CIRCULATING-FUEL NUCLEAR REACTOR

Abstract

A circulating-fuel nuclear reactor comprising: a reactor core chamber having an inlet and an outlet for fluid fuel; a heat exchanger configured to receive fluid fuel from the reactor core chamber via the outlet, to transfer heat from the fluid fuel, and to return the fluid fuel to the reactor core chamber via the inlet; a flow regulator operable to vary an operational flow rate of fluid fuel through the heat exchanger; and a control module configured to cause the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger to maintain an operational temperature of the fluid fuel within a predetermined range.

Claims

1. A circulating-fuel nuclear reactor comprising: a reactor core chamber having an inlet and an outlet for fluid fuel; a heat exchanger configured to receive fluid fuel from the reactor core chamber via the outlet, to transfer heat from the fluid fuel, and to return the fluid fuel to the reactor core chamber via the inlet; a flow regulator operable to vary an operational flow rate of fluid fuel through the heat exchanger; and a control module configured to cause the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger to maintain an operational temperature of the fluid fuel within a predetermined range.

2. The circulating-fuel nuclear reactor according to claim 1, wherein the operational temperature of the fluid fuel is dependent on reaction conditions in the reactor core chamber, wherein the circulating-fuel nuclear reactor (1) further comprises a sensor module operable to measure a parameter indicative of reaction conditions in the reactor core chamber, and wherein the control module is configured to cause the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger in response to detecting a change in reaction conditions in the reactor core chamber based on an output from the sensor module.

3. The circulating-fuel nuclear reactor according to claim 2, wherein the sensor module is operable to measure a parameter indicative of reaction kinetics in the reactor core chamber, and wherein the control module is configured to cause the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger in response to detecting a slowdown or a speedup in the nuclear reaction in the reactor core chamber based on an output from the sensor module.

4. The circulating-fuel nuclear reactor according to claim 2, wherein the sensor module is operable to measure a parameter indicative of the operational temperature of the fluid fuel within the nuclear reactor, and wherein the control module is configured to cause the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger in response to determining, based on an output from the sensor module, that the operational temperature of the fluid fuel is above or below a critical temperature value.

5. The circulating-fuel nuclear reactor according to claim 4, wherein the operational temperature is the temperature of the fluid fuel at the inlet to the reactor core chamber, and wherein the control module is configured to cause the flow regulator to increase the operational flow rate of fluid fuel through the heat exchanger in response to determining, based on the output from the sensor module, that the temperature of the fluid fuel at the inlet is below a critical inlet temperature value.

6. The circulating-fuel nuclear reactor according to claim 2, wherein the sensor module is operable to measure a parameter indicative of a level of fissile material in the fluid fuel, and wherein the control module is configured to cause the flow regulator to increase the operational flow rate of fluid fuel through the heat exchanger in response to detecting a reduction in the level of fissile material in the fluid fuel based on an output from the sensor module.

7. The circulating-fuel nuclear reactor according to claim 1, wherein the circulating-fuel nuclear reactor comprises a clock, and wherein the control module is configured to cause the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger as a function of time.

8. A method of operating a circulating-fuel nuclear reactor, the circulating-fuel nuclear reactor comprising: a reactor core chamber having an inlet and an outlet for fluid fuel; a heat exchanger configured to receive fluid fuel from the reactor core chamber via the outlet, to transfer heat from the fluid fuel, and to return the fluid fuel to the reactor core chamber via the inlet; a flow regulator operable to vary the operational flow rate of fluid fuel through the heat exchanger; and a control module to control operation of the flow regulator; wherein the method comprises: the control module causing the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger to maintain an operational temperature of the fluid fuel within a predetermined range.

9. The method according to claim 8, wherein the operational temperature of the fluid fuel is dependent on reaction conditions in the reactor core chamber, wherein the circulating-fuel nuclear reactor comprises a sensor module operable to measure a parameter indicative of reaction conditions in the reactor core chamber, and the method comprises: the control module causing the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger in response to detecting a change in reaction conditions in the reactor core chamber based on an output from the sensor module.

10. The method according to claim 9, wherein the sensor module is operable to measure a parameter indicative of reaction kinetics in the reactor core chamber, and the method comprises: the control module causing the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger in response to detecting a slowdown or a speedup in the nuclear reaction in the reactor core chamber based on an output from the sensor module.

11. The method according to claim 8, wherein the sensor module is operable to measure a parameter indicative of the operational temperature of the fluid fuel within the nuclear reactor, and the method comprises: the control module causing the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger in response to determining, based on an output from the sensor module, that the operational temperature of the fluid fuel is above or below a critical temperature value.

12. The method according to claim 11, wherein the operational temperature is the temperature of the fluid fuel at the inlet to the reactor core chamber, and the method comprises: the control module causing the flow regulator to increase the operational flow rate of fluid fuel through the heat exchanger in response to determining, based on the output from the sensor module, that the temperature of the fluid fuel at the inlet is below a critical inlet temperature value.

13. The method according to claim 8, wherein the circulating-fuel nuclear reactor comprises a clock and the method comprises: the control module causing the flow regulator to vary the operational flow rate of fluid fuel through the heat exchanger as a function of time.

14. The circulating-fuel nuclear reactor according to claim 1, wherein the fluid fuel is a molten fuel salt.

15. The method according to claim 8, wherein the fluid fuel is a molten fuel salt.

16. A computer program comprising instructions to cause a control module of a circulating-fuel nuclear reactor to carry out the method according to claim 8.

17. A non-transitory computer-readable medium storing, or a data carrier signal carrying, the computer program according to claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0057] FIG. 1 is a schematic illustration of a Molten Salt Fast Reactor (MSFR);

[0058] FIG. 2 is a schematic cross-section through a portion of a reactor core chamber of the MSFR of FIG. 1;

[0059] FIG. 3 is a schematic illustration of a control module of the MSFR in communication with a computer readable memory;

[0060] FIG. 4 is a schematic illustration of a method of operating an MSFR;

[0061] FIG. 5 is a plot of inlet temperature (T.sub.in) and outlet temperature (T.sub.out) of molten fuel salt flowing through the inlet and outlet of the reactor core chamber, as a function of time, as calculated for a model MSFR operating under two different fractions of a nominal molten fuel salt flow rate;

[0062] FIG. 6 is a plot of inlet temperature (T.sub.in) and outlet temperature (T.sub.out), and the effective neutron multiplication factor (k.sub.eff), as a function of molten fuel salt flow rate in a model MSFR;

[0063] FIG. 7 is a plot of inlet temperature (T.sub.in), outlet temperature (T.sub.out) and average reactor core chamber temperature (T.sub.average) as a function of time for a model MSFR subjected to a step increase in molten fuel salt flow rate; and

[0064] FIG. 8 is a plot of the fraction of nominal power output by a model MSFR as a function of molten fuel salt flow rate expressed as a fraction of a nominal fuel salt flow rate.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0065] FIG. 1 provides a schematic illustration of a Molten Salt Fast Reactor (MSFR) 1, which is an example of a circulating-fuel nuclear reactor. The MSFR includes a reactor core chamber 2 for receiving molten fuel salt (i.e. a fluid fuel) containing fissile material for sustaining a nuclear chain reaction. The reactor core chamber 2 is in fluid communication, by way of pipes 3, with a heat exchanger 4 such that molten fuel salt may be transferred between the reactor core chamber 2 and the heat exchanger 4. The MSFR 1 further includes a variable flow rate pump 5 for pumping molten fuel salt from the reactor core chamber 2 out of the reactor core chamber 2 at an outlet 6, through the heat exchanger 4, and back into the reactor core chamber 2 at an inlet 7, around a reactor flow loop indicated generally by arrows 8 and 9. In other examples, the pump may be located at any position within the flow loop.

[0066] The MSFR 1 also includes a generator heat exchanger 10 in fluid communication, by way of pipes 11, with the heat exchanger 4. A pump 12 is provided for pumping coolant salt from the generator heat exchanger 10, through the heat exchanger 4, and back to the generator heat exchanger 10, around a generator heat exchange flow loop indicated generally by arrows 13 and 14. The heat exchanger 4 operates to transfer heat from the molten fuel salt in the reactor flow loop to the coolant salt in the generator heat exchange flow loop for subsequent transfer by the generator heat exchanger 10 to a generator (not shown) for generating electricity.

[0067] A reactor flow loop access point 15 is provided between the heat exchanger 4 and the reactor core chamber 2, the access point being accessible for extracting fuel salt from the reactor flow loop for transfer to a chemical processing plant (indicated by transfer vehicle 16) and for introducing fresh or processed fuel salt and/or fissile material into the reactor flow loop.

[0068] An emergency escape pipe 18 connects the reactor flow loop to emergency dump tank 19 by way of a freeze plug 20. The freeze plug 20 is made of solidified fuel salt and normally blocks passage of molten fuel salt from the reactor flow loop to the emergency dump tank.

[0069] The MSFR 1 is an example of a Molten Salt Reactor (MSR). In an example of use, molten fuel salt having a composition of LiF(77.5%)-HNF(22.35%), where HNF are heavy nuclei fluorides including fissile uranium-233 (.sup.233U) and fertile thorium-232 (.sup.232Th), is pumped through the reactor core chamber 2 and around the reactor flow loop as indicated in FIG. 1. Although in the simplified diagram of FIG. 1 only one inlet, one outlet and one heat exchanger are shown, in practice there may be a plurality of heat exchangers (for example twelve or sixteen) angularly arranged around the reactor core chamber 2. Each of the plurality of heat exchangers may be connected to a single inlet and a single outlet located, respectively, above and below the reactor core chamber 2, or instead each of the plurality of heat exchangers may be connected to a respective one of a plurality of inlets evenly spaced around the reactor core chamber 2 towards its bottom end and a respect one of a plurality of outlets evenly spaced around the reactor chamber 2 towards its top end. Each heat exchanger may be associated with an individually-controllable variable flow pump for pumping molten fuel salt out of the reactor core chamber from the corresponding outlet, through the heat exchanger, and back into reactor core chamber through the corresponding inlet. For example, each heat exchanger may be a 187 MWth heat exchanger and each pump may be configured to pump molten fuel salt, under normal conditions, at a nominal flow rate of about 0.1 to about 2.5 m.sup.3/s, such that the transition period of the fuel salt inside the reactor core chamber (not including time spent travelling through the heat exchangers) is about 2 to about 35 s. Nominal flow rates as low as about 0.04 m.sup.3/s or as high as about 7 m.sup.3/s may be achievable in alternative MSR designs, with reactor core chamber transition periods up to about 60 s.

[0070] When in operation, as the molten fuel salt passes through the reactor core chamber 2 (shown in more detail in FIG. 2), fissile material (in particular uranium-233) in the salt undergoes nuclear fission, releasing heat. Accordingly, the temperature of the molten fuel salt tends to increase as it travels through the reactor core chamber 2 from the inlet 7 to the outlet 6. As the molten fuel salt is pumped through the heat exchanger 4, heat is transferred from the molten fuel salt to the coolant salt pumped around the generator heat exchange flow loop. The generator heat exchanger 10 transfers heat from the coolant salt to the generator, where it is used to heat water to generate steam and drive a turbine (not shown), thereby generating electricity.

[0071] In the MSFR, neutron absorber or moderator rods (such as control rods) are not available for controlling the nuclear reaction in the event of an inadvertent speedup in the reaction rate. Use of control rods, in particular, in a circulating-fuel nuclear reactor is undesirable because they obstruct fuel salt circulation and increase the number of structural components subjected to irradiation. Instead of control rods, three methods of controlling criticality in the MSFR reactor core chamber have been proposed up to now.

[0072] The first criticality control method which has been proposed is passive criticality control through the strong negative temperature coefficient of reactivity, .sub.T, of the molten fuel salt. Since .sub.T is negative, an increase in fuel salt temperature tends to lead to a reduction in effective neutron multiplication factor, k.sub.eff, which in turn leads to a reduction in neutron flux and therefore a reduction in reactor power. As the reactor power reduces, so does the fuel salt temperature. Accordingly, the MSFR is considered to be an inherently safe reactor design, as small fluctuations in temperature are self-correcting and k.sub.eff tends to stabilise at a value of 1.

[0073] The second criticality control method which has been proposed is fuel salt reprocessing. As the nuclear reaction proceeds, fissile material in the fuel salt is used up and fission products are generated. The main fission products generated in the MSFR are noble gases and soluble fission products such as soluble lanthanides, which are considered to be neutron poisoning isotopes since they (due to their relatively large neutron absorption cross-sections) absorb neutrons generated on fission which could otherwise go on to cause new fission events. In fuel salt reprocessing, noble gases are removed from the molten fuel salt by bubbling helium gas through the salt, while soluble fission products are removed by pyrochemical processing.

[0074] Fuel salt reprocessing, by removing neutron poisoning isotopes, can be used to increase k.sub.eff, compensating for the reduction in k.sub.eff which would otherwise occur as fission products build up in the system and the fissile material is depleted. k.sub.eff can also be increased by introduction of new fissile material at the access point 15. Alternatively, k.sub.eff can be reduced by deliberately introducing non-fissile salts, or lower enrichment fuel salts, into the reactor at the access point 15.

[0075] However, fuel salt reprocessing is carried out in a chemical processing plant which, although it may be located onsite, requires extraction of molten fuel salt from the reactor flow loop through the access point 15 prior to processing, as well as reintroduction of fresh fuel salt back into the reactor flow loop. A continuous supply and handling of active isotopes is therefore required. Such a mechanism may therefore be unsuitable for use in emergency situations, or in the event of a failure of the chemical plant or unavailability of fresh fissile isotopes.

[0076] The third criticality control mechanism which has been proposed is provided by the melt plug 20 formed from solidified fuel salt. In the event of the core overheating, the melt plug 20 melts and allows fuel salt to escape from the reactor core chamber 2 into the drain tank 19 under gravity. The drain tank is air cooled and has a deeply subcritical geometry such that the nuclear reaction in the tank is suppressed. This method therefore only provides criticality control in extreme situations when the reactor overheats. An additional drawback of this method of criticality control is that, in the event that the melt plug 20 melts, the reactor core chamber 2 requires refueling before it is possible to restart the nuclear reaction.

[0077] In addition to being configured to implement the three criticality control mechanisms described hereinabove, the MSFR of the present invention also provides a fourth criticality control mechanism in the form of the variable flow rate pump 5. This new control mechanism is based on the discovery that the flow rate of molten fuel salt around the reactor flow loop has a direct effect on the temperature of the fuel salt. In particular, and as explained in more detail below, increasing the flow rate of molten fuel salt around the reactor flow loops tends to cause an increase in the temperature of molten fuel salt entering the reactor core chamber 2 at the inlet 7 and a reduction in the temperature of molten fuel salt exiting the reactor core chamber 2 at the outlet 6. In contrast, reducing the flow rate of molten fuel salt around the reactor flow loops tends to cause a reduction in the temperature of molten fuel salt entering the reactor core chamber 2 at the inlet 7 and an increase in the temperature of molten fuel salt exiting the reactor core chamber 2 at the outlet 6. Accordingly, adjustment of the flow rate of molten fuel salt around the reactor flow loop can be used to compensate for changes in the reaction conditions in the reactor core chamber which result in changes in the temperature of the fuel salt in different locations in the reactor.

[0078] As shown in FIG. 1, the MSFR 1 is provided with a control module 21 operatively connected to the variable flow rate pump 5. The control module 21 is configured to control operation of the variable flow rate pump 5 so as to vary the flow rate at which molten fuel salt flows out of the reactor core chamber 2, through the heat exchanger 4, and back into the reactor core chamber 2, around the reactor flow loop. In particular, the control module 21 is configured to operate the variable flow rate pump 5 to vary the flow rate at which molten fuel salt flows around the reactor flow loop to maintain an operational temperature of the fuel salt within a predetermined range. Accordingly, the control module 21 is able to maintain criticality by adjusting the molten fuel salt flow rate in response to changes in the reaction conditions in the reactor core chamber (such as changes in the reaction kinetics, which may be characterised by k.sub.eff or , changes in the temperature of the fuel salt at any given location, or changes in the amount of fissile material circulating within the nuclear reactor).

[0079] In order to detect changes in the reaction conditions, the MSFR 1 includes a sensor module 22. In the particular embodiment shown in FIG. 1, the sensor module 22 includes a temperature sensor operable to measure the temperature of molten fuel salt entering the reactor core chamber 2 at the inlet 7. The temperature sensor may be a temperature sensor of any type known in the field, for example a high-temperature thermocouple. The control module 21 is operatively connected to the sensor module 22 for receiving measurements of the temperature of the molten fuel salt at the inlet 7. In alternative embodiments, the sensor module 22 may include any type of sensor suitable for measuring parameters indicative of the nuclear reaction conditions in the reactor core chamber 2, such as a neutron detector (e.g. a proportional counter, a fission chamber such as a uranium fission chamber, a self-powered neutron detector, a MicroMegas detector, a liquid scintillation detector or an ionization chamber), a calorimeter, a mass flow rate sensor (e.g. a Coriolis flow meter, which may function as a density sensor), a volume flow rate sensor, or a chemical sensor such as a spectrometer, and the control module 21 may be operatively connected to the sensor module 22 for receiving measurements of the said parameters. Conditions in the generator heat exchange flow loop may also be indicative of the reaction conditions since the heat transfer to the generator heat exchange flow loop is a function of the reaction conditions and the flow rate through the heat exchanger. Accordingly, in some examples, the sensor module may be provided in the generator heat exchange flow loop or in the generator and may be configured to monitor a temperature, pressure or dryness fraction at any particular location around the flow loop, or a power output or rotary speed of the generator.

[0080] In the embodiment shown in FIG. 1, the control module 21 is configured to control operation of the variable flow rate pump 5 in response to measurements output by the sensor module 22. In particular, the control module 21 is configured to control operation of the variable flow rate pump 5 in response to measurements of the temperature of the molten fuel salt at the inlet 7 received from the sensor module 22. In more detail, as shown schematically in FIG. 3, the control module 21 includes a processor 23 in communication with a computer readable medium 25 containing computer executable program instructions 24 for controlling operation of the variable flow rate pump 5 differently dependent on the measurements received from the sensor module 22.

[0081] As outlined in FIG. 4, the control module is configured: to receive an output from the sensor module 22 indicative of the temperature of the molten fuel salt at the inlet 7 (block 100); to compare the temperature of the molten fuel salt at the inlet 7 to a critical inlet temperature value (block 101); and, if the temperature of the molten fuel salt at the inlet 7 is less than the critical inlet temperature value, to operate the variable flow rate pump 5 (block 102) to increase the flow rate of molten fuel salt around the reactor flow loop, thereby increasing the temperature of the molten fuel salt at the inlet. By setting the critical inlet temperature value close to the melting temperature (T.sub.M) of the fuel salt (for example, to a value of T.sub.M+50 C.), solidification of the fuel salt at the inlet 7 can be avoided.

[0082] The particular details of the control algorithm implemented by the control module 2 will depend on the parameter measured by the sensor module 22. For example, in embodiments in which the sensor module 22 contains a temperature sensor operable to measure the temperature of the molten fuel salt at the outlet 6, the control module 2 may be configured: to receive an output from the sensor module 22 indicative of the temperature of the molten fuel salt at the outlet 6; to compare the temperature of the molten fuel salt at the outlet 6 to a critical outlet temperature value; and, if the temperature of the molten fuel salt at the outlet 6 is greater than the critical outlet temperature value, to operate the variable flow rate pump 5 to increase the flow rate of molten fuel salt around the reactor flow loop, thereby reducing the temperature of the molten fuel salt at the outlet 6 and increasing the temperature of the molten fuel salt at the inlet 7. The critical outlet temperature value may be a predetermined function of the flow rate and heat exchanger conditions, such as the temperature of the fluid acting as a heat sink in the heat exchanger.

[0083] The control module 21 may be configured to operate the variable flow rate pump 5 to increase or reduce the flow rate of molten fuel salt around the reactor flow loop in response to measurements, provided by the sensor module 22, of any parameters indicative of changes in the reaction conditions in the reactor core chamber 2, in order to directly or indirectly maintain the operating temperature within a predetermined range, and thereby avoid supercriticality or freezing of the fuel salt within the reactor. A control procedure which features temperature in the feedback loop or objective function may be considered to directly maintain the operating temperature, whereas control procedures based on other parameters related to the reaction conditions and thereby indirectly related to the operational temperature, may be considered to be methods of indirectly maintaining the operating temperature. Accordingly, it will be appreciated that operational temperature need not be directly monitored in a control procedure in order to be maintained within a predetermined range.

[0084] In some embodiments, the control module 2 may be configured to operate the variable flow rate pump 5 independently of any output from the sensor module 22. For example, the control module 2 may include a clock (not shown) and the control module 2 may be configured to operate the variable flow rate pump 5 to increase the flow rate of the molten fuel salt around the reactor flow loop continuously or discontinuously as a function of time. A continuous increase in the flow rate of the molten fuel salt as the reactor operates may be used to compensate for a predicted continuous decrease in the amount of fissile material circulating within the reactor as the fissile material is used up in the nuclear reaction. Alternatively, a discontinuous (i.e. discrete or step-change) increase in the flow rate of the molten fuel salt may be used to increase the temperature of the molten fuel salt at the inlet 7 at a time at which it is predicted that the temperature of the molten fuel salt will fall below the critical inlet temperature value. In such examples, the operational temperature may be predictable as a function of time, such that a control procedure based only on time may be configured to maintain the operational temperature within a predetermined range.

[0085] FIGS. 4 to 8 provide examples of the effect of varying molten fuel salt flow rate in a model MSFR. These results are based on neutronics and thermal-hydraulics coupled calculations using the neutronics simulation code SERPENT-2, which is based on three-dimensional continuous-energy Monte Carlo reactor physics burnup methods (see, for example, J. Leppnen. Serpenta Continuous-energy Monte Carlo Reactor Physics Burnup Calculation Code. VTT Technical Research Centre of Finland (Jun. 18, 2015)), and the generic thermal-hydraulics solver OpenFOAM (available from The OpenFOAM Foundation Ltd, England).

[0086] The model MSFR included a reactor core chamber 2.5 m in height and 1.25 m in radius connected to sixteen heat exchanger loops. The power output was 1 GWe (at 40% thermodynamic efficiency) and the operating temperature was (630 C.). The fuel salt was LiFThF.sub.4UF.sub.4, having a melting temperature of 565 C., a density of 4.3 g/cm.sup.3 at 630 C. and a temperature-independent specific heat capacity of 1391 J/kgK, and including 22 mol. % of heavy nuclear (ThF.sub.4 and UF.sub.4) with 3 mol. % .sup.233U. The reactor contained 20 m.sup.3 of fuel salt. The fuel cycle period was 6.8 s and, for each cycle of fuel salt, the time spent in the reactor core chamber was 3.4 s and the time spent in the heat exchangers was 3.4 s. The heat exchangers were designed to pump the fuel salt at a nominal flow rate of 3.61 m.sup.3/s. The fuel salt temperature at the inlet to the reactor core chamber was calculated based on the average outlet temperature and the flow rate. If the outlet temperature and the flow rate were not sufficient provide 3 GW power output, the inlet temperature was assumed to be 850 K, above the melting temperature of the fuel salt (823 K).

[0087] FIG. 5 shows the temperature of the fuel salt calculated at the inlet and the outlet of the reactor core chamber as a function of time following a reduction (lines with circles) and an increase (unmarked lines) in the flow rate of the fuel salt through the heat exchangers relative to the nominal flow rate. In particular, the flow rate was reduced to 0.8 of the nominal value and increased to 1.3 of the nominal value. As can be seen from FIG. 5, reducing the flow rate leads to an increase in the temperature at the reactor core chamber outlet and a reduction in the temperature at the reactor core chamber inlet. In contrast, increasing the flow rate leads to a reduction in the temperature at the outlet and an increase in the temperature at the inlet.

[0088] These results can be understood as follows. The temperature of the fuel salt increases as it passes through the reactor chamber core due to fission reactions. When the fuel salt is circulated at a lower flow rate, it spends more time in the core and, therefore, more time fissioning. Consequently, the temperature of the fuel salt is increased as it passes through the core chamber relative to fuel salt flowing at the nominal flow rate. Accordingly, the outlet temperature increases. At the same time, since the flow rate is the same in the core chamber and in the heat exchangers, at lower flow rates, the fuel salt spends more time in the heat exchangers in contact with coolant salt and so more heat is transferred from the fuel salt to the generator heat exchange flow loop. This leads to a reduction in the temperature of the fuel salt leaving each heat exchanger at the corresponding inlet to the core chamber.

[0089] Since the fuel salt has a negative temperature coefficient of reactivity, fluctuations in k.sub.eff, and thus fluctuations in the temperature, tend to reduce over time. This is confirmed by FIG. 6 which shows that k.sub.eff tends to stabilise around a value of 1 despite large changes in the fuel salt flow rate.

[0090] As the amount of fissile material in the fuel salt is used up, a decrease in k.sub.eff is generally observed. As the nuclear reaction slows down, the average temperature in the reactor core chamber decreases, as shown in FIG. 7. Given constant conditions in the generator heat exchange flow loop, both the inlet and the outlet temperatures fall at the same rate. If the outlet temperature is allowed to drop below the melting temperature of the fuel salt, the salt will solidify, and the power output from the reactor will drop until fresh fissile material can be added or the fuel can be chemically reprocessed.

[0091] However, as shown in FIG. 7, a discrete increase in fuel salt flow rate (by 30% relative to the nominal flow rate) as the melting temperature is approached results in an increase in the inlet temperature, such that solidification of the fuel salt is delayed until such time as reprocessing or replacement of fuel is feasible. The lifetime of the reaction can therefore be extended by controlling the flow rate of the fuel salt. In addition, the results shown in FIG. 8 indicate that nominal power output can be maintained for constant fuel salt flow rates from about 0.7 to 1.3 of nominal flow rate. Similar behaviour at higher flow rates is expected. At very low flow rates (below about 70% of nominal flow rate), the suppression of k.sub.eff due to the increased temperature results in a reduced power output.

[0092] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.