What are molten salts?
...and why are they of interest?
Molten salt refers to any salt heated to its liquid state. For nuclear applications chloride or fluorides are usually preferred since they have physical and chemical properties that lend themselves to use in the nuclear industry:
- Liquid over a wide range of temperatures (few 100°C to over 1000°C)
- Excellent heat transfer characteristics
- High radiation tolerance
- Low vapour pressure
- Typically low capital and environmental cost
- High solubility for relevant materials
- Act as an electrolyte enabling electrochemistry
The first three of these make them well suited as coolants for nuclear reactors (e.g. Karios Power MSR]) providing high quality heat for power generation (e.g. electricity production) and for use in industry (e.g. hydrogen and synthetic fuel production. The same properties are used in the solar industry to store solar energy, the hot salt acting like a battery. The same principle can be used to manage fluctuations in demand with excess heat stored in salt tanks until required (e.g. Grid Reserve, Moltex Energy, Hyme], Terra Power).
The low vapour pressure, even at relatively high temperatures, offers a significant safety and cost advantage over other conventional nuclear coolants such as steam since there can be no explosions due to pressure and therefore there is no requirement for expensive pressure vessels. Furthermore, they are typically safe to handle (once cooled) and non-toxic so do not have the handling limitations of liquid metals.
Molten salts have a high solubility of nuclear materials: uranium, plutonium and thorium, and dissolve a significant proportion of the fission products that are generated during fission, including actinide activation products. This means that they can act as a carrier for the fuel – acting as both a coolant and a fuel. The ability to operate with a liquid, mobile fuel is a feature unique to MSRs. While this presents challenges it represents a paradigm shift in reactor technology, enabling online refuelling and removal of waste without disruption to reactor operation.
The ability of molten salt to conduct electricity allows an electrical potential to be applied across the salt such that dissolved material can be preferentially deposited at electrodes. This application of electrochemistry enables either fuel or contaminants to be selectively recovered from the salt. The salt is not changed in the process and can therefore be reused indefinitely. Specifically, this can be used to recover uranium, plutonium and actinide species from irradiated fuel (c.f. reprocessing), or remove fission product contaminants from the salt, potentially during online clean-up. Conventional chemical methods (e.g. precipitation reaction and filtration) for removal of are also possible.
