Freshly discharged spent fuel is many orders of magnitude more radioactive than the source material because of the fission products generated: https://en.wikip

Freshly discharged spent fuel is many orders of magnitude more radioactive than the source material because of the fission products generated: https://en.wikipedia.org/wiki/Nuclear_fission_product https://en.wikipedia.org/wiki/Nuclear_fission_product That's also why an explosive breach of an operating reactor, like at Chernobyl, is far more dangerous than an explosive accident at a uranium mining site. The danger of radioactive materials is inversely proportional to their half-lives, so acute danger from fission products lasts only a few centuries. But it's pretty important to contain those fission products until said centuries have passed.