The dependence of in-reactor UO2 densification on temperature and microstructure

The in-reactor changes in size of sintered UO₂ are analyzed by superposing matrix swelling and pore shrinkage. At first pore shrinkage dominates and results in densification, an effect which has been known for a few years. With regard to shrinkage mechanisms one should distinguish between very fine pores and coarser pores, the latter being responsible for the majority of total porosity in well sintered UO₂. For this coarser porosity a two-step mechanism is postulated: first, the generation of vacancies by the fission fragments traversing the pores, second, the migration of these vacancies to effective sinks (grain boundaries) producing densification. Based on this assumption the dependence of densification on fission rate, temperature, pore size distribution and grain size is evaluated. There is a transient temperature above which the generation of vacancies is rate controlling and below which the vacancy migration becomes the rate controlling mechanism. At temperatures much lower than the transient temperature no densification should be observed.