In Situ Strain Analysis of Cr-coated Zircaloy-4 Under Simulated LOCA Conditions

Abstract

An understanding of the primary deformation mechanisms for nuclear fuel cladding materials during postulated accident conditions is necessary for development of mechanistic models for materials qualification and lifetime prediction. Digital image correlation was used to measure strain and strain rates of Zry-4 and Cr-coated Zry-4 during a simulated loss of coolant accident. Zircaloy-4 and Cr-coated zircaloy-4 claddings were internally pressurized to 9.66 MPa and rapidly heated at 5°C/s in flowing steam until cladding rupture. The addition of a Cr coating increased the cladding rupture temperature and delayed the onset of deformation for the underlying substrate relative to the uncoated substrate. The onset of cladding deformation was correlated with overcoming the yield stress for Zry-4, where primary deformation was linked to creep deformation. A transition in the deformation mechanism was found from 2% strain rate until cladding rupture, likely indicative of plasticity-dominated failure. The present work demonstrates how Cr coatings can provide increased mechanical stability during temperature transients, yet the underlying deformation mechanisms are not impacted. Modelling efforts for Cr/Zry-4 under accident scenarios are expected to have similar mechanistic parameters as bare Zry-4 yet with increased performance at higher temperatures.