EFFECT OF GRAIN-SIZE ON RECRYSTALLIZATION IN HIGH BURNUP FUEL PELLETS

The effect of grain size on recrystallized structure formation in the outer region of high burnup UO2 fuel pellets was studied by optical mi croscopy, SEM, EPMA, XRD and TEM. Specimens were prepared from three k inds of fuels with different grain size (the standard pellet (grain si ze: 9 mu m), the undoped large-grained pellet (51 mu m) and the alumin o-silicate-doped large-grained pellet (45 mu m)), irradiated up to an average pellet burnup of 60 GWd/t in the Halden Reactor. The TEM obser vations showed that recrystallized structures were formed in a region from the middle to the edge (relative radius: r/r(0) = 0.7-1.0) of all fuel pellets, though they were less likely to form in the undoped lar ge-grained pellet and the alumino-silicate-doped large-grained pellet than in the standard one. This result agreed qualitatively with the re sults obtained from optical microscope observations of the whole pelle t region, SEM fractographs, and measurements of Xe concentration in th e fuel matrix by EPMA. The effects of grain size and irradiation tempe rature on recrystallized structure formation were discussed in connect ion with fission damage accumulation. (C) 1997 Elsevier Science B.V.