An alternative explanation for evidence that xenon depletion, pore formation, and grain subdivision begin at different local burnups

In order to interpret the recent observation that xenon depletion, pore for mation, and grain subdivision occur successively at increasing local burnup s, a rate-theory-based model is used to investigate the nucleation and grow th of cavities during low-temperature irradiation of UO2 in the presence of irradiation-induced interstitial-loop formation and growth. Consolidation of the dislocation structure takes into account the generation of forest di slocations and capture of interstitial dislocation loops. The loops accumul ate and ultimately evolve into a low-energy cellular dislocation structure. The cell walls have been previously identified as recrystallization nuclei . The calculations indicate that nanometer-size bubbles are associated with this cellular dislocation structure while the observed micron-size bubbles are presumed to be either preexisting pores deformed by adjacent grains an d/or new pores formed in the new recrystallized grain-boundary junctions. S ubsequent to recrystallization, gas released from the recrystallized grains feeds the preexisting pores and the recrystallized grains may appear to fo rm a preferential concentration of subdivided grains around the growing por es. This picture is illustrated in a sequence of photomicrographs of irradi ated U3O8. (C) 2000 Elsevier Science B.V. All rights reserved.