The role of the pellet rim on fission gas release at extended burnup

In the past decade the discharge burnup of PWR fuel assemblies has steadily been increasing and in the meantime has reached maximum assembly average values of 55 MWd/kgU. Higher target burnups require higher fuel enrichments. Fuel rods with higher enrichment are operated at higher average power and hence higher fuel temperatures over a longer period of time. The fractional fission gas release of such fuel rods increases with burnup from about 7 % at 50 MWd/kgU to about 12 % at a rod average burnup of 75 MWd/kgU. The fission gas release characteristic is being discussed and correlated with fuel structural changes with emphasis on the pellet rim. (author)