Contribution to the description of the absorber rod behavior in severe accident conditions: An experimental investigation of the Ag–Zr phase diagram

Abstract Most pressurized water reactor (PWR) absorber rods are composed of an Ag–In–Cd (SIC) alloy inside a stainless steel (SS) cladding, themselves inserted into a Zircaloy tube. During a severe accident, the SIC alloy which melts at 800 °C does not practically interact with SS. However, the cladding failure results from its internal pressurization and its eutectic interaction with Zircaloy and occurs at temperatures greater than 1200 °C. The subsequent interaction between the SIC melt and the Zircaloy has a strong impact on the quantities of aerosols released into the primary circuit and finally on the iodine chemistry. Accurate knowledge of the Ag–Zr system is a prerequisite to address this issue. Within this concern, our experimental work is focused both on the investigation of the Ag–Zr phase diagram and on the determination of the thermodynamic properties of the intermetallic compounds in the system. Two intermetallic compounds (AgZr and AgZr2) were identified. Ag–Zr cast alloys with a Ag/Zr ratio of 1:1 elaborated using an arc-melting furnace, once annealed, contained only a single phase AgZr. From metallographic observations, it appears that AgZr2 likely forms by the peritectic reaction from liquid and the bcc (βZr) phase. The partial enthalpies of solution of silver and zirconium in aluminum were experimentally determined at 723 °C in order to determine the enthalpies of formation of the intermetallic compounds. For silver solution calorimetry in aluminum bath, our measurements were successful and in agreement with the previous data. Yet, this study shows that liquid aluminum should not be used as a solvent for zirconium below 1000 °C.