b'Analytical microscopy on pre- and post-LOCA tested fuel rods has revealed insight on how microstructure may affect post-LOCA fuel fragmentation.burnup and temperature on theseand allow for mobilization of FMPs samples. A SEM survey of all samplesand the formation of LAGBs. While showed trends in porosity as a func- results are still under reinvestiga-tion of the radial position along thetion due to resolution, they suggest fuel pin. The position is defined byburnup also effects distribution the ratio of the radial position (r) toand mobility of FMPs across the the radius of the fuel pellet (r 0 ) suchcentral region of the fuel. Addition-that the fuel cladding interface isally, the largest FMPs were found defined as r/r 0~1. The samples exam- near the fuel/cladding interface, ined showed a decreasing number ofand again at approximately r/r 0pores at the fuel/cladding interface~0.5 (central restructured region of as a function of LOCA testing. Poresthe fuel). Work has been done to are more dense but smaller close toquantify Xe bubbles in STEM maps the cladding, then again at around r/ taken at select regions across the r 0~ 0.5. Between this at r/r 0~0.75 andfuel pin. Xe bubbles are distributed again close to the fuel center at r/r 0~non-uniformly across the fuel pin, 0.3, there is an increase in averageoften appearing at or near grain pore area, but a decrease in the totalboundaries. No obvious monotomic pore numbers in that region. In therelationship was observed between regions where the pores are smallerFMPs and Xe bubbles in the samples but denser, pores are highly circular.tested. Taken broadly, however, Five metal precipitates (FMPs) wereFMPs are often associated with fine observable in the HBu structureXe bubbles. Xe bubbles are larger of some of the as-irradiated andfarther from the cladding, likely due post-LOCA samples via SEM, butto the higher temperature profile due to resolution deficiencies wasand lower generation rate driving not seen in all samples. The FMPsthe nucleation and growth process in the central regions of the fueldifferently. A model has been devel-began appearing in the restructuredoped to quantify the number of atoms central region. The results fromin Xe bubbles to aid with future fuel SEM support the theory that there ismodeling efforts of Xe evolution and increased diffusion of FMPs at highersafety testing responses.temperatures. It appears defects and FMPs in the hotter central region of the fuel have higher mobility 2022|AFC ACCOMPLISHMENTS 29'