Strontium-90 is indeed a product of nuclear fission, and it is bad for a variety of reasons, namely a short half life of 28.8 years, which is too long to say 'it is gone before crossing the road' and too short for 'well, and how many decays per second was this again?', and a troubling tendency of the body to treat it like calcium, e.g. accumulate it into bones and like.