[en] Detailed PIE has been carried out on three fuel assemblies irradiated in BWRs to 30 - 35 GWd/t. Our data base for fission gas release (FGR), has been extended to 39 GWd/t in rod burnup and to 43 GWd/t in pellet burnup. The FGR rate data measured on the three assemblies showed large scattering from 0% to 25%, as the previously reported data of less than 30 GWd/t. These scattering data can be related with the maximum powers they experienced beyond 10 GWd/t. The FGR rate seems to depend mainly on maximum powers, however, only a little on burnups. Micro gamma scanning and EPMA results revealed that local FGR rate varied radially in the pellet for a high FGR rate rod, i.e. almost 0% at the outer region and almost 100% at the center region. There existed a narrow transient band between them and local FGR rate showed a rapid change there. Ceramography and SEM observations showed that the local FGR rate variations related closely to pellet micro structural changes. At the center region, many large pores were found on the grain boundaries, connecting to each other and providing tunnels for gas release. The tunnel formation may do a key role to increase the local FGR rate. No remarkable changes were found at the outer region, except a thin pellet outer surface layer (pellet rim). The micro structure of the transient band indicated that the process of pore growth and tunnel formation related with the local FGR rate increase. The tunnels seem to control local FGR rate since their occurrence. Their effect, especially historical effect on FGR, should be considered in fuel behaviour analysis codes. The pellet rim showed to have very different structure. The original structure disappeared and very fine pores appeared. The observed rim structure looked still retaining most of fission gas. However, the numerical density of the pores is so high that pore connections and tunnel formations may occur and induce additional FGR. However, further investigations are required to evaluate the rim effect on FGR at high burnup. (author). 7 refs, 6 figs, 2 tabs