Control of rare gas bubble formation in molten salt fission reactors.

Abstract

A molten salt nuclear fission reactor includes a core having a plurality of fuel tubes. Each fuel tube includes 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 contacts the gas space during reactor operation. The reactor also includes a fuel salt cooling system configured to cool 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 cools the fuel salt in each fuel tube at all points except the respective gas interfaces during reactor operation. [Equation 9] It is configured to be TIFF2023526083000010.tif33117.

Description

[Technical field]

[0001] The present invention relates to nuclear fission reactors, and more particularly to reactor designs and operating methods for controlling the formation of gas bubbles in the fuel salt of a molten salt nuclear fission reactor. [Background technology]

[0002] Nuclear reactors using molten salt as fuel (MSR) have been known since the 1960s, when the American molten salt reactor experiments were successful. Many designs for molten salt reactors have been considered since then. Such reactors fall into two classes:

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

[0004] In Class 2 static molten salt reactors, the molten salt fuel is contained within fuel tubes, often formed into fuel tube assemblies, and moves within the tubes only by natural convection; the fuel remains within the core and is not pumped out of the core. A second fluid (e.g. coolant salt) passes through the fuel tubes and removes heat from the core. The fuel salt remains in the core throughout its operational life and is refreshed by removing spent fuel assemblies and replacing them with new ones - essentially the same as for fuel assemblies containing solid fuel elements. This second class of MSR is described in GB2508537 and equivalents.

[0005] Molten-salt reactors inevitably produce noble gases as fission products, and those gases have low solubility in the molten-salt fuel. One of these gases is xenon-135, the strongest known neutron absorber, and therefore has a significant impact on the reactivity of the reactor core, so it is important for the safety of the reactor that the migration of noble gases out of the active core is highly predictable.

[0006] In Class 1 pumped molten salt reactors, these gases can be removed in a variety of ways, including sparging the fuel salts with helium.

[0007] Managing noble gases in Class 2 molten salt reactors is a greater challenge. Gases can diffuse from the molten salt fuel into the gas space above the fuel, which is essentially outside the core, so the effect on reactivity is small. However, they can also form gas bubbles on the inner surface of the fuel tubes. Such bubbles can be induced to detach from the tubes by physical impact or other effects, causing many bubbles to simultaneously rise up the fuel salt into the gas space, greatly increasing core reactivity.

[0008] This is an unacceptable safety vulnerability and therefore it is necessary to prevent the formation of gas bubbles in the fuel lines. Summary of the Invention

[0009] According to a first aspect of the invention, there is provided a molten salt fission reactor. The reactor comprises a core having a plurality of fuel tubes. Each fuel tube comprises 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 in contact with a gas space during operation of the reactor. The reactor also 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 for extracting heat from the coolant liquid. The fuel salt cooling system is configured to cool the fuel salt in each fuel tube at all points except for said respective gas interfaces during operation of the reactor.

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[0010] Therefore, the solubility of helium is lowest at the gas interface.

[0011] 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 near the gas interface is lower than its solubility elsewhere in the fuel tube, thereby ensuring that the noble gas does not reach saturation concentrations outside of this region near the gas interface, and therefore gas bubbles do not form.

[0012] The solubility of a noble gas in a fuel salt is a function of: Fuel salt properties Gas properties (xenon is particularly important due to its high neutron absorption rate) The solubility of noble gases in molten salt increases as the temperature of the salt increases. - Pressure of molten salts whose solubility is roughly proportional to pressure The latter two factors are the easiest to control.

[0013] 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, gas bubbles will not form and the gas will exit the fuel salt by diffusing across the gas interface. This low temperature can be achieved by one or more of the following mechanisms: - Suppression of fission in the top region of the fuel tubes using neutron absorbers or reflectors Ensuring that the coolant temperature is low enough in the top region of the fuel tube to maintain a low temperature within the fuel salt Deflecting the natural convection flow of fuel salt in the fuel tube prevents hot salt generated in the high power region of the fuel tube from rising to the salt region close to the gas interface

[0014] However, pressure effects are also very important in such systems: the fuel salt is a dense liquid, and the hydrostatic pressure of the liquid column means that at a constant temperature the solubility of the noble gases increases at lower levels within the fuel salt column.

[0015] Thus, it is possible to achieve the goal of having minimum gas solubility at the upper salt / gas interface even if the fuel salt temperature is somewhat lower further down the salt column.

[0016] The balance of temperature and pressure effects determines the safe operating envelope of a nuclear reactor.

[0017] According to a second aspect of the present invention there is provided a method of operating a molten salt nuclear fission reactor according to the first aspect, comprising determining, during operation of the reactor, a temperature of the fuel salt at all points in the fuel salt within each fuel tube, except at the respective gas interfaces, by:

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[0018] Therefore, the solubility of helium in the fuel salt is lowest at the gas interface.

[0019] The same is true for rare gases other than helium. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 shows a static MSR with downward coolant flow. [Diagram 2] FIG. 2 shows a static MSR with upward coolant flow. [Diagram 3] FIG. 3 shows a static MSR with a slow upward flow of coolant salt. [Figure 4] FIG. 4 shows a graph of the temperature along a cross section at the bottom of the tube. [Figure 5A] FIG. 5A shows fuel salt top cladding temperature contours. [Figure 5B] FIG. 5B is a diagram showing coolant outlet temperature contours. [Figure 6] FIG. 6 illustrates a detailed view of a fuel tube of a static MSR with upward coolant flow using a displacement geometry to cool the fuel-gas interface. [Figure 7] Figure 7 is a graph showing the cross-sectional minimum fuel temperature along with the height and temperature locus where the saturation concentration is equal to the fuel surface. The loci are shown for several surface pressures. [Figure 8] 1 is a graph showing the cross-sectional minimum fuel temperature along with the locus of height and temperature where the saturation concentration is equal at the fuel surface. The locus is shown for various salt properties. [Figure 9] Figure 9 is a graph showing the cross-sectional minimum fuel temperature along with the height and the locus of temperature where the saturation concentration is equal to the fuel surface. The location of the displacement geometry is marked to show the effect on the fuel temperature. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] In developing the safety case for static molten salt reactors as described in GB2508537, it was found that there are significant potential hazards associated with the behaviour of noble gases in molten salt fuelled reactors which are not adequately addressed in current designs.

[0022] The mechanism by which severe transients in reactor power can occur centers around the presence of gas bubbles in the reactor core, especially in the critical region (i.e., the region where the density of fissile isotopes is sufficient for self-sustaining nuclear reactions to occur during operation). If the concentration of gas dissolved in the fuel remains below its saturation concentration, no bubbles will form and the effect of dissolved gas on reactor physics is straightforward to calculate. However, if bubbles form, much more complicated phenomena can occur.

[0023] Xenon gas dispersed in a solution of fuel salt has a greater neutron absorption effect than the same amount of gas in bubbles. This is because xenon-135 is such a strong absorber that the xenon in the center of a bubble containing Xe-135 is substantially shielded from neutrons by the bubble itself. Thus, when bubbles form from a supersaturated solution of gas in fuel salt, neutron absorption is significantly reduced, which in turn leads to an undesirable and uncontrollable increase in the reactivity of the core, which can lead to a rapid and unmanageable increase in reactor power.

[0024] Similar concerns arise with other noble gases because gas bubbles can displace or displace fuel, which can lead to a large increase in reactivity if a significant amount of bubbles in the reactor are displaced at once and rise to the surface from within the critical region of the fuel tube.

[0025] Looking at previous data in light of these findings, an important clue that a problem may exist came from the operation of the molten salt reactor experiment. Report ORNL4396 (p17) reported a significant frequency of apparently random blips of around 10% of the reactor power. Despite much investigation, no clear explanation for these power fluctuations was ever reached. However, the behavior of gases in the fuel salt circuit was thought to be the cause, and an empirical solution was achieved that eliminated the blips by adjusting pump pressure and gas sparging. The molten salt reactor experiment was operated at very low power, less than 10 MW. Commercial reactors operate at radically higher powers. Because the magnitude of the xenon effect is proportional to power, this potentially means that fluctuations that may only be an annoyance at very low powers could become a significant hazard at higher powers. The molten salt reactor experiment had the ability to vary the helium sparging rate to control the xenon bubble problem. In a molten salt reactor such as that described in GB2508537, such a capability does not exist as gas loss from the fuel salt can only occur by passive diffusion into the gas space above the fuel salt.

[0026] The problem of supersaturation can be greatly exacerbated by the actual fission process in the fuel salt.

[0027] At a fuel output of 100kW / L, the number of fissions per second can be calculated as follows: Fission energy = 3.2e -11 Joule / Nuclear Fission Reactor power = 1100,000 J / sec / liter of fuel salt Fission rate = 100,000 / 3.2e -11 =3.12e +15 Fission / Fuel Salt Litre Each fission produces 2 fission fragments, which are high-energy nuclei traveling at a significant fraction of the speed of light. Thus, 6e per second per liter of salt. +15 Such energetic particles are produced, their typical path length in moderately dense matter being about 20 μm.

[0028] A bubble with a diameter of 10 μm has a volume of 4 / 3*3.14*1e compared to a sphere with a diameter of 20 μm. -12 =4e -12 The fission fragments produced within a litre have about a 25% chance of colliding, so the bubble is subjected to around 1000 collisions per second.

[0029] The energy of such fission fragments is several orders of magnitude higher than the surface energy of the bubbles, resulting in the temperature of the medium through which the particles pass reaching tens of thousands of degrees. Therefore, the gas content is likely to re-dissolve in the salt and destroy the bubbles. Thus, a very high level of gas supersaturation in the molten salt can be maintained as long as fission continues. Thus, a feedback loop between the rate of fission and the reactivity of the core is established, which can easily lead to instability.

[0030] Other potential phenomena include the deposition of gas bubbles on solid surfaces in the core. Such bubbles can accumulate for some time and be displaced from the solid surface by shock waves, vibrations, or disturbances in the fuel salt flow. In that case, a large amount of neutron absorbing material can leave the core in a short time, resulting in a sudden increase in the reactivity of the core and potentially a damaging power surge. Furthermore, the removal of void volume from the core replaced by fissile fuel salt (i.e., increasing the average fuel salt density in the critical region) can lead to another increase in reactivity.

[0031] While these phenomena are a concern in all molten salt reactors, they are of particular concern in the static molten salt reactor class, in which the fuel salt flows relatively slowly under natural convection and therefore has a greater chance of gas bubbles forming and accumulating on the inner surfaces of the fuel tubes than in reactors in which the salt is pumped at relatively high velocities.

[0032] It is evident that in molten salt reactors in which gas bubbles are permitted to form in critical regions of the core during operation, it is exceptionally difficult to prevent unanticipated and dangerous effects from occurring, and it is therefore highly desirable to maintain the concentration of dissolved gases in the molten salt in the core of such reactors below saturation concentrations, thereby preventing the formation of gas bubbles in the core regions.

[0033] The solubility of noble gases in molten salts increases with increasing temperature, the opposite of the behavior of gases in water (RNL-2931 Reactor Chemistry Division annual progress report, January 31, 1960). As a result, the gas dissolves in the salt where there is a gas / salt interface, where the salt is at its hottest. Thus, as the salt cools, the gas becomes supersaturated in the salt and tends to form bubbles.

[0034] Furthermore, as fission gases are continually produced in the fuel salt, the concentration of these gases in the fuel salt can increase until they are off-gassing at the same rate as they are being produced. In a well-mixed salt where the concentration is the same everywhere, off-gassing can occur where solubility is lowest, and if this is not at the surface, gas bubbles will then come out of solution below the fuel.

[0035] In the following discussion, we propose to tailor the design of the molten salt reactor so that the salt in contact with the gas phase has a lower gas solubility than any other point in the fuel, and thus the dissolved gas diffuses across the salt interface without forming bubbles.

[0036] Most molten salt reactor designs, including that described in GB2508537, utilize the increased buoyancy of the molten salt as it heats up to drive partial or complete circulation of the molten salt. This necessarily requires that the salt flow direction is upwards. Unfortunately, however, this also means that the temperature of the coolant salt will be highest at the top of the fuel tubes, making it difficult to avoid the fuel salt in that region being hotter and therefore having a higher solubility for the noble gases, especially at the bottom of the tubes where the coolant is at a lower temperature, than further down the core.

[0037] Therefore, one way to reduce the temperature of the fuel salt near the gas interface is to reverse the direction of the coolant salt flow vertically downwards, causing the temperature of the coolant salt to increase as it flows downward through the core. This has the disadvantage of opposing the forces of natural convection, but it ensures that the upper regions of the flow circuit are at a lower temperature.

[0038] Where it is not possible to reverse the coolant flow direction, for example in a static molten salt reactor where the coolant flow is by natural convection, it is still possible to ensure that the upper surface of the fuel salt is the point at which the gas solubility of the fuel salt is lowest. This can be achieved by one or a combination of the following methods: Reducing heat generation in the fuel salt near the top of the tube by shielding it with neutron absorbing material Introduce a secondary flow of cold coolant salt to cool the top region of the fuel tube and the fuel salt. 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, so that gas convection cools the upper surface of the fuel salt even though the coolant outside the tube is hotter at that surface level. This gas space cooling can be accomplished by a secondary flow of coolant salt, by having the portion of the fuel tube containing gas at the surface of the coolant salt appear in a lower temperature region, or by actively passing coolant gas through the gas space in the fuel tube. Baffles are placed a short distance below the fuel salt surface to prevent bulk convection from the hot region from reaching the surface from regions further down the tube, while allowing slow mixing of the molten salt and gas diffusion through the baffles to the surface. · Maintaining a relatively slow flow rate of coolant therefore ensures that the bottom tube wall, where the coolant temperature is lowest, will be intermediate in temperature between the bulk fuel salt and the coolant salt, which will be higher than the surface temperature of the fuel salt at the top of the tube. In this way, the gas space above the fuel in contact with the fuel tube above the fuel salt level will have the coolant at its lowest temperature, but may be cooler than the tube wall in contact with the hot fuel salt. Provide an insert to displace salt from the center of the tube to the surface and nearby areas, leaving only a thin periphery of salt in contact with the walls. This thin band of salt has the same cooling surface area as before, but the volume that produces heat is now much smaller. The remaining salt is therefore kept cool by the coolant. Adding insulation to the bottom of the fuel tubes to increase the temperature of the fuel inside Injecting coolant above the bottom of the fuel tube, leaving the lower section uncooled

[0039] 1 shows a static MSR with coolant flowing downwards. The reactor comprises vertical fuel tubes 101 containing fuel salt 102, optionally separated by a moderating structure (not shown) of graphite or other moderating material. Coolant 103 is pumped around a circuit with the core and heat exchanger 104, thus flowing downwards 105 through the fuel tubes, ensuring that the fuel salt at the top of the fuel tubes, i.e. adjacent the gas space 106, is at the lowest temperature within the fuel tubes due to a combination of the reduced power density due to the location of the fuel salt at the outer edge of the core and the lower temperature of the coolant in contact with the top of the fuel tubes.

[0040] Figure 2 shows a static MSR with coolant flowing upwards. The static molten salt reactor has a vertically oriented fuel tube 205, where the top gas-filled portion 207 of the fuel tube emerges from the circulating coolant salt 203 in a region cooler than the coolant salt. The coolant salt circulates through the core and heat exchanger 208 by natural convection 204. The gas space above the coolant salt is cooled by a cooling gas system with a cooling gas inlet 201 and a cooling gas outlet 202, which cools the upper portion of the fuel tube and then the gas inside the fuel tube. The convection cell is a gas-flowing cell in which the cooling gas descends the outer region of the interior of the fuel tube, is heated by contact with the top surface of the fuel salt, and then ascends the center of the fuel tube. This cools the top layer of the fuel salt, creating the required low-temperature gas / salt interface and maintaining the fuel salt below the saturated gas concentration in the bulk of the fuel salt.

[0041] FIG. 3 shows a static MSR with a slow upward flow of coolant salt. The reactor comprises vertical fuel tubes 301 containing fuel salt 302, which may optionally be separated by a moderating structure of graphite or other moderating material. The coolant salt 303 circulates upwards 305 through the core and downwards through a heat exchanger 304, at a relatively slow flow rate and only by natural convection. The coolant flow rate and 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 exiting the top of the core. This means that the gas space 306 is cooler than any of the fuel salt due to cooling by the coolant exiting the core. The temperature of the top surface of the fuel salt is lower than anywhere else in the fuel salt, due to the combined effect of the cooling of the top fuel salt layer by the hot coolant and the cooling of its surface by gas convection in the gas space above the coolant.

[0042] 4 is a graph of temperature along a cross section of the bottom of the tube. The graph shows the temperature in three regions of the coolant salt: the tube bottom 410, the bottom tube wall 420, and the fuel salt 430. As can be seen from the graph, the temperature of the coolant salt at the bottom of the tube is lower than the temperature of the tube wall, which is lower than the temperature of the fuel salt. The coolant temperature at the top of the fuel tube 401 is lower than the temperature of the bottom tube wall, which means that dissolved gases will preferentially vent into the gas space at the top of the tube rather than forming bubbles in the tube wall.

[0043] The relative temperatures of the fuel tube walls, the coolant salt at the bottom of the tube, and the coolant salt at the top of the tube depend on the dimensions of the fuel tube, the flow rate of the coolant salt, the temperature of the coolant salt entering the core, and the power density of the nuclear reaction. In particular, models known to be capable of testing the viability of solutions include the following: Average power density in fuel salt Diameter of each fuel tube Height of each fuel tube · The annular thickness of the coolant salt around the fuel tube (i.e. how much space there is for the coolant to flow around the tube) Coolant inlet temperature Coolant outlet temperature - Fuel tube cladding temperature at the bottom of the fuel tube

[0044] This model can be used in two ways, either by assuming that the tube produces heat uniformly along its length (up to the level of the top of the fuel salt) for a simpler model, or by considering vertical gradients in energy production in the fuel tube for a more accurate model. For example, the simpler model can be used to identify target parameter values ​​that can then be confirmed with a more accurate model or with experiments.

[0045] For a given power density, coolant salt inlet temperature, fuel tube length, fuel tube diameter, and annular 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 and therefore can be used to calculate the temperature of the cladding under the bottom of the fuel tube.

[0046] As a further example, by fixing all but two of the model's input variables (e.g., fixing the power density, inlet temperature, and fuel tube length), a graph of coolant exit temperature versus cladding temperature can be plotted as dependent on the other two variables (in this case fuel pin diameter and annular thickness). An example of this is shown in Figures 5A and 5B, where Figure 5A shows a contour of the cladding temperature at the top of the fuel salt, and Figure 5B shows a contour of the coolant exit temperature, with the addition of a bold 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 tube cladding temperature, and therefore the region of the graph that corresponds to the required relationship that the tube wall temperature at the bottom of the tube is higher than the coolant salt temperature at the top of the tube.

[0047] Other simulation, modeling or prototyping methods and techniques known in the art may alternatively be used to determine the required parameters.

[0048] It will be understood that this analysis is necessary only for the simulation. In the case of an actual furnace, the temperatures can simply be measured to determine if they are in the correct relationship.

[0049] Optionally, a neutron absorbing structure can be inserted anywhere inside or outside the fuel tube, from just below the surface of the fuel salt to partway into the gas space of the fuel tube, so that fission is suppressed in the top layer of the fuel salt.

[0050] FIG. 6 shows a detailed view of a static MSR fuel tube with an upward coolant flow 606 using a displacement geometry 602 to cool the fuel-gas interface 607. Heat generated in the fuel salt 604 can only be removed through the fuel tube walls 605. For most fuels, heat is generated volumetrically, so this leads to a large temperature increase in the center of the fuel tube. In the thin annular region 608, the volume of fuel generating heat decreases sharply, but the surface area of ​​the fuel tube walls remains the same as in a tube without the displacement geometry, resulting in a fuel-gas interface temperature that is much closer to the coolant temperature than the rest of the fuel.

[0051] The general equation for the solubility of a gas in a fluid is given by Henry's law: a =P*H cp where c a is the solubility in mol / cc, P is the partial pressure of the gas at the surface in the atmosphere (1 atm = 101325 Pa), H cp is the Henry solubility of the fluid. This is H cp H (T) =H o *exp[-ΔH / R*(1 / T-1 / T o )], where H(T) is the updated Henry's constant for temperature T, H o is the reference temperature T o where ΔH is the enthalpy of the solution, and R is the gas constant of the gas involved. Note that the variables ΔH and H have completely different quantities and should not be confused.

[0052] The surface of a bubble formed deep in the salt is the hydrostatic pressure of the salt at that depth. This means that the partial pressure of the gas inside the bubble is equal to the hydrostatic pressure (for a single gas). Therefore, when calculating the saturation concentration in the subsurface region, the hydrostatic pressure of the salt at that point is used rather than the surface pressure of the salt.

[0053] FIG. 7 shows the temperature of the fuel salt 701 along the height of a fuel pin cooled by an upward flow. o =873.15K,H o=1.94×10 -8 mol / cc / atm, ΔH = 353390 J / kg, and R = 63.33 J / kg / K (this is the solubility behavior of xenon gas measured in a 53-47 mol% NaF-ZrF4 mixture used as an example), the salt at 1 Bar pressure and temperature of 879.7 K has a melting point of 2.0079 × 10 -8 Saturates at a gas concentration of mol / cc.

[0054] The fuel depth is 1800mm and the average density is 3181.7kg / m 3 Therefore, the absolute pressure at the base of the pin is 1.561 Bar. The temperature of the salt at the base of the pin is 825.3 K, so the fuel there, despite being cooler, is 2.0633 × 10 -8 It saturates at high concentrations of mol / cc.

[0055] For the fuel at the bottom of the tube to be saturated with the same gas concentration as at the top, the temperature at the bottom must be

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[0056] Therefore, to ensure that the xenon does not come out of solution except at the gas interface, the furnace is constructed as follows.

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[0057] A temperature locus 702 is shown in Figure 7. This locus shows the temperature to which the fuel salt must drop at that height to match its solubility at the fuel surface with a surface pressure of 1 Bar. A second locus 703 is shown for different absolute pressures at the surface of 0.5 Bar.

[0058] The solubility limits described by loci 702 and 703 are relative to the fuel surface, so for a constant H o In a well-mixed system, where the fuel surface is saturated and cannot dissolve more gas, the trace shows the temperature that must drop in the remainder of the tube for the fuel to become saturated.

[0059] While the above example focuses on xenon as an exemplary gas of concern, it may be possible to design a furnace that can tolerate limited formation of xenon bubbles but cannot tolerate formation of lower atomic number noble gas bubbles while operating safely. For such a furnace, the above analysis can be performed but with the gas constant R substituted for the gas constant of helium, neon, argon or krypton, and the enthalpy of solution ΔH substituted for the enthalpy of the solution of that gas with a molten salt. The gas constant R (J / kgK) is higher for the lighter noble gases, as is ΔH (J / kg), but at a slower rate of increase. This means that a furnace that meets this criterion for one noble gas (e.g. argon) will also meet this criterion for all lower atomic number noble gases (e.g. helium, neon) as the value of R / ΔH increases and thus the minimum temperature T2 decreases.

[0060] FIG. 8 shows how the enthalpy ΔH of the solution affects the saturation temperature trajectory with traces 802-809, where 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).

[0061] For many combinations of noble gases and molten salts, ΔH can be obtained from the literature. If no such previous measurements are available, ΔH can be determined experimentally by the following process: 1. Measure the solubility of a gas in a molten salt at several different temperatures and several different pressures. 2. Henry's Law constant H for each temperature cp Find 3.H vs. Temperature cp The resulting plot is plotted as a function of temperature. cp Fitting the expected variation of (calculated from Henry's law and the Van't Hoff equation).

[0062] Each of these steps is considered in more detail below.

[0063] The solubility of a gas in a molten salt at a given temperature and pressure can be measured by any suitable means. One example is described in W. R. Grimes, N. V. Smith, and G. M. Watson, J. Phys. Chem. 62, 862 (1958), where the gas solubility of a salt can be measured by saturating a sample of the salt with a pure stream of the test gas while maintaining conditions at the temperature and pressure of the desired measurement point. The salt sample can then be isolated and sparged with another gas to remove the dissolved test gas. The level of the test gas in the exit stream can then be measured and the saturation concentration calculated. Grimes et al. describe the Henry's constant, H cp The symbol K is used for this.

[0064] Henry's constant H cp is the slope of the graph of solubility versus pressure at a given temperature, or it can be obtained from a single measurement by dividing the solubility measurement by the pressure (though, as always, taking the slope from multiple measurements improves accuracy).

[0065] Once for a series of temperatures cpOnce found, the plot of the Henry's constant H as a function of temperature can be calculated using the Van't Hoff equation.

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[0066] The above derivation is the same regardless of the units used (unless temperature is measured in a system such as Kelvin which treats absolute zero as 0). Solubility is usually expressed in moles per unit volume (often expressed as mol / cm 3 ) and H o is usually expressed in moles per cubic centimeter per atmosphere (mol / cm 3 / atm) or equivalent. ΔH is measured in joules / kilogram. (Note the units of ΔH and R; ΔH is often given in cal / mol and should be converted for use with the SI value of R.)

[0067] ΔH for a given gas / molten salt pair may be independent of temperature and pressure (if the gas is a gas and the molten salt is a liquid). Thus, the choice of temperature and pressure values ​​used to determine ΔH should not affect the final result. However, suitable values ​​of pressure would be, for example, the maximum and minimum operating pressures of the furnace and their midpoints, or 0.5 atm, 1 atm, 1.5 atm. Suitable values ​​of temperature would be, for example, the maximum and minimum operating temperatures and their midpoints, or 100°, 200°, and 300° above the melting point of the molten salt.

[0068] Figures 9a and 9b show the effect of a displacement geometry of the type shown in Figure 6, simulated with CFD. The fuel temperature 901 drops in the area affected by the geometry and approaches the coolant temperature 902. The saturation temperature locus 903 remains below the fuel temperature 901 at all points within the tube. Figure 9b better shows the bottom 904 and top 905 edges of the geometry, and the fuel surface 906.

Claims

1. 1. A molten salt nuclear fission reactor having a core having a plurality of fuel tubes, Each fuel tube: a fuel salt which is a molten salt of one or more fissile isotopes; a gas interface of the fuel salt, the surface of the fuel salt which 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 having a coolant tank containing a coolant liquid in which the fuel tubes are at least partially immersed and a heat exchanger for exchanging heat from the coolant liquid; Equipped with The fuel salt cooling system, during operation of the reactor, cools the fuel salt in each fuel tube at all points except the gas interface thereof, [006] where: T １ is the temperature of the fuel salt at the gas interface, T ２ is the temperature of the fuel salt at the measurement point, P １ is the absolute pressure at the gas interface, P ２ is the absolute pressure at the measurement point, R Ｈｅ is the gas constant for helium, ΔH Ｈｅ is the enthalpy of solution of helium in the fuel salt; Molten Salt Fission Reactor.

2. The fuel salt cooling system, during operation of the reactor, cools the fuel salt in each fuel tube at all points except the gas interface thereof, [0070] where: R Ｘ is the gas constant for the noble gas, ΔH Ｘ is the enthalpy of solution of the noble gas in the fuel salt; The noble gas is either neon, argon, krypton or xenon; 2. The molten salt nuclear fission reactor of claim 1.

3. The temperature T of the fuel salt at each gas interface １ is the temperature T of the fuel salt in all other regions of each of the fuel tubes. ２ Lower than 2. The molten salt nuclear fission reactor of claim 1.

4. the coolant liquid is pumped downwardly when it contacts the fuel tubes during operation of the reactor; 2. The molten salt nuclear fission reactor of claim 1.

5. each fuel tube having an upper section containing a respective gas space; at least a portion of the upper section projects into a cooling gas space located above the coolant liquid during operation of the furnace; the molten salt nuclear fission reactor further comprises a gas cooling system configured to cool the cooling gas space; 2. The molten salt nuclear fission reactor of claim 1.

6. each fuel tube includes a baffle extending across the fuel tube and immersed in the fuel salt; 2. The molten salt nuclear fission reactor of claim 1.

7. the coolant liquid circulates through the fuel tubes by natural convection only; 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 liquid at the top of a critical region; 2. The molten salt nuclear fission reactor of claim 1.

8. an upper section of each fuel tube is shielded with neutron absorbing material such that, in use, heat generation of the fuel salt in the upper section of each fuel tube is less than in the remainder of the fuel salt in the fuel tube; 2. The molten salt nuclear fission reactor of claim 1.

9. the cooling system is configured to direct a secondary flow of coolant salt to a top region of the fuel tube.

2. The molten salt nuclear fission reactor of claim 1.

10. each fuel tube includes a displacement element extending from the gas interface into the fuel salt; the displacement element is configured to displace the fuel salt from a central axis of the fuel tube.

2. The molten salt nuclear fission reactor of claim 1.

11. an area at the bottom of each fuel tube being more thermally isolated than other areas of each fuel tube; 2. The molten salt nuclear fission reactor of claim 1.

12. the cooling system is configured such that a bottom area of ​​each fuel tube is not directly cooled by the coolant liquid; 2. The molten salt nuclear fission reactor of claim 1.

13. 1. A method for operating a molten salt nuclear fission reactor, comprising: the molten salt nuclear fission reactor comprises a core having a plurality of fuel tubes; Each fuel tube: a fuel salt which is a molten salt of one or more fissile isotopes; a gas interface of the fuel salt, which is the surface of the fuel salt 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 having a coolant tank containing a coolant liquid in which the fuel tubes are at least partially immersed and a heat exchanger for exchanging heat from the coolant liquid; Equipped with The method includes, during operation of the reactor, determining, for all points of the fuel salt in each fuel tube except for the respective gas interfaces: [0080] maintaining a temperature of the fuel salt such that: T １ is the temperature of the fuel salt at the gas interface, T ２ is the temperature of the fuel salt at the measurement point, P １ is the absolute pressure at the gas interface, P ２ is the absolute pressure at the measurement point, R Ｈｅ is the gas constant for helium, ΔH Ｈｅ is the enthalpy of solution of helium in the fuel salt; method.

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