heat engine

Abstract

A closed working fluid heat engine (100) for use in generating power in a fusion power system (200) includes a primary heat exchanger (102) coupled to a primary heat source (106) and a secondary heat exchanger (108) coupled to a secondary heat source (110). The secondary heat source is thermally independent of the primary heat source, and the primary heat source is hotter than the secondary heat source. The heat engine further includes a main turbine (104) and an auxiliary turbine (116). The turbines are arranged in parallel flow, with a parallel flow junction located between the primary and secondary heat exchangers. The main turbine receives a working fluid heated by the secondary heat exchanger and the primary heat exchanger, while the auxiliary turbine receives a working fluid heated by the secondary heat exchanger. The heat engine further includes a first recuperator (124) configured to thermally couple a working fluid outlet from the main turbine to a working fluid inlet to the primary heat exchanger. Suitable nuclear power generation systems and methods for operating the same are also described.

Description

[Technical Field]

[0001] The present disclosure relates generally to thermal power transfer and power generation systems, and more particularly to thermal power transfer and power generation systems associated with the extraction of electrical power from nuclear fusion reactors (i.e., power systems as part of a fusion power plant), particularly tokamak reactor / power plants. Even more particularly, the present disclosure relates to thermal power transfer and power generation systems based on a modified Brayton cycle. [Background technology]

[0002] One of the major challenges facing nuclear fusion technology concerns how to convert the energy generated by a fusion reactor into usable electricity. Particularly in the context of tokamak reactors, closed working fluid heat engines operating according to the (closed) Brayton cycle have been considered as potentially advantageous power generators.

[0003] One of the challenges in efficiently utilizing heat from fusion reactors, especially tokamak reactors, in Brayton / closed-cycle gas turbines is that fusion reactors often have one or more sources of excess low-grade (i.e., low-temperature) heat. While the high-grade (i.e., high-temperature) heat output from the reactor can be properly utilized to power a heat engine, the problem remains of how to deal with the low-grade heat that is simultaneously produced. One simple solution is to simply consider this heat (and the energy used to generate it) as waste heat, but of course this has an impact on power generation efficiency. Summary of the Invention [Problem to be solved by the invention]

[0004] It is therefore desirable to develop a power delivery system that efficiently utilizes all of the heat produced by a fusion reactor, and in particular integrates the excess low-grade heat from the reactor into the power generation cycle. [Means for solving the problem]

[0005] The present invention is defined according to the independent claims. Additional features will be apparent from the dependent claims and the description herein. The embodiments described but not included in the claims should be interpreted only as examples useful for a better understanding of the invention.

[0006] These exemplary embodiments are provided with the goal of addressing at least some of the difficulties faced by current approaches to thermal power transfer from fusion reactors, whether such difficulties are specifically mentioned above or will be understood from the discussion herein.

[0007] Thus, in one aspect of the present invention, there is provided a heat engine for use in generating power in a fusion power plant. The heat engine comprises a primary heat exchanger coupled to a primary heat source and a secondary heat exchanger coupled to a secondary heat source, the heat exchangers configured to heat a working fluid of the heat engine. The primary and secondary heat sources are thermally independent, with the primary heat source being at a higher temperature than the secondary heat source. Suitably, the primary heat source may provide high-grade heat (e.g., heat from an outer blanket component) while the secondary heat source may provide low-grade heat (e.g., heat from an inner first wall). The heat engine further comprises a main turbine and an auxiliary turbine configured in parallel flow. A parallel-flow junction is provided between the primary and secondary heat exchangers, such that the main turbine receives working fluid heated by the secondary and primary heat exchangers (in series), and the auxiliary turbine receives working fluid heated by the secondary heat exchanger. The heat engine further includes a first recuperator configured to thermally couple the working fluid outlet from the main turbine to the working fluid inlet to the primary heat exchanger.

[0008] Advantageously, the auxiliary turbine maximizes heat recovery from the power plant's reactor by integrating energy from a secondary (low-grade) heat source into the power generation cycle.

[0009] In one example, the branch point includes a controllable valve configured to control the flow rate of working fluid sent from the secondary heat exchanger to the main and auxiliary turbines. Advantageously, the flow rates to the main and auxiliary turbines can be optimized based on the operating parameters of the heat engine, such as inlet / outlet temperatures, the primary coolant used, and heat distribution between components. Additionally, cooling capacity (i.e., temperature) can affect the feasibility of this cycle. In one example, a temperature sensor can be coupled to the working fluid inlet to the primary heat exchanger, and the valve can be controlled to reduce the flow rate to the primary heat exchanger if the temperature of the inlet to the primary heat exchanger exceeds a predetermined value (e.g., 500°C).

[0010] Furthermore, by providing an auxiliary turbine on the parallel working fluid path, operational uncertainties are reduced. If the heat distribution between reactor components or the outlet temperature (in other words, the difference between the primary and secondary heat sources) is not as expected, the system can still integrate all of the reactor heat into the working fluid by manipulating the distribution and specific temperatures of the two turbines. Similarly, the technique further reduces design uncertainties for STEP reactors and similar reactors. If the heat distribution between reactor components or the outlet temperature changes during further design of a particular reactor, the system can still integrate all of the reactor heat into the working fluid by manipulating the distribution of the two turbines.

[0011] In one example, the heat engine includes a second recuperator configured to thermally couple the outlet from the main turbine to the inlet to the auxiliary turbine. In this manner, enthalpy can be removed from the outlet stream of the main turbine and transferred to the auxiliary turbine inlet rather than becoming waste heat. The recuperator therefore provides a final heating step for the working fluid sent to the auxiliary turbine. Furthermore, by manipulating the temperature of the low-pressure CO2 stream at the outlet of the recuperator, the amount of heat added to the auxiliary turbine stream can be effectively specified. Such temperature manipulation, in addition to controlling the flow rate to each turbine, allows for further optimization of power generation.

[0012] In one example, the heat engine further comprises a combustion burner configured to heat the working fluid received by the main turbine. Advantageously, such an arrangement provides an auxiliary heating system for maximizing power output during operation (i.e., by adapting to temperature changes of the primary and secondary heat sources).

[0013] Suitably, in one related aspect of the invention, there is provided a fusion power generation system incorporating the heat engine described above. More specifically, the fusion power generation system includes a nuclear reactor having a primary heat source and a secondary heat source, the secondary heat source being cooler than the primary heat source during normal reactor operation, a primary heat exchanger coupled to the primary heat source, and a secondary heat exchanger coupled to the secondary heat source, the heat exchangers configured to heat a working fluid. The system further includes a main turbine and an auxiliary turbine configured in parallel flow. A parallel flow junction is located between the primary and secondary heat exchangers, such that the main turbine receives working fluid heated by the secondary and primary heat exchangers (in series), while the auxiliary turbine receives working fluid heated by the secondary heat exchanger.

[0014] In one example, the reactor is a tokamak reactor, and suitably the primary heat source comprises heat from at least one of the reactor blanket (e.g., a lithium breeder blanket) and the outer first wall, while the secondary heat source comprises heat from at least one of the inner first wall and the inner shield.

[0015] Additionally, some reactors may include a tertiary heat source that is cooler than the primary heat source (and cooler than the secondary heat source). Suitably, the system may further include a tertiary heat exchanger (a heat exchanger suitably configured to heat the working fluid) coupled to the tertiary heat source, the tertiary heat exchanger being arranged in flow series with the secondary heat exchanger to preheat the working fluid prior to the secondary heat exchanger.

[0016] In this case, the tertiary heat source may consist of heat from the divertor components of a tokamak reactor.

[0017] In yet another related aspect of the present invention, there is provided a method for operating a closed working fluid heat engine in a fusion power generation system, the power generation system including a nuclear reactor generating a primary heat source and a secondary heat source cooler than the primary heat source, the method including heating a working fluid using a secondary heat exchanger coupled to the secondary heat source, controllably splitting the working fluid flow heated by the secondary heat exchanger into parallel fluid paths, further heating the working fluid in one parallel fluid path using a primary heat exchanger coupled to the primary heat source and using the working fluid heated by both the primary and secondary heat exchangers to power a main turbine, and using the working fluid heated by the secondary heat exchanger in the other parallel path to power an auxiliary turbine, and recombining the working fluid discharged from the main and auxiliary turbines via at least one compressor into a common flow path coupled to an inlet to the secondary heat exchanger.

[0018] For a better understanding of the present disclosure, reference will now be made, by way of example only, to the accompanying drawings in which: [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a schematic flow diagram of a closed Brayton cycle. [Figure 2] 1 is a schematic flow diagram of an example heat engine based on a modified Brayton cycle. [Figure 3] 1 is a schematic flow diagram of another example heat engine based on a modified Brayton cycle. [Figure 4] 1 is a schematic flow diagram of another example heat engine based on a modified Brayton cycle. [Figure 5] FIG. 1 is a schematic diagram of an example fusion power generation system. DETAILED DESCRIPTION OF THE INVENTION

[0020] At least some of the following example embodiments provide heat engines for use in generating electricity from fusion power plants, and in particular provide improved thermal power transfer systems based on modified Brayton cycles. Additionally, other advantages and improvements may become apparent from the embodiments discussed herein.

[0021] Figure 1 shows a basic schematic flow diagram of a closed Brayton cycle. A working fluid (gas) is compressed by a compressor, increasing the temperature and pressure of the working fluid. This high-pressure working fluid is further heated (ideally isobaric) by a heat exchanger, and the heated fluid is then used to spin a turbine to generate useful external work, such as generating electricity. The turbine is an expander, which causes the working fluid to reduce its pressure at the turbine outlet and then pass through another heat exchanger to remove excess heat. The working fluid is then returned to the compressor, continuing the cycle.

[0022] FIG. 2 shows a schematic flow diagram of a heat engine 100 based on a modified (improved) Brayton cycle, which has been developed specifically for use in a tokamak-type nuclear reactor, such as the applicant's Spherical Tokamak for Energy Production "STEP" project. The heat engine uses a working fluid in a closed-cycle configuration. Preferably, the working fluid is a gas, more preferably supercritical carbon dioxide. Suitably, the working fluid network structure shown in FIG. 2 can be considered to be formed from appropriately arranged piping, inlets, outlets, valves, and other such fluid-coupling components, as will be familiar to those skilled in the art.

[0023] The heat engine 100 comprises a primary heat exchanger 102 and a main turbine 104 for generating external work by passing a (heated) working fluid through the primary turbine 104. The primary heat exchanger 102 is suitably coupled to a primary heat source 106, which may be considered to provide a high-grade temperature. In this way, the working fluid may be suitably heated by the primary heat exchanger 102 using energy from the primary heat source 106. In the context of a tokamak, and in particular STEP, the primary (high-grade) heat source 106 may be heat from a blanket module surrounding the reactor's vacuum vessel and / or heat from a first wall outside the vacuum vessel. In this case, the primary heat (i.e., high-grade temperature) supplied to the primary heat exchanger is preferably at least 500°C (Celsius), more preferably at least 600°C.

[0024] The heat engine 100 further comprises a secondary heat exchanger 108 suitably coupled to a secondary heat source 110. In this way, the working fluid can be suitably heated by the secondary heat exchanger 108 using energy from the secondary heat source 110. The secondary heat source 110 can be considered as providing low-grade heat. In other words, the secondary heat source 110 provides a lower temperature than the primary heat source 106. In the context of a tokamak, and particularly STEP, the secondary heat source 110 can be heat from the inner first wall of the vacuum vessel and / or the inner radiation shield. In this case, the secondary heat (i.e., the low-grade temperature) provided to the secondary heat exchanger is typically substantially below 300°C.

[0025] At least a portion of the working fluid output from the secondary heat exchanger 108 is input to the primary heat exchanger 102. In this way, the secondary heat exchanger 108 preheats the working fluid input to the primary heat exchanger 102, thereby making heating of the working fluid by the primary heat exchanger 102 more efficient. Suitably, the main turbine 104 receives the working fluid output from the primary heat exchanger 102 that has been heated by both the secondary heat exchanger and the primary heat exchanger. In other words, the secondary heat exchanger 108, the primary heat exchanger 102, and the main turbine 104 are connected in a serial flow path 112. By way of example, in the case of STEP, the temperature of the working fluid output after the primary heat exchanger and therefore input to the main turbine 104 (i.e., after the heating stage) may be greater than 500°C. The working fluid outflow 114 from the main turbine 104 (which may be in the temperature range of, for example, 300-500°C) then passes through an appropriate (recovery) flow loop and returns to the inflow of the secondary heat exchanger 108. The components of this working fluid recovery loop are discussed below.

[0026] Additionally, the heat engine 100 includes an auxiliary turbine 116 configured in parallel flow with the main turbine. More specifically, a parallel-flow junction 118 in the heat engine 100 is located between the primary heat exchanger 102 and the secondary heat exchanger 108. In this manner, the auxiliary turbine 116 is also isolated from the primary heat exchanger 102 (by being located on a separate fluid path 120 from the fluid path 112), and thus receives working fluid heated by the secondary heat exchanger 108 but not by the primary heat exchanger 102. An outlet 122 from the auxiliary turbine is combined with the outlet 114 from the main turbine and follows the same recovery loop back to the secondary heat exchanger 108.

[0027] Suitably, the junction 118 may comprise a controllable valve configured to control the flow rate of the working fluid routed from the secondary heat exchanger 108 to the main turbine 104 (via the primary heat exchanger 102) and the auxiliary turbine 116. In this manner, the flow rate of the working fluid along paths 112 and 120 may be appropriately controlled based on desired operating parameters of the system.

[0028] In one example, the flow split may be changed in response to temperature changes in the primary heat source 106 and the secondary heat source 110 (e.g., to compensate for a lower primary or secondary temperature).

[0029] In another example, the flow division between the two turbines can be based on the power output of the turbines. That is, the flow division can be appropriately optimized for energy production by varying the flow rate to each of the turbines. In particular, by utilizing a computer simulation of the heat engine 100, it is possible to calculate the flow rates for each of the paths 112, 120 that will result in maximum performance (i.e., power production) from the turbines based on actual input data of the primary and secondary heat sources (e.g., fluid temperatures, among other operating parameters of the heat engine 100).

[0030] Furthermore, this flow division may be varied depending on the operating parameters of the heat engine, for example to ensure safe and efficient operation of the heat engine. One example parameter is the temperature of the working fluid, and suitable temperature sensors (not shown) may be positioned at various locations throughout the heat engine 100. In particular, a sensor positioned to measure the inlet temperature to the primary heat exchanger 102 may be utilized to control the branch point valve 118 to reduce the flow to the primary heat exchanger 102 if the temperature of the incoming working fluid exceeds a predetermined value (an excessively high inlet temperature could reduce the efficiency of the primary heat exchanger 102).

[0031] As shown, the heat engine 100 may further include one or more recuperators. A recuperator is a heat exchanger positioned to thermally couple the turbine inlet path to the turbine outlet path, allowing for recovery of excess heat from the turbine outlet (i.e., enthalpy removal from the turbine exit flow). In this manner, inlet heating may be more efficient and waste heat may be reduced.

[0032] The first recuperator 124 may be configured to thermally couple the working fluid outlet 114 from the main turbine to the working fluid inlet 126 of the primary heat exchanger 102. Suitably, the junction 118 between the flow path 112 and the flow path 120 is located before the first recuperator 124 (i.e., between the first recuperator 124 and the secondary heat exchanger 108). In other words, the first recuperator 124 is part of the flow path 112 and therefore in series with the secondary heat exchanger 108 and the primary heat exchanger 102.

[0033] The second recuperator 128 may be configured to thermally couple the outlet 114 from the main turbine 104 to the inlet 130 of the auxiliary turbine 116. Suitably, this second recuperator is in series flow with the auxiliary turbine 116 (along path 120). Such a recuperator may significantly increase the temperature of the auxiliary turbine inlet (e.g., up to 450°C) above what would be achievable solely from the heat supplied to the working fluid by the secondary heat source 110. In other words, the second recuperator 128 is provided to maximize the heat delivered to the auxiliary turbine 116, thereby improving its performance. Furthermore, the second recuperator 128 provides a useful auxiliary heat dump to the outlet flow 114, since in situations where the primary heat source is excessively hot, it is undesirable to recuperate heat to the inlet 126 via the first recuperator 124.

[0034] A third recuperator 132 may be configured to thermally couple a combined working fluid outlet 134 from the main turbine 104 and the auxiliary turbine 116 to an inlet 135 to the secondary heat exchanger 108 .

[0035] Some nuclear reactors may include more than two low-grade heat sources, and therefore, suitably, heat engine 100 may further include a tertiary heat exchanger 136 suitably coupled to a tertiary heat source 138. Tertiary heat source 138 is suitably at a lower temperature than primary heat source 106, and preferably also at a lower temperature than secondary heat source 110. In the context of tokamaks, and particularly STEP, tertiary heat source 138 may be heat from divertor components of the reactor.

[0036] The tertiary heat exchanger 136 is disposed in series with the secondary heat exchanger 108 (optimally via the third recuperator 132 ), thereby preheating the working fluid prior to the secondary heat exchanger 108 .

[0037] The combined effluent / outlet flow 134 from the two turbines passes through one or more recuperators discussed above (not all of these recuperators need be present, but preferably at least one provides enthalpy removal from the turbine outlet, similar to FIG. 1 ), then passes through the (main) compressor 140 and then back to the secondary heat exchanger 108 (or tertiary heat exchanger 136, if present), thereby repeating the cycle. That is, the outlet flows from the main turbine 104 and auxiliary turbine 116 recombine into a common flow path 134 that is connected to the inlet 135 of the secondary heat exchanger 108 through at least one compressor, as in a conventional Brayton cycle.

[0038] Optionally, the present disclosure provides for incorporating an oxy-combustion direct cycle by further modifying the Brayton cycle. In this case, the heat engine 100 includes a combustion burner 142 that burns a fuel consisting of natural gas and oxygen. The combustion burner 142 provides non-fusion-based additional heating to the working fluid entering the main turbine 104. The recovery loop may be adapted to accommodate the oxy-combustion element, suitably by providing a condenser 144, with excess H2O and CO2 (entrained by combustion) extracted via appropriate fluid paths 147, 148.

[0039] Additionally, a fourth recuperator 150 may be positioned to thermally couple the combined turbine exit flow 134 to the input of the secondary heat exchanger 108 or the tertiary heat exchanger 136, as appropriate.

[0040] 3 illustrates an alternative configuration of the heat engine 100 of FIG. 2 , in which the system 100 includes a bypass 152 for the second recuperator 128. Specifically, either or both of the outlet flow 114 from the main turbine 104 or the inlet 130 to the auxiliary turbine 116 may include the bypass 152. The bypass 152 is particularly advantageous in situations where unexpectedly high-temperature, low-grade heat may be encountered, providing a way to prevent excessively high temperatures in the inlet 130 to the auxiliary turbine 116 by blocking heat transfer from the main turbine outlet flow 114. Suitably, a temperature sensor may be coupled to the secondary heat source 110, or preferably to the working fluid path 120 (i.e., the inlet to the second recuperator 128), and data from this sensor may be used to determine whether the system could benefit from isolation from the second recuperator 128 and to control the bypass accordingly.

[0041] 4 shows another embodiment of the heat engine 100 including a set of auxiliary turbines 116 (here 116a and 116b). Preferably, each turbine 116a, 116b in the set 116 may be configured to operate at a different optimum fluid temperature, and preferably, the auxiliary turbines 116 are arranged in parallel. In this case, a branch point 154 is provided between the first auxiliary turbine 116a and the second auxiliary turbine 116b, such that the first auxiliary turbine 116a has its inflow thermally coupled to the main turbine outlet flow 114, while the second auxiliary turbine receives fluid only from the secondary heat exchanger 108.

[0042] Suitably, for each of the examples of FIGS. 2-4, the main turbine 104 and the auxiliary turbine 116 are coupled to generators suitable for generating electrical power that is distributed to a suitable electrical infrastructure 156 .

[0043] FIG. 5 illustrates an example fusion power system 200 (ie, a system for generating electrical power) incorporating the example heat engine 100.

[0044] In this case, the fusion power system comprises a nuclear reactor 201 having a primary heat source 206 and a secondary heat source 210. As noted above, the primary heat source 206 can be considered the high-grade heat source, and the secondary heat source can be considered the low-grade heat source, and therefore the primary heat source will be hotter than the secondary heat source (at least during normal operation of the reactor).

[0045] In a preferred example, the reactor 201 is a tokamak. Suitably, the primary heat source 206 consists of heat from at least one of the reactor blanket 203 and the outer first wall 205 (i.e., the components facing the exterior of the reactor 201), while the secondary heat source consists of heat from at least one of the inner first wall 207 and the inner (radiation) shield 209 (i.e., the components facing the interior of the reactor 201 proximate the central cylindrical "Z" axis of the tokamak). Suitably, the reactor 201 may further comprise a tertiary heat source 238, which may consist of heat from the divertor 211.

[0046] Suitably, each heat source 206, 210, 238 may be coupled to a respective heat exchanger (primary, secondary, tertiary) of the heat engine 100 described above to transfer thermal power from the reactor 201 to the turbine of the heat engine for generating electricity therefrom.

[0047] In summary, exemplary embodiments of improved thermal power transfer systems for fusion power plants have been described.

[0048] The exemplary embodiments described are adaptable and controllable to various nuclear reactors, allowing for more efficient utilization of the heat generated by the reactor. In particular, the exemplary embodiments are capable of achieving high efficiency in fusion reactors with a high proportion of low-grade heat.

[0049] These exemplary devices may be manufactured industrially. Industrial applications of these exemplary embodiments will become apparent from the discussion herein. Furthermore, the exemplary embodiments described herein are easy to manufacture and simple to use.

[0050] While preferred embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that changes can be made therein without departing from the scope of the invention as defined in the claims and described above.

[0051] Attention is directed to all articles and documents related to this application that are filed contemporaneously or previously hereto and that are open to public inspection herewith, and the contents of all such articles and documents are incorporated herein by reference.

[0052] All of the features disclosed herein and / or all of the steps of any method or process disclosed herein may be combined in any combination, except for combinations where at least some of such features and / or steps are mutually exclusive.

[0053] Each feature disclosed in this specification, unless expressly stated otherwise, may be replaced by alternative features serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

[0054] The invention is not limited to the details of the foregoing embodiments, and the scope of the invention extends to any novel feature or any novel combination of features disclosed herein, or any novel feature or any novel combination of method or process steps disclosed herein. [Explanation of symbols]

[0055] 100 Heat Engine 102 Primary heat exchanger 104 Main Turbine 106 Primary heat source 108 Secondary heat exchanger 110 Secondary heat source 112 serial flow path 114 Working fluid outflow (from main turbine) 116 Auxiliary Turbine 116a First auxiliary turbine 116b Second auxiliary turbine 118 Branch points, branch point valves 120 Fluid Path 122 Outflow (from auxiliary turbine) 124 First Recuperator 126 Working fluid inflow (primary heat exchanger) 128 Second Recuperator 130 Inflow (to auxiliary turbine) 132 Third Recuperator 134 Working fluid outflow (combined) 135 Inflow (to secondary heat exchanger) 136 Tertiary heat exchanger 138 Tertiary heat source 140 (Main) Compressor 142 Combustion Burner 144 Condenser 147 Fluid Path 148 Fluid Path 150 Fourth Recuperator 152 Bypass 154 Branching Point 156 Electrical Infrastructure 200 Nuclear Fusion Power Generation System 201 Nuclear reactor 203 Reactor Blanket 205 outer first wall 206 Primary heat source 207 Inner first wall 209 Inner (radiation) shield 210 Secondary heat source 211 Diverter 238 Tertiary heat source

Claims

1. 1. A heat engine for use in generating electricity in a fusion power system, comprising: a primary heat exchanger coupled to a primary heat source and a secondary heat exchanger coupled to a secondary heat source, the secondary heat source being thermally independent of the primary heat source and having a higher temperature than the secondary heat source, the primary heat exchanger and the secondary heat exchanger being configured to heat a working fluid; a main turbine and an auxiliary turbine configured to be in parallel flow, a parallel flow branch point being disposed between the primary heat exchanger and the secondary heat exchanger, the main turbine receiving working fluid heated by the secondary heat exchanger and the primary heat exchanger, and the auxiliary turbine receiving working fluid heated by the secondary heat exchanger; a first recuperator configured to thermally couple a working fluid outlet from the main turbine to a working fluid inlet to the primary heat exchanger; A heat engine comprising:

2. The heat engine of claim 1 , wherein the branch point comprises a controllable valve configured to control the flow rate of working fluid from the secondary heat exchanger to the main turbine and the auxiliary turbine.

3. 3. The heat engine of claim 2, further comprising a temperature sensor coupled to a working fluid inflow to the primary heat exchanger, the valve being controlled to reduce flow to the primary heat exchanger when a temperature of the inflow to the primary heat exchanger exceeds a predetermined value.

4. 4. The heat engine of claim 3, wherein the branch point is located between the first recuperator and the secondary heat exchanger.

5. 5. A heat engine according to any one of claims 1 to 4, further comprising a second recuperator configured to thermally couple the outflow from the main turbine to the inflow to the auxiliary turbine.

6. 6. The heat engine of claim 5, further comprising a bypass of the second recuperator at least one of an outlet from the main turbine and an inlet to the auxiliary turbine.

7. 7. The heat engine of claim 1, further comprising a third recuperator configured to thermally couple a combined working fluid outlet from the main turbine and the auxiliary turbine to a working fluid inlet to the secondary heat exchanger.

8. 8. The heat engine of claim 1, further comprising a tertiary heat exchanger configured to heat the working fluid, the tertiary heat exchanger being coupled to a tertiary heat source that is cooler than the primary heat source.

9. 9. The heat engine of claim 8, wherein the tertiary heat exchanger is disposed in flow series with the secondary heat exchanger to preheat working fluid prior to the secondary heat exchanger.

10. 10. A heat engine according to claim 8 or 9 when dependent on claim 7, wherein the third recuperator is disposed between the secondary heat exchanger and the tertiary heat exchanger.

11. 11. A heat engine according to any one of claims 1 to 10, wherein the auxiliary turbine is the first turbine of a set of auxiliary turbines.

12. 12. The heat engine of claim 11, wherein each turbine of the auxiliary turbine set is configured to operate at a different fluid temperature.

13. 13. A heat engine according to any one of claims 1 to 12, further comprising a combustion burner configured to heat a working fluid received by the main turbine.

14. A nuclear fusion power generation system, a nuclear reactor having a primary heat source and a secondary heat source, the secondary heat source being thermally independent of the primary heat source, the primary heat source being hotter than the secondary heat source during normal operation; a primary heat exchanger coupled to the primary heat source and a secondary heat exchanger coupled to the secondary heat source, the primary heat exchanger and the secondary heat exchanger configured to heat a working fluid; a main turbine and an auxiliary turbine configured to be in parallel flow, a parallel flow branch point being disposed between the primary heat exchanger and the secondary heat exchanger, the main turbine receiving working fluid heated by the secondary heat exchanger and the primary heat exchanger, and the auxiliary turbine receiving working fluid heated by the secondary heat exchanger; a first recuperator configured to thermally couple a working fluid outlet from the main turbine to a working fluid inlet to the primary heat exchanger; A nuclear fusion power generation system comprising:

15. 15. The fusion power system of claim 14, wherein the primary heat source comprises heat from at least one of a reactor blanket and an outer first wall.

16. 16. The fusion power system of claim 14 or 15, wherein the secondary heat source comprises heat from at least one of the inner first wall and the inner shield.

17. 17. The fusion power system of claim 14, wherein the reactor further comprises a tertiary heat source that is cooler than the primary heat source, and the system further comprises a tertiary heat exchanger coupled to the tertiary heat source configured to heat the working fluid, the tertiary heat exchanger being arranged in flow series with the secondary heat exchanger to preheat the working fluid prior to the secondary heat exchanger.

18. 18. The fusion power system of claim 17, wherein the tertiary heat source comprises heat from a divertor.

19. 1. A method for operating a closed working fluid heat engine in a nuclear fusion power generation system, the power generation system including a nuclear reactor generating a primary heat source and a secondary heat source, the secondary heat source being thermally independent from the primary heat source and having a higher temperature than the secondary heat source, the method comprising: heating the working fluid using a secondary heat exchanger coupled to the secondary heat source; controllably splitting the working fluid stream heated by the secondary heat exchanger into parallel fluid paths; in one parallel fluid path, further heating the working fluid using a first recuperator coupled to a working fluid outlet from a main turbine and a primary heat exchanger coupled to the primary heat source, and operating the main turbine using the working fluid heated by the primary heat exchanger, the secondary heat exchanger, and the first recuperator; In another parallel path, using the working fluid heated by the secondary heat exchanger to operate an auxiliary turbine; recombining the working fluid discharged from the main turbine and the auxiliary turbine via at least one compressor into a common flow path coupled to an inlet of the secondary heat exchanger; A method comprising:

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