Analysis of fuel rod behavior during loss-of-coolant accidents using the BISON code: Fuel modeling developments and simulation of integral experiments

In the first part of this work, we discussed BISON fuel performance code developments for Zircaloy cladding behavior under loss-of-coolant accident (LOCA) conditions and validation to separate-effects experiments. In this paper, we present modeling developments for UO₂ fuel behavior during LOCAs and validation to integral experiments. Code developments are related in particular to modeling axial relocation of fuel fragments during cladding ballooning. Code validation is performed against three integral fuel rod LOCA tests from the Halden IFA-650 series, covering fresh and pre-irradiated fuel rods with and without significant axial fuel relocation observed. Calculated results are systematically compared to experimental data of fuel rod inner pressure evolution during the LOCA transients, time to cladding burst and post-test cladding diameter profile. Comparisons in terms of rod pressure evolution and time to burst appear adequate. Also, the ability to reproduce axial relocation of fuel fragments is demonstrated. Calculated cladding diameter profiles are shown to be considerably sensitive to the selection of the burst failure criterion, highlighting the difficulty to accurately evaluate cladding strains at burst during postulated LOCA scenarios with fuel performance codes.