Fission Products (by Element) - Caesium-133, 134, 135, 137

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. IAEA map

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. Cs-134 has a half-life of 2 years and may be a major source of gamma radiation in the first few years after discharge.

Caesium-135 is a long-lived fission product with much weaker radioactivity. Neutron capture inside the reactor diverts much of the xenon-135 that would otherwise decay to Cs-135.