Once caesium enters the ground water, it is deposited on soil surfaces and removed from the landscape primarily by particle transport.
Caesium-134 undergoes beta decay (β−), producing 134Ba directly and emitting on average 2.23 gamma ray photons (mean energy 0.698 MeV).
Except in the Molten salt reactor, where 135Cs is created as a completely separate stream outside the fuel (after the decay of bubble-separated 135Cs).
The low decay energy, lack of gamma radiation, and long half-life of 135Cs make this isotope much less hazardous than 137Cs or 134Cs.
Because of this, much of the 135Xe produced in current thermal reactors (as much as >90% at steady-state full power)[11] will be converted to extremely long-lived (half-life on the order of 1021 years) 136Xe before it can decay to 135Cs despite the relatively short half life of 135Xe.
A nuclear reactor will also produce much smaller amounts of 135Cs from the nonradioactive fission product 133Cs by successive neutron capture to 134Cs and then 135Cs.
[12] Disposal of 135Cs by nuclear transmutation is difficult, because of the low cross section as well as because neutron irradiation of mixed-isotope fission caesium produces more 135Cs from stable 133Cs.
It constitutes most of the radioactivity still left from the Chernobyl accident and is a major health concern for decontaminating land near the Fukushima nuclear power plant.
Because these elements are volatile and can diffuse through nuclear fuel or air, caesium is often created far from the original site of fission.