A recent article at Decouple examined the CANDU reactor as a potential destination for reprocessed uranium (RepU) from LWR spent fuel. The concept is straightforward: CANDU reactors run on natural uranium, and the U-235 remaining in spent LWR fuel exceeds the natural uranium threshold, so RepU blended with depleted uranium to produce a natural uranium equivalent (NUE) should work as CANDU fuel without re-enrichment. A demonstration at Qinshan in China proved the concept neutronically but ran into serious fabrication problems. Those problems have been taken as evidence that the pathway is impractical. I think that conclusion is wrong, and the reason comes down to chemical feedstock.

Problem Encountered at Qinshan

The Qinshan fabricators tried to dry-blend RepU oxide powder with depleted uranium oxide powder. The two powders had incompatible particle morphologies — different grain sizes, different sintering behaviors — and wouldn’t blend uniformly. The fix was to dissolve both in acid and co-convert them back to UO2, which worked at demonstration scale but reintroduced aqueous chemistry into a fabrication process whose simplicity is central to the CANDU value proposition. The program stalled there.

The key question is whether this problem is inherent to RepU as a CANDU feedstock, or whether it’s an artifact of the chemical form in which the RepU arrived. The answer is the latter — and the chemical form is not fixed by physics. It depends on how the reprocessing was done upstream.

RepUF6 as a New Starting Point

Fluoride volatility reprocessing of LWR spent fuel reacts the spent fuel with fluorine gas. Uranium forms volatile UF6 and separates from the non-volatile fluorides of the fission products and actinides. After cleanup to remove NpF6 and noble metal hexafluorides by fractional distillation and selective trapping, what remains is a purified RepUF6 stream.

UF6 is the standard commodity form in which uranium moves through the entire front end of the nuclear fuel cycle. Every UO2 fuel fabrication plant converts UF6 to oxide powder as its first processing step, through well-established routes that produce powder of controlled and consistent particle characteristics. When RepUF6 is converted freshly to UO2 this way, the powder characteristics are set by that conversion step — not by the prior history of the material. Bring the depleted uranium diluent through the same process, and the two powders match. The grain size problem that defeated Qinshan doesn’t arise, because the feedstock arrives in the right chemical form to begin with. The wet chemistry step that looked like an unwelcome complication at Qinshan is, from a UF6-based fabrication process, simply normal fuel fabrication.

The Other Output Stream

The UF6 that volatilizes during fluoride reprocessing represents roughly 95% of the metal mass of LWR spent fuel. The remaining 5% — the non-volatile fluoride residue — contains the plutonium, minor actinides, and fission products. After removal of the more problematic fission products, this residue is natural feedstock for a molten salt reactor. The transuranics are already in fluoride form, already mixed, and don’t need to be fabricated into solid fuel. They dissolve into the MSR’s fluoride salt and are progressively consumed as fuel. The long-lived actinides that make geological disposal a deep-time problem get transmuted rather than buried. The waste that ultimately leaves the system is shorter-lived and smaller in volume than what a once-through cycle sends to a repository.

The Complete Picture

Fluoride volatility reprocessing of LWR spent fuel produces two streams that map cleanly onto two reactor technologies. The RepUF6 stream — 95% of the spent fuel mass — could go to CANDU as NUE fuel, potentially fabricated without the powder problems that stalled Qinshan. The actinide fluoride residue goes to a molten salt reactor for fission and transmutation. CANDU and MSR don’t have to be competing visions. They could be complementary destinations for the two output streams of a single reprocessing step, linked by a common fluoride chemistry, together accomplishing what neither achieves alone: substantial energy recovery from spent fuel and substantial reduction of the long-lived waste burden.