New burst criteria for loss of coolant accidents radiological consequences assessment

During a loss of coolant accident (LOCA), elevated temperatures and depressurization of the primary circuit can lead to cladding ballooning and burst. In the frame of the R2CA project (Reduction of Radiological Consequences of design basis and design extension Accidents), funded in HORIZON 2020 and coordinated by IRSN (France), new burst models were developed to better evaluate the number of failed rods during a LOCA. A large literature review was performed to build various best estimate burst criteria as well as lower and upper envelopes and a conservative strain limit reduction approach. The impact of the cladding state was partially considered (only the hydrogen impact was quantified) due to a lack of data. Experimental data were reassessed with advanced scanning methods, providing a deeper understanding of the cladding deformation occurring during ballooning and burst. A series of validation tests were carried out with the DRACCAR and FRAPTRAN codes to compare the burst parameters (in this case temperature and strain) calculated with the new and the original burst criteria. The temperature calculated using the new burst limits was better reproduced with DRACCAR, but was more conservative with FRAPTRAN, than the values obtained using the original burst criteria of the codes. Burst strain was typically underestimated in both codes when using the new burst limits. The new criteria are therefore well suited to give a best-estimate or somewhat conservative estimate of the number of failed rods with respect to assessing the radiological consequences of a LOCA. The need to disentangle the impact of parameters like the heating mode or the type of test (single or bundle) was outlined. More work with advanced methods is needed to evaluate test type impacts and uncertainties.