Calculations on the in-pile behavior of fission gas in oxide fuels

Abstract

Model calculations on fission gas behavior and fuel swelling during reactor operation are presented. In its present form, the model allows us to quantitatively calculate the contribution of single gas atom diffusion to the overall fission gas behavior, a contribution which was previously frequently neglected. Thus, the model includes gas atom diffusion, precipitation into a constant number of bubbles of variable size, multiple fission induced resolution of gas atoms from bubbles and precipitation at grain boundaries which are regarded to be deep sinks. Numerical solutions have been obtained for many different sets of parameters for typical irradiation conditions in both fast and thermal reactors. The equated quantities are the concentrations of gas in dynamical solution, in intragranular bubbles and at grain boundaries, as well as the local swelling rate. As a typical example, some calculations for a fast flux irradiation experiment (DS irradiation) with mixed (U, Pu) oxides are given which show that experimental swelling curves can be reasonably well predicted with acceptable parameters. Conversely, the comparison of calculations and experimental results can serve to better determine such important parameters as effective in-pile diffusion coefficients. In addition, the relative importance of gas bubble mobility and sweeping phenomena etc. can be evaluated in this way.