Modelling of grain face bubbles coalescence in irradiated UO2 fuel

Two different mechanisms of grain face bubbles coalescence in irradiated UO2 fuel are critically analysed and further developed. Coalescence of face bubbles due to their random migration is reconsidered on the base of the general theory for evolution of two-dimensional array of bubbles and shown to be effective for steady irradiation conditions and apparently insufficient for description of high-temperature annealing regimes. On the contrary, the improved mechanism of coalescence by growth and impingement of randomly distributed bubbles predicts rather weak decrease of face bubbles concentration under steady state irradiation conditions, however, becomes effective for simulation of high-temperature annealing tests with noticeable gas release. On this base, the general coalescence model which includes superposition of the two complementary mechanisms is formulated and proposed for implementation in the MFPR code.