Pore size distribution for the optimization of dimensional behaviour of UO2 pellets

Based on theoretical modelling and experimental results, it is possible to define a pellet density and pore size distribution which allows the optimization of the dimensional behavior by compensation of matrix swelling and densification. The amount of fine pores should be as small as possible. The volumetric amount and the pore size of the coarse pores should be such, that their lifetime is comparable with the time to reach the final burn-up, and that their shrinkage rate is comparable to the matrix swelling rate. For a target burn-up of 45 MWd/kg(U), a pore volume of 4.5 % with an average pore diameter of about 5 μm would represent optimized structural parameters. UO2 pellets fabricated with UO2-powder from the AUC-conversion process containing additions of U3O8 automatically fulfill these conditions. (Auth.)