About the modelling of fuel thermal conductivity degradation at high burn-up accounting for recovering processes with temperature

In BWR and PWR commercial nuclear reactors, the average fuel discharge burn-up has been increased from 35 to 52 GWd/tM over the last 20 years. Furthermore, a large scale of experimental irradiation programmes have shown the capability of the actual design to achieve more than 72 GWd/tM if more appropriate zircaloy alloys allow to limit the corrosion process and hydrogen pick-up of the cladding. One must account for the consequence of the burn-up increase on the material properties. In particular, the fuel material which is obviously the place for the fission process can no more be considered as pure UO2. The fuel conductivity is progressively degraded. Subsequently, a part of the fuel conductivity degradation can be recovered if the fuel material sustains an increase of temperature during a period of its history. A modelling is proposed to account for the burn-up degradation, very similar to the model developed previously, but with a recovery term based on the lattice parameter considered as a marker for the fuel disorder. (author)