Fission gas release mechanisms operating in water reactor fuel in power transients

Local Xe concentrations in grains and grain boundary porosities were obtained as a function of the relative radius on pellets of transient tested, high burnup BWR and PWR fuels. The techniques used were X-ray fluorescence analysis and electron probe microanalysis (EPMA). The PWR fuel had a burnup of 3.4% fissions of initial metal atoms (FIMA) (31 GW·d/t U) and was held for 28 h at a transient terminal level of 32 kW/m. The BWR fuel with 4.3% FIMA (39 GW·d/t U) was held at a terminal level of 42 kW/m for 14 h. Considerable amounts of gas in the grain boundary porosities were found at certain radial positions, up to 40% of the Xe generated for the PWR and up to 20% for the BWR fuel. The Xe distribution within single grains in the PWR fuel was measured by EPMA. On the basis of these observations, the mechanisms of transient fission gas release from high burnup fuel are discussed. It is shown that for both types the gas release is limited by the tunnel formation process and not by diffusion of gas from the grain interior to the grain boundary. (author)