STABLE SALT REACTOR | FUEL & FUEL CYCLE


It is a fast spectrum reactor, which has no moderator to slow down the neutrons produced during fission. Without a moderator, more fissile material is needed to achieve criticality, but the reactor will also consume all the higher actinides produced in conventional moderated reactors. This is important because spent nuclear fuel contains


two distinct classes of radioactive waste products. ● Fission products form when nuclei that absorb neutrons split into smaller atoms. These products are highly radioactive but short-lived. After 300 years, they decay to a radioactivity similar to that of mined uranium.


● Higher actinides form when nuclei absorb neutrons and become atoms that are heavier than uranium, such as plutonium, americium and curium. These products are highly radioactive and long-lived. For up to a million years, they dominate the radioactivity of the spent fuel and drive the need for enormously expensive deep geological repositories.


The SSR uses higher actinides as fuel, producing energy while converting them. So this novel design can clean up the nuclear waste from current reactors, which use uranium as fuel and in doing so, accumulate higher actinides as long-lived waste. The SSR will still have fission products, but these are much shorter-lived and this helps give nuclear energy the social acceptability it needs. First, it is necessary to extract those higher actinides


from spent fuel. In Canada, the dominant nuclear reactor is the CANDU reactor, which was developed in Canada and uses non-enriched uranium fuel. Because it uses natural uranium, it achieves a relatively low burnup, about one fifth of that achieved by reactors using enriched uranium. For any given amount of energy produced, CANDU reactors therefore produce five times the mass of spent fuel compared to other reactor types. Its spent fuel contains about one third of the higher actinides found in spent enriched fuel. Traditional spent fuel reprocessing (aqueous


reprocessing) is intended to produce pure plutonium. But separating plutonium from other higher actinides, which are chemically very similar, is a complex and expensive process. So reprocessing is not extensively used. Processing the large mass of spent CANDU fuel through conventional aqueous reprocessing would be utterly uneconomic. Moltex has therefore invented a new way to extract higher actinides from spent fuel safely. This is the Waste To Stable Salt (Watss) process.


Watss: turning nuclear waste into fuel What makes the Watss process economically viable is that the SSR reactor does not need high-purity fuel. It needs the higher actinides as fissile material, but they can be mixed with both unused uranium and lanthanide fission products. Highly radioactive, long-lived CANDU waste enters this


process. What comes out is: ● A small volume (about 1/100th of the input spent fuel)


of highly radioactive but relatively short-lived salt, produced during the extraction process. Depending on the country, the salt can be disposed of in conventional deep geological repositories (about 500m deep) or down 5km deep boreholes in geologically stable rock. It could also be used in a heat battery and non-radioactive fission products could be recovered as a source of rare earth metals.


● Zircaloy sheathing material that could be reused as an alloying element in a metal fuel or as a redox agent in a liquid fuel or coolant, or disposed of as intermediate level waste.


● Depleted uranium, with very low radioactivity and negligible heat generation, that can be safely and inexpensively stored until the uranium has a value that makes it worth recycling. There are also a number of intermediate and long-term storage options available in the event this by-product is not recycled.


● Fuel for the SSR, which can be recycled until all the higher actinides are consumed.


Corrosion challenge Engineering has largely solved the problem of metal corrosion in water and air. The solution has been to find ways to form stable oxide layers on steels — better known as stainless steels — which almost halt corrosion. But molten salts are notorious for dissolving such oxide layers. Many molten salt reactor designers resort to advanced alloys containing very high levels of nickel, which is less easily corroded. But those alloys have no track record of use in the nuclear industry, which may add many years to the development timeline. Nickel also produces helium when irradiated by neutrons, which makes the alloy brittle. Moltex has a different approach, enabled by its fuel-in- tube design. Adding small amounts of metallic zirconium to each fuel tube has the effect of scavenging any oxidising species in the salt. Because fission products include about a third of all chemical elements, there are many such species. In chemical terms, the zirconium locks the redox


potential of the salt to that of zirconium metal. That redox potential is so strongly reducing that the thermodynamically stable form of iron and chromium is actually the metal. There is no driving force to extract the metal into the salt as with chromium or iron chlorides. This means we can use standard, well understood iron/ chromium ferritic steels, with no nickel. The downside is that the zirconium metal will migrate


through the salt to deposit in the coldest part of the system. This makes it impossible to use in pumped molten salt reactors, but this problem does not arise in Moltex’s design as it is not a pumped molten salt reactor. ■


Below: Hierarchy of controls


Elimination Physically remove the hazards Moltex approach Substitution


Replace the hazards Engineering controls


Post-Chernobyl


Isolate people from the hazards Admin controls


Change the way people work PPE


Protect people with personal protective equipment


www.neimagazine.com | February 2022 | 31


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