Control of inert gas bubble formation in molten salt reactors

Abstract

A molten salt fission reactor. The reactor includes a reactor core including a plurality of fuel tubes. Each fuel tube contains a fuel salt and a gas interface. The fuel salt is a molten salt of one or more fissile isotopes. The gas interface is a surface of the fuel salt that is in contact with a gas space during reactor operation. The reactor also includes a fuel salt cooling system configured for cooling the fuel salt. The cooling system includes a heat exchanger and a coolant tank. The coolant tank contains a coolant liquid in which the fuel tubes are at least partially immersed. The heat exchanger is for extracting heat from the coolant liquid. The fuel salt cooling system is configured such that, during reactor operation, for all points within the fuel salt within each fuel tube except at the respective gas interface:

Description

TECHNICAL FIELD

[0001] The present invention relates to fissile reactors. In particular, the present invention relates to reactor designs and methods for controlling the operation of a molten salt reactor in order to control the formation of gas bubbles within the fuel salt of the reactor. BACKGROUND

[0002] Nuclear reactors using molten salts as fuel (MSRs) have been known since the successful operation of the United States Molten Salt Reactor Experiment in the 1960s. Since then, many designs of molten salt reactors have been conceived. Such reactors fall into two categories.

[0003] In the first category of pumped MSRs, the molten salt fuel is actively pumped between a reactor chamber, where the fuel goes critical and generates fission heat, and a heat exchanger, where the heat is transferred to another fluid, typically another molten salt that does not contain fissile elements ("coolant salt"), and used to generate electricity.

[0004] In the second category of static molten salt reactors, the molten salt fuel is located within fuel tubes, typically formed into fuel tube assemblies, and moves within the tubes by natural convection only, with the fuel remaining within the reactor core without being pumped outside of the core. A second fluid, such as a coolant salt, flows through the fuel tubes and removes heat from the core. The fuel salt remains in the core throughout its operational lifetime, being replaced by removing a spent fuel assembly and replacing it with a new assembly, much as is the case with fuel assemblies containing solid fuel elements. The second category of MSRs is described in GB 2508537 and its equivalents.

[0005] Molten salt reactors inevitably produce noble gases as fission products, and these gases have low solubility in the molten salt fuel. It is critical to the safety of the reactor that the movement of the noble gases from the active reactor core is highly predictable, as one of these gases is xenon 135, which is the known strongest neutron absorber and so has a significant impact on the reactivity of the core.

[0006] In the first category of pumped molten salt reactors, these gases can be removed in a number of ways, including sparging the fuel salt with helium.

[0007] The challenge of managing noble gases is greater in the second category of molten salt reactors. The gases can diffuse out of the molten salt fuel into a gas space above the fuel, where they have only a small reactivity impact since they are essentially outside of the reactor core. However, it is also possible that they will form bubbles on the inner surface of the fuel tubes. These bubbles can be triggered to detach from the tubes due to physical impact or other influences; as a large number of bubbles rise simultaneously, the fuel salt enters the gas space and causes a large increase in the reactivity of the core.

[0008] This would be an unacceptable safety breach, so it is necessary to ensure that bubble formation does not occur in the fuel tubes. SUMMARY

[0009] According to a first aspect of the application, there is provided a molten salt fission reactor. The reactor comprises a reactor core comprising a plurality of fuel tubes. Each fuel tube contains a fuel salt and a gas interface. The fuel salt is a molten salt of one or more fissile isotopes. The gas interface is a surface of the fuel salt which is in contact with a gas space during reactor operation. The reactor further comprises a fuel salt cooling system configured to cool the fuel salt. The cooling system comprises a heat exchanger and a coolant tank. The coolant tank contains a coolant liquid in which the fuel tubes are at least partially immersed. The heat exchanger is used to extract heat from the coolant liquid. The fuel salt cooling system is configured such that, during reactor operation, for all points within the fuel salt within each fuel tube except at the respective gas interface:

[0010]

[0011] where:

[0012] T1 is the temperature of the fuel salt at the gas interface;

[0013] T2 is the temperature of the fuel salt at the measurement point;

[0014] P1 is the absolute pressure at the gas interface;

[0015] P2 is the absolute pressure at the measurement point;

[0016] R He is the gas constant for helium;

[0017] AH He is the solubility enthalpy of helium in the fuel salt.

[0018] Thus, the solubility of helium is lowest at the gas interface.

[0019] The fuel salt cooling system is configured such that, during reactor operation, the solubility of the noble gas (e.g. helium as described above, or any other noble gas from helium to xenon) in the fuel salt close to the gas interface is lower than its solubility elsewhere in the fuel tube. This ensures that the noble gas does not reach a saturated concentration except in the region close to the gas interface, and thus does not form bubbles.

[0020] The solubility of the noble gas in the fuel salt is a function of:

[0021] • the properties of the fuel salt

[0022] • the nature of the gas (xenon is of particular interest due to its higher neutron absorption)

[0023] • the temperature of the fuel salt, wherein the solubility of the noble gas in the molten salt increases as the molten salt temperature increases

[0024] • the pressure of the molten salt, wherein the solubility is approximately directly proportional to the pressure

[0025] The last two factors are the most easily controlled.

[0026] If the temperature of the fuel salt at the top of the tube can be maintained at a level lower than the temperature below the fuel column, then bubbles will not form and the gas will leave the fuel salt by diffusion at the gas interface. This low temperature can be achieved by one or more of the following mechanisms:

[0027] • suppression of nuclear fission in the top region of the fuel tube by use of a neutron absorber or reflector

[0028] • ensuring that the temperature of the coolant in the top region of the fuel tube is sufficiently low to maintain a low temperature in the fuel salt

[0029] • deflecting the natural convection of the fuel salt within the tube, thereby preventing the hot salt produced in the high power region of the fuel tube from rising to the region of the salt near the gas interface

[0030] However, the pressure effect is also highly relevant in such a system. The fuel salt is a dense liquid and the hydrostatic pressure of a column of liquid means that, at constant temperature, the solubility of an inert gas within a column of fuel salt increases at lower levels.

[0031] Thus, even if the fuel salt temperature is slightly lower below the salt column, the goal of having a minimum gas solubility at the upper salt / gas interface can be achieved.

[0032] The balance of the temperature and pressure effects determines the safe operating range of the nuclear reactor.

[0033] According to a second aspect of the present invention, there is provided a method of operating a molten salt fission reactor according to the first aspect. The temperature of the fuel salt is maintained such that, during operation of the reactor, for all points within the fuel salt in each fuel tube other than at the respective gas interface:

[0034]

[0035] wherein:

[0036] T1 is the temperature of the fuel salt at the gas interface;

[0037] T2 is the temperature of the fuel salt at the measurement point;

[0038] P1 is the absolute pressure at the gas interface;

[0039] P2 is the absolute pressure at the measurement point;

[0040] R He is the gas constant for helium;

[0041] AH He is the solubility enthalpy of helium in the fuel salt.

[0042] Thus, the solubility of helium in the fuel salt is lowest at the gas interface.

[0043] This can be done equivalently for inert gases other than helium. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 A static MSR with downward coolant flow is shown.

[0045] Figure 2 A static MSR with upward coolant flow is shown.

[0046] Figure 3 A static MSR with slow upward coolant salt flow is shown.

[0047] Figure 4 is a plot of the temperature profile along the cross section of the bottom portion of the tube.

[0048] Figure 5A Contour plot of cladding temperature at the top of the fuel salt is shown.

[0049] Figure 5B Contour plot of coolant outlet temperature is shown.

[0050] Figure 6 A detailed view of the fuel tube in a static MSR with upward coolant flow is shown, which uses a displacement geometry to cool the fuel gas interface.

[0051] Figure 7 is a plot of the lowest fuel temperature in the cross section as a function of height, and the locus where the saturation concentration will equal the temperature of the fuel surface. Several surface pressure loci are shown.

[0052] Figure 8 is a plot of the lowest fuel temperature in the cross section as a function of height, and the locus where the saturation concentration will equal the temperature of the fuel surface. Various salt properties loci are shown.

[0053] Figure 9 is a plot of the lowest fuel temperature in the cross section as a function of height, and the locus where the saturation concentration will equal the temperature of the fuel surface. The location of the displacement geometry is marked to show its effect on the fuel temperature. Detailed Implementation

[0054] In developing a safety case study of a static molten salt reactor as described in GB2508537, we have identified serious potential hazards in molten salt fuel reactors that are not adequately addressed in current designs, related to the behavior of inert gases.

[0055] The potential mechanism by which reactor power can experience significant transients revolves around the presence of bubbles within the reactor core, particularly in the critical region (i.e., the region where the fission isotope density is sufficient to allow for a self-sustaining nuclear reaction during operation). If the concentration of gases dissolved in the fuel remains below its saturation level, bubbles will not form, and the effects of dissolved gases on reactor physics are easily calculated. However, if bubbles do form, more complex phenomena can emerge.

[0056] Xenon dispersed in a fuel salt solution will have a stronger neutron absorption effect than the same amount of gas in bubbles. This is because xenon-135 is a very strong absorber, so the xenon at the center of a bubble containing xenon-135 will be essentially shielded from neutrons by the bubble itself. Therefore, the formation of bubbles from a supersaturated solution of gas in the fuel salt will greatly reduce neutron absorption, leading to an undesirable and uncontrolled increase in core reactivity, which will result in a rapid and potentially unmanaged increase in reactor power.

[0057] Other inert gases will also present similar problems because any bubbles will displace the fuel. If a large number of bubbles within the reactor are simultaneously displaced and rise from the critical region of the fuel lines to the surface, this could lead to a significant increase in reactivity.

[0058] When these findings are reviewed in conjunction with previous data, a key clue is that there may be a problem with the operation of molten salt reactor experiments. In report ORNL 4396 (page 17), noticeable random flickering was reported at a significant frequency at approximately 10% of reactor power. Despite extensive investigation, these power fluctuations have never been definitively explained. However, it is believed that the behavior of the gas in the fuel salt loop is the cause, and an empirical solution to eliminate the flickering has been achieved by adjusting pump pressure and gas injection. Molten salt reactor experiments are operated at very low power (<10 MW). Commercial reactors will operate at much higher power. Since the magnitude of the xenon effect is proportional to power, this potentially means that fluctuations that may be merely stimulating at very low power could become serious hazards at higher power. In molten salt reactor experiments, there is the ability to change the helium injection rate to control the xenon bubble problem. This capability is absent in molten salt reactors such as those described in GB2508537, where gas loss from the fuel salt can only occur through passive diffusion into the gas space above the fuel salt.

[0059] The actual nuclear fission process in the fuel salt can greatly exacerbate the problem of supersaturation.

[0060] At a power of 100 kW / liter of fuel, the number of fissions per second can be calculated as follows:

[0061] Fission energy = 3.2e-11 Joules per fission

[0062] Reactor power = 100,000 Joules / second / liter of fuel salt

[0063] Fission rate = 100,000 / 3.2e-11 = 3.12e+15 fissions per liter of fuel salt

[0064] Each fission produces 2 fission fragments, which are high-energy nuclei traveling at a significant fraction of the speed of light. Thus, 6e+15 of these high-energy particles are produced per liter of salt per second. Their typical path length in a medium of moderate density is on the order of 20 um.

[0065] A bubble of 10 um diameter has a probability of about 25% of being hit by any fission fragment produced within a spherical volume of 20 um diameter, which is 4 / 3*3.14*1e-12 = 4e-12 liters. Thus, the bubble will be hit by about 1000 times per second.

[0066] The energy in such a fission fragment is orders of magnitude higher than the surface energy of the bubble and causes the medium through which the particle passes to reach temperatures of tens of thousands of degrees. Thus, this can lead to the destruction of the bubble, with the gas content re-dissolving in the salt. Thus, the gas in the molten salt can be kept at a very high supersaturation level as long as fission continues. Thus, a feedback loop between the fission rate and the reactivity of the reactor core can be established and can easily lead to instabilities.

[0067] Other potential phenomena include the deposition of gas bubbles on solid surfaces within the reactor core. Such bubbles can accumulate for a while and be dislodged from the solid surface by shock waves, vibrations, or flow disturbances in the fuel salt. In this case, a large number of neutrons absorbed by the material can leave the core in a short time, causing a sudden increase in the reactivity of the core and possibly a damaging power surge. In addition, the removal of void volume from the core replaced with fissioning fuel salt (i.e., increasing the average fuel salt density within the critical region) can cause a further increase in reactivity.

[0068] While these phenomena are of concern in all molten salt reactors, they are of particular concern in the static molten salt reactor class, as the fuel salt in such reactors flows under natural convection at a relatively low velocity, and as a result, bubbles are more likely to form and accumulate on the inner surface of the fuel tubes than in reactors where the salt is pumped at a relatively higher velocity.

[0069] It is apparent that it is very difficult to ensure that no unintended hazardous consequences occur in a molten salt reactor when the formation of bubbles within the critical region of the reactor core is allowed to occur during operation. It is therefore highly desirable to keep the concentration of dissolved gas in the molten salt in the core of such a reactor below its saturation concentration, so that the formation of bubbles within the core region is not possible.

[0070] The solubility of inert gases in molten salts increases with increasing temperature, as opposed to the behavior of gases in water (ORNL-2931 Reactor Chemistry Department Annual Progress Report, January 31, 1960). As a result of this, at the gas / salt interface where the salt is at its highest temperature, the gas will dissolve in the salt. As the salt cools, the gas can thus become supersaturated in the salt and bubbles are easily formed.

[0071] Furthermore, as fissile gases are continuously produced in the fuel salt, the concentration of these gases in the fuel salt will rise until venting occurs at the same rate as production. In well-mixed salt, the concentration is the same everywhere, and venting will occur at the place where solubility is lowest - if this is not the surface, then bubbles will come out of the solution in the fuel lower.

[0072] In the following description, we suggest arranging the design of a molten salt reactor so that the salt in contact with the gas phase has a lower solubility of the gas than any other point in the fuel. The dissolved gas will thus diffuse out of the salt through the interface, without forming bubbles.

[0073] Most molten salt reactor designs - including the one described in GB 2508537 - take advantage of the increased buoyancy of the molten salt when heated to drive the circulation of the molten salt partially or completely. This necessarily requires the direction of the salt flow to be upwards. Unfortunately, however, this also means that the coolant salt temperature is highest at the top of the fuel tubes, which makes it difficult to avoid the fuel salt in this region being hotter, and thus having a higher solubility of inert gas than the solubility below the core, especially at the bottom of the tubes where the coolant temperature is lower.

[0074] A way to reduce the temperature of the fuel salt near the gas interface is therefore to reverse the direction of flow of the coolant salt to be vertically downwards, so that the coolant salt temperature increases as it flows downwards through the core. This has the disadvantage of being opposite to the natural convection forces, but ensures that the upper region of the flow loop is at a lower temperature.

[0075] In cases where the coolant flow direction cannot be reversed, such as in a static molten salt reactor where coolant flow is by natural convection, it is still possible to ensure that the upper surface of the fuel salt is the point in the fuel salt where gas solubility is lowest. This can be achieved by one or a combination of the following methods:

[0076] • Reducing the heat generation in the fuel salt by screening the fuel salt near the top of the tube with a neutron absorbing material

[0077] • Introducing a secondary coolant salt flow to cool the top region of the fuel tube and the fuel salt

[0078] • Cooling the gas space above the fuel salt in the tube to a temperature lower than the lowest temperature of the tube wall anywhere in the tube, causing the convection of gas to cool the upper surface of the fuel salt, despite the coolant outside the tube being hotter at that surface level. This cooling of the gas space can be achieved by supplementing the flow of cooler coolant salt, entering the region of lower temperature in the portion of the fuel tube containing gas above the surface of the coolant salt, or actively passing cooled gas into the gas space in the fuel tube.

[0079] • Placing a baffle a small distance below the surface of the fuel salt so that the bulk convection from the hot region below the tube does not reach the surface, while still allowing the molten salt to mix slowly and gas to diffuse to the surface through the baffle

[0080] • Maintaining a relatively slow coolant flow rate so that at the bottom of the tube, the wall is at the lowest coolant temperature, which is between the temperature of the bulk fuel salt and the temperature of the coolant salt, which has an intermediate temperature that is higher than the surface temperature of the fuel salt at the top of the tube. In this method, it is possible that the gas space above the fuel is in contact with the fuel tube above the fuel salt level, cooler than the wall of the tube where the coolant is at its lowest temperature but in contact with the hot fuel salt.

[0081] • Providing inserts to move the salt away from the center of the tube in the region near and at the surface, leaving only a thin perimeter of salt in contact with the wall. This thin band of salt will have the same cooled surface area as before, but the volume that generates heat is very small. Therefore, the remaining salt will be cooled by the coolant.

[0082] • Adding insulation to the bottom of the fuel tube to increase the temperature of the inner fuel

[0083] • Injecting coolant above the bottom of the fuel tube while leaving the lower section uncooled

[0084] Figure 1A static MSR with downward coolant flow is shown. The reactor consists of vertical fuel tubes 101 containing fuel salt 102, which can optionally be spaced apart by a moderator structure of graphite or other moderator material (not shown). Coolant 103 is pumped around a loop including the reactor core and heat exchanger 104, such that the coolant flows down 105 through the fuel tubes, ensuring that the fuel salt at the top of the fuel tubes (i.e., immediately adjacent to the gas space 106) is the coolest in the fuel tube, due to a combination of the reduced power density at the location of the outer edge of the reactor core and the lower temperature of the coolant in contact with the top of the fuel tube.

[0085] Figure 2 A static MSR with upward coolant flow is shown. The static molten salt reactor has vertically oriented fuel tubes 205, with the top gas-filled portion 207 of the fuel tubes entering a region of lower coolant salt temperature from the circulating coolant salt 203. The coolant salt is circulated through the core and heat exchanger 208 by natural convection 204. The gas space above the coolant salt is cooled by a cold gas system including a cold gas inlet 201 and a cold gas outlet 202, in turn cooling the upper section of the fuel tubes, which in turn cool the gas within the fuel tubes. The result of the convection cell is that cold gas falls from the outer regions within the fuel tubes, is heated by contact with the upper surface of the fuel salt, and then rises to the center of the fuel tube. This results in cooling of the uppermost layer of fuel salt, creating the low temperature gas / salt interface needed to keep the fuel salt below the saturation gas concentration in the bulk of the fuel salt.

[0086] Figure 3 A static MSR with slow upward coolant salt flow is shown. The reactor consists of vertical fuel tubes 301 containing fuel salt 302, which can optionally be spaced apart by a moderator structure of graphite or other moderator material. The coolant salt 303 is circulated through the core upward 305 and through the heat exchanger downward by natural convection only, with a relatively slow flow rate. The flow rate of the coolant and the power density in the fuel salt are such that the tube wall temperature at the bottom of the tube is higher than the temperature of the coolant emerging from the top of the core. This means that the gas space 306 will be cooler than any of the fuel salt, due to cooling by the coolant emerging from the core. The temperature of the upper surface of the fuel salt is lower than anywhere else in the fuel salt, due to a combination of cooling the uppermost layer of fuel salt by the hot coolant and cooling this surface by convection of gas in the gas space above the coolant.

[0087] Figure 4is a plot of the temperature along the cross-section of the bottom portion of the tube. The plot shows the temperature of three regions - the coolant salt at the bottom of the tube 410, the bottom tube wall 420, and the fuel salt 430. As can be seen from the plot, the temperature of the coolant salt at the bottom of the tube is lower than the temperature of the tube wall, which in turn is lower than the temperature of the fuel salt. The temperature of the coolant at the top of the fuel tube 401 is lower than the temperature of the bottom tube wall - meaning that dissolved gas will preferentially vent into the gas space at the top of the tube, rather than forming bubbles on the tube wall.

[0088] The relative temperatures of the fuel tube wall, the coolant salt at the bottom of the tube, and the coolant salt at the top of the tube will depend on the fuel tube size, the flow rate of the coolant salt, the coolant salt temperature entering the core, and the power density of the nuclear reaction. In particular, a model that has been found to be able to test the viability of solutions includes:

[0089] • the average power density in the fuel salt

[0090] • the diameter of each fuel tube

[0091] • the height of each fuel tube

[0092] • the thickness of the coolant salt annulus around the fuel tube (i.e. how large the space is for coolant to flow around the tube)

[0093] • the coolant inlet temperature

[0094] • the coolant outlet temperature

[0095] • the temperature of the fuel tube cladding at the bottom of the fuel tube

[0096] The model can be used in two ways: one way is to assume that the tube generates heat uniformly along its length (up to the level of the top of the fuel salt) for a simpler model; the other way is to account for the vertical gradient of energy generation in the fuel tube for a more accurate model. For example, the simpler model can be used to identify target parameter values, which are then checked with the more accurate model or experiment.

[0097] For a given power density, coolant salt inlet temperature, fuel tube length, fuel tube diameter, and annulus thickness, the coolant outlet temperature and the temperature of the fuel tube at the top of the fuel salt can be calculated. The difference between the coolant outlet temperature and the temperature of the fuel tube cladding at the top of the fuel salt is equal to the difference between the coolant inlet temperature and the temperature of the fuel tube cladding at the bottom. The coolant inlet temperature is a defined parameter, so it can be used to calculate the temperature of the cladding at the bottom of the fuel tube.

[0098] As another example, by fixing all input variables in the model except for two (e.g., fixed power density, inlet temperature, and fuel pipe length), a graph of coolant outlet temperature and cladding temperature can be plotted based on the other two variables (in this case, fuel pin diameter and annular thickness). Figure 5A and Figure 5B An example is shown in the figure, where, Figure 5A The contour lines of the cladding temperature at the top of the fuel salt are shown. Figure 5B The contour lines for the coolant outlet temperature are shown, with the additional thick line 801 indicating the region where the temperature at the top of the cladding is 1000°C. This can be used to determine the difference between the coolant temperature and the fuel line cladding temperature, and thus to determine which regions of the graph correspond to the desired relationship where the pipe wall temperature at the bottom of the pipe is greater than the coolant salt temperature at the top of the pipe.

[0099] Alternatively, other simulation, modeling, or prototyping methods and techniques known in the art can be used to determine the required parameters.

[0100] It will be clear that this analysis is for simulation purposes only—for a real reactor, temperatures can be simply measured to determine if they are in the correct relationship.

[0101] Alternatively, the neutron-absorbing structure can be inserted anywhere inside or outside the fuel tube, from directly below the surface of the fuel salt to above the gas space in the fuel tube, thereby suppressing fission in the uppermost layer of the fuel salt.

[0102] Figure 6 A detailed view of the fuel tube in a static MSR with an upward coolant flow 606 is shown, which uses displacement geometry 602 to cool the fuel-gas interface 607. The heat generated in the fuel salt 604 can only be removed through the fuel tube wall 605. In most fuels, this results in a significant temperature rise at the center of the fuel tube because heat is generated there in a volumetric manner. In the thin annular region 608, the volume of heat-generating fuel decreases sharply, but the surface area of ​​the fuel tube wall remains the same as in a tube without displacement geometry, resulting in a fuel-gas interface temperature much closer to the coolant temperature than the rest of the fuel.

[0103] The general equation for the solubility of gases in fluids is given by Henry's Law: c a =P*H cp , where c a Solubility is expressed in mol / cc, P is the partial pressure of the gas at the surface in the atmosphere (1 atm = 101325 Pa), and H... cp It is the Henry's law solubility of the fluid. This can be determined by adding H... cp Replace with to achieve temperature dependence, where H(T) is the updated Henry's constant at temperature T, H o is the Henry's constant at a reference temperature T o , ΔH is the solubility enthalpy, and R is the gas constant for the gas in question. Note that the "H" in the variables ΔH and H are completely different quantities and should not be confused.

[0104] Any bubble surface formed at a depth in the salt will be at the hydrostatic pressure of the salt at that depth. This means that the partial pressure of the gas in the bubble will equal the hydrostatic pressure (for a single gas). Thus, when calculating the saturation concentration in the region below the surface, the salt hydrostatic pressure at that point is used instead of the salt surface pressure.

[0105] Figure 7 The temperature of the fuel salt 701 along the height of the fuel pin, which is cooled by upward flow, is shown. To illustrate this example, if T o = 873.15 °K, H o = 1.94*10 -8 mol / cc / atm, ΔH = 353390 J / kg, and R = 63.33 J / kg / K (this is the xenon solubility behavior measured in a 53-47 mol% NaF-ZrF4 mixture, used as an example), then the salt at a pressure of 1 Bar and a temperature of 879.7 °K will be saturated at a gas concentration of 2.0079*10 -8 Mol / cc.

[0106] Since the depth of the fuel is 1800 mm, the average density is 3181.7 kg / m 3 , and the absolute pressure at the bottom of the pin is 1.561 Bar. The temperature of the salt at the bottom of the pin is 825.3 °K, so, despite the lower temperature, the fuel will be saturated at a higher concentration of 2.0633*10 - 8 Mol / cc.

[0107] To have the fuel at the bottom of the tube saturated at the same gas concentration as the top, the temperature at the bottom would have to be equal to the temperature where P1 and T1 are the pressure and temperature at the surface of the fuel, and P2 and T2 are the pressure and temperature at the bottom of the tube. This relationship applies to any depth in the fuel. For the example shown in Figure 7 the limiting temperature at the bottom of the fuel is 822.0 °K. A lower surface pressure of 0.5 Bar allows the limiting temperature to be lower than 786.4 °K.

[0108] Thus, to ensure that xenon does not escape from the solution, unless at the gas interface, the reactor is configured so that It should be noted that the example discussed earlier, T2 > T1, will always satisfy this relationship because the right hand side will always be less than T1.

[0109] The trajectory 702 of temperature is shown as Figure 7 The trajectory shows the temperature to which the fuel salt will have to fall at this elevation for the solubility of the fuel surface to match the surface pressure of 1 Bar. A second trajectory 703 is shown for a different absolute pressure at the surface of 0.5 Bar.

[0110] Since the solubility limits described by the trajectories 702 and 703 are relative to the fuel surface, they are not affected by the constant H o .

[0111] In a well-mixed system where the fuel surface is saturated, unable to dissolve more gas, the trajectory shows the temperature to which the fuel must fall in the rest of the pipe to also reach saturation.

[0112] While the examples above have focused on xenon as an exemplary gas of interest, it is possible to design a reactor that can tolerate limited xenon bubble formation while still operating safely, but not the formation of bubbles of lower atomic number noble gases. For such a reactor, the above analysis can be performed, but with the gas constant R for the gas instead of the gas constant for helium, neon, argon, or krypton, and with the enthalpy of solution, ΔH, instead of the enthalpy of the solution of the gas with the fuel salt. The higher the gas constant R (in J / kg°K) for the lighter noble gas, the higher ΔH (in J / kg) is, although it rises at a slower rate. This means that any reactor that meets the criteria for a certain noble gas (e.g., argon) also meets the criteria for all noble gases of lower atomic number (e.g., helium and neon) because the value of R / ΔH will increase, so the minimum temperature T2 will decrease.

[0113] Figure 8 Trajectories showing how the enthalpy of solution, ΔH, affects the saturation temperature are shown, where trajectories 802 through 809 are for ΔH = 150 kJ / kg (802), 200 kJ / kg (803), 250 kJ / kg (804), 300 kJ / kg (805), 350 kJ / kg (806), 400 kJ / kg (807), 450 kJ / kg (808), and 500 kJ / kg (809), respectively.

[0114] Many combinations of noble gas and molten salt ΔH are available from the literature. If such measurements are not available previously, ΔH can be experimentally determined by the following procedure:

[0115] 1. Measure the solubility of the gas in the molten salt at several different pressures at each of several different temperatures;

[0116] 2. Calculate the Henry's Law constant H for each temperature. cp ;

[0117] 3. Draw H cp The curve is plotted against temperature, and the resulting curve is fitted to (calculated according to Henry's Law and van Hoff's equation) H. cp With the expected change in temperature.

[0118] Each of these steps will now be considered in more detail.

[0119] The solubility of a gas in molten salt at a given temperature and pressure can be measured by any suitable method. An example is described in W. Grimes, N.S.mith, and G.W. Watson in J. Phys. Chem. 62, 862 (1958), where the gas solubility in the salt can be measured by allowing the salt sample to be saturated with a pure stream of the test gas while maintaining the conditions at the desired temperature and pressure. The salt sample can then be separated and sprayed with different gases to remove the dissolved test gas. The level of the test gas in the outlet stream can then be measured, and the saturation concentration calculated. Note that Grimes et al. used the symbol K to denote the Henry's Law constant H. cp .

[0120] Henry's Law constant H cp It is the gradient of the solubility versus pressure curve at a given temperature, or the Henry's Law constant H can be obtained from a single measurement by dividing the solubility measurement by the pressure. cp (Although, as always, obtaining gradients from multiple measurements will improve accuracy).

[0121] Once H has been determined for a set of temperatures cp The curve of the Henry's constant H, which depends on temperature, will follow van Hoff's equation:

[0122]

[0123] Among them, H o The reference temperature T o The value of H is given by [reference temperature], where R is the gas constant of the gas. The reference temperature and H can be compared... o The corresponding reference value is selected as one of the measurement points, and then the parameter ΔH can be changed to obtain the best fit (e.g., the measurement value is the minimum of the total squared errors between the point and the curve). The ΔH value that produces the best fit can then be used to determine the temperature relationship of the specific gas / molten salt pair mentioned above.

[0124] Regardless of the units used, the above derivation remains the same (assuming the temperature is measured in a system such as Kelvin, which treats absolute zero as 0). Solubility is usually expressed as moles per unit volume (typically mol / cm³). 3 ), H o It is usually expressed as moles per cubic centimeter per atmosphere (mol / cm³) 3 / atm) or equivalent units. ΔH is measured in joules per kilogram (note the units of ΔH and R—ΔH is usually given in cal / mol, so it should be converted to the SI value of R).

[0125] The ΔH for a given gas / molten salt pair is independent of temperature and pressure (assuming the gas is gas and the molten salt is liquid). Therefore, the choice of temperature and pressure values ​​used to determine ΔH should not affect the final result. However, suitable pressure values ​​could be, for example, the reactor's maximum and minimum operating pressures and their midpoint, or 0.5 atm, 1 atm, and 1.5 atm. Suitable temperature values ​​could be, for example, the highest and lowest operating temperatures and their midpoint, or 100, 200, and 300 degrees above the melting point of the molten salt.

[0126] Figure 9 a and Figure 9 b shows a simulation using CFD. Figure 6 The effect of the type of displacement geometry shown is as follows: The fuel temperature 901 decreases in the region affected by this geometry, bringing it closer to the coolant temperature 902. The saturation temperature trajectory 903 remains below the fuel temperature 902 at all points in the tube. Figure 9 b better illustrates the bottom edge 904 and top edge 905 of the geometry, as well as the fuel surface 906.

Claims

1. A molten salt fission reactor, comprising: a reactor core, the reactor core comprising a plurality of fuel tubes, each fuel tube containing: a fuel salt, the fuel salt being a molten salt of one or more fissile isotopes; a gas interface of the fuel salt, the gas interface of the fuel salt being a surface of the fuel salt that is in contact with a gas space during operation of the reactor; a fuel salt cooling system, the fuel salt cooling system being configured to cool the fuel salt, the fuel salt cooling system comprising: a coolant tank, the coolant tank containing a coolant liquid, the fuel tubes being at least partially submerged in the coolant liquid, and a heat exchanger, the heat exchanger being for extracting heat from the coolant liquid; wherein: the fuel salt cooling system is configured to, during operation of the reactor, for all points within the fuel salt within each fuel tube except at the respective gas interface: wherein: T1 is a temperature of the fuel salt at the gas interface; T2 is a temperature of the fuel salt at the measurement point; P1 is an absolute pressure at the gas interface; P2 is an absolute pressure at the measurement point; R He is the gas constant for helium; ΔH He is the solubility enthalpy of helium in the fuel salt.

2. The molten salt fission reactor of claim 1, wherein, the fuel salt cooling system is configured to, during operation of the reactor, for all points within the fuel salt within each fuel tube except at the respective gas interface: wherein: R X is the gas constant for an inert gas; ΔH X is the enthalpy of dissolution of the inert gas in the fuel salt; the inert gas is one of neon, argon, krypton or xenon.

3. The molten salt fission reactor of claim 1, wherein, the temperature T1 of the fuel salt at each gas interface is less than the temperature T2 of the fuel salt in all other regions of the respective fuel tube.

4. The molten salt fission reactor of claim 1, wherein, the coolant liquid is pumped such that, during operation of the reactor, the coolant liquid flows downward when in contact with the fuel tubes.

5. The molten salt fission reactor of claim 1, wherein: each fuel tube comprises an upper section, the upper section containing a respective gas space; at least a portion of the upper section protrudes into a coolant gas space above the coolant liquid during operation of the reactor; the molten salt fission reactor further comprises a gas cooling system, the gas cooling system being configured to cool the coolant gas space.

6. The molten salt fission reactor of claim 1, wherein, each fuel tube comprises a baffle submerged in the fuel salt, the baffle extending across the fuel tube.

7. The molten salt fission reactor of claim 1, wherein, the coolant liquid is circulated through the fuel tubes only by natural convection, and the flow rate of the coolant liquid and the power density of the fuel salt are such that the temperature of the wall of the fuel tube at the bottom of the fuel tube is greater than the temperature of the coolant at the top of the critical region.

8. The molten salt fission reactor of claim 1, wherein, the upper section of each fuel tube is shielded with a neutron absorbing material such that, in use, the heat generation of the fuel salt within the upper section of each fuel tube is less than the heat generation of the remaining fuel salt within the fuel tube.

9. The molten salt fission reactor of claim 1, wherein, the cooling system is configured to direct a secondary flow of coolant salt to a top region of the fuel tube.

10. The molten salt fission reactor of claim 1, wherein, each fuel tube comprises a displacement element, the displacement element extending from the gas interface into the fuel salt, the displacement element being configured to displace fuel salt from a central axis of the fuel tube.

11. The molten salt fission reactor of claim 1, wherein, a region at the bottom of each fuel tube is more thermally insulated than other regions of each fuel tube.

12. The molten salt fission reactor of claim 1, wherein, The cooling system is configured such that a region at a bottom of each fuel tube is not directly cooled by the coolant liquid.

13. A method of operating a molten salt fission reactor, wherein, The molten salt fission reactor comprises: a reactor core comprising a plurality of fuel tubes, each fuel tube containing: a fuel salt, the fuel salt being a molten salt of one or more fissile isotopes; a gas interface of the fuel salt, the gas interface of the fuel salt being a surface of the fuel salt that is in contact with a gas space during operation of the reactor; a fuel salt cooling system configured to cool the fuel salt, the fuel salt cooling system comprising: a coolant tank containing a coolant liquid, the fuel tubes being at least partially immersed in the coolant liquid, and a heat exchanger for extracting heat from the coolant liquid; the method comprising maintaining a temperature of the fuel salt such that, during operation of the reactor, for the fuel salt within each fuel tube: wherein: T1 is a temperature of the fuel salt at the gas interface; T2 is a temperature of the fuel salt at a measurement point; P1 is an absolute pressure at the gas interface; P2 is an absolute pressure at the measurement point; R He is the gas constant for helium; ΔH He is the solubility enthalpy of helium in the fuel salt.

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