Before revision: Caesium-133, 134, 135, 137 Caesium-137, with a half-life of 30 years, is the main medium-lived fission product, along with Sr-90.Cs-137 is the primary source of penetrating gamma radiation from spent fuel until 300 years or more after discharge.It is the most significant radioisotope left in the area around Chernobyl.Distribution of Surface Ground Contamination by Caesium-137 Released in the Chernobyl Accident and Deposited in the Byelorussian SSR, the Russian SSR, and the Ukrainian SSR (December 1989), IAEA, 1991 Caesium-134 is found in spent nuclear fuel but is not produced by nuclear weapon explosions, as it is only formed by neutron capture on stable Cs-133, which is only produced by beta decay of Xe-133 with a half-life of 3 days.
After revision: Caesium-133, 134, 135, 137 Caesium-134 is found in spent nuclear fuel but is not produced by nuclear weapon explosions, as it is only formed by neutron capture on stable Cs-133, which is only produced by beta decay of Xe-133 with a half-life of 3 days.