Byproducts of
nuclear fusion and
nuclear fission. These
processes are used to
generate energy, to either generate
electricity through a rather
crude and
inefficient system of
conversion1, or as a
means of destruction (nuke).
The actual waste consists of water poisoned by radiation (see below), and spent uranium, which is the source of nuclear power.
1 Energy released by fission or fusion is released as heat, which is used to boil water, which is turned into steam, which turns turbines, which in turn generate electricity. Approximately 90% of the power originally generated is lost this way, but no alternate means (read: practical) of harnessing nuclear power have yet been discovered.