Reactor core and reactor

Abstract

A reactor core assembly for a nuclear generator, the reactor core assembly comprising: a plurality of fuel elements containing nuclear fuel to form a fuel assembly; a plurality of coolant pipes comprising a phase change material, said plurality of coolant pipes being positioned proximate to, or integrally formed with, the plurality of fuel elements for transferring heat away from the plurality of fuel elements; a moderator assembly comprising a plurality of moderator elements, each moderator element comprising a moderator material wherein the moderator elements of the moderator assembly are separated from the fuel elements by a sealed cavity comprising a vacuum.

Description

REACTOR CORE AND REACTORTECHNICAL FIELD

[0001] The present invention relates to nuclear reactor systems and nuclear reactors for power production and propulsion. The application also includes subject matter for nuclear core architecture which may be suitable for use with existing nuclear and power generation infrastructure.BACKGROUND

[0002] Any references to methods, apparatus or documents of the prior art are not to be taken as constituting any evidence or admission that they formed, or form part of the common general knowledge.

[0003] Nuclear reactors include nuclear cores that produce decay thermal energy after shut down. The amount of decay thermal energy produced after shutdown is proportional to the fuel power generation history and power density characterizing the nuclear core. To avoid overheating of the nuclear fuel in any location of the core, decay heat energy must be transferred from the core using redundant heat transfer mechanisms. Most if not all currently known mechanisms are supported by complex systems external to the vessel and structures designed to contain the core.

[0004] Passive and active safety systems developed externally to the vessel housing the core require a complex system of redundant piping, valves, heat exchangers, as well as pumps / blowers and ancillary power and control cabling networks (i.e. required to provide motive-electric power and control for active systems). Currently known safety systems are somewhat unsuitable for small nuclear reactor designs and there is a need for improving passive and active heat systems particularly for small scale nuclear reactors.SUMMARY OF INVENTION

[0005] In an aspect, the invention provides a reactor core assembly for a nuclear generator, the reactor core assembly comprising: a plurality of fuel elements containing nuclear fuel to form a fuel assembly; a plurality of heat pipes comprising a phase change material, said plurality of heat pipes being positioned proximate to the plurality of fuel elements for transferring heat away from the plurality of fuel elements; a moderator assembly comprising one or more moderator elements, each moderator element comprising a moderator material wherein the moderator elements of the moderator assembly are separated from the fuel elements by a sealed cavity comprising a vacuum.

[0006] In an embodiment, the plurality of fuel elements, the plurality of heat pipes, the moderator assembly are contained within a sealed containment vessel.

[0007] In an embodiment, the sealed cavity is formed within an internal volume of said sealed containment vessel.

[0008] In an embodiment, the moderator material is adapted to undergo density change in response to change of temperature and / or change of phase.

[0009] In an embodiment, the reactor core further comprises a thermosyphon being thermodynamically coupled with the moderator assembly for allowing heat transfer from the moderator via a heat exchanging surface.

[0010] In an embodiment, the plurality of heat pipes extends outwardly from the reactor core, the plurality of heat pipes being thermodynamically coupled with a power generation module to convert heat into electricity.

[0011] In an embodiment, the sealed cavity comprises a surrounding shell said shell comprising a neutron shielding layer.

[0012] In an embodiment, the sealed containment vessel comprises a gamma shielding vessel to surround said plurality of fuel rods.

[0013] In an embodiment, the gamma shielding vessel comprises gamma shielding material.

[0014] In an embodiment, the moderator material comprises a liquid phase moderator material and / or a solid moderator material.

[0015] In an embodiment, the reactor further comprising a high temperature thermal storage module located outside the reactor core, the high temperature thermal storage module being arranged relative to the heat pipes extending from the reactor core for enabling transfer of excess heat from the heat pipes when the heat being conducted via the heat pipes exceeds a preset threshold.

[0016] In an embodiment, the high temperature thermal storage module is operatively coupled to an auxiliary emergency cooling module via one or more variable conductance heat pipes.

[0017] In an embodiment, the variable conductance heat pipes and thermodynamically coupled with an auxiliary cooler module for receiving heat from the variable conductance heat pipes and reject said heat to the surroundings of the auxiliary cooler module.

[0018] In an embodiment, the reactor further comprises a low temperature thermal storage module located outside the reactor core, the low temperature thermal storage module being arranged relative to the heat pipes extending from the reactor core for enabling transfer of excess heat from the heat pipes when the heat being conducted via the heat pipes is below a preset threshold.

[0019] In an embodiment, the reactor further comprises a power conversion module configured to receive thermal energy from the heat pipes to generate mechanical energy and a power generation module coupled with the power conversion module configured to receive mechanical energy and convert said mechanical energy to generate electricity.

[0020] In another aspect, the invention provides a reactor core assembly for a nuclear generator, the reactor core assembly comprising: a plurality of fuel elements containing nuclear fuel to form a fuel assembly; a plurality of coolant pipes comprising a coolant fluid material, said plurality of coolant pipes being positioned proximate to the plurality of fuel elements for transferring heat away from the plurality of fuel elements; a moderator assembly comprising one or more moderator elements, each moderator element comprising a moderator material wherein the moderator elements of the moderator assembly are separated from the fuel elements by a sealed cavity comprising a vacuum.

[0021] In an embodiment, the plurality of fuel elements, the plurality of coolant pipes, the moderator assembly are contained within a sealed containment vessel.

[0022] In an embodiment, the sealed cavity is formed within an internal volume of said sealed containment vessel.

[0023] In an embodiment, the moderator material is adapted to undergo density change in response to change of temperature and / or change of phase.

[0024] In an embodiment, the reactor core further comprises a thermosyphon being thermodynamically coupled with the moderator assembly for allowing heat transfer from the moderator via a heat exchanging surface.

[0025] In an embodiment, the plurality of coolant pipes extends outwardly from the reactor core, the plurality of coolant pipes being thermodynamically coupled with a power generation module to convert heat into electricity.

[0026] In an embodiment, the sealed cavity comprises a surrounding shell said shell comprising a neutron shielding layer.

[0027] In an embodiment, the sealed containment vessel comprises a gamma shielding vessel to surround said plurality of fuel rods.

[0028] In an embodiment, the gamma shielding vessel comprises gamma shielding material.

[0029] In an embodiment, the moderator material comprises a liquid phase moderator material and / or a solid moderator material.

[0030] In an embodiment, the reactor further comprises a high temperature thermal storage module located outside the reactor core, the high temperature thermal storage module being arranged relative to the coolant pipes extending from the reactor core for enabling transfer of excess heat from the heat pipes when the heat being conducted via the coolant pipes exceeds a preset threshold.

[0031] In an embodiment, the high temperature thermal storage module is operatively coupled to an auxiliary emergency cooling module via one or more variable conductance coolant pipes.

[0032] In an embodiment, the variable conductance coolant pipes and thermodynamically coupled with an auxiliary cooler module for receiving heat from the variable conductance coolant pipes and reject said heat to the surroundings of the auxiliary cooler module.

[0033] In an embodiment, the reactor further comprises a low temperature thermal storage module located outside the reactor core, the low temperature thermal storage module being arranged relative to the coolant pipes extending from the reactor core for enabling transfer of excess heat from the coolant pipes when the heat being conducted via the coolant pipes is below a preset threshold.

[0034] In an embodiment, the reactor further comprises a power conversion module configured to receive thermal energy from the coolant pipes to generate mechanical energy and a power generation module coupled with the power conversion module configured to receive mechanical energy and convert said mechanical energy to generate electricity.

[0035]

[0036] In accordance with a second aspect of the invention there is provided a reactor core assembly for a nuclear generator, the reactor core assembly comprising: a plurality of fuel assemblies, each fuel assembly including a liquid fuel element wherein the liquid fuel element is configured self-regulate its temperature to enable the liquid fuel element to operate at a desired temperature; and a moderator assembly comprising one or more moderator elements, each moderator element comprising a moderator material wherein the moderator elements of the moderator assembly are separated from the liquid fuel element of each fuel assembly by a sealed cavity comprising a vacuum.

[0037] In an embodiment, each of the liquid fuel elements and the moderator assembly are contained within a sealed containment vessel.

[0038] In an embodiment, the sealed cavity is formed within an internal volume of said sealed containment vessel.

[0039] In an embodiment, the moderator material is adapted to undergo density change in response to change of temperature and / or change of phase.

[0040] In an embodiment, the reactor core further comprises a thermosyphon being thermodynamically coupled with the moderator assembly for allowing heat transfer from the moderator via a heat exchanging surface.

[0041] In an embodiment, the sealed cavity comprises a surrounding shell said shell comprising a neutron shielding layer.

[0042] In an embodiment, the sealed containment vessel comprises a gamma shielding vessel to surround said plurality of fuel rods.

[0043] In an embodiment, the gamma shielding vessel comprises gamma shielding material.

[0044] In an embodiment, the moderator material comprises a liquid phase moderator material and / or a solid moderator material.BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows:Figure 1 is a simplified schematic diagram of a small scale nuclear reactor system 1000 in accordance with a preferred embodiment.Figure 2 is a full scale expanded view of the small scale nuclear reactor 1000.Figures 3 to 7 illustrate various possible embodiments of the fuel assembly 110. Each configuration has been denoted by 100A, 100B, 100C, 100D and 100E.Figure 7A illustrates yet another configuration of the fuel assembly 110F.Figures 8 and 9 illustrate the use of a heavy water 122 and BeO 124 being used for the moderator assembly 120 in two different arrangements.Figures 10 and 11 illustrate the use of light water 122 being used for the moderator assembly in two different arrangements in the moderator 120.Figures 12 and 13 illustrate the use of heavy water 122 (only) being used for the moderator assembly in two different arrangements in the moderator 120. Figure 13A is an axial sectional view of the nuclear reactor system 1000.Figures 14 to 16 illustrate three different embodiments 100A, 100B and 100C of the reactor core 100.Figure 17 provides an illustration of potential heat losses in the nuclear reactor system 1000.Figures 18 and 19 illustrate heat transfer mechanisms in the nuclear reactor system 1000.Figure 20 is a schematic illustration of the “shielding” effect.Figure 21 also illustrates the direction of heat transfer from the reactor core of the nuclear reactor system 1000.Figure 22 is yet another schematic illustration of the nuclear reactor system 1000) indicating the working of the auxiliary cooling arrangement.Figures 23 and 24 illustrate changes in reactor temperature over time.DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0046] Figures 1 and 2 illustrate schematic views of a small-scale nuclear reactor 1000 in accordance with a preferred embodiment. Persons skilled in the art would appreciate that the scope of the nuclear reactor described herein is not limited merely by the output of the nuclear reactor and that the general inventive concept may be applicable to large scale nuclear reactors which have not been discussed in great detail. Whilst the preferred embodiment is a nuclear reactor with an output of 100kWe, such an output capacity is not limiting in any manner.

[0047] The nuclear reactor 1000 is preferably suitable for generation of electrical power or provide propulsion in a compact and transportable configuration with a safe form factor. For ease of explanation, the nuclear reactor 100 will be described with reference to the following key systems:• Reactor Core Assembly 100 which includes a fuel assembly 110 and a moderator 120;• Primary Heat or Coolant Pipe Loop 200;• Thermal Management System 300;• Power Conversion & Generation Module 400;• Power Management System 500;• Control System 600; and• Structural and Mechanical Interfaces 700

[0048] The reactor core assembly 100 comprises a fuel assembly 110 with plurality of fuel elements in the form of fuel tubes 112 (also referred to interchangeably as fuel rods or fuel pins throughout the specification) containing nuclear fuel to form a fuel assembly. In the preferred embodiment, low enriched uranium fuel rods with a low neutron absorption and high temperature cladding are used. These fuel rods may have solid or liquid cores. Doppler broadening and temperature induced density changes inthe reactor core 100 are key passive and inherently present reactivity feedback mechanisms which will be discussed in the foregoing sections. Molten fill or other fluids including liquids and gases 114 may be used for facilitating heat transfer from the fuel rods 112 to the primary heat loop 200 (that consists of coolant pipes 210 discussed in further detail, below). The combination of the fuel rods 112 and the molten fill 114 may be collectively referred to as the fuel assembly 110.

[0049] The reactor core 100 also comprises a moderator assembly 120 comprising a plurality of moderator elements. Each of the moderator elements comprises a moderator material (light or heavy water or organic carbon-based material). The moderator elements of the moderator assembly are separated or isolated from the fuel tubes by a sealed cavity 116 comprising a vacuum for maintaining a temperature gradient between the fuel tubes and the moderator elements. In some alternative embodiments, the sealed cavity 116 may be filled partial atmospheres of some gases. In at least some embodiments, the fuel rods 112 might be positioned within a vacuum evacuated tube to provide the vacuum isolation between the moderator elements of the moderator assembly 120 and the fuel assembly 110. Figures 3 to 7 illustrate various possible embodiments of the fuel assembly 110. Each configuration has been denoted by 100A, 100B, 100C, 100D and 100E. Figure 3 illustrates a fuel rod 112 with a single heat I coolant pipe 210 being located coaxially within the internal volume of the fuel rod 112. It should be noted that throughout this specification, the terms “heat pipe” and “coolant pipe” may be used interchangeably and refer to the same component that is configured to transfer heat away from the fuel rod 112 or to cool the fuel rod 112, as necessary. Figure 5 illustrates multiple coolant pipes 210 being positioned within the internal volume of the fuel rod 112. Figure 7 illustrates yet another embodiment in which multiple coaxial pairs of fuel rods 112 and coolant pipes 210 are shown. Figures 4 and 6 illustrate further embodiments in which multiple and discrete fuel rods 114 and coolant pipes 210 provided within a cylindrical volume with fill material 114 being disposed within the gaps between the fuel rods 112 and the coolant pipes 210. A sealed vacuum cavity 116 is provided in each of these configurations shown in Figures 3 to 7. The vacuum cavity generally surrounds the fuel rods 112 to form the vacuum isolation between the fuel assembly 110 and the moderator assembly 120. Figure 7A illustrates yet another embodiment in which a centrally located moderator 120 is surrounded by nuclear fuel 112. Coolant pipes 210 are located withinto draw heat from the nuclear fuel 112 during use. The vacuum isolation 116 forms a sealed cavity to isolate the central moderator assembly 120 from the surrounding nuclear fuel 112.

[0050] For ease of explanation, the fuel rod configuration 110A has been used to explain the working of the reactor system 1000 in further reactor core configurations shown in Figures 8 to 16. Whilst only the fuel rod configuration 110A has been shown in Figures 8 to 16, any of the other fuel rod configurations 110B to 110E may be utilized in these reactor configurations. Figures 8 and 9 illustrate the use of a heavy water 122 and BeO 124 being used for the moderator assembly 120 in two different arrangements. Figures 10 and 11 illustrate the use of light water 122 being used for the moderator assembly in two different arrangements in the moderator 120. Figures 12 and 13 illustrate the use of heavy water 122 (only) being used for the moderator assembly in two different arrangements in the moderator 120. In any of the described embodiments, other moderators such as heavy water, organic fluid and / or metal hydrides or graphite may be used in other alternative embodiments.

[0051] Figures 14 to 16 illustrate three different embodiments 100A, 100B and 100C of the reactor core 100. Figure 13A provides an axial sectional view of the reactor system 1000. Once again, whilst only the fuel rod configuration 110A has been shown in Figures 14 to 16, any of the other fuel rod configurations 110B to 110E may be utilized in these reactor configurations without departing from the spirit and scope of the invention. Figure 14 illustrates the use of light water or an organic material as a moderator material. Figure 15 illustrates the use of heavy water and BeO as moderator material. Figure 16 illustrates the use of only heavy water as moderator material. The combination of the moderator assembly 120 and the fuel rod assembly 110 is surrounded by a reflector assembly 132 that comprises heavy water (light water, graphite, beryllium, hydrocarbons, natural uranium, lead or lead containing materials) based reflector which forms a part of the neutron reflection system. The reflector assembly 132 may be formed with BeO, stainless steel, FeCrAI alloys in other alternative embodiments. It should be understood that a hexagonal lattice (such as those shown in Figures 9, 11 and 13) may be utilized in the reactor configurations shown in Figures 14 to 16.

[0052] The reactor core assembly 100 comprises a reaction vessel within which the fuel assembly 110 and the moderator assembly 120 are housed with the vacuum isolation being present between the fuel assembly 110 and the moderator assembly 120 for allowing the secondary heat transfer between the fuel assembly 110 and the moderator assembly 120. The vacuum isolation is provided by a sealed cavity that preferably surrounds the fuel rods 112. The sealed cavity is surrounded by an outer neutron reflector 132 with an outer neutron shield 134 made from boron carbide. Other materials such as concrete, borated water, leaded water etc. may also be used. A further outer gamma shield 140 is also provided. The role of the reflector system and the gamma shield 140 has been discussed in the foregoing sections.

[0053] The outer containment vessel may be made from armoured steel to maintain the reactor core and protect against external forces.

[0054] Referring to Figure 20, heat is generated in the moderator assembly 120 through heat transfer mechanisms that can cross the vacuum barrier or vacuum cavity. This is predominantly, in descending order of magnitude; infrared (radiation), neutronic heating, gamma heating. Infrared radiation through “two brown body” radiation transfer with the heat transfer from the fuel to the reactor vessel via conduction before radiating cross the vacuum with the hot body (reactor vessel) temperature being approximately 750°C and the moderator vessel temperature being 100°C. Heat is also generated in the moderator assembly 120 due to neutron interaction and it is generally accepted about 2.5% of the fission power is transferred to the moderator due to neutron thermalisation. Heat generated from gamma emissions is expected to be negligible in the moderator due to the moderator being mainly transparent to gamma. Secondary heating of moderator from reflector walls or inner reflectors or shields from gamma induced heating. Steam nucleation occurs in the moderator at liquid saturation temperature and moves through the thermosyphon where it rejects heat to atmosphere via a heat exchanger surface, condenses and returns to the moderator via a condenser.

[0055] Referring to Figure 21, heat is generated in a Tertiary heat removal system through heat transfer mechanisms that can cross the vacuum barrier in the reactor core 100. This is predominantly, in descending order; gamma, infrared, neutronradiation. Heat generated from gamma emissions is expected to be in the gamma shielding, where heavy atomic mass particles absorb the gamma radiation generated from fission and that absorption is converted to heat. This is generally considered to be 2% of the fission power. This heat is conducted into the containment vessel and then, via convection, lost to the surroundings / atmosphere. Same process applies to infrared and neutron radiation except the heat is initially transferred first into the inner neutron shield (low Z) before conduction in the gamma shield and so forth.

[0056] The Primary Coolant Pipe Loop 200 comprises a plurality of coolant pipes 210 that are positioned proximate to the fuel assembly 110 to transfer heat from the fuel assembly 110. Heat is predominately (93.5%) generated by nuclear thermal spectrum fission in the moderated, low enriched fuelled reactor core and to a minor (6.5%) but important extent by fission product decay. Heat is removed predominately via passive thermal conduction and convection into low pressure, high temperature (-800C) sodium heat pipes 210.

[0057] The coolant pipes 210 extend out of the reactor core assembly 100 and the reactor system utilizes a thermal storage system 300 that further comprises of a high temperature storage module and a low temperature storage module. The high temperature thermal storage module is arranged relative to the coolant pipes 210 extending from the reactor core 100 for enabling transfer of excess heat from the coolant pipes 210 when the heat being conducted via the coolant pipes exceeds a preset threshold. The high temperature thermal storage module is operatively coupled to an auxiliary emergency cooling module 312 via variable conductance cooling pipes 314. The variable conductance heat pipes 314 are thermodynamically coupled with the auxiliary cooler module 312 for receiving heat from the variable conductance heat pipes 314 and reject said heat to the surroundings.

[0058] In the preferred embodiment, the high temperature storage module 310 comprises miscible gap phase change material thermal connected to the primary coolant pipes that is designed to have a set point approximately 50°C above the nominal system temperature setpoint. The Variable Conductance Coolant Pipes 314 are designed to not conduct heat effectively at the nominal system setpoint but designed to “turn on” as the operating temperature reaches a higher setpoint (+50°C).The high temperature storage module 310 is designed to remove excess heat from the reactor system 1000 to allow time for reactivity changes to lower core power and prevent an over temperature event.

[0059] The low temperature storage module 320 is also located outside the reactor core 100. The low temperature thermal storage module 320 is arranged relative to the coolant pipes 210 extending from the reactor core 100 for enabling transfer of excess heat from the coolant pipes 210 when the heat being conducted via the coolant pipes is below a preset threshold. The low temperature storage module 320 is designed to add heat into the system to allow time for reactivity changes to increase core power and prevent an under-temperature event.

[0060] Referring to Figures 18 and 19 heat is predominately (93.5%) generated by nuclear thermal spectrum fission in the (light / heavy) water moderated, low enriched fuelled reactor core 100 and to a minor (6.5%) but important extent by fission product decay. Heat is removed predominately via passive thermal conduction and convection into low pressure, high temperature (-800C) sodium coolant pipes 210. The coolant pipes 210 transfer the heat from the evaporator zone to potentially three condenser zones, two thermal storage miscible gap alloys (a low temp and high temp zone) and / or directly into supercritical carbon dioxide via a heat exchanger. The heated supercritical carbon dioxide drives a turbine, exhaust heat after the turbine is recuperated and any residual heat is further removed by a cooler ready to be reused to complete the cycle.The sodium filled coolant pipes 210 are the primary heat removal mechanism of the reactor core 100 and are in direct contact with the fuel and fill assemblies inside the reactor vessel. The coolant pipes 210 operate at the reactor temperature and at the low vapour pressure of sodium. Another novel feature of the coolant pipes 210 in present reactor system 100 is the provision of three distinct condenser sections axially along the coolant pipe for passive thermal control of reactor power and temperature. The first section to the over temperature protection thermal storage system 310 (designed to absorb heat +50C above nominal conditions to prevent over temperature events), the second section 320 to the under temperature protection thermal storage system (designed to release heat - 50C below nominal conditions to prevent under temperature events) and the final section to the primary heat exchanger, wherenominally 250-270 kWt is transferred to the supercritical carbon dioxide Brayton cycle working fluid.

[0061] The supercritical carbon dioxide at 650C (800C / 1000C), 20 MPa, in the preferred embodiment, is driven through the turbine, which spins a high-speed generator to generate 100kWraw electrical AC power. The exhaust lower temperature (-500C) and pressure (8 MPa) working fluid is then passed through a recuperator (by 100C, 20 MPA working fluid from the compressor to preheat and recoup energy to the new working fluid heading toward the primary heat exchanger) and a cooler (dry or wet cooled to atmosphere) to bring the working fluid back to its starting state 50C, 8MPa. From where it is compressed to 20MPa and pumped through the recuperator and heater before repeating the cycle above.

[0062] In a preferred embodiment the reactor further includes a power conversion and generation module 400 configured to receive thermal energy from the coolant pipes to generate mechanical energy and a power generation module coupled with the power conversion module configured to receive mechanical energy and convert said mechanical energy to generate electricity.

[0063] The supercritical carbon dioxide (sCO2) Brayton cycle is an innovative, high- efficiency thermodynamic cycle that utilizes carbon dioxide in a supercritical state as the working fluid. The cycle has the following key components; h eate r / p rim ary heat exchanger, Turbine-Generator, Recuperator, Cooler, Compressor-Motor.

[0064] Heater I Primary Heat Exchanger: The heater raises the temperature of the supercritical CO2 by adding heat energy to the system from the primary loop coolant pipes exiting the reactor core. This is where the working fluid receives the thermal energy necessary to perform work in the turbine. The heater efficiently transfers heat to the sCO2 without causing excessive pressure drops or thermal stresses by using a novel radial inflow design. Materials used must withstand high temperatures and pressures.

[0065] Turbine-Generator: The heated, high-pressure sCO2 expands through the turbine, converting thermal energy into mechanical work. The turbine drives directly ahigh-speed generator (-100,000 RPM), producing raw AC electricity. Turbines must be designed to handle the high pressures and temperatures of sCO2 (20-30 MPa, 650C or greater), requiring materials that can withstand such conditions and maintain efficiency. Further the balance between rotor dynamics and thermal management is key to reliable operation.

[0066] Recuperator: A heat exchanger that recovers heat from the turbine exhaust and uses it to preheat the sCO2 entering the heater. This significantly improves the cycle's thermal efficiency by reducing the amount of external heat required. The recuperator must efficiently transfer heat between the two streams of CO2 without mixing them. It must also handle the high pressures and temperatures involved.

[0067] Cooler: After expanding in the turbine and passing through the recuperator, the sCO2 is cooled in the cooler, condensing it back to a supercritical fluid state before it is fed into the compressor. This process rejects waste heat to the environment. The choice of cooling medium can vary, including air, water, or other coolants, depending on environmental and economic considerations. The cooler must efficiently reject heat from the sCO2 while minimizing pressure drops. It also needs to be designed for the specific cooling medium and environmental conditions.

[0068] Compressor-Motor: The compressor increases the pressure of the cooled sCO2, preparing it to absorb heat in the heater again. This component is critical for maintaining the cycle's pressure and flow rates. The compressor is typically driven by an electric motor, which represents a significant portion of the cycle's energy input. Compressor design focuses on efficiency and reliability, as it must handle supercritical CO2 at varying temperatures and pressures. Material selection and aerodynamic design are critical to minimize energy losses and wear. Thermal management and cooling are key to the motor design.

[0069] The Power Management System 500 in the reactor 1000- is crucial for ensuring the efficient, reliable, and safe operation of the CUPS. This system manages the flow of electrical power from the generator to the grid or load, handling conversion, conditioning, and distribution processes. A well-designed power management system is essential for maximizing the benefits of an sCO2 Brayton cycle power generationfacility, ensuring that the generated power is efficiently, reliably, and safely delivered to the grid or end-users.The Power Management System has the following key components:

[0070] AC Bus from the Generator: The AC bus serves as the primary interface between the generator and the power management system. It collects alternating current (AC) electricity generated by the turbine-driven generator and distributes it to the power conversion systems. The AC bus must be capable of handling the voltage and current levels produced by the generator. It also needs protection systems to manage faults, overloads, and fluctuations in power quality.

[0071] Converter (AC to DC): The converter transforms AC electricity from the generator into direct current (DC) electricity. This conversion is necessary for energy storage, DC distribution systems, or when interfacing with certain types of loads that require DC power. Converters can be rectifiers, using diodes or thyristors, or more complex systems like active front-end converters using IGBTs (Insulated Gate Bipolar Transistors) for better control over power quality and efficiency. The converter design focuses on efficiency, reliability, and the ability to handle the power levels from the generator. It also includes features for power quality management, such as filtering harmonics.

[0072] DC Bus: The DC bus acts as a hub for the DC electricity within the power management system. It connects the output of the converter with the inputs of the inverter, energy storage systems, or DC loads. The DC bus must be designed to minimize electrical losses and handle the expected current and voltage levels. It also requires protective devices to manage short circuits, overvoltages, and other fault conditions.

[0073] Inverter (DC to AC): The inverter converts DC electricity back into AC electricity at the required voltage and frequency for distribution to the grid or AC loads. This component is essential for integrating power from the sCO2 Brayton cycle into conventional power systems. Inverters are variable-frequency drives that offer precise control over the output power's voltage, frequency, and phase. Inverter designprioritizes efficiency, power quality, and reliability. It must produce AC electricity that meets grid standards for voltage, frequency, and harmonics. Advanced control strategies, such as maximum power point tracking (MPPT) and grid synchronization, are also potentially important features.

[0074] The reactor system 1000 comprises a Control System 600 which is hypothesized to be walk away safe that integrates advanced and inherent passive safety features (stable temperature setpoint, strong negative reactivity feedbacks, large thermal inertia, large reactivity and temperature limit margins), as well as automated active controls, to manage its operations. The Control System 600 continuously monitors reactor parameters (such as temperature, pressure, flow rates and neutron flux), cooling system status, and power conversion efficiency to adjust secondary loop sCO2 coolant flow (primary power control), control drums (temperature setpoint control on startup, shutdown and burnup adjustment), and other system components in real-time. The system is designed with redundancy, diversity, and fail-safe principles to enhance reliability and safety.

[0075] Reactor Control. Neutron Flux and Reactivity Control: The primary function of the reactor control system is to maintain the reactor core's neutron flux within safe and efficient operational limits. Control Drum operations set the stable temperaturecriticality setpoint during startup, compensate temperature drift for fuel burnup during operations and used for normal reactor shutdown and SCRAM. The strong negative temperature / density feedback characteristics of the reactor; in concert with the thermal management system, greatly assists and enables passive load following (reactor power changes) in normal operations without the use of control drum operations. This rapid passive inherently safe reactor power down in all emergency situations also occurs with active SCRAM measures as redundancy to prevent over temperature (meltdowns) situations.

[0076] Cooling Systems Control. Primary Cooling System Management: The control system manages the flow of coolant pipe sodium coolant indirectly through the reactor core via heat removal in the secondary sCO2 loop. Adjustments in sCO2 working fluid flow rates (compressor power adjustment) to match end user power load requirements is the primary means of reactor power control setting. When the reactor is at thenominal temperature setpoint and an increase in load requirements is detected by the control system, the control system responds by increasing sCO2 flow rates in the secondary loop, which removes more heat from the coolant pipe primary loop, which drops the temperature of the core, which lowers the radiation signal to the moderator, which lowers void fraction and increase density in the moderator leading to 4 main negative feedbacks to kick in. first, the fuel density decreases increasing likelihood of fissions. Second, fuel doppler narrowing occurs leading to less neutron absorptions and more fissions. Third, Moderator void fraction decreasing, leading to more thermalisation of neutrons and increased fissions. Lastly, moderator density leading to more optimal thermalisation of neutrons and increased fissions. These feedbacks work in a stable underdamped fashion until a new fission power setting is achieved. The same is achieved in reverse if a new lower power setting is required. Temperatures, Pressures, flow rates, rotary machinery speed and power are monitored to ensure correct system function and availability status.

[0077] Secondary Cooling and Heat Exchangers: moderator secondary cooling system continuously removes heat from the reactor via radiative heat transfer, neutron moderation related heat transfer, neutronic and gamma heating of (non-moderator) materials, and then through the system thermosyphons. This is a closed system where temperatures and pressures are monitored to ensure correct system function and availability status.

[0078] Emergency Cooling: In the event of abnormal operation or failure of the primary cooling system, the control system passively activates emergency cooling systems to ensure the reactor core remains cool and safe via heat absorption into the thermal management system and if temps reach 50C above nominal the variable conductance coolant pipes activate to removal additional heat from the system with significant margin above the decay heat generation profile.

[0079] Power Conversion System Control. Efficiency Optimization: The control system optimizes the power conversion process to maximize efficiency. This involves controlling turbine inlet temperatures, pressures, and flow rates. Load Following: For CUPSs connected to the electric grid, the control system adjusts the power output to follow demand variations. Safety and Protection: The control system includes safetyprotocols to shut down the power conversion system safely in case of malfunctions or deviations from normal operating conditions.

[0080] Integrated Control and Safety Systems: Digital Control Systems: The CUPS utilizes digital control systems with advanced sensors, actuators, and computer algorithms for precise control over all aspects of reactor operation, cooling, and power conversion.

[0081] Redundancy and Diversity: Key components of the CUP system are designed with redundancy (multiple copies) and diversity (different methods) to ensure that failure of one component does not compromise the reactor's safety or operation.

[0082] Human-Machine Interface (HMI) and Operator Interaction. HMI Design: The control system provides operators with a user-friendly interface to monitor reactor status, control operations, and respond to alarms or abnormal conditions on site and remotely.

[0083] Automation and Manual Override: While the reactor safety system is highly passive, inherently safe with backup automated active provisions, provisions for manual override are available for operators to intervene in the reactor's control if necessary, ensuring ultimate human control over critical safety functions.

[0084] The power management system requires a sophisticated control system to manage the flow of power through the system, respond to changes in load or generation capacity, and ensure stability and efficiency. This includes real-time monitoring, protective relaying, and automated control algorithms. Comprehensive protection systems are necessary to detect and respond to electrical faults, overloads, and other abnormal conditions. This includes circuit breakers, fuses, and protective relays.

[0085] The power management system requires a sophisticated control system to manage the flow of power through the system, respond to changes in load or generation capacity, and ensure stability and efficiency. This includes real-time monitoring, protective relaying, and automated control algorithms. Comprehensiveprotection systems are necessary to detect and respond to electrical faults, overloads, and other abnormal conditions. This includes circuit breakers, fuses, and protective relays.Efficiency and Reliability Enhancements

[0086] Energy Storage Integration: Incorporating energy storage systems, such as batteries or supercapacitors, can enhance the flexibility and efficiency of the power management system, allowing for increase system response, and improved power quality.

[0087] Advanced Control Strategies: Implementing advanced control strategies, such as predictive control and adaptive algorithms, can further optimize the efficiency and reliability of the power management system.

[0088] The control system of the CUPS is a sophisticated, yet simple, integration of technology, safety, and operational efficiency, designed to meet the unique requirements of modular and small-scale nuclear operations. Through passive inherent safety features, advanced automation, and rigorous engineering practices, CUPS aim to set new standards for nuclear power safety and flexibility.

[0089] Cybersecurity Measures: Given the critical nature of nuclear power systems, the control system is designed with robust cybersecurity measures to protect against unauthorized access and cyber threats.

[0090] The reactor system 1000 is a thermal spectrum reactor, that may utilize LEU fuel (potential to use thorium for neutron flux flattening), a water (or hydrocarbon) based moderator, water (or hydrocarbon) based reflector, Boron Carbide neutron shield and a Lead gamma shield. A novel feature of this reactor is that the moderator is isolated from the fuel / reactor vessel by a vacuum, limiting interaction between the two via thermal radiation only. The neutronics performance has many similarities to that of a Boiling Water Reactor (BWR) with respect to neutron spectrum, void fraction, doppler broadening negative reactivity feedback, and passive load following with heat removal, however; with the exception of the cooling being primarily driven by passivecoolant pipes to drive a Brayton cycle and that the water in the moderator system is not directly used for power conversion purposes.

[0091] In the reactor system 1000, the fuel assemblieslOO, composed of low enriched uranium fuel, are where the fission process occurs. When a neutron is absorbed by a fissile nucleus (such as U-235), it may cause the nucleus to become unstable and split into two smaller nuclei (fission fragments), releasing a significant amount of energy, 2- 3 new neutrons, and gamma radiation. These newly released neutrons can then go on to cause more fission events, sustaining the chain reaction.

[0092] Water serves as both a secondary in core coolant and a moderator in the reactor system 1000. The moderation process is crucial because fast neutrons released from fission events are too energetic to efficiently cause further fission. Water molecules slow these neutrons to thermal energies (becoming thermal neutrons), at which point they have a higher probability of causing fission. The effectiveness of moderation is influenced by the temperature and density of the water, which change as the water heats up and boils.

[0093] Reactivity in a nuclear reactor 1000 describes the state of the neutron population over time; it must be carefully controlled to ensure the reactor operates safely and efficiently. The characteristics of the reactor system 1000 is that the stable critical temperature setpoint is set at reactor’s "Beginning of Cycle” (BOC) with the control drums having a maximum excess reactivity with margin for SCRAM, as the fuel is burnt up the control drums adjust providing more reactivity to compensate.

[0094] In normal operation, the control drums are only used for startup, fuel burn up adjustments and shutdown of the reactor to set reactor temperature (and initial reactor power from background losses), whilst the reactor power, once thermal equilibrium is reach, is controlled through the reactor heat removal flow via changes in the supercritical carbon dioxide flow rates in the power conversion system.

[0095] Changing (increasing or decreasing) the flow of sCO2 in the power conversion system and thus the heat removal through the core is the normal and convenient method for controlling power from approximately 10% to 100% reactor power. Whenoperating on the so-called "100% rod line", power may be varied from approximately 30% to 100% of rated power by changing the sCO2 compressor power. As heat removal through the core is increased, core temperature will drop, less thermal radiation will be transferred to the moderator and steam bubbles ("voids") are more quickly removed from the moderator, the amount of liquid water in the moderator increases, neutron moderation increases, more neutrons are slowed to be absorbed by the fuel, and reactor power increases. As heat removal through the core is decreased, core temperature will increase, more thermal radiation will be transferred to the moderator, steam voids remain longer in the moderator, the amount of liquid water in the moderator decreases, neutron moderation decreases, fewer neutrons are slowed enough to be absorbed by the fuel, and reactor power decreases. Thus the reactor system 1000 is expected to have a strong negative void coefficient. Further to this effect, doppler broadening neutron absorption by U-238, fuel density decreases and moderator density decreases occurs as core temperatures increase and vice versa when core temperatures decrease, further adding to the negative reactivity reactor coefficient.

[0096] The system 1000 may have inherent negative feedback mechanisms and large relative thermal inertia provided by the Thermal Storage System that help stabilize the reactor operation. This effect acts as a natural safety feature, providing inertia to the system, enhancing stability.

[0097] These feedback mechanisms are fundamental to the safe operation of the reactor system 1000, allowing the reactor to respond automatically to changes in operating conditions without the need for external intervention. The negative feedback provided by the void coefficient and Doppler coefficient helps to stabilize the reactor's power output and prevent conditions that could lead to overheating or other unsafe conditions. Each of these mechanisms plays a crucial role in reactor stability and safety.

[0098] The distribution of power (and thus, neutron flux) within the reactor core is not uniform, affected by factors like fuel burnup, control rod positions, and moderator conditions. Ensuring a stable and safe power distribution requires careful design and operational practices.

[0099] Whilst instantaneous power fluctuations can be larger than the nominal system rated 300kWt, very rapidly these power fluctuations are limited by the physical limitations of heat removal by the primary coolant pipe system and the secondary sCO2 Brayton cycle system.

[0100] The CUPS has a number of shielding and containment measures to prevent environmental exposure of radiation from the reactor; these include fuel cladding, reactor pressure vessel, a neutron shield, gamma shield and an armoured outer containment vessel.

[0101] Electrical Process Overview

[0102] The electrical system has novel integrated turbine-generator and compressor-motor system coupled with standard electrical power conditioning equipment such as the rectifier and inverter.

[0103] The high speed generator is coupled directly with the turbine shaft. The raw 150kW AC power is rectified to DC power before being inverted into the correct AC power voltage, current and frequency by the power management system for dispatch to end user system. Approximately 50kW of electrical power is required to power the compressor and the reactor control system, its drives and various sensors. Net 100kWe is the useable power from the reactor system 1000 in the present embodiment.

[0104] Figure 25 shows a further embodiment of a fuel assembly 110G having a similar configuration to the fuel assembly of Figure 3, discussed above. In particular, the fuel assembly includes a fuel rod 112 with a single heat I coolant pipe 210 being located coaxially within the internal volume of the fuel rod 112. A sealed vacuum cavity 116 is further provided between the fuel rod and the moderator assembly. In particular, the vacuum cavity generally surrounds the fuel rods 112 to form the vacuum isolation between the fuel assembly 110G and the moderator assembly 120.

[0105] The embodiment shown in Figure 25 further includes a cladding layer 113 provided within the vacuum cavity 116 and which separates the fuel rod 112 fromthe vacuum cavity 116. The cladding layer 113 may be configured to hold the vacuum cavity 116 and limit pressure on the inner coolant pipe 210 during use of the fuel assembly. The vacuum cavity may be sealed via a vacuum evacuated tube that essentially creates a barrier between the fuel assembly 110G and the moderator assembly 120.

[0106] An even further embodiment of a fuel assembly 110H is shown in Figure 26. Although the fuel assembly 110G may comprise of components similar to that of Figures 3 to 7 and 25, a major difference of this embodiment is that the fuel assembly comprises a liquid fuel element 115. The liquid fuel element 115 may be a suitable liquid fuel that is configured to provide the features and functions provided by the fuel rod 112 and the coolant pipes 210 of these other embodiments. In other words, in this embodiment the coolant pipe is no longer in use, and the fuel rod is filled with a liquid fuel which is configured to be used as a fuel component and coolant I phase change component of the fuel assembly 11 OH.

[0107] The fuel assembly 11 OH further comprising a cladding layer 113 provided between the liquid fuel element 115 and the vacuum cavity 116. The vacuum cavity 116 may be the same vacuum cavity as described with reference to Figures 3 to 7 and 25.

[0108] Accordingly, the fuel assembly 11 OH shown in Figure 26 is configured so that the coolant pipe and the fuel rod are integrated into a single cavity as a liquid fuel element with the vacuum still being retained so as to isolate the fuel element from the moderator. By combining the fuel rod and the coolant pipe into a single liquid fuel element, the liquid fuel element may serve as a nuclear fuel with inherent self-cooling properties. By combining the roles of the fuel rod and the phase change material (coolant pipes), the overall efficiency of the fuel assembly within the reactor may be improved.

[0109] Accordingly, it is an aspect of the description to provide a reactor core assembly for a nuclear generator. The reactor core assembly may comprise: a plurality of fuel elements containing nuclear fuel to form a fuel assembly; a plurality of coolant pipes comprising a phase change material, and which coolant pipes are configured totransferring heat away from the plurality of fuel elements; a moderator assembly comprising a plurality of moderator elements, each moderator element comprising a moderator material wherein the moderator elements of the moderator assembly are separated from the fuel elements by a sealed cavity comprising a vacuum. It should be appreciated that in some embodiments the fuel elements the liquid fuel element may perform both the role of the fuel rod, discussed above, and the coolant pipes (or phase change material). However, it is an important aspect of the invention that the liquid fuel element (or the separate components in the case in which they are not integrated to form a single liquid fuel element) be separated from the moderator elements via a vacuum.

[0110] By having the vacuum provided between the fuel element and the moderator elements, safety of the reactor core assembly of the nuclear generator may be improved by, for example, reducing heat transfer, preventing unwanted chemical reactions, and mitigating radiation damage to the moderator during normal operation and potential accident scenarios.

[0111] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. The term “comprises” and its variations, such as “comprising” and “comprised of” is used throughout in an inclusive sense and not to the exclusion of any additional features.

[0112] It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect.

[0113] The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted by those skilled in the art.

Claims

CLAIMS1 . A reactor core assembly for a nuclear generator, the reactor core assembly comprising: a plurality of fuel elements containing nuclear fuel to form a fuel assembly; a plurality of heat pipes comprising a phase change material, said plurality of heat pipes being positioned proximate to the plurality of fuel elements for transferring heat away from the plurality of fuel elements; a moderator assembly comprising one or more moderator elements, each moderator element comprising a moderator material wherein the one or more moderator elements of the moderator assembly are separated from the fuel elements by a sealed cavity comprising a vacuum.

2. A reactor core assembly in accordance with claim 1 wherein the plurality of fuel elements, the plurality of heat pipes, the moderator assembly are contained within a sealed containment vessel.

3. A reactor core assembly in accordance with claim 2 wherein the sealed cavity is formed within an internal volume of said sealed containment vessel.

4. A reactor core assembly in accordance with any one of claims 1 to 3 wherein the moderator material is adapted to undergo density change in response to change of temperature and / or change of phase.

5. A reactor core assembly in accordance with any one of the preceding claims further comprising a thermosyphon being thermodynamically coupled with the moderator assembly for allowing heat transfer from the moderator via a heat exchanging surface.

6. A reactor core assembly in accordance with any one of the preceding claims wherein the plurality of heat pipes extends outwardly from the reactor core, theplurality of heat pipes being thermodynamically coupled with a power generation module to convert heat into electricity.

7. A reactor core assembly in accordance with any one of the preceding claims wherein the sealed cavity comprises a surrounding shell said shell comprising a neutron shielding layer.

8. A reactor core assembly in accordance with claim 2 or claims 3 to 7 when dependent upon claim 2 wherein the sealed containment vessel comprises a gamma shielding vessel to surround said plurality of fuel rods.

9. A reactor core assembly in accordance with claim 8 wherein the gamma shielding vessel comprises gamma shielding material.

10. A reactor core in accordance with any one of the preceding claims wherein the moderator material comprises a liquid phase moderator material and / or a solid phase moderator material.

11. A reactor comprising the reactor core assembly in accordance with any one of the preceding claims, the reactor further comprising a high temperature thermal storage module located outside the reactor core, the high temperature thermal storage module being arranged relative to the heat pipes extending from the reactor core for enabling transfer of excess heat from the heat pipes when the heat being conducted via the heat pipes exceeds a preset threshold.

12. A reactor according to claim 11 wherein the high temperature thermal storage module is operatively coupled to an auxiliary emergency cooling module via one or more variable conductance heat pipes.

13. A reactor according to claim 12 wherein the variable conductance heat pipes and thermodynamically coupled with an auxiliary cooler module for receiving heat from the variable conductance heat pipes and reject said heat to the surroundings of the auxiliary cooler module.

14. A reactor according to any one of claims 11 to 13 further comprising a low temperature thermal storage module located outside the reactor core, the low temperature thermal storage module being arranged relative to the heat pipes extending from the reactor core for enabling transfer of excess heat from the heat pipes when the heat being conducted via the heat pipes is below a preset threshold.

15. A reactor according to any one of claims 11 to 14 further comprising a power conversion module configured to receive thermal energy from the heat pipes to generate mechanical energy and a power generation module coupled with the power conversion module configured to receive mechanical energy and convert said mechanical energy to generate electricity.

16. A reactor core assembly for a nuclear generator, the reactor core assembly comprising: a plurality of fuel elements containing nuclear fuel to form a fuel assembly; a plurality of coolant pipes comprising a phase change material, said plurality of coolant pipes being positioned proximate to the plurality of fuel elements for transferring heat away from the plurality of fuel elements; a moderator assembly comprising one or more moderator elements, each moderator element comprising a moderator material wherein the one or more moderator elements of the moderator assembly are separated from the fuel elements by a sealed cavity comprising a vacuum.

17. A reactor core assembly in accordance with claim 15 wherein the plurality of fuel elements, the plurality of coolant pipes, the moderator assembly are contained within a sealed containment vessel.

18. A reactor core assembly in accordance with claim 16 wherein the sealed cavity is formed within an internal volume of said sealed containment vessel.

19. A reactor core assembly in accordance with any one of claims 15 to 17 wherein the moderator material is adapted to undergo density change in response to change of temperature and / or change of phase.

20. A reactor core assembly in accordance with any one of claims 15 to 18 further comprising a thermosyphon being thermodynamically coupled with the moderator assembly for allowing heat transfer from the moderator via a heat exchanging surface.

21. A reactor core assembly in accordance with any one of claims 15 to 19 wherein the plurality of coolant pipes extends outwardly from the reactor core, the plurality of coolant pipes being thermodynamically coupled with a power generation module to convert heat into electricity.

22. A reactor core assembly in accordance with any one of claims 15 to 20wherein the sealed cavity comprises a surrounding shell said shell comprising a neutron shielding layer.

23. A reactor core assembly in accordance with claim 16 or claims 17 to 21 when dependent upon claim 16 wherein the sealed containment vessel comprises a gamma shielding vessel to surround said plurality of fuel rods.

24. A reactor core assembly in accordance with claim 22 wherein the gamma shielding vessel comprises gamma shielding material.

25. A reactor core in accordance with any one of claims 15 to 23 wherein the moderator material comprises a liquid phase moderator material and / or a solid phase moderator material.

26. A reactor comprising the reactor core assembly in accordance with any one of claims 15 to 24, the reactor further comprising a high temperature thermal storage module located outside the reactor core, the high temperature thermal storage module being arranged relative to the coolant pipes extending from thereactor core for enabling transfer of excess heat from the coolant pipes when the heat being conducted via the coolant pipes exceeds a preset threshold.

27. A reactor according to claim 25 wherein the high temperature thermal storage module is operatively coupled to an auxiliary emergency cooling module via one or more variable conductance coolant pipes.

28. A reactor according to claim 26 wherein the variable conductance coolant pipes and thermodynamically coupled with an auxiliary cooler module for receiving heat from the variable conductance coolant pipes and reject said heat to the surroundings of the auxiliary cooler module.

29. A reactor according to any one of claims 25 to 27 further comprising a low temperature thermal storage module located outside the reactor core, the low temperature thermal storage module being arranged relative to the coolant pipes extending from the reactor core for enabling transfer of excess heat from the coolant pipes when the heat being conducted via the coolant pipes is below a preset threshold.

30. A reactor according to any one of claims 25 to 28 further comprising a power conversion module configured to receive thermal energy from the coolant pipes to generate mechanical energy and a power generation module coupled with the power conversion module configured to receive mechanical energy and convert said mechanical energy to generate electricity.

31. A reactor core assembly for a nuclear generator, the reactor core assembly comprising: a plurality of fuel assemblies, each fuel assembly including a liquid fuel element wherein the liquid fuel element is configured self-regulate its temperature to enable the liquid fuel element to operate at a desired temperature; and a moderator assembly comprising one or more moderator elements, each moderator element comprising a moderator material wherein themoderator elements of the moderator assembly are separated from the liquid fuel element of each fuel assembly by a sealed cavity comprising a vacuum.

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