Molten salt coolant for nuclear reactors

Abstract

Regarding molten salts used in contact with graphite in nuclear reactors, there is a need for them to be easily maintained under strong reducing conditions without causing graphite degradation. [Solution] Use of a molten salt containing aluminum trifluoride and sodium fluoride as a primary coolant for a nuclear fission reactor, wherein the molten salt comes into contact with graphite and aluminum metals during the operation of the nuclear fission reactor.

Description

Technical Field

[0001] The present invention relates to a cooling system for a nuclear reactor. In particular, the present invention relates to a molten salt coolant used in a nuclear reactor.

Background Art

[0002] Nuclear reactors that use molten salts as fuels or reactor coolants have been known for a long time. Such salts have several important properties, which are outlined in depth by Williams et al. (ORNL / TM-2006 / 12), who recommend that, like many other authors, it is necessary to consider salt mixtures containing LiF, BeF2, NaF, ZrF4, RbF, and KF (see Tables A and B in the above reference).

[0003] Molten salt reactors that utilize graphite as a moderator are known to face issues with the interaction between the molten salt and graphite. They are particularly troubled by special problems when the redox state of the molten salt is maintained strongly enough on the reducing side so that chromium is not extracted from the steel. Under such conditions, many suitable molten salts react with graphite. For example, ZrF4 reacts readily with graphite to form stable carbides under reducing conditions as described by Scott (PCT / GB2016 / 053861), and like the widely proposed FLIBE salt and BeF2.

Summary of the Invention

Problems to be Solved by the Invention

[0004] Therefore, there is a need for molten salts used in contact with graphite in a nuclear reactor to be easily maintained under strong reducing conditions without causing degradation of the graphite.

Means for Solving the Problems

[0005] In a first aspect of the present invention, a use is provided for a molten salt comprising aluminum trifluoride and sodium fluoride as a primary coolant for a nuclear fission reactor, wherein the molten salt comes into contact with graphite and aluminum metals during the operation of the nuclear fission reactor.

[0006] A second aspect of the present invention provides a fission reactor comprising a core and a primary coolant system. The core comprises one or more containment units containing fissile fuel. The primary coolant system comprises a primary coolant configured to contact the containment units and remove heat from the core. The primary coolant is a molten salt comprising aluminum trifluoride and sodium fluoride. The fission reactor comprises aluminum metal and graphite, the graphite being arranged to contact the primary coolant during the operation of the reactor.

[0007] A third aspect of the present invention provides a fission reactor comprising a heat exchanger and a fuel salt system. The fuel salt system comprises a core and fuel salt, configured such that the fuel salt flows between the core and the heat exchanger. The fuel salt is a molten salt comprising aluminum trifluoride, sodium fluoride, and chlorides or fluorides of elements having fissile isotopes. [Brief explanation of the drawing]

[0008] [Figure 1] This is a phase diagram of a mixture of AlF3 and NaF. [Figure 2] This is a schematic diagram of an exemplary reactor. [Figure 3] This is a schematic diagram of another exemplary reactor. [Modes for carrying out the invention]

[0009] A reactor in which a molten salt coolant is in contact with graphite is cooled by a molten salt containing aluminum trifluoride (AlF3) and sodium fluoride (NaF). The coolant is further in contact with metallic aluminum in a bulk molten or dispersed state. The molten salt may be used as is if the coolant salt is transporting heat from a stationary nuclear fuel structure, or, if the coolant is also fissile fuel in the reactor, it may be mixed with a fluoride of an element having fissile isotopes, such as actinide fluoride.

[0010] Metallic aluminum may be added directly or formed in situ by the addition of a reducing agent such as sodium metal, by an electrolytic process, or by other known processes.

[0011] A eutectic mixture of AlF3 and NaF (approximately 55 mol% NaF and approximately 45 mol% AlF3) has a particularly desirable combination of properties, as shown in Figure 1. Its melting point is approximately 700°C, making it a practical coolant for use in temperature ranges above 750°C. Instead of increasing the operating temperature range, the proportion of AlF3 can also be changed, which is preferable because even a slight reduction in the proportion of AlF3 can lower the vapor pressure of the salt. Small amounts (e.g., up to 10 mol%) of other compounds may also be present in the molten salt. For example, the proportion of NaF may be 45 to 65 mol%, and the proportion of AlF3 may be 35 to 55 mol% (with a total upper limit of 100%, and if the AlF3 + NaF ratio is less than 100%, the remainder is formed from other salts). At these concentrations, the melting point of the AlF3 / NaF mixture is maintained below 900°C, eliminating the possibility of sodium vapor construction due to the formation of metallic sodium in a strongly reducing mixture above the boiling point of sodium. The vapor pressure is controllable at 900°C (0.05 mbar), but above that temperature it rises rapidly, potentially requiring exhaust or other gas control measures.

[0012] The eutectic AlF3 / NaF salt has low viscosity, decreasing from 1.2 cP at 750°C to 1 cP at 830°C, making it close to an ideal coolant with flow properties similar to liquid water.

[0013] The neutron absorption rates are particularly low for Na, F, and Al, as they have thermal neutron cross-sections of 531 mb, 9.6 mb, and 230 mb, respectively.

[0014] A particularly important property of these salt systems is that they can dissolve more than 1% of aluminum oxide without significantly increasing viscosity. In relation to the reactor coolant, if the fluorine produced is neutralized, the salt can absorb a considerable amount of oxygen without significantly altering its properties. Therefore, the presence of a substantial amount of aluminum metal in the system allows the molten salt to remain non-corrosive to the metal even if there is substantial leakage of water or oxygen into the system. Thus, an active process that maintains a low redox state can be replaced by a simple passive presence of aluminum metal that effectively neutralizes the produced fluorine. Since aluminum melts at a lower temperature than the AlF3 / NaF eutectic, aluminum is usually in a liquid state. This is desirable because it prevents the formation of a passivation layer of aluminum oxide on the surface of the metal.

[0015] However, contrary to suggestions in the literature, in this use of aluminum metal in a system, it has been observed that NaF cannot be interchanged with other alkali metal salts when graphite is also present. When substituted with lithium, potassium, or rubidium fluorides, alkali metals in metallic form are produced, which intercalate into graphite, rapidly degrading its structure. While we do not wish to be bound by theory, the specific resistance of graphite to NaF salts in this scenario is thought to be a result of a thermodynamic barrier to sodium intercalation in graphite preventing the formation of a damaging concentration of sodium in the molten salt. On the other hand, a positive thermodynamic driving force for the intercalation of other alkali metals would shift the equilibrium between aluminum and those metals toward the formation of metals, resulting in a high level of intercalation.

[0016] LiF, KF, or RbF may be acceptable in salts if they are not the main component (i.e., they are present in amounts less than NaF). At such low concentrations, the ratio of these contaminating salts to the graphite surface is such that they would be depleted before significant damage to the graphite occurs.

[0017] A second factor contributes to the stability of graphite in reducing molten salts: carbide formation. In the case of zirconium fluoride, the reaction between the reducing zirconium species and graphite causes the graphite structure to collapse. Aluminum fluoride forms carbides upon contact with aluminum metal, but experience in aluminum smelters has shown that these carbides form a stable surface coating that does not degrade the graphite.

[0018] Therefore, the combination of AlF3 and NaF is particularly suitable for applications in graphite, including nuclear reactors, because both elements Al and Na interact well with graphite. On the other hand, substituting either one with a similar fluoride causes damage to the graphite.

[0019] Figure 2 is a schematic diagram of an exemplary reactor. The reactor has a core 201, which includes one or more containment units 202 containing fissile fuel. The containment units may be fuel rods containing solid fissile fuel or molten salt fissile fuel, or any other suitable fissile fuel containment vessel used in a reactor where the coolant is separated from the fissile fuel. The reactor further has a cooling system for cooling the fissile fuel, the cooling system having a primary coolant, which is a molten salt consisting of AlF3 and NaF as described above. In this example, the cooling system has a heat exchanger 203, and the arrows indicate the flow of coolant between the heat exchanger and the core. The reactor further has a graphite member in contact with the primary coolant—a control rod 204 in this example—and a system 205 that brings the primary coolant into contact with aluminum metal. This system may be merely a region of the coolant channel where the aluminum metal is present, and / or may include a unit for adding a reducing agent such as sodium, and / or an electrolytic system.

[0020] Figure 3 is a schematic diagram of another exemplary reactor. The reactor comprises a core 301 and a cooling system for cooling the reactor. The primary coolant of the cooling system is the reactor's fuel salt, a molten salt containing AlF3, NaF, and fluorides of elements having fissile isotopes. The cooling system comprises a heat exchanger 303. The reactor further comprises a graphite member in contact with the primary coolant—in this example, a control rod 304—and a system 305 for bringing the molten salt into contact with the aluminum metal. This system may be merely a region of the coolant channel where the aluminum metal is present, and / or may include a unit for adding a reducing agent such as sodium, and / or an electrolytic system.

[0021] While AlF3 has been mentioned as a potential component of coolant salts in existing documents, these previous discussions have not addressed the aforementioned issues of preventing contamination of the graphite structure while ensuring adequate oxygen absorption.

[0022] Some properties of AlF3 are described in the aforementioned paper by Williams et al. (ORNL / TM-2006 / 12), but this is only a table of properties and does not suggest that they are salts suitable for nuclear use. Specifically, the NaF / AlF3 system is mentioned in Table 8 of this paper as having a melting point of 1000 °C with a composition of 75% NaF / 25% AlF3, but the need to mitigate the production of sodium gas is not discussed. Also, there is no disclosure regarding the contact between the coolant and graphite and aluminum.

[0023] Reliable reports on molten salts for nuclear reactors (Thoma, ORNL-2548) describe many potential salt systems, but do not include AlF3 salts.

[0024] Holcomb et al. (ORNL / TM-2010 / 156) state that the use of AlF3 is widespread in aluminum smelting (p3), but their paper (Table 9, p42) only records the thermal conductivity of the same 75 mol% NaF / 25 mol% AlF3 salt mentioned by Williams and is only mentioned once. There is no mention of its suitability as a nuclear reactor coolant or other special problems arising from its use (e.g., interaction with graphite).

[0025] AlF3-based salts have been proposed for use in the reprocessing of spent nuclear fuel, for example, in the fluoride volatility process for uranium recovery, proposed by Carr et al. (ORNL 4574) and Thoma (ORNL-3594). There is no mention in any of the papers regarding the use of AlF3 in nuclear reactor cooling.

[0026] The World Nuclear Society (https: / / www.world-nuclear.org / information-library / current-and-future-generation / molten-salt-reactors.aspx) mentions AlF3 salts as secondary coolants, stating, "In industrial applications, molten fluoride salts (perhaps simply cryolite-Na-Al fluoride) are preferred interfacial fluids in the secondary circuit between a nuclear heat source and any chemical plant. The aluminum smelting industry provides ample experience in safely managing them." However, there is no mention of these as reactor coolants.

[0027] Laurenty (LM-LS Experiment: Study on Corrosion Control of Liquid Fluoride Salts by Liquid Alkali Metals, Report UCBTH-06-002, 2006) mentions Al and AlF3 as part of a corrosion control system for molten salt coolants in improved high-temperature reactors, but rejects them as inappropriate (p30), clearly discouraging their use.

[0028] Benson et al. (WO2019 / 231971, WO2020 / 123509, 2020 / 123513) list AlF3 as one of many salts that can be incorporated into molten salt reactors, but do not recognize any particular advantages to using this salt. Benson has not considered using aluminum metal to maintain the reduced state of the salt, and therefore has not considered the above-mentioned issues regarding the reaction between the salt and graphite.

Claims

1. The use of a molten salt containing aluminum trifluoride and sodium fluoride as a primary coolant for a nuclear fission reactor, The molten salt comes into contact with graphite and aluminum metal during the operation of the nuclear fission reactor. The molten salt used contains 45 to 65 mol% sodium fluoride and 35 to 55 mol% aluminum trifluoride.

2. The use according to claim 1, wherein the molten salt further contains fissile isotopes, and the use of the molten salt is used as both the primary coolant and fuel salt of the fission reactor.

3. The use according to claim 1, wherein the molten salt is located at the eutectic point of the aluminum trifluoride and the sodium fluoride.

4. The aluminum metal is formed in situ, as described in claim 1.

5. The aforementioned aluminum metal is Addition of a reducing agent to the molten salt, Addition of metallic sodium to the molten salt, Electrolysis of the molten salt The use according to claim 4, formed by any one of the following.

6. A nuclear fission reactor, A reactor core having one or more containment units containing fissile fuel, A primary coolant system having a primary coolant, wherein the primary coolant is configured to come into contact with the containment unit and remove heat from the reactor core, It has, The primary coolant is a molten salt containing aluminum trifluoride and sodium fluoride. The fission reactor has graphite, which is arranged to be in contact with the primary coolant during the operation of the reactor. The fission reactor is a fission reactor having a system that provides contact between an aluminum metal and the molten salt during the operation of the reactor.

7. The fission reactor according to claim 6, wherein the molten salt is at the confocal of the aluminum trifluoride and the sodium fluoride.

8. The system that provides contact between the aluminum metal and the molten salt is Aluminum metal member in contact with the molten salt, A reducing agent supply unit configured to supply a reducing agent or sodium metal to the molten salt, and An electrolytic unit configured to electrolyze the molten salt, A nuclear fission reactor according to claim 6, comprising one or more of the following:

9. A nuclear fission reactor, Heat exchanger, A fuel salt system having a reactor core and fuel salt, wherein the fuel salt is configured to flow between the reactor core and the heat exchanger, It has, The fuel salt is a molten salt containing aluminum trifluoride, sodium fluoride, and chlorides or fluorides of elements having fissile isotopes. The molten salt comprises 45 to 65 mol% sodium fluoride and 35 to 55 mol% aluminum trifluoride. The fission reactor has graphite, and the graphite is arranged so that it is in contact with the molten salt during the operation of the fission reactor. The fission reactor has a system that provides contact between the aluminum metal and the molten salt during the operation of the fission reactor.

10. The nuclear fission reactor according to claim 9, wherein the molten salt is located at the eutectic point of the aluminum trifluoride and the sodium fluoride.

11. The system that provides contact between the aluminum metal and the molten salt is Aluminum metal member in contact with the molten salt, A reducing agent supply unit configured to supply a reducing agent or sodium metal to the molten salt, An electrolytic unit configured to electrolyze the molten salt, A nuclear fission reactor according to claim 9, comprising one or more of the following:

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