CONCERNING THE DEVELOPMENT OF GRAIN FACE BUBBLES AND FISSION GAS RELEASE IN UO-2 FUEL
A series of scanning electron micrographs showing the development of gas bubbles on the grain faces in a transient tested BWR fuel are presented. The observations indicate that gas bubbles nucleate on metallic precipitate particles and that bubble migration is prevented by the precipitates. X-ray fluorescence and electron probe microanalysis results show an accumulation of Xe on the grain boundaries at intermediate radial positions and an increasing in grain'' retention towards the centre of the fuel. These findings are ascribed to the presence of high compressive forces in the fuel and an increase in the rate of thermal resolution with increase in temperature. Under transient conditions the magnitude of the local mechanical restraint and the existence of macroscopic pressure gradients are important in determining the behaviour of grain face bubbles and thus fission gas release. In high pressure fields both the bubbles and precipitates grow by Ostwald Ripening. The bubble growth is impeded so that no bubble coalescence occurs. In a non-uniform pressure field bubble coalescence can take place on grain boundaries with low restraint pressure. This leads to gas escape tunnels at grain edges. Intergranular cracks which form when the mechanical restraint is removed at the end of the transient may provide further escape paths for fission gas.
Bibliographic Reference: ENLARGED HALDEN MEETING, LOEN (NORWAY), MAY, 8-13, 1988, AVAILABILITY: CEC-LUXEMBOURG, DG-XIII-C-3, POB 1907, MENTIONING PAPER EN 34086 ORA
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Record Number: 1989126106600 / Last updated on: 1989-06-01
Available languages: en