Predicting the flow stress of Zircaloy-4 under in-reactor accident conditions

During loss of coolant accident (LOCA) and reactivity-initiated accident (RIA),
nuclear fuel rods experience high heating rates that change the microstructure and properties of zirconium cladding materials. The aim was to determine how different fast heating rates affect the kinetics of transformation and whether its consequences on the material yield stresses in the dual-phase region could be predicted. Phase fraction and texture of cold-rolled Zircaloy-4 were thus measured at fast heating rates similar to those experienced during LOCA and RIA by in-situ high-energy synchrotron x-ray diffraction; the yield stress was also
measured at different heating rates to the same dual-phase temperature by electrical resistance method. Crystal plasticity finite element model was used to simulate the flow stresses with the inputs of the measured phase fraction and texture. The model is calibrated using the flow stress at the heating rate of 10ºCs-1 and used to predict the flow stress at the heating rate of 50ºCs-1 at the same temperature. A significant shift of the β transus temperature was identified for fast heating rates in the range of 10-100oCs-1. There is great similarity in texture at different heating rates. In the dual-phase temperature region, the α
texture is almost identical to the starting texture and the β texture is nearly random. Yield stresses of cold-rolled Zircaloy-4 decrease at faster heating rate at the same temperature of 920ºC. A good agreement was found between the experimental and predicted flow stress at different heating rates, indicating this methodology has potential to be used to predict flow behavior in a fast transient regime.