Investigation of the PCMI failure of pre-hydrided Zy-4 cladding during Reactivity Initiated Accidents with ALCYONE and OFFBEAT fuel performance codes

This paper presents simulations of Reactivity Initiated Accident on PWR fuel rods performed with two fuel performance codes: ALCYONE and OFFBEAT. These simulations were carried out in the framework of the first phase of the High-Burnup Experiments of Reactivity Initiated Accident (HERA) project of the Nuclear Energy Agency (NEA). The focus of this project is the hydride assisted cladding failure during the Pellet-Clad Mechanical Interaction phase of the transient. In the simulations performed with ALCYONE and OFFBEAT, cladding mechanical behavior is simulated with an anisotropic viscoplastic law dependent on temperature, strain rate and hydrogen content. Clad failure is predicted with a hoop strain criterion, taking into account the impact of strain-rate, temperature, hydrogen content and hydride rim thickness and the ductility recovery that occurs at moderate or elevated temperature due to the dissolution of hydrides. The validation of the proposed failure criterion is based on 2D-(rθ) simulations performed with ALCYONE that allows to explicitly model cracking of the hydride rim at the cladding periphery and the resulting strain localization in the remaining sound clad ligament beneath it. The results of the simulations performed with ALCYONE 1.5D or OFFBEAT 2D-(rz) are in good agreement with the ALCYONE 2D-(rθ) results, thus indicating that the proposed failure criterion is able to account for the detrimental effect of the hydride rim in ALCYONE 1.5D or OFFBEAT simulations. Finally, the calculated enthalpies at failure as a function of the hydrogen content/hydride rim thickness are shown to be consistent with previous experimental results obtained in the NSRR test facility.