Modelling of irradiated UO2 fuel behaviour under transient conditions

On the basis of a critical assessment of the Booth-Speight model for the fission gas release from irradiated fuel, a more consistent mechanistic approach is developed for consideration of the microstructure evolution and fission gas release under conditions of power ramp tests. A new model that modifies the Nelson model for gas atom resolution from bubbles and explains the observed formation of a new population of large bubbles, is developed and implemented in the MFPR code. Superposition of various mechanisms of gas transport: gas atom diffusion, bubbles random and biased migration, and gas sweeping by grain boundaries, results in a reasonable prediction of the measured fractional gas release and microstructure evolution, observed in the ramp tests.