difference across the thermoelectric generator 80 which can be used to generate an electrical current to supply power to a load.

[0035] The thermoelectric generators 80 have excellent reliability and long lifespans and the nuclear fission power plant 10 can continue to operate in the event of one or more thermoelectric generators 80 failing. Furthermore, the thermoelectric generator and heat pipe arrangement removes the need for intermediate fluid loops and radiator infrastructure which increases the simplicity and reduces the weight of the system.

[0036] The nuclear fission power plant 10 may be deployable from a stowed configuration (e.g., in which the nuclear power plant may be stowed within a rocket for transportation) to a deployed configuration (e.g., in which the nuclear power plant may be operated to generate electricity). For example, the plurality of thermoelectric generators 80 and the second portions 42 of the heat pipes 40 may be deployable from a stowed configuration to a deployed configuration in which the second portions 42 of the heat pipes 40 may fan out with respect to one another. The deployability of the nuclear fission power plant 10 may be at least partially enabled by the heat pipes 40 having flexible connections between the first and second portions 41, 42, e.g. such that the second portions 42 can be folded into the stowed configuration.

[0037] Once deployed, the nuclear fission power plant 10 may be configured such that the thermoelectric generators 80 are spaced apart from the unmoderated nuclear reactor core 20. The thermoelectric generators 80 may be spaced apart from the unmoderated nuclear reactor core 20 by 5 metres, by 10 metres or more. This may allow the thermoelectric generators 80 to be spread out over a greater area and it may increase the radiative capacity by being further from the unmoderated nuclear reactor core 20. The spacing of the thermoelectric generators 80 from the unmoderated nuclear reactor core 20 may be achieved by the second portions 42 of the heat pipes 40 having a certain length without any thermoelectric generators 80.

[0038] The present disclosure also relates to a kit of parts for the nuclear fission power plant 10. The kit of parts may comprise at least some of the above-described components. The kit of parts may be configured for placement within a rocket to be launched into space. The kit of parts may also be configured at least partially for extra-terrestrial assembly. Once deployed in space, the kit of parts may automatically assemble or may be assembled with the assistance of a robot, an astronaut or any other space operative.

[0039] Wth reference to Figure 4, the present disclosure also relates to a method 200 for the nuclear fission power plant 10. The method 200 comprises controlling the nuclear fission power plant 10. The control of the nuclear fission power plant 10 may be at least partially carried out remotely, for example on a lunar base, from Earth or any other location.

[0040] The method 200 controls the nuclear fission power plant 10 such that in a first action 210, the nuclear fission power plant 10 generates heat with the unmoderated nuclear reactor core 20. In a second action 220, the nuclear fission power plant 10 transfers heat from the unmoderated nuclear reactor core using the plurality of heat pipes 40. In a third action 230, the nuclear fission power plant 10 transfers heat from the second portion 42 of the heat pipes 40 to the plurality of thermoelectric generators 80. In a fourth action 240, the nuclear fission power plant 10 radiates heat from the cold second end of each of the thermoelectric generators. In a fifth action 250, the nuclear fission power plant 10 generates electricity by virtue of the temperature gradient across the thermoelectric generators 80.

[0041] The present disclosure advantageously provides a nuclear power plant that is passive and with very few moving parts. The nuclear power plant is also quick to manufacture and assemble, which is an important factor in space applications.

[0042] Furthermore, as the nuclear reactor core is unmoderated it operates as a fast nuclear reactor. As a result, the nuclear reactor core can operate at a higher temperature, which improves the heat output. This can further compensate for the less efficient thermoelectric generators. The fast nuclear reactor core may also be more robust because it has minimal graphite, which tends to be fragile and therefore requires additional measures to prevent it from being damaged during rocket launch.

[0043] The heat pipes also promote a flat temperature distribution within the reactor core, which improves reactor performance. The resulting improvement in heat output further compensates for the less efficient thermoelectric generators. The overall efficiency of the nuclear power plant is therefore maintained at an acceptable level.

[0044] In addition, the present disclosure provides a lightweight and compact design.

For example, the rotatable drums minimize the volume required since the occupied space does not change when the drums are actuated. The heat pipes also provide a simple but effective design that allows a modular arrangement. The overall package size and weight is therefore lower.

[0045] Moreover, not having linearly actuated control rods saves weight, reduces complexity and provides a more robust arrangement. The present disclosure is more resistant to damage during rocket launch by not having linearly actuated control rods. By contrast, linearly actuated control rods are essentially cantilevers with a distal end that must be protected from high vibration loads encountered during rocket launch.

[0046] The heat pipe and thermoelectric generator arrangement is also resilient due to the high level of redundancy since a particular heat pipe or thermoelectric generator can fail without affecting operation of the remaining components.

[0047] Various examples have been described, each of which feature various combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

CLAIMS1. A nuclear fission power plant configured for extra-terrestrial use, the nuclear fission power plant comprising: an unmoderated nuclear reactor core, the unmoderated nuclear reactor core comprising an unmoderated fuel; a neutron reflector disposed around a periphery of the unmoderated nuclear reactor core; a plurality of primary neutronic control elements; a plurality of heat pipes, each heat pipe comprising a first portion extending within the unmoderated nuclear reactor core and a second portion extending beyond an end of the unmoderated nuclear reactor core; and a plurality of thermoelectric generators, wherein a plurality of thermoelectric generators are coupled to each heat pipe, each thermoelectric generator having a hot first end coupled to the second portion of a corresponding heat pipe and a cold second end configured to radiate heat.
2. The nuclear fission power plant of claim 1, wherein the unmoderated fuel comprises High Assay Low Enriched Uranium (HALEU).
3. The nuclear fission power plant of claim 1 or 2, wherein the unmoderated fuel is enriched to substantially 19.75% uranium-235.
4. The nuclear fission power plant of any preceding claim, wherein the unmoderated fuel comprises uranium dioxide, UO2.
5. The nuclear fission power plant of any preceding claim, wherein the unmoderated fuel comprises pellets of fuel stacked within a metal tube.
6. The nuclear fission power plant of any of the preceding claims, wherein the heat pipes comprise a heat pipe working fluid comprising an alkali metal.
7. The nuclear fission power plant of any of the preceding claims, wherein the plurality of primary neutronic control elements comprise rotatable control drums disposed around the periphery of the unmoderated nuclear reactor core.
8. The nuclear fission power plant of any of the preceding claims, further comprising at least one linearly movable control rod (54).
9. The nuclear fission power plant of any of the preceding claims, wherein the heat pipe second portions fan out with respect to one another.
10. A kit of parts for a nuclear fission power plant configured for extra-terrestrial use, the kit of parts being configured at least partially for extra-terrestrial assembly and comprising: an unmoderated nuclear reactor core, the unmoderated nuclear reactor core comprising an unmoderated fuel; a neutron reflector configured to be disposed around a periphery of the unmoderated nuclear reactor core; a plurality of primary neutronic control elements; a plurality of heat pipes, each heat pipe, when the nuclear fission power plant is assembled, comprising a first portion extending within the unmoderated nuclear reactor core and a second portion extending beyond an end of the unmoderated nuclear reactor core; and a plurality of thermoelectric generators, wherein a plurality of the thermoelectric generators are couplable to each heat pipe, each thermoelectric generator having a hot first end couplable to the second portion of a corresponding heat pipe and a cold second end configurable to radiate heat.
11. The kit of parts of claim 10, further comprising a linearly movable control rod configured to be insertable into a cavity within the unmoderated nuclear reactor core.
12. A method for a nuclear fission power plant configured for extra-terrestrial use, the method comprising controlling the nuclear fission power plant to: generate heat with an unmoderated nuclear reactor core comprising an unmoderated fuel, wherein a neutron reflector is disposed around a periphery of the unmoderated nuclear reactor core, and the nuclear fission power plant comprises a plurality of primary neutronic control elements; transfer heat from the unmoderated nuclear reactor core using a plurality of heat pipes, each heat pipe comprising a first portion extending within the unmoderated nuclear reactor core and a second portion extending beyond an end of the unmoderated nuclear reactor core; transfer heat from the second portion of the heat pipes to a plurality of thermoelectric generators; wherein a plurality of the thermoelectric generators are coupled to each heat pipe, each thermoelectric generator having a hot first end coupled to the second portion of a corresponding heat pipe and a cold second end; radiate heat from the cold second end of each of the thermoelectric generators; and generate electricity by virtue of a temperature gradient across the thermoelectric generators.