I'm not really sure if this falls into the denied suggestions ideas since this isnt radiation nor radiation steam, however if it does then delete this thread.
My idea is every so often the nuclear reactor would corrupt some water around it into radioactive waste which must be removed or it will drasitcally heat up the reactor. It could be removed with a buckit or a block similar to a pump from a miner, which would store it into radioactive cells, theese could be destroyed in the recycler or by some other way (cant think of a better way) or they could be used similar to a lava buckit to heat up the reactor, or they could be emptied into a buckit to be used to make mob traps, anything inside the radioactive waste would either be mutated or damaged, an example is if a pig enters the waste it turns into a zombie pigman, in the same way lightning morphs pigs.
Of course it seems like this is all a downside so why would anyone want radioactive waste? Well it might also be usefull, with the pump you could instantly remove it so it doesnt really have a downside if your water source blocks are high above the reactor, so you could use the radioactive cells to heat up the reactor, usefull for breeding uranium or getting the heat up cheaply (since it is a renewable source) for increased EU/t. It would act like lava in the aspect it flows slowly and doesn't travel far, as well as it cant be made infinate with 2 source blocks.
Another idea is a radioactive cell could be mixed with a rubber sheet to form a radioactive sheet, it would slow things a bit more than rubber and damage anything that walks over it.
If radioactive waste is in direct contact with wood it will slowly erode it, like fire. Maybe it would do the same to stone, so something like obsidian would be needed to contain it in mob traps.
, when its good working it will happen less many times but if you haven't set it up right it will happen a lot.
) to some other block (less strong) like mossycobble stone.