Patent Application
20180286525
Molten salts are known to be highly corrosive to metals because of their ability to dissolve protective oxide layers from the metal. The more chemically reactive species in the metal then dissolve as metal salts in the molten salt.
In the case of molten halide salts, corrosion can be reduced by contacting the salt with a more reactive sacrificial metal. For steels, relevant sacrificial metals are zirconium, titanium or other metals as described in WO 2015/140495. In many cases however this approach is impractical due to migration of the sacrificial metal from one area in contact with the molten salt to another. This can occur by simple dissolution/redeposition or by galvanic transfer. There remains a need to a way to control corrosion where continuous contact of a sacrificial metal with the molten salt is impractical.
According to a first aspect, there is provided a molten halide salt mixture for use in a nuclear fission reactor. The molten halide salt mixture comprises a reactive metal halide salt. The reactive metal halide salt is a halide salt of a reactive metal. The reactive metal has a Pauling electronegativity between 1.2 and 1.7, and at least one other halide salt of higher valence than the reactive metal halide salt. The reactive metal salt is at a concentration sufficient to prevent corrosion of metals in contact with the molten halide salt mixture and insufficient to cause deposition of the reactive metal at an operating temperature of the nuclear fission reactor.
According to a further aspect, there is provided a nuclear fission reactor comprising a molten halide salt fissile fuel, wherein the molten halide salt fissile fuel is a molten halide salt according to the first aspect.
According to a further aspect, there is provided a nuclear fission reactor comprising a molten halide salt coolant, wherein the molten halide salt coolant is a molten halide salt according to the first aspect.
According to a further aspect, there is provided a method of reducing corrosion of metals by a molten halide salt mixture. The method comprises including a reactive metal halide salt in the molten halide salt mixture. The reactive metal halide salt is a halide salt of a reactive metal. The reactive metal has a Pauling electronegativity between 1.2 and 1.7, and at least one other halide salt of higher valence than the reactive metal halide salt. The reactive metal salt is at a concentration sufficient to prevent corrosion of metals in contact with the molten halide salt mixture and insufficient to cause deposition of the reactive metal at an operating temperature of the nuclear fission reactor.
Further specific embodiments are defined in claim 2 et seq.
It has now been discovered that addition of small amounts of low valency halides of reactive metals to molten halide salt mixtures has remarkably high efficacy in preventing corrosion of metals. Without being restricted by theory, it is believed this is due to the combination of the naturally low redox potential of the low valency halide with the extreme stability of the oxides of the metals. The latter results in oxygen from air or water contamination of the molten salt being sequestered in a non reactive form.
Reactive metals which are suitable for use are those with at least two stable halides of different valencies, and with a Pauling electronegativity between 1.2 and 1.7. Metals with an electronegativity above 1.7 will generally not provide an anticorrosive effect, and metals with an electronegativity below 1.2 are likely to cause unwanted redox reactions within the molten salt (e.g. reducing sodium salts to their metal form). For example, zirconium (ZrF2, ZrF4), titanium (TiF2, TiF4), and vanadium (VF2, VF3) would be suitable. The reactive metal salt which is used to prevent corrosion is then the lower valency halide salt of the reactive metal. In particular, the stable monovalent and divalent halide salts of zirconium, titanium, and vanadium are suitable, e.g. ZrF2, ZrCl, TiF2, VF2.
In order to prevent deposition of the pure metal on surfaces in contact with the molten salt mixture, the concentration of the reactive metal halide should be insufficient to cause such deposition at the operating temperature of the molten halide salt mixture. The maximum concentration will depend on the reactive metal salt used, the other metal salts in the molten halide salt mixture, the temperature, and other factors. In general, the maximum concentration will be larger if the reactive metal salt contains the same metal as another salt in the molten salt mixture (e.g. where the higher valency halide salt of the reactive metal is present in the molten salt mixture, such as VF2 in a molten salt containing VF5).
For the rest of this disclosure, zirconium difluoride is used as an exemplary reactive metal halide salt. However, it will be appreciated that the embodiments presented can be achieved with other reactive metal halide salts as described above.
The zirconium difluoride can be added to the salt directly, or generated in situ by dissolving small amounts of metallic zirconium in a fluoride containing molten salt. This is particularly useful as an approach where a significant component of the molten salt is zirconium tetrafluoride but can be applied to any molten halide salt mixture.
The zirconium difluoride concentration in the molten salt will fall over time as oxygen or water enters the molten salt, resulting in formation of zirconium oxide and zirconium tetrafluoride.
The zirconium difluoride concentration may be monitored electrochemically and additional zirconium metal or zirconium difluoride added to maintain the zirconium difluoride level.
Alternatively, a solid zirconium metal rod or other structure can be intermittently immersed in the molten salt for a period sufficient to replenish the zirconium difluoride concentration but not long enough to raise the zirconium difluoride concentration to the point where deposition of zirconium on surfaces exposed to the molten salt will occur.
A further alternative is to continuously contact the zirconium metal with a portion of the molten salt which is cooled to a lower temperature than the bulk of the molten salt that contacts the other metal surfaces. As the equilibrium concentration of zirconium difluoride in contact with zirconium metal rises with temperature, this prevents redeposition of the zirconium on surfaces in contact with the molten salt.
A similar approach can be used with chloride or mixed halide salts. With chloride salt systems, zirconium monochloride is the species added. Zirconium monochloride can be prepared by reaction of zirconium tetrachloride with zirconium metal but it will in most cases be convenient to introduce it to the molten salt system by contacting the salt with zirconium metal as described for zirconium difluoride.
Other monovalent or divalent zirconium halide salts may be used to equivalent effect in other salt mixtures.
The range of concentrations for which the zirconium salt will not deposit zirconium metal is dependent on the temperature of the salt—at lower temperatures, the allowable concentration is lower. At typical molten salt temperatures, a range of 0.1% to 2% zirconium halide will be appropriate (and similar ranges are appropriate for titanium and vanadium halides), but the skilled person will readily be able to determine whether a given concentration will cause deposition at the operating temperature of their application for the salt, and whether the concentration will be sufficient to prevent corrosion of metals in contact with the molten salt (i.e. to maintain a low redox state of the molten salt).
Molten halide salt mixtures for use as fissile fuel salts or coolant salts in a nuclear fission reactor could be adapted using the above disclosure to reduce corrosion in such a reactor.
An exemplary reactor where a zirconium halide is used in the coolant salt is shown in
The reactor further comprises a source of zirconium halide 2001, and a sensor 2002. The sensor 2002 is configured to determine a concentration of zirconium halide in the coolant salt 102. If the zirconium halide concentration is below a threshold (determined to keep the concentration of zirconium halide sufficient to reduce corrosion as described above), then additional zirconium halide is added from the source 2001. The source may directly add zirconium halide, or it may add zirconium metal (e.g. by addition of metal pellets which then dissolve, or by temporarily immersing zirconium metal in the molten salt coolant). The amount of zirconium halide added is determined such that the concentration does not rise sufficiently to cause zirconium metal to deposit on components in contact with the coolant salt.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1517835.3 | Oct 2015 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2016/053861 | 12/8/2016 | WO | 00 |
