b'more, this process offers the possi- thermal annealing, the model shows bility to gather in situ data as wella less satisfactory agreement with as post irradiation or transient datathe experimental data. Finally, a UO 2such as thermal conductivity, specificMiniFuel test matrix is proposed to heat, fission gas diffusion andhelp to extend the models opera-release. However, accelerating fueltional range and validate the new qualification is not solely reliant onFGR model capabilities to higher generating large amounts of data butburnups and transient conditions also on developing an informed testand will provide valuable means to matrix designed to rapidly generatevalidate BISONs FGR model. Lastly, impactful data. This process is relianttwo major mechanisms behind the on fuel performance codes, such asexperimentally observed FG burst BISON, to evaluate MiniFuel irradia- release are currently not accounted tions using existing material models.for in the model. First, the forma-This process pinpoints model/ tion of high burnup structures data gaps, identifies desired irradia- (HBS) results in a significant drop in tion conditions, and subsequentlythe average grain size, allowing for supports model validation andmore FG at grain faces because of development. This work describesshorter diffusion times. Second, the the use of BISON to perform aoverpressurization of HBS bubbles number of sensitivity studiesleads to important fragmentation designed to understand conditionsduring large and rapid temperature that lead to FGR according to thevariations. Therefore, further model model predictions under steady-statedevelopment in BISON is necessary isothermal irradiation conditionsto better capture transient FGR. and temperature transient condi- Moreover, further irradiation experi-tions. The model is applied to aments on UO 2MiniFuel specimen at UO 2MiniFuel disk and shows anhigher burnups and fuel temperature overall good qualitative agreementwill provide valuable means to with experimental FGR annealingvalidate BISONs FGR model.tests under different temperature conditions. It also accounts well for microstructural effects on FGR. When quantitatively compared with FGR data from previously irradiated 103 MWd/kgU UO 2disks under 2021|AFC ACCOMPLISHMENTS 75'