GB2602339A - Molten salt coolant for nuclear reactor - Google Patents

Molten salt coolant for nuclear reactor Download PDF

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Publication number
GB2602339A
GB2602339A GB2020536.5A GB202020536A GB2602339A GB 2602339 A GB2602339 A GB 2602339A GB 202020536 A GB202020536 A GB 202020536A GB 2602339 A GB2602339 A GB 2602339A
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Prior art keywords
molten salt
reactor
aluminium
salt
graphite
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GB2020536.5A
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GB202020536D0 (en
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Richard Scott Ian
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Individual
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Priority to GB2020536.5A priority Critical patent/GB2602339A/en
Publication of GB202020536D0 publication Critical patent/GB202020536D0/en
Priority to EP21770241.4A priority patent/EP4211703A1/en
Priority to US18/044,483 priority patent/US20230343474A1/en
Priority to CN202180061733.4A priority patent/CN116250045A/en
Priority to PCT/EP2021/074301 priority patent/WO2022053396A1/en
Priority to KR1020237012081A priority patent/KR20230062648A/en
Priority to JP2023515586A priority patent/JP2023539930A/en
Priority to CA3192159A priority patent/CA3192159A1/en
Publication of GB2602339A publication Critical patent/GB2602339A/en
Pending legal-status Critical Current

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    • CCHEMISTRY; METALLURGY
    • C09DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09KMATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K5/00Heat-transfer, heat-exchange or heat-storage materials, e.g. refrigerants; Materials for the production of heat or cold by chemical reactions other than by combustion
    • C09K5/08Materials not undergoing a change of physical state when used
    • C09K5/10Liquid materials
    • C09K5/12Molten materials, i.e. materials solid at room temperature, e.g. metals or salts
    • GPHYSICS
    • G21NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21CNUCLEAR REACTORS
    • G21C15/00Cooling arrangements within the pressure vessel containing the core; Selection of specific coolants
    • G21C15/28Selection of specific coolants ; Additions to the reactor coolants, e.g. against moderator corrosion
    • GPHYSICS
    • G21NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21CNUCLEAR REACTORS
    • G21C3/00Reactor fuel elements and their assemblies; Selection of substances for use as reactor fuel elements
    • G21C3/42Selection of substances for use as reactor fuel
    • G21C3/44Fluid or fluent reactor fuel
    • G21C3/54Fused salt, oxide or hydroxide compositions
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E30/00Energy generation of nuclear origin
    • Y02E30/30Nuclear fission reactors

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Plasma & Fusion (AREA)
  • General Engineering & Computer Science (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Combustion & Propulsion (AREA)
  • Thermal Sciences (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Electrolytic Production Of Metals (AREA)

Abstract

The present invention relates to a nuclear fission reactor cooled with a molten salt comprising aluminium fluoride (AlF3) and sodium fluoride (NaF), wherein during operation of the reactor the molten salt is in contact with graphite, e.g. graphite control rods 204, and metallic aluminium in a bulk molten or dispersed state. The molten salt may be used to carry heat from immobile nuclear fuel structures 202 to a heat exchanger 203 or, alternatively, may be mixed with chlorides or fluorides of an element having fissile isotopes, e.g. actinide fluorides, so that it also acts as a fuel salt (see Fig. 3). A system 205 for contacting the molten salt with aluminium metal may simply comprise an aluminium component, e.g. a region of the coolant channel where aluminium metal is present. Alternatively, it may comprise a unit for forming the metallic aluminium in situ by an electrolytic process or by the addition of a reducing agent such as sodium metal.

Description

Molten Salt Coolant for Nuclear Reactor
Field of the Invention
The present invention relates coolant systems for nuclear reactors. In particular, the invention relates to molten salt coolants for use in nuclear reactors.
Background
Nuclear reactors using molten salts as fuel or reactor coolant have been known for many years. There are several vital properties for such salts which were reviewed in depth by Williams et al (ORNUTM-2006/12) who, like many other authors recommended that salt mixtures containing [IF, BeF2, NaF, ZrF4, RbF and KF should be considered (see tables A and B in the above reference).
Molten salt reactors utilising graphite as moderator are known to suffer problems of interaction of the molten salt with graphite. They suffer a particular problem when the redox state of the molten salt is maintained sufficiently strongly reducing so as not to extract chromium from steels. Many suitable molten salts react with graphite under such conditions. ZrF4 for example, which is particularly easy to maintain in a reducing condition as described by Scott (PCT/GB2016/053861) reacts readily with graphite forming stable carbides, as does the widely proposed FLI BE salt with BeF2.
There is a need therefore for a molten salt for use in contact with graphite in nuclear reactors which can be maintained readily in a strongly reducing condition without causing degradation of the graphite.
Summary
According to a first aspect of the present invention, there is provided the use of a molten salt comprising aluminium trifluoride and sodium fluoride as a primary coolant for a fission reactor, wherein the molten salt is in contact with graphite and with aluminium metal during operation of the fission reactor.
According to a second aspect of the invention, there is provided a nuclear fission reactor. The nuclear fission reactor comprises a reactor core and a primary coolant system. The reactor core comprises one or more containment units containing fissile fuel. The primary coolant system comprises a primary coolant and is configured such that the primary coolant is in contact with the containment units and removes heat from the reactor core. The primary coolant is a molten salt comprising aluminium trifluoride and sodium fluoride. The nuclear fission reactor comprises aluminium metal and graphite arranged such that the aluminium metal and the graphite are in contact with the primary coolant during operation of the reactor.
According to a third aspect of the invention, there is provided a nuclear fission reactor. The nuclear fission reactor comprises a heat exchanger and a fuel salt system. The fuel salt system comprises a reactor core and a fuel salt, and is configured such that the fuel salt flows between the reactor core and the heat exchanger. The fuel salt is a molten salt comprising aluminium trifluoride, sodium fluoride, and a chloride or fluoride of an element having a fissile isotope.
Brief Description of the Drawinps
Figure 1 is a phase diagram of an AlF3 and NaF mixture.
Detailed Description
A nuclear reactor in which a molten salt coolant is in contact with graphite is cooled with a molten salt comprising aluminium trifluoride (AIF3) and sodium fluoride (NaF) with the coolant further in contact with metallic aluminium in the bulk molten or dispersed states. The molten salt may be used as is, where the coolant salt carries heat from immobile nuclear fuel structures, or may be mixed with fluorides of elements having fissile isotopes, e.g. actinide fluorides, where the coolant is also the fissile fuel of the reactor.
The metallic aluminium may be added as such or formed in situ by addition of a reducing agents such as sodium metal, by electrolytic processes, or by other known processes.
The eutectic mixture of AIF3 and NaF (approx. 55% moN/0 NaF and 45mo1% AIF3) has a particular desirable combination of properties as shown in figure 1. Its melting point is approximately 700°C making it a practical coolant for operating temperature ranges above 750°C. Variations in the proportion of AlF3 are possible, at the expense of raising the operating temperature range and can be desirable as reducing the AIF3 proportion even marginally can lower the vapour pressure of the salt. Small amounts (e.g. up to 10 mol%) of other compounds may also be present within the molten salt. For example, the proportion of NaF may be 45 to 65 mol°/0, and the proportion of AlF3 may be 35 to 55 moN/0 (capped at a total of 100%, and with the remainder being formed from other salts if the proportion of AlF3+NaF is less than 100%). These concentrations will keep the melting point of the AlF3/NaF mixture below 900°C, eliminating potential build-up of sodium vapour due to the production of metallic sodium in a strongly reducing mixture above the boiling point of sodium. The vapour pressure is manageable (0.05mbar) at 900°C, but rapidly increases above that temperature and would require venting or other gas control measures.
The eutectic AlF3/NaF salt has a low viscosity falling from 1.2cP at 750°C to 1cP at 830°C making it a close to ideal coolant with flow properties similar to liquid water.
Its neutron absorbance is particularly low as Na, F and Al have respectively thermal neutron cross sections of 531mb, 9.6mb and 230mb.
A particularly important property of these salt systems is that they can dissolve over 1% of aluminium oxide without increasing their viscosity significantly. In the context of nuclear reactor coolants it allows the salt to absorb substantial amounts of oxygen without major changes in its properties provided that the resulting fluorine produced is neutralised. The presence of substantial quantities of aluminium metal in the system therefore allows the molten salt to remain non corrosive to metals despite substantial leakage of water or oxygen into the system. Active processes to maintain the low redox state can thus be replaced with simple passive presence of aluminium metal, which will effectively neutralise the fluorine produced. Since aluminium melts at a lower temperature than the AlF3/NaF eutectic, the aluminium will normally be in the liquid state which is desirable as it prevents formation of a passivafing layer of aluminium oxide on the surface of the metal.
However, it has been found that this use of aluminium metal within the system means that, in contrast to suggestions in the literature, NaF is not interchangeable with other alkali metal salts when graphite is also present. Replacement with lithium, potassium or rubidium fluoride results in the production of the alkali metal in metallic form, which intercalates into the graphite, rapidly damaging its structure. While not wishing to be bound by theory, we believe that the particular resistance of graphite to the NaF salt in this scenario is a result of the thermodynamic barrier to sodium intercalation in graphite preventing formation of damaging concentrations of sodium in the molten salt. In contrast, the positive thermodynamic driving force for intercalation of the other alkali metals results in the equilibrium between aluminium and those metals shifting towards production of the metals with the result that high levels of intercalation take place.
LiF, KF or RbF can be tolerated in the salt provided that they are not the primary component (i.e. they are present in lower amounts than NaF). At such low concentrations, the ratio of those contaminating salts to graphite surface is such that they are depleted before significant graphite damage occurs.
There is a second factor in stability of graphite in reducing molten salts. This is formation of carbides. In the case of zirconium fluoride the reaction of the reducing zirconium species with graphite destroys the graphite structure. Aluminium fluoride, in contact with aluminium metal, does form a carbide, however experience in aluminium smelters has shown that this carbide forms a stable surface coating which does not degrade the graphite.
The combination of AIF3 and NaF is thus uniquely suited to the application in graphite containing nuclear reactors as both the Al and Na elements interact benignly with the graphite while replacement of either with comparable fluorides results in graphite damage.
Figure 2 is a schematic diagram of an exemplary reactor. The reactor comprises a core 201 containing 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 may be any other suitable fissile fuel containment for use in a reactor where the coolant is separate from the fissile fuel. The reactor further comprises a cooling system for cooling the fissile fuel, the cooling system comprising a primary coolant which is a molten salt composed of AlF3 and NaF as described above. In this example, the coolant system comprises a heat exchanger 203, and the arrows show coolant flow between the heat exchanger and the core. The reactor further comprises graphite components in contact with the primary coolant -in this example the control rods 204 -and a system 205 for contacting the primary coolant with aluminium metal. This system may be simply a region of the coolant channel where aluminium metal is present, and/or may include a unit for the addition of a reducing agent such as sodium, and/or an electrolysis system.
Figure 3 is a schematic diagram of an alternative exemplary reactor. The reactor comprises a core 301, and a cooling system for cooling the reactor. The primary coolant of the cooling system is also the fuel salt for the reactor, and is a molten salt comprising AlF3, NaF, and a fluoride of an element having a fissile isotope. The cooling system comprises a heat exchanger 303. The reactor further comprises graphite components in contact with the primary coolant -in this example the control rods 304-and a system 305 for contacting the molten salt with aluminium metal. This system may be simply a region of the coolant channel where aluminium metal is present, and/or may include a unit for the addition of a reducing agent such as sodium, and/or an electrolysis system.
While AlF3 has been mentioned in passing in existing documents as a potential constituent of a coolant salt, these previous discussions have not addressed the issues described above of ensuring adequate oxygen absorption while also preventing contamination of graphite structures.
Some properties of AlF3 are mentioned in the Williams et al (ORNL/TM-2006/12) paper mentioned earlier, but only in tables of properties and with no suggestion that they are appropriate salts for nuclear use. Specifically, the system NaF/AIF3 is mentioned in table 8 of this paper as having a composition of 75% NaF/25% AlF3 with a melting point of 1000°C, without any discussion of the need to mitigate production of sodium gas.
Additionally, there is no disclosure of contact between the coolant and graphite and aluminium..
The definitive report regarding molten salts for nuclear reactors (Thoma, ORNL-2548) lists a large number of potential salt systems but includes no AIF3 salts.
Holcomb et al (ORNLJTM-2010/156) while mentioning the use of AlF3 in aluminium smelting (p3) where it is extensively use, mention it only once in their paper (table 9, p42) only to record the thermal conductivity of the same 75 moN/0 NaF/25mo1°/0 AIF3 salt mentioned in Williams, with no mention of its suitability as a nuclear reactor coolant, or the other particular issues which arise from that use (e.g. the interaction with graphite).
AlF3 based salts have been proposed for use in reprocessing of spent nuclear fuel, for example by Carr et al (ORNL 4574) and Thoma (ORNL-3594) in the fluoride volatility process for recovery of uranium. No mention in either paper was made of AlF3 use in nuclear reactor cooling.
The World Nuclear Association (https://www. world-nuclearorg/information-librarv/current-and-future-qeneration/molten- salt-reactors.aspx) mention AlF3 salt as secondary coolants thus "In industrial applications molten fluoride salts (possibly simply ctyolite -Na-Al fluoride) are a preferred interface fluid in a secondary circuit between the nuclear heat source and any chemical plant. The aluminium smelting industry provides substantial experience in managing them safely" but does not mention them as primary reactor coolants.
Laurenty (The LM-LS experiment: investigating corrosion control in Liquid Fluoride Salts by Liquid alkali Metal, Report UCBTH-06-002, 2006) mentions Al and AlF3 as part of a corrosion control system for molten salt coolant in the Advanced High Temperature Reactor but rejects them as unsuitable (p30), which is clearly teaching away from their use.
Benson et al (W02019/231971, W02020/123509, 2020/123513) list AlF3 as one of a large number of salts that could be incorporated into molten salt nuclear reactors but ascribe no particular advantages of the use of that salt. Benson does not consider the use of aluminium metal to maintain the reduction state of the salt, and therefore does not consider the issue described above of the reaction of the salt with graphite.

Claims (11)

  1. CLAIMS: 1. Use of a molten salt comprising aluminium trifluoride and sodium fluoride as a primary coolant for a fission reactor; wherein the molten salt is in contact with graphite and with aluminium metal during operation of the fission reactor.
  2. 2. Use according to claim 1, wherein the molten salt further comprises fissile isotopes, and comprising using the molten salt as both the primary coolant and the fuel salt of the fission reactor.
  3. 3. Use according to any preceding claim, wherein the molten salt contains 45 to 65 mol% sodium fluoride and 35 to 55 molVo aluminium trifuoride.
  4. 4. Use according to any preceding claim, wherein the mixture is at a eutectic point for the salt mixture.
  5. 5. Use according to any preceding claim, wherein the aluminium metal is formed in situ.
  6. 6. Use according to claim 5, wherein the aluminium metal is formed by one of: the addition of a reducing agent to the molten salt; the addition of metallic sodium to the molten salt; electrolysis of the molten salt.
  7. 7. A nuclear fission reactor comprising: a reactor core, the reactor core comprising one or more containment units containing fissile fuel; a primary coolant system comprising a primary coolant, configured such that the primary coolant is in contact with the containment units and removes heat from the reactor core wherein: the primary coolant is a molten salt comprising aluminium trifluoride and sodium fluoride; the nuclear fission reactor comprises graphite arranged such that the graphite is in contact with the primary coolant during operation of the reactor; and the nuclear fission reactor comprises a system for providing contact between aluminium metal and the molten salt during operation of the reactor.
  8. 8. A nuclear fission reactor, the reactor comprising: a heat exchanger; a fuel salt system comprising a reactor core and a fuel salt, and configured such that the fuel salt flows between the reactor core and the heat exchanger; wherein: the fuel salt is a molten salt comprising aluminium trifluoride, sodium fluoride, and a chloride or fluoride of an element having a fissile isotope; the nuclear fission reactor comprises graphite arranged such that the graphite is in contact with the primary coolant during operation of the reactor; and the nuclear fission reactor comprises a system for providing contact between aluminium metal and the molten salt during operation of the reactor.
  9. 9. A nuclear fission reactor according to claim 7 or 8, wherein the molten salt contains 50 to 60 mol% aluminium trifluoride and 40 to 50 mol% sodium fluoride.
  10. 10. A nuclear fission reactor according to any one of claims 7 to 9, wherein the mixture is at a eutectic point for the salt mixture.
  11. 11. A nuclear fission reactor according to any one of claims 7 to 10, wherein the system for providing contact between aluminium metal and the molten salt comprises one or more of: an aluminium metal component 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 electrolysis unit configured to electrolyse the molten salt.
GB2020536.5A 2020-09-09 2020-12-23 Molten salt coolant for nuclear reactor Pending GB2602339A (en)

Priority Applications (8)

Application Number Priority Date Filing Date Title
GB2020536.5A GB2602339A (en) 2020-12-23 2020-12-23 Molten salt coolant for nuclear reactor
EP21770241.4A EP4211703A1 (en) 2020-09-09 2021-09-02 Molten salt coolant for nuclear reactor
US18/044,483 US20230343474A1 (en) 2020-09-09 2021-09-02 Molten salt coolant for nuclear reactor
CN202180061733.4A CN116250045A (en) 2020-09-09 2021-09-02 Molten salt coolant for nuclear reactor
PCT/EP2021/074301 WO2022053396A1 (en) 2020-09-09 2021-09-02 Molten salt coolant for nuclear reactor
KR1020237012081A KR20230062648A (en) 2020-09-09 2021-09-02 Molten Salt Coolant for Nuclear Reactors
JP2023515586A JP2023539930A (en) 2020-09-09 2021-09-02 Molten salt coolant for nuclear reactors
CA3192159A CA3192159A1 (en) 2020-09-09 2021-09-02 Molten salt coolant for nuclear reactor

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GB2020536.5A GB2602339A (en) 2020-12-23 2020-12-23 Molten salt coolant for nuclear reactor

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GB202020536D0 GB202020536D0 (en) 2021-02-03
GB2602339A true GB2602339A (en) 2022-06-29

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2014119429A (en) * 2012-12-19 2014-06-30 Toshiba Corp Molten salt reactor
US20150340109A1 (en) * 2012-06-26 2015-11-26 Commissariat A L'energie Atomique Et Aux Energies Alternatives Process for separating at least one first chemical element e1 from at least one second chemical element e2, involving the use of a medium comprising a specific molten salt
WO2020123509A1 (en) * 2018-12-10 2020-06-18 Alpha Tech Research Corp. Eutectic salts

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20150340109A1 (en) * 2012-06-26 2015-11-26 Commissariat A L'energie Atomique Et Aux Energies Alternatives Process for separating at least one first chemical element e1 from at least one second chemical element e2, involving the use of a medium comprising a specific molten salt
JP2014119429A (en) * 2012-12-19 2014-06-30 Toshiba Corp Molten salt reactor
WO2020123509A1 (en) * 2018-12-10 2020-06-18 Alpha Tech Research Corp. Eutectic salts

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