[ExcelSaga]

Tritium. In addition to the points made by other replies, current fusion is (mostly) D-T, and we’re nowhere near breeding T in the reactor, so it has to be made in fission plants. Tritium is very expensive, and a significant risk for nuclear weapons proliferation.

[GCU-Empiricist]

This will probably end up pushing us to Lithium fusion pretty fast, the fuel is a lot more common and the temperature requirement is only a little hotter.

That said nuclear proliferation from tritium is a lot less of a risk than from uranium. Anyone can put together a nuclear event with sufficient enriched uranium, pinching tritium to fuse using a fission event, well it's a significant technical hurdle.