What happens to used nuclear fuel

Spent fuel comes out of the core intensely radioactive and thermally hot. It goes first into a pool at the reactor for several years, where water provides both cooling and shielding. Once the heat output has fallen enough, it can move into dry casks — steel and concrete containers sitting on a concrete pad, passively cooled by air.

Almost every operating reactor in the world is surrounded by these casks, because the decision about what comes next has been deferred nearly everywhere.

Reprocessing dissolves the fuel and chemically separates uranium and plutonium from the fission products. Orano's La Hague plant, at 1,700 tonnes a year, is the largest in the world and underpins France's use of MOX fuel. Russia reprocesses at Mayak. Japan has been building Rokkasho since 1993 and has postponed its completion more than twenty times, which leaves its entire closed-cycle policy resting on a plant that has never commercially operated. The UK reprocessed for decades and has now stopped entirely.

Reprocessing does not make the waste problem disappear. It reduces the volume of material needing deep disposal and recovers usable fuel, but it produces vitrified high-level waste that still requires a repository, and it separates plutonium, which carries proliferation concerns that the United States found decisive when it abandoned commercial reprocessing in the 1970s.

Direct disposal means putting the fuel underground and leaving it. Finland got there first: Onkalo, 400 metres into granite at Olkiluoto, is built, has been tested with dummy fuel, and is waiting on an operating licence. Sweden's Forsmark repository, using the same KBS-3 copper canister method, began construction after its 2022 licence. Canada selected a site in 2024 after a fifteen-year consent-based process. France, Switzerland and the UK are years from a licence, and Cigéo is not expected before 2050.

Everyone else is storing. Some of it is centralised — Sweden's Clab holds the country's entire inventory in one building, the Netherlands' HABOG is designed for a hundred years, Ukraine commissioned a central store in 2023 during a war to stop shipping fuel to Russia. But most of the world's used fuel simply sits in dry casks on a concrete pad beside the reactor that made it.

The United States is the extreme case. It has the largest spent fuel inventory in the world and no disposal route at all: Yucca Mountain took two decades and billions of dollars, never received a single assembly, and lost its funding in 2010. The federal government now pays damages to utilities for failing to collect fuel it is legally obliged to take.