Assessment of the radial extent and completion of recrystallisation in high burn-up UO2 nuclear fuel by EPMA

Questions concerning the correctness of the Xe concentration measured in the recrystallised UO(2) grains and the reliability of the values obtained for the width of the zone of recrystallisation in the fuel are answered. It is argued that since the Cs concentration is correct and since both Xe and Cs are determined simultaneously and under the same conditions the measured Xe concentration is also correct. In addition, experimental evidence is presented to show that EPMA provides a true indication of the zone width of rerrystallisation because it detects islands of recrystallised fuel that are not taken into account in the optical microscopy results so far reported. EPMA results contained in the paper show that the transition zone between the normal UO(2) structure and the fully developed high burn-up structure extends from about 60 to roughly 120 GWd/tU and that the width of the zone containing recrystallised fuel increases considerably with the pellet burn-up. (C) 1999 Elsevier Science B.V. All rights reserved.