Nuclear waste produced in the 20^th and 21^st centuries will require active management by future generations well into the 22^nd century and beyond. Current U.S. law on direct placement in an underground repository requires a start next year on the politically infeasible siting of a second permanent underground repository east of the Mississippi river. Each repository would be sealed after the emplaced fuel is air cooled for about 300 years. In order of increasing technical challenge and cost, three alternative options for spent nuclear reactor fuel management are these:

1. Air-cooled storage on aging pads for 100 years. .
2. Prompt reprocessing to make mixed plutonium-uranium oxide (MOX) fuel to be burnt only once in existing reactors.
3. Multiple rounds or continuous reprocessing and burning in liquid-metal-cooled reactors to substantially reduce the required final repository volume.