Interpretation of high-burnup fuel annealing tests

The growth of the porosity in high-burnup fuel is of particular interest when considering the effect of fission gas retention within the high-burnup structure (HBS). A mechanistic model of porosity growth under annealing conditions for light water reactor (LWR) UO2 fuel with typical stereological parameters of the HBS has been developed. The model takes into account both multipore and surface interactions that lead to fission gas release from the HBS porosity. We have applied the model to an HBS annealing experiment and found that reasonable agreement with the experimental gas release curve can be achieved for a vacancy diffusion enthalpy close to that measured experimentally. A comparison of the model behaviours at different pore growth rates is performed. We found that the model exhibits porosity evolution characteristics independent of the growth rate, specifically the existence of a maximum porosity, a continuous gas release, and a decreasing pore number density. A general comparison between the model results and in-pile data indicates that up to 250MWd/kgU, most of the gas is retained within the HBS. (author)