- 35 A great number of radioisotopes have been added to the oceans from the detonation of nuclear devices especially in the vicinity of Bikini and Eniwetok Atolls. The kinds of radioisotopes produced are determined largely by the type of detonation -- fission or fusion. Previous to 1952 nuclear detonations were exclusively of the fission type but since then there also have been detonations of the fusion and the fission-fusion-fission type. The greatest number of radioisotopes is produced by the detonation of a fission type device. In the fission process the nuclei ot atoms of ue>> or Pu239 are split when struck by neutrons. In addition to the release of energy at the time of fission, the two parts of the original atom become isotopes of two new elements that are approximately one-half the weight of the original atom. The nuclei of the newly-formed isotopes are unstable usually because of an excess of neutrons, hence are radioactive. (Because the neutron- proton ratio in the nucleus of stable light elements is less than in heavy elements and, nearly all of the newly- formed isotopes retain the neutron-proton ratio of the original atom (U°5> or Pu259), there is generally an excess of neutrons in the nuclei of the newly-formed isotopes)