My understanding was that waste, or indeed any radioactive substance, is either highly radioactive for a short period of time, or not very radioactive for a lon

My understanding was that waste, or indeed any radioactive substance, is either highly radioactive for a short period of time, or not very radioactive for a long time. A mole of stuff has a set number of spare neutrons, and it can release them either quickly or slowly. If it releases them quickly ("highly radioactive"), we can probably get power from it somehow.