Xenon-133 is a radioactive noble gas produced during nuclear fission that poses no concern at the detected level.

FORBES: EPA Sounds an 'All-Clear' (so far) for Western U.S.

Fusion is the process of forcing the nuclei of atoms so close together that they fuse into a nucleus of a new element. (Fission, of course, is the opposite: energy produced by splitting nuclei.) The new, fused nucleus weighs less than the original two.

FORBES: A Nuclear Hybrid

Reactors that exploit nuclear fission (in which energy is generated by splitting uranium atoms) have produced thousands of tonnes of spent fuel and other radioactive by-products.

ECONOMIST: Nuclear waste