Caesium-135

Caesium-135 is one of the isotopes of caesium. It is mildly radioactive, undergoing low-energy beta decay to barium-135 with a half-life of 2.3 million years. It is one of only 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 LLFPs. ¹³⁵Cs's low decay energy, lack of gamma radiation, and long half-life, make this isotope much less hazardous than Cs-137 or Cs-134.

Its precursor Xenon-135 has a high fission product yield, such as 6.3333% for U-235 and thermal neutrons, but also has the highest known thermal neutron neutron capture cross section of any nuclide, so much of the Xe-135 produced in current thermal reactors (as much as >90% at steady-state full power [1]) will be converted to stable Xenon-136 before it can decay to Cs-135. Less or no Xe-135 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 Caesium-133 by successive neutron capture to Cs-134 and then Cs-135.

¹³⁵Cs's thermal neutron capture cross section and resonance integral are 8.3±0.3 and 38.1±2.6 barns respectively. [2] Disposal of Cs-135 by nuclear transmutation is difficult, because of the low cross section, because neutron irradiation of mixed-isotope fission caesium produces more Cs-135 from stable Cs-133, and because the intense medium-term radioactivity of Cs-137 makes handling difficult. [3]

See also

• Isotopes of caesium
• Isotopes of xenon
• ANL factsheet