I was asked to speak at the Platts Nuclear Energy Conference in Washington, DC, on February 10, as part of their panel on “New Approaches to Advanced Reactor Design.”

I began my talk by emphasizing how the generation of “nuclear waste” has become a political problem, with spent fuel sites dotting the country, but that most of this spent fuel is unconsumed uranium and relatively short-lived fission products. Only a small percentage is long-lived transuranic nuclides that have special disposal challenges. The plan to put this spent nuclear fuel in Yucca Mountain runs up against the reality that Yucca Mountain would already be filled to its statutory capacity. Above-ground interim storage is a credible solution to buy time for a better approach. For seventy years, the overwhelming majority of the nuclear community has believed that fast breeder reactors using plutonium was the strategy that we should take, but with thorium and liquid-fluoride reactor technology, there is another and I believe better approach. We should fission the transuranic nuclides in a liquid-fluoride reactor jacketed with a thorium-bearing salt. This will produce uranium-233 even as transuranic nuclides are permanently destroyed. Uranium-233 can then be used to start up sustainable liquid-fluoride thorium reactors (LFTRs) that do not produce transuranic nuclear waste. Existing uranium-233 inventories at Oak Ridge and Idaho National Laboratory should also be preserved.

Flibe’s LFTR design occupies a unique corner of the design space of molten-salt reactors, different from the other MSR designs being offered.

I thought that this slide would be helpful for people to understand how the different MSR concepts compare to one another. It places neutron spectrum along the x-axis and nuclear fuels (in a continuum from thorium to uranium) across the y-axis. The thermal spectrum MSR concepts can be seen on the left-hand side. In the upper left-hand corner there’s Flibe Energy’s LFTR concept as well as the Chinese TMSR, the two approaches that are trying to achieve the admirable goal of true operation on thorium through breeding. Then there are uranium designs that are still thermal spectrum. Terrestrial Energy’s IMSR uses uranium fuel, and ThorCon’s design uses both uranium and thorium, but predominantly relies on uranium. But uranium breeding is impossible in the thermal spectrum so neither design has good fuel utilization.

Outside of the thermal designs there is Transatomic’s WAMSR design, which is intermediate in its spectrum and uses only uranium. And at the uranium-fueled, fast-spectrum corner of the design space, we find the chloride reactor designs: TerraPower’s MCFR, Elysium’s design, and the Moltex design. These reactors use a fast neutron spectrum to achieve uranium breeding, since uranium can only breed in the fast spectrum.

An outlier is the European MSFR design, which uses thorium fuel yet has a “fast-ish” spectrum, since it still uses fluoride salts. Fluorides tend to moderate neutrons more than chlorides and it’s difficult to achieve a truly fast spectrum in any fluoride reactor.