Fuel performance code to code comparative analysis for the OECD/NEA MPCMIV benchmark

The recently developed Multi-Physics Pellet Cladding Mechanical Interaction Validation (MPCMIV) benchmark includes dedicated transient fuel performance exercises. In this work, three fuel performance codes (BISON, OFFBEAT, FAST) are used to perform supporting studies for the benchmark. The exercises consist of a three-year long base irradiation of a father rod in a boiling water reactor followed by a cold ramp transient for a fuel rodlet refabricated from the father rod. For the base irradiation, the results obtained are satisfactory in comparison to the measurements, with some discrepancies observed in the cladding outer diameter for OFFBEAT and BISON, which can be explained by the oxidation models implemented in both codes. Concerning the cold ramp, which consists of a very fast power increase with the linear heat rate going from zero to its maximal value in just a few seconds, all the codes tend to underpredict the cladding axial elongation temporal evolution. The observed discrepancies between predictions and measurements are both in the maximal amplitude and shape of the cladding axial elongation temporal evolution. This suggests that the phenomenology is not predicted accurately. Using a multi-physics coupling (Griffin, BISON, THM), involving reactor-physics, thermal-hydraulic, and fuel performance, the ramp is investigated and an estimation of the LHR is obtained. The OFFBEAT model is then updated with the new LHR. The cladding axial elongation is predicted with significant better agreement compared to the measurements. In single physics fuel performance modeling, the linear heat rate is obtained by calorimetric technique, which is not suited for fast transient, while in the multi-physics model, the linear heat rate is predicted instead by the multi-physics model and then is used as a source term in the fuel performance code. Analyzing further the obtained results, plastic strains mainly axially and with an amplitude one order of magnitude lower than the total strain at peak transient are observed on a small part of the cladding. Future work will focus on improving the BISON model by implementing frictional contact and to use the model for more multi-physics studies as well.