In-reactor UO2 densification

In previous papers the in-pile dimensional behaviour of UO2 was analyzed by superposing matrix swelling and pore shrinkage. The total porosity was subdivided into several pore size classes, ranging from submicroscopic pores to those visible in the microscopic range, which were treated separately. For the coarser porosity a two step process was postulated: first, the generation of vacancies by the fission fragments transversing the pore, second, the migration of these vacancies to effective sinks (grain boundaries). Based on these assumptions the dependence of the pore shrinkage on fission rate, temperature, grain size and pore size distribution was evaluated. Several irradiation results have shown that in less stable fuel, early densification by the annihilation of the submicroscopic porosity may contribute a large fraction to the total densification effect. The topic of this paper is to evaluate in greater detail the behaviour of the fine sized porosity. The same basic principles are applied as in the models mentioned before. (Auth.)