Evaluation of FRAPCON-4.0’s uncertainties predicting PCMI during power ramps

Abstract The Pellet-Cladding Mechanical Interaction (PCMI) is a fuel failure-preceding mechanism and, as a consequence, a relevant aspect when the fuel rod performance is modelled. Given the current trend towards more flexible modes of operation, PCMI studies are getting focused on operational power ramps. This study assesses FRAPCON-4.0′s uncertainties predicting PCMI under ramping conditions. A set of ramps from a Halden test performed on high burnup fuel has been analysed in terms of cladding elongation. An important error between data and predictions has been obtained (overestimation higher than 50% at the end of ramps), which cannot be fully explained by the uncertainty associated to power and in-code models. However, if the rigid pellet approximation adopted in FRAPCON-4.0 is removed and a formulation for fuel creep is introduced, the code response is notably enhanced to the extent that the uncertainty from this approximation allows explaining the error found.