The Moltex Reactor and used CANDU Fuel

Frank Greening, 9 Dec 24

From a MOLTEX Technical Report, issued May 2022:

THE FUEL

The reactor core comprises an array of fuel tubes in a graphite matrix, which fills most of the tank. Each tube sits in a separate channel, within which a molten salt primary coolant circulates.

FUEL SALT

The SSR-U is a fluoride or chloride salt reactor with separate fuel and coolant salts. The fuel is in the form of molten low-enriched uranium fluoride or chloride salt (6% enrichment).

What I find most interesting about this information on the Moltex SMR is that the fuel is enriched uranium, even though we have been led to believe that the Moltex reactor can run on used CANDU fuel, which is certainly not enriched, but depleted in U-235.

For example, consider this announcement by Moltex dated October 3, 2024:

“The SSR-W was specifically engineered to efficiently reuse and consume recycled nuclear waste,” said Moltex CEO Rory O’Sullivan. “This breakthrough research, the result of years of collaboration, clearly demonstrates that ability.” According to this research, the SSR-W can recycle used fuel indefinitely, producing a minimum of 6,000 MW of clean energy from Canada’s existing CANDU reactor fuel without the need for new fuel imports.

Unfortunately, Molex is not very forthcoming about how much fuel their reactor will use. I am going to conservatively assume that 20 tonnes of enriched uranium will be needed for the first year of reactor operation. Based on information I have found in a number of reports on the cost of enriching uranium to 6% U-235, I estimate this will cost about $1 million per tonne. By comparison, the production cost of CANDU fuel is about $250,000 per tonne.

But there are other hidden costs to the production of enriched uranium, one of the most significant being the cost of disposing of the depleted uranium generated by the U-235 enrichment process. Thus, the production of 1 kg of U-235 enriched to 6% generates 12 kg of depleted uranium tailings waste. The uranium enrichment process begins with the conversion of uranium oxide to uranium hexafluoride, UF₆, which is gaseous above 64 ⁰C. UF₆ is very toxic and chemically reactive so it is necessary to convert the depleted UF₆ back to a solid uranium form for safe disposal – adding substantially to the cost of producing enriched fuel for the Moltex reactor.

But there’s still more bad news for the Moltex SMR! Thus, I quote from an article in the May 2023 issue of the Bulletin of the Atomic Scientists by J. Kang et al. entitled: “Canadian reactors that “recycle” plutonium would create more problems than they solve”, where we read:

Moltex’s fresh fuel will consist of potassium chloride, uranium chloride, and plutonium chloride, with some unspecified actinides and lanthanides. Using the expected distribution of plutonium and uranium, and using the ratio of the atomic masses of chlorine and plutonium, one can conclude that the reactor would need about 392 kg of plutonium as fuel every year. In a 2021 presentation, Moltex also mentioned that the average fuel assembly resides for 6.3 years in the reactor. This means that the initial loading for the reactor to start operating would require roughly 2.4 tons of plutonium.

To obtain the 2.4 tons of plutonium required in the startup fuel for a single 300 megawatt-electric (MWe) Moltex reactor, around 577 tons of CANDU spent fuel would have to be processed. In addition, a further 94 tons of CANDU spent fuel must go through Moltex’s waste-to-stable-salts chemical process to produce the necessary fuel for each year of operations.