Extension of the OFFBEAT fuel performance code to finite strains and validation against LOCA experiments

The mechanical analysis of nuclear fuel behavior under base-irradiation conditions has traditionally been performed adopting the small-strain approximation. However, many cases of interest for fuel behavior involve the occurrence of large rod deformations that overcome the validity limits of the small-strain approach such as the cladding ballooning during Loss Of Coolant Accidents, the Pellet-Cladding Mechanical Interaction in case of a Missing Pellet Surface or bending experiments on spent fuel rods. For this reason, the mechanical framework of the open-source fuel performance code OFFBEAT is extended to finite strains. To allow for validation against cladding burst experiments, high-temperature models tailored for LOCA regime including creep, Zircaloy phase-transition and cladding burst criteria are also implemented in the code and described in the present work. The paper presents the implementation and the verification of the large-strain mechanical solver implemented in OFFBEAT, together with the results of the validation performed against the PUZRY and the IFA-650.2 LOCA experiments.