Lord!... not picking on you, just the lack of information people deal with.

MOX = Mixed Oxide

When they realised they had no where to store all the waste... they upped the ante, by 'blending' Uranium with 'Plutonium' to put it back into the reactor, whilst awaiting somewhere to put it, this is highly toxic, and was a 'stop gap'.. it is WAY more toxic than the original fuel intended... it's no missing somewhere....

BTW they can't go look for it, even with robots as everything dies getting close to it.....