 Complete the following phrase:  Caesium-137 , or radiocaesium, is a radioactive isotope of caesium that is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nuclear weapons. Trace quantities also originate from natural fission of uranium-238. It is among the most problematic of the short-to-medium-lifetime fission products. When suddenly released at high temperature, as in the case of the 