What happens to the uranium nobody wanted

This is not a step in the chain. It is the branch that leaves it. Enrichment splits its feed into a small enriched product and a much larger depleted stream, and the depleted stream is the majority of the uranium ever mined.

Why it cannot simply be left as it is. Uranium hexafluoride is convenient for enrichment because it becomes a gas at a low temperature, and dangerous for storage for the same reason. It reacts with atmospheric moisture to produce hydrogen fluoride, so a corroded cylinder in a yard is a chemical hazard well before it is a radiological one. Deconversion converts it to uranium oxide, a stable ceramic-like powder, and captures the fluorine as hydrofluoric acid, which is sold.

Two plants do this at commercial scale, both attached to enrichment sites: Orano's W plant at Tricastin and Urenco ChemPlants at Capenhurst. The United States built one at Portsmouth and one at Paducah to work through a Cold War inventory of some 700,000 tonnes.

Is it waste? Legally it usually is not, which is convenient, because a country that classified its tails as waste would acquire an enormous disposal liability overnight. It is held instead as a resource, on the argument that a fast reactor fleet could one day use it, which is true and has been true for fifty years. Some of it does get used: depleted uranium goes into radiation shielding, aircraft counterweights and armour-piercing ammunition.

The honest description is that the largest single stream of material in the nuclear fuel chain is sitting in cylinder yards awaiting a decision, exactly like the spent fuel at the other end, and attracting a fraction of the attention.