Behaviour of fission gas in the rim region of high burn-up UO2 fuel pellets with particular reference to results from an XRF investigation

XRF and EPMA results for retained xenon from Battelle's high burn-up effect s program are re-evaluated. The data reviewed are from commercial low enric hed BWR fuel with burn-ups of 44.8-54.9 GWd/tU and high enriched PWR fuel w ith burn-ups from 62.5 to 83.1 GWd/tU. It is found that the high burn-up st ructure penetrated much deeper than initially reported. The local burn-up t hreshold for the formation of the high burn-up structure in those fuels wit h grain sizes in the normal range lay between 60 and 75 GWd/tU. The high bu rn-up structure was not detected by EPMA in a fuel that had a grain size of 78 mu m although the local burn-up at the pellet rim had exceeded 80 GWd/t U, It is concluded that fission gas had been released from the high burn-up structure in three PWR fuel sections with burn-ups of 70.4, 72.2 and 83.1 GWd/tU, In the rim region of the last two sections at the locations where X RF indicated gas release the local burn-up was higher than 75 GWd/tU. (C) 1 999 Published by Elsevier Science B.V. All rights reserved.