SUMMARY

The swelling mechanisms of U3Si2 under neutron irradiation in reactor conditions are not unequivocally known. The limited experimental evidence that is available suggests that the main driver of the swelling in this material would be fission gases accumulation at crystalline grain boundaries. The steps that lead to the accumulation of fission gases at these locations are multiple and complex. However, gradually, the gaseous fission products migrate by diffusion . Upon reaching a grain boundary, which acts as a trap, the gaseous fission products start to accumulate, thus leading to formation of bubbles and hence to swelling.

Thus, for very large grains, diffusing fission gases may not reach the boundary and remain, effectively, in solution or in the form of clusters of a few gas atoms. It follows that the formation of bubbles would be enhanced by the proximity of grain boundaries, i.e., by small initial grain size or by the subdivision of large grains into numerous small ones.

Therefore, a quantitative model of swelling requires the incorporation of phenomena that increase the presence of grain boundaries and decrease grain sizes, thus creating sites for bubble formation and growth. It is assumed that grain boundary formation results from the conversion of stored energy from accumulated dislocations into energy for the formation of new grain boundaries.

This thesis attempts to develop a quantitative model for grain subdivision in U3Si2 based on the above mentioned phenomena to verify the presence of this mechanism and to use in conjunction with swelling codes to evaluate the total swelling of the pellet in the reactor during its lifetime. Due to the lack of experimental data for U3Si2, this model will be validated using known and estimated behavior of UO2 and compared with high burnup structure formation in this material.