GB2598739A

GB2598739A - Molten salt coolant for nuclear reactor

Info

Publication number: GB2598739A
Application number: GB2014181.8A
Authority: GB
Inventors: Richard Scott Ian
Original assignee: Individual
Current assignee: Individual
Priority date: 2020-09-09
Filing date: 2020-09-09
Publication date: 2022-03-16
Also published as: GB202014181D0

Abstract

The present invention relates to the use of a molten salt comprising aluminium fluoride (AlF3) as the primary coolant in a nuclear fission reactor. The molten salt may further comprise a monovalent metal fluoride chosen from lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF) and rubidium fluoride (RbF). The molten salt may be used to carry heat from immobile nuclear fuel structures to a heat exchanger or, alternatively, may be mixed with a chloride or fluoride of a fissile isotope so it acts as both the primary coolant and the fuel salt of the reactor. For moderated reactors operating in the thermal neutron spectrum, a eutectic mixture of approximately 55 mol% of sodium fluoride and 45 mol% of aluminium fluoride may be used.

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). Some properties of AlF3 are mentioned in this paper 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°/0 AlF3 with a melting point of 1000C, an entirely impractical melting point for use in a nuclear reactor. This paper therefore clearly teaches away from the use of AIF3 salts in the nuclear context.

The definitive report regarding molten salts for nuclear reactors (Thoma, ORNL-2548) lists a large number of potential salt systems but includes no AlF3 salts.

Holcomb et al (ORNUTM-2010/156) while mentioning the use of AIF3 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 a 75 mol% NaF/25mo1% AIF3 salt with no mention of its suitability as a nuclear reactor coolant.

AIF3 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/informationlibrary/current-and-future-generationimolten-s alt-reactors.aspx) mention AIF3 salt as secondary coolants thus "In industrial applications molten fluoride salts (possibly simply cryolite -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).

It is clear, and perhaps surprising, that AlF3 based salts, which are probably the most used molten salt systems in industry through their use (as cryolite mixtures) in aluminium smelting have never been suggested as nuclear reactor coolants despite 70 years of intense interest in such molten salt cooled reactors.

In developing a high temperature fluoride salt cooled version of the static molten salt reactor described in GB 2508537 we have surprisingly discovered that aluminium fluoride mixtures with sodium, potassium or rubidium fluorides have properties that make them of great value as reactor coolants.

Summary

The present invention is defined in the appended claims.

Brief Description of the Drawings

Figure 1 is a phase diagram of an AIF3 and NaF mixture

Detailed Description

A nuclear reactor is cooled with a molten salt comprising aluminium trifluoride (AIF3) and a monovalent metal fluoride chosen from LiF, NaF, KF, RbF. The molten salt may be used as is, where the coolant salt carries heat from immobile nuclear fuel structures, or may be mixed with actinide fluorides where the coolant is also the fissile fuel of the reactor.

For moderated reactors operating in the thermal neutron spectrum, the eutectic mixture of AIF3 and NaF (approx. 55% mol% NaF and 45mo1% AIF3 has a desirable combination of properties as shown in figure 1 Its melting point is approximately 700C making it a practical coolant over temperature ranges above 750C. Variations in the proportion of AIF3 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.

It has a low viscosity falling from 1.2cP at 750C to 1cP at 830C making it a close to ideal coolant with flow properties not dissimilar 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.

Aluminium fluoride has significant advantages over zirconium fluoride which has frequently been suggested as a component of primary reactor coolants. Aluminium undergoes neutron activation to Al-28 which decays rapidly (half life of 2.24 minutes) to stable, non radioactive Si-28. In contrast, zirconium is activated to the radioactive isotopes Zr-93 with a half life of 1.6 million years and Zr-95 with half life of 64 days. Coolant salt containing zirconium therefore represents both a short term and long term radiological hazard while aluminium does not.

An alternative to NaF in this system is rubidium fluoride which has a low thermal neutron cross section of 380mb but is substantially more expensive. The mixture with RbF does have the advantage however of a lower melting point of 486C at 48.4% AIF3. Mixtures of NaF and RbF have intermediate melting points which may be useful in certain applications For fast neutron spectrum reactors, the same salt systems can be used but the system KF/AIF3 becomes a low cost alternative. It has a relatively low melting point at -5500 but has low absorption in the fast neutron spectrum and has a lower inelastic scattering cross section than NaF and therefore allows a harder neutron spectrum to be achieved which can improve the breeding performance of the reactor.

A particularly important property of these salt systems is that they can dissolve quite a high concentration of aluminium oxide without increasing their viscosity significantly. That high solubility is the key to using such systems in aluminium smelting where aluminium oxide must be dissolved in the electrolyte. 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.

This allows a uniquely advantageous way of utilising this salt. The salt is maintained in contact with a pool of aluminium metal. With a melting point of 6600, the aluminium will typically be in the molten form under the nuclear reactor conditions although with the lower melting point salts use in the solid state is possible. Any oxygen or oxides of metals such as Fe or Cr, or moisture that enters the coolant system will react with the AlF3 producing aluminium oxide. The resulting fluorine or hydrogen fluoride released will react rapidly with the molten or solid aluminium producing more AlF3. The fact that the aluminium oxide is soluble in the molten salt means that filtration systems to remove precipitated material are not required, which is a major simplification of the reactor design.

A further advantage of this salt over alternative salts is its compatibility with graphite, which is often used as moderator in nuclear reactors. The extensive history of use of AIF3 salts in aluminium smelters lined with graphite demonstrates that compatibility while, for example ZrF4 based salts under reducing conditions will readily react with graphite destroying its structural integrity. Without wishing to be bound by theory, we consider this difference relates to the much smaller atomic diameter of aluminium compared to zirconium so that the layer of aluminium carbide produced on graphite is much more stable that that of zirconium carbide.

Claims

CLAIMS: 1. Use of a molten salt comprising aluminium trifluoride as a primary coolant for a fission reactor.
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. Use according to any preceding claim, wherein the molten salt further comprises one or more of: lithium fluoride; sodium fluoride; potassium fluoride; rubidium fluoride.
4. Use according to any preceding claim, wherein the mixture is at a eutectic point for the salt mixture.
5. Use according to any preceding claim, wherein the molten salt is in contact with aluminium metal during operation of the fission reactor.
6. Use according to any preceding claim, wherein the molten salt is in contact with graphite during operation of the reactor. 25
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.
8. A nuclear fission reactor according to claim 7, wherein the primary coolant further comprises one or more of: lithium fluoride; sodium fluoride; potassium fluoride; rubidium fluoride.
9. A nuclear fission reactor according to any preceding claim, wherein the mixture is at a eutectic point for the salt mixture.
10. A nuclear fission reactor according to any preceding claim, wherein the primary cooling system is configured such that the molten salt is in contact with aluminium metal during operation of the reactor.
11. A nuclear fission reactor according to any preceding claim, wherein the primary cooling system is configured such that the molten salt is in contact with graphite during operation of the reactor.
12. A molten salt 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 and a chloride or fluoride of a fissile isotope.