Relationship between porosity of UO2 sintered pellets and technological manufacturing parameters

Knowledge of porosity of sintered pellets as well as of processes of density increasing have a special importance for evaluating the radial temperature distribution, the plastic can deformation and implicitly of the interaction pellet-can, due to solid matrix swelling. For fuel elements with collapsing (CANDU) cans these aspects are of little importance in case of low burnups because the fuel has a high density (>96% DT) and the matrix swelling is relatively small. In this case the risk of can failure is relatively low. But, when the burnup is high, the interaction can-pellet increases and consequently the failure risk increases too. As the process of density increase is dependent on the porosity distribution the densification analysis requires knowing the pore microstructure of the sintered pellets as well as its effect upon densification and fission gas accommodation. Evaluation of the fine porosity fraction, responsible for the fast densification at the fuel introduction into the channel is a very important data for computation codes simulating the reactor processes. This work presents the results obtained in experiments of producing sintered pellets with controlled porosity utilised to extend the nuclear fuel burnup. Such pellets were obtained by two methods: 1. utilization of DUA and polyvinyl alcohol as pore forming agents; 2. modification of the production technology parameters, i.e., the calcination temperature and calcination duration of the ammonium diuranate. Tests of re-sintering and mechanical resistance as well as irradiation tests of experimental fuel elements with UO2 pellets of controlled porosity were performed. Results were presented as plots, histograms, photographs and tables. In conclusion the frequency of small and large pore occurrence can be controlled as well as the density, grain sizes, the pore total volume, the distribution of the pore volume as a function of pore radius, etc