Power cycle system

Abstract

A power cycle system 100, suitable for an intermittent heat source 10, comprises means for carrying a working fluid. An indirect heating means 118 is configured to indirectly heat the working fluid us

Description

Field of the Invention

[01] The present disclosure relates generally to power cycle systems for generating electricity in a power plant, and more specifically to a power cycle system for use with a fusion reactor; in particular, a tokamak type reactor / power plant. Yet more specifically, the present disclosure relates to a power cycle system based on a modified Brayton or Rankine cycle. Background

[02] One of the great challenges facing nuclear fusion technology is how to convert the energy generated by a fusion reactor into useable electricity. In the context of tokamak type reactors particularly, a closed working fluid heat engine operating according to a (closed) Brayton or Rankine cycle - i.e., a closed cycle gas turbine operating with supercritical, subcritical or transcritical fluid - has been postulated as a potentially advantageous electrical generator.

[03] However, existing power cycle technologies for fusion reactors face a problem of how to handle reactor uncertainty in heat split (quantity and quality), intermittency (pulsed operation), start-up I shut down, trip and dynamics (rapid load ramp rate), and so on.

[04] One solution to some of these problems utilises a thermal storage whereby heat is stored in a suitable salt composition. However, the thermal storage loses heat over time and can be inefficient to heat and maintain, particularly as high temperature salt compositions which are typically used can be highly corrosive. Also, the thermal response to deliver heat to the power cycle is very slow, and so while it can be useful in a startup phase, it is unsuitable for dealing with operations uncertainty. Finally thermal storage is finite in nature, which is particularly challenging when trying to develop a system as uncertain as fusion energy and prototypic.

[05] Another approach utilises an external steam Rankine or Brayton engine to smooth power performance. However, thermal response time is limited by the thermal inertia of the steam boiler, again making such a system unsuitable for responding to short term operational uncertainty.

[06] Hence it is desirable to develop a power cycle system which may more readily adapt to reactor conditions. Summary

[07] The present invention is defined according to the independent claims. Additional features will be appreciated from the dependent claims and the description herein. Any embodiments which are described but which do not fall within the scope of the claims are to be interpreted merely as examples useful for a better understanding of the invention.

[08] The example embodiments have been provided with a view to addressing at least some of the difficulties that are encountered with current approaches to power cycle systems for in a fusion reactor, whether those difficulties have been specifically mentioned above or will otherwise be appreciated from the discussion herein.

[09] In particular, the example embodiments provide a closed loop Brayton or Rankine cycle (supercritical, subcritical or transcritical) configured to indirectly convert heat from an (intermittent) fusion reactor into electricity, with the cycle being augmented by a backup heat source to directly add heat into the cycle. That is, the same power cycle is used, whether it is directly heated or indirectly heated. Plasma ramps up >1 GW thermal very rapidly, and so the backup heat source that directly adds heat into the cycle prepares, and / or maintains the power cycle prior to receiving the heat from plasma (heating the components, rolling the turbine etc.), thereby reducing the thermal expansion / stresses to prolong component lifetime; hence operations are not hindered by the power cycle ramping limits.

[10] Accordingly, in one aspect of the invention there is provided a power cycle system for an intermittent heat source, for example a fusion power plant. The power cycle system comprises means for carrying a working fluid, indirect heating means configured to indirectly heat the working fluid using the intermittent heat source (e.g., heat from a reactor, such as a tokamak), direct heating means configured to directly heat the working fluid, and a turbine configured to generate electrical power using heated working fluid. The system comprises a first mode of operation in which working fluid is indirectly heated using the indirect heating means while the direct heating means is thermally decoupled from the working fluid, and a second mode of operation in which the working fluid is heated using the direct heating means while the indirect heating means is substantially thermally decoupled from the working fluid. Optionally, the system may operate in a third mode in which the both the indirect heating means and direct heating means are active and thermally coupled to the working fluid. In this way the described system may readily adapt to operational uncertainty of a reactor, as well as providing other advantages.

[11] In an example, the means for carrying working fluid comprises a common flow path for carrying working fluid when the system is configured in both of the first and second modes. Suitably, at least some of the same power cycle components may be utilised regardless of mode of operation, thereby reducing overall complexity and footprint of the system. In a related example, the system comprises a first set of controllable valves which are open in the first mode and closed in the second configuration, and a second set of controllable valves which are open in the second mode and closed in the first mode. Opening and closing the first and second sets of valves opens and closes dedicated indirect mode flow paths and direct heat mode flow paths which are coupled to the common path.

[12] In an example, the indirect heating means comprises a set of heat exchangers thermally couplable to the intermittent heat source which outputs different grades of heat, wherein the set comprises a primary heat exchanger thermally couplable to a primary, high temperature, heat source, a second heat exchanger thermally couplable to a secondary, medium temperature, heat source, and a third heat exchanger thermally couplable to a tertiary, low temperature, heat source. In this way the power cycle may be configured to efficiently operate with heat output from a fusion reactor which outputs multiple grades of heat (i.e., different amounts and quality). In further examples, a fourth (or more) heat sources may also be coupled to the system by suitable heat exchangers; for example, recaptured waste heat.

[13] In an example, the direct heating means comprises a combustor configured to receive a fuel, which may be hydrogen, carbon or hydrocarbon, and oxygen.

[14] In an example, the working fluid comprises at least one of di-nitrogen, argon, helium, air, Xenon, Neon, CH4, 02 or water, though preferably the working fluid comprises primarily carbon dioxide. In particular, a purity level of carbon dioxide may be at least 50%, and further preferably at least about 85% to 99%.

[15] In an example, the working fluid comprises a binary or tertiary mixture of carbon dioxide and at least one of CeFe, C4F8, C2H3N, TiCL4, NO2, SO2, SiCk, WCb, WFe, UFe to facilitate condensing of the working fluid even at higher ambient temperature (e.g., ~50°C).

[16] In a related aspect of the invention there is provided a fusion power plant comprising a fusion reactor and the power cycle system of any preceding claim, wherein the power cycle system is operated in the first mode when the fusion reactor is in an operational mode. The power cycle system may be operated in the second mode during a startup phase of the nuclear reactor, a shutdown phase of the nuclear reactor, or an interrupt in operation of the nuclear reactor. In an example, the power plant may comprise a controller configured to receive a signal corresponding to a status of the reactor, and (automatically) change the power cycle from operating in the first mode to operating in the second mode, or vice versa, responsive to the received signal.

[17] As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word “about”, even if the term does not expressly appear. The term “about”, or substantially, when used herein means +1- 5% of the stated value. Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein, and the terms “from” and “to” a pair of values are intended to indicate such values are included in the range. Singular encompasses plural and vice versa. Additionally, although the present invention has been described in terms of “comprising”, the processes, materials, and coating compositions detailed herein may also be described as “consisting essentially of’ or “consisting of’. Brief Description of the Drawings

[18] For a better understanding of the present disclosure reference will now be made by way of example only to the accompanying drawings, in which:

[19] Fig. 1 shows a schematic of an example fusion power plant;

[20] Fig. 2 shows a schematic of an example fusion power plant including an improved power cycle system;

[21] Fig. 3 shows a detailed schematic of the example power cycle system. Detailed Description

[22] At least some of the following example embodiments provide a power cycle system for use in generating electricity from a fusion power plant - in particular an improved power cycle system based on a modified Brayton cycle. Other advantages and improvements may also be apparent from the discussed embodiments herein.

[23] Figure 1 shows an example arrangement for a fusion power plant 10. The power plant comprises a reactor 12 and a power cycle 14 to convert heat from the reactor 12 into electrical energy. In the present examples the reactor 12 is assumed to be a tokamak type reactor, for example the Spherical Tokamak for Energy Production ‘STEP’ project. The tokamak type reactor 12 may comprise a variety of heat sources which provide different grades of temperature.

[24] For example, a primary heat source supplying a primary heat, which may be regarded as high grade temperature, may be heat from a blanket module surrounding the vacuum vessel of the reactor, and / or from an outboard first wall of the vacuum vessel. Here the primary heat may be at least 500°C (degrees centigrade), and possibly at least 600°C. Heat from a secondary heat source supplying a secondary heat (low grade), may be heat from an inboard first wall of the vacuum vessel and / or inboard radiation shield. Here the secondary heat may be about 300°C or lower, and in general will be always lower than the primary heat. Some reactors may comprise a third heat source supplying tertiary heat (low grade), which may be heat from heat from a divertor component of the reactor, and maybe lower than the second heat. Here the tertiary heat may be above 150°C, and in general will be always lower than the primary and secondary heats. Heat lower than 150°C may be regarded as waste heat from parts of the plant such as heating and current drive, (potentially from) cryogenic plant etc. In some examples such heat may also be utilised as a fourth (or further) heat source.

[25] It should however be appreciated that the present techniques are not limited to tokamak type reactors. The present disclosure may be readily applied to any fusion reactor where startup phases, shutdown phases, reactor intermittence, and so on, can be expected, and any reactor where heat grade and the number of temperature levels vary.

[26] The power cycle 14 is formed from a closed Brayton (or Rankine) cycle. A working fluid (liquid or gas) is compressed by a compressor 16 to increase the temperature and pressure of the working fluid. The high pressure working fluid is heated (ideally isobarically) by a heat exchanger 18, and then the heated fluid is used to turn a turbine 20 to produce useful external work - i.e., generating electricity. The turbine 20 is an expander such that the working fluid reduces temperature and pressure on the outlet side of the turbine before being passed through a cooler 22 to remove any excess heat. The working fluid is then directed back into the compressor for the cycle to continue.

[27] The power plant of course comprises suitable grid infrastructure 24 to transfer electrical power generated by the turbine 22 to an external energy grid 26 (e.g., the national grid). Optionally, the power plant 10 may also comprise a thermal storage 28, which may be configured to (slowly) add heat into the power cycle 14 in certain circumstances; for example, when the reactor 12 is undergoing maintenance, or during a startup phase Moreover, thermal storage (or buffer) helps add thermal intertia to enable a transition between operating modes of the reactor.

[28] Figure 2 shows an example arrangement for a fusion power plant 10 comprising an improved power cycle 100 based on an augmented (improved) Brayton cycle.. The working fluid is preferably a gas, for example di-nitrogen, argon, helium, air, water, or preferably carbon dioxide (which may be further preferably supercritical). In some examples the working fluid may be a binary or tertiary mixture of carbon dioxide with other organic based working fluids such as Hydrofluorocarbons (e.g., CeFe, C4F8), C2H3N, TiCL4, NO2, SO2, SiCk, WCb, WFe, UFe; such mixtures facilitate condensing of the working fluid even at higher ambient temperature (~50°C), thereby reducing the pumping power. The working fluid may be suitably transported by various fluidically coupling means including a network of piping, inlets, outlets, valves, and other suitable components as would be familiar to those in the art.

[29] Similar to Fig. 1, the power cycle 100 also comprises indirect heating means 118, such as a heat exchanger, configured to indirectly heat the working fluid using heat from the reactor 12 of the fusion power plant 10, as well as an electric generator 120 configured to generate electrical power using heated working fluid using a turbine.

[30] The cycle 100 also comprises direct heating means 102 configured to directly heat the working fluid. The direct heating means 102 may be a combustor which is suitably provided with fuel and an oxidant. Preferably the fuel is carbon free (i.e., not a fossil fuel), such as hydrogen or any other green fuel, but may (in non-preferred examples) comprise hydrocarbons. Preferably average levels of carbon dioxide in the working fluid are about 95% or greater, with the remaining percentage formed from impurities such as Argon, N2, 02 and possibly trace amounts of SOx and Nox. It will of course be appreciated that the carbon dioxide level may vary throughout the cycle, for example being about 97% or greater at a pump (or compressor) component 122 and about 90% to 95% at a turbine (generator) component 120. In general, carbon dioxide purity should be above about 50%. Operating the entire cycle with a carbon dioxide purity in similar ranges (e.g., between 85% and 99% purity) allows for the establishment of quicker chemical equilibrium when making mode changes in the power cycle (see below). Operating with higher carbon dioxide purity is possible but requires the addition of a purification system (not shown) and results in slower chemical equilibration time.

[31] The combustion products of the direct heating means are primarily water and CO2 as combustion is conducted with near pure oxygen rather than air. The water level reaches a about 4% at the generator (turbine) 120 inlet, which is removed. There is also a possibility of having some small amount of un-combusted CO and CH4 remain in the system. The CO2 (and other combustion products) may be removed from the cycle for subsequent sequestration.

[32] Suitably, the same power cycle that is designed to accept indirect heat from the fusion reactor may be augmented to include an auxiliary direct heat process. In particular, the power cycle system 100 is configurable to adapt between a fusion mode and a direct heat mode. In the fusion mode - otherwise termed a first mode or first configuration herein - the fusion reactor 12 is active and used to indirectly heat the working fluid via heating means 118 (e.g., a heat exchanger); hear the power cycle system operates in a closed-loop configuration. That is, the heating means 118 are thermally coupled to the working fluid to provide heat to the working fluid. In the fusion mode the direct heating means 102 may be deactivated (i.e., not in use). By contrast, in the direct heat mode - otherwise termed a second mode, second configuration, or an oxycombustion mode herein - the direct heating means 102 are configured to be active to directly heat the working fluid. In this second mode, the indirect heating means 118 are substantially inactive. For example, the indirect heating means may be thermally decoupled from the working fluid 100. Here the power cycle operates in a semi-closed-loop configuration.

[33] In some examples the power cycle 100 may also comprise a dual mode, otherwise termed a third mode or configuration herein, in which the both the indirect heating means and direct heating means may be active in providing heat to the working fluid.

[34] It will be appreciated that switching between the modes may be performed manually, for example by a power plant operator, or may be automatic. For example, the power plant may comprise a control unit (not shown) configured to monitor a status of the reactor 12 and change the operational mode (first or second) of the power cycle 100 responsive to a status signal.

[35] Combining the two modes of operation within a single power cycle provides many advantages for nuclear fusion power.

[36] The power cycle 100 is particularly suitable in cases where there is pulsed heat coming from the fusion reactor 12 in short duration, so that the direct heat can be operated to smooth over operation between pulses.

[37] Using the same power cycle components during fusion mode and backup oxy-combustion mode of operation facilitates a quick transition between modes as the components are already hot and running, thereby minimising the thermal stresses.

[38] The oxy-combustion mode can be utilised to preheat the fusion reactor 12 and other components thereof such as a breeder blanket and fluid(s) therein. Relatedly, the back-up oxy-combustion heat source 102 may be activated during a startup phase of the reactor 12 to start up the power cycle 100 (i.e., preheat) with the power cycle 100 later being switched to accept heat indirect heat from the fusion reactor 12 (i.e., fusion mode).

[39] The back-up oxy-combustion heat source 102 may be configured to come online when the fusion reactor 12 shuts down or trips (interrupts) to keep the machine either continue running in an island mode (i.e., until fusion is restored) or shutdown the reactor 12 in a controlled manner (again, thereby reducing thermal stresses on the power cycle 100 components). Likewise, uncertainty linked to prototypic operations including lot of trips, but the components may be spared from undergoing severe stresses as the combustion cuts in. When there is a trip of fusion heat, the combustion recovers the operation of the power cycle and keep the power cycle running to enable restarting the cycle whilst also reducing the stresses.

[40] The example power cycle 100 allows for load following potential with the aid of back-up oxy-combustion heat source.

[41] The direct heat mode may be utilised as a black start capability, as the electricity generated from the backup oxy-combustion can be used to supply all of the power plant 100 parasitic loads during start-up.

[42] In dual mode operation, the back-up oxy-combustion heat source 102 may be utilised to balance out uncertainty in reactor performance.

[43] Finally, the same power cycle is utilized to capture heat from the fusion reactor 12 with the combustor turned off (i.e., first mode operation), thereby eliminating a need for redundant auxiliary power cycles or components (e.g., reducing a size of required thermal storage), thereby significantly reducing the potential power plant footprint and capital costs

[44] These are but some of the advantages. It will be appreciated that there may be other advantages not listed here.

[45] Figure 3 shows the example power cycle 100 in more detail. Here, the indirect heating means 118 comprises a set of heat exchangers 118 configured to heat the working fluid at various points in the power cycle 100. A first heat exchanger 104 in the set is thermally coupled to the primary heat 11 OH of the reactor 12 (i.e., a high temperature, high grade heat), a second heat exchanger 106 in the set is thermally coupled to the secondary heat 110M of the reactor 12 (i.e., mid temperature, low grade heat), and a third heat exchanger 108 in the set is thermally coupled to the tertiary heat 110L of the reactor 12 (i.e., low temperature, low grade heat).

[46] Each of the heat exchangers 104-108 may themselves be part of a subset of heat exchangers. In this example, the second heat exchanger 106 thermally coupled to the second heat of the reactor 12 comprises a pair of such heat exchangers 106a, 106b.

[47] The thermal coupling between the reactor 12 and set of heat exchangers 118 may be direct or indirect. For example, as shown the reactor 12 may heat a set of fluid loops corresponding to each member of the set of heat exchangers 118.

[48] The power cycle 100 comprises a common flow path 130 through which working flow during all modes of operation. That is, working fluid flows through network of pipes, etc, 130 in both the first mode and second mode of operation (and also third mode of operation). As can be seen, the common path 130 includes main turbine 120 (i.e., electric generation), cooler 122 (which may have a corresponding condenser 123), and pump / compressor 116.

[49] The common flow loop 130 may also include an auxiliary turbine 132 which is configured in flow parallel with the main turbine (path 131). That is, the auxiliary turbine 132 is not in a direct flow path with the primary heat exchanger 104 (by being arranged on separate fluid path 120 to the fluid path 112), so that the auxiliary turbine 116 does not receive working fluid heated by the first heat exchanger 104. Suitably the auxiliary turbine 132 may form part of the same electrical generator as the main turbine 120 or may be coupled to a separate electrical generator.

[50] The common flow loop 130 may also include one or more recuperators; recuperators are heat exchangers arranged to thermally couple turbine inflow paths with turbine outflow paths, which allows excess heat to be recovered from the turbine outflows (i.e., to remove enthalpy from a turbine exit stream). In this way the inflow heating may be made more efficient, and less heat wasted.

[51] A first recuperator 134, also termed a high temperature recuperator, may be configured to thermally couple working fluid outflow with working fluid inflow to the primary heat exchanger 104. A second recuperator 136, also termed a low temperature recuperator, may be configured to thermally couple the outflow form the main turbine to the inflow path of the (optional) auxiliary turbine 132. Suitably, the second recuperator forms a series flow (along path 120) with the auxiliary turbine 132. Such a recuperator may significantly increase the temperature of the auxiliary turbine inflow.

[52] The power cycle 100 also comprises a fusion mode flow path 138 corresponding to the fusion mode of operation; that is, a working fluid loop corresponding to the first mode / configuration. In this mode a set of first valves 140 which are coupled to the common flow path 130 are controlled to be open (e.g., by an operator or automatic control unit). Suitably, opening the valves 140 allows for working fluid to flow through each heat exchanger in the set of heat exchangers 118. Most importantly, there are valves providing access for working fluid to the primary heat exchanger 104 . Similarly, the valves 140 may complete the parallel path 131 for the auxiliary turbine 132. Put another way, the first set of valves 140 may be used to define a first controllable flow path for the working fluid.

[53] In this first configuration, a corresponding set of second valves 142 coupled to the common flow path 130 are controlled to be closed. This second set of valves 142 relate to a combustion mode path 144, so that closing these valves thereby isolates the combustion mode path 144 from the common path 130. Suitably, in this configuration the director heater 102 may be controlled to be off.

[54] Conversely, in the direct heat (second) mode of operation, the second set of valves 142 are controlled to be opened and the direct heater 102 turned on. The first set of valves 140 are controlled to be closed. Put another way, the second set of valves 142 may be used to define a second controllable flow path for the working fluid. In some examples, the first set of valves 140 may not be completely closed, but controlled to a trickle setting whereby a smaller volume of working fluid than normal may pass through the valves to access the set of heat exchangers 118. This may be desirable so that working fluid in the heat exchangers is not completely isolated and allowed to cool. Herein it is taken that ‘isolating’ the indirect heating means 118 (e.g., heat exchangers 104-108) includes allowing for a trickle setting.

[55] Suitably, closing the first set of valves 140 and opening the second set of valves 142 bypasses the indirect heating means 118 of the power cycle 100, while directing working fluid to the direct heat source 102 for direct heating.

[56] In summary, exemplary embodiments of an improved power cycle system for a nuclear fusion power plant have been described. The described exemplary embodiments enable multimode operation of the power cycle in orderto better meet operational demand of the fusion reactor and power plant, notably utilising the same power cycle components for both a direct heat and indirect heat configuration.

[57] The example apparatus may be manufactured industrially. An industrial application of the example embodiments will be clear from the discussion herein. Additionally, the described exemplary embodiments are convenient to manufacture and straightforward to use.

[58] Although preferred embodiment(s) of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined in the claims and as described above.

[59] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[60] All of the features disclosed in this specification, and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[61] Each feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[62] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification, or to any novel one.tur or any novel combination, of the steps of any method or process so disclosed. 24 04 24

Claims

1. A power cycle system for a fusion reactor, comprising:means for carrying a working fluid;indirect heating means configured to indirectly heating the working fluid using heat from the fusion reactor;direct heating means configured to directly heat the working fluid;a turbine configured to generate electrical power using heated working fluid;wherein the power cycle system comprises a first mode in which the system is operated in a closed loop configuration in which the indirect heating means is used to indirectly heat the working fluid while the direct heating means is thermally decoupled from the working fluid, and a second mode in which the system is operated in a semi-closed loop configuration in which the direct heating means is used to heat the working fluid while the indirect heating means is substantially thermally decoupled from the working fluid.

2. The system of claim 1 wherein, in the first mode, the direct heating means are deactivated.

3. The system of claim 1 or 2, further comprising a third mode in which the both the indirectheating means and direct heating means are active and thermally coupled to the working fluid.

4. The system of any preceding claim, wherein the means for carrying working fluid comprisesa common flow path for carrying working fluid when the system is configured in both of the firstand second modes.

5. The system of any preceding claim, wherein the means for carrying working fluid comprises a first set of controllable valves which are open in the first mode and closed in the second mode.

6. The system of any preceding claim, wherein the means for carrying working fluid comprises a second set of controllable valves which are open in the second mode and closed in the first mode.24 04 247. The system of any preceding claim, wherein the indirect heating means comprises a set of heat exchangers thermally couplable to the fusion reactor, wherein the set comprises a primary heat exchanger thermally couplable to a primary, high temperature, heat source of the fusion reactor.

8. The system of claim 7, wherein the set of heat exchangers comprises a second heat exchanger thermally couplable to a secondary, medium temperature, heat source of the fusion reactor.

9. The system of claim 8, wherein the set of heat exchangers comprises a third heat exchanger thermally couplable to a tertiary, low temperature, heat source of the fusion reactor.

10. The system of any preceding claim, wherein the direct heating means comprises a combustor configured to receive a fuel and an oxidant.

11. The system of claim 10, wherein a fuel for the combustor comprises hydrogen, hydrocarbon, ammonia or other carbonaceous fuels.

12. The system of any preceding claim, wherein the working fluid comprises at least one of dinitrogen, argon, helium, air, Xenon, Neon, CH4, O2, or water.

13. The system of any preceding claim, wherein the working fluid comprises carbon dioxide.

14. The system of claim 13, wherein a purity level of carbon dioxide is at least 50%15. The system of claim 13 or 14, wherein a purity level of carbon dioxide is in a range of about85% to 99%.

16. The system of any of claims 13 to 15, wherein the working fluid further comprises at least one of C6F6, C4F8, C2H3N, TiCL4, NO2, SO2, SiCI4, WCI6, WF6, UF6.24 04 2417. A fusion power plant comprising a fusion reactor and the power cycle system of any preceding claim, wherein the power cycle system is operated in the first mode when the fusion reactor is in an operational mode.

18. The power plant of claim 17, wherein the power cycle system is operated in the second mode during a startup phase of the fusion reactor.

19. The power plant of claims 17 and 18, wherein the power cycle is operated in the first and second mode simultaneously.

20. The power plant of claim 17 or 19, wherein the power cycle system is operated in the second mode during a shutdown phase of the fusion reactor, or an interrupt in operation of the fusion reactor.

21. The power plant of any of claims 17 to 20, further comprising a controller configured to receive a signal corresponding to a status of the fusion reactor, and change the power cycle from operating in the first mode to operating in the second mode, or vice versa, responsive to the received signal.

22. The power plant of any of claims 17 to 21, further comprising a thermal buffer coupled to a primary heating loop of the power cycle system.

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