Tritium formation and elimination in light-water nuclear plants

In light-water reactors, the tritium balance should be considered from the points of view of both the operational constraints and environmental pollution. In pressurized water reactors using boric acid in solution for reactivity control, the amounts of tritium formed in the primary circuit are worthy of note. The estimates of the tritium production in a hypothetical 1000-MW(e) reactor are discussed. In the tritium build-up, it is still difficult to estimate what percentage is represented by the tritium formed by fission in the fuel owing to diffusion through cladding. The tritium balances in different working nuclear power stations are consequently of interest. However, the tritium produced by ternary fission in the fuel is always much more abundant, and remains almost entirely confined in the uranium oxide if the fuel is clad with Zircaloy. The annual quantity stored in the fuel elements is more than 20 times larger than that of the free tritium built up in the primary circuit water of a reactor. It amounts to about 12,400Ci in the hypothetical reactor. In the reprocessing plants operating at present, tritium is distributed in the bulk of the effluents, and most of it is released to the environment. In view of the increasing quantities of highly irradiated fuel to be reprocessed, it seems necessary to develop separation processes. Development work and tests have been undertaken jointly by the CEA and Saint-Gobain Techniques Nouvelles in order to contain the tritium in the high-activity part of the plant and to keep the tritiated effluent volume small, i.e. about 300 litres per tonne of reprocessed uranium. It is then possible to envisage either a storage that will allow the radioactivity to decrease or detritiation treatment. Such detritiation processes have been estimated by CEA assuming a daily output of 1500 litres of water containing 2.3Ci/litre of tritium, the desired decontamination factor being 100. (author)