Isotopes of Caesium - Caesium-135

Caesium-135

Caesium-135 is a mildly radioactive isotope of caesium, undergoing low-energy beta decay to barium-135 with a half-life of 2.3 million years. It is one of the 7 long-lived fission products and the only alkaline one. In nuclear reprocessing, it stays with Cs-137 and other medium-lived fission products rather than with other long-lived fission products. The low decay energy, lack of gamma radiation, and long half-life of 135Cs make this isotope much less hazardous than 137Cs or 134Cs.

Its precursor 135Xe has a high fission product yield (e.g. 6.3333% for 235U and thermal neutrons) but also has the highest known thermal neutron neutron capture cross section of any nuclide. Because of this, much of the 135Xe produced in current thermal reactors (as much as >90% at steady-state full power) will be converted to stable 136Xe before it can decay to 135Cs. Little or no 135Xe will be destroyed by neutron capture after a reactor shutdown, or in a molten salt reactor that continuously removes xenon from its fuel, a fast neutron reactor, or a nuclear weapon.

A nuclear reactor will also produce much smaller amounts of 135Cs from the nonradioactive fission product Cs-133 by successive neutron capture to 134Cs and then 135Cs.

The thermal neutron capture cross section and resonance integral of 135Cs are 8.3 ± 0.3 and 38.1 ± 2.6 barns respectively. Disposal of Cs-135 by nuclear transmutation is difficult, because of the low cross section as well as because neutron irradiation of mixed-isotope fission caesium produces more Cs-135 from stable Cs-133. In addition, the intense medium-term radioactivity of Cs-137 makes handling of nuclear waste difficult.

• ANL factsheet