A study on the reaction kinetics of Zircaloy-4 with H2/H2O mixture

Through-wall defects resulting from debris fretting, grid to rod fretting, and PCI can develop in the fuel rod during reactor operation. In this case, coolant water enters inside fuel rod through the defect hole. Then it flashes into steam in the gap and the steam causes not only oxidation of inner surface of cladding but also generation of hydrogen takes place in the gap continuously as the cladding corrosion reaction. Thus, the gap is filled with hydrogen/steam mixture. It is well know that the hydrogen/steam pressure ratio, P(H2)/P(H2O), is higher than certain critical value, βCr, the oxide no longer plays a protective role and massive hydriding occurs. This henomenon is called the secondary hydriding degradation. The critical ratios are known to be order of 102-106 for zirconium alloy depending on specimen geometry, temperature, and total pressure. Few studies, however, on the critical value have been reported in case of Zircaloy-4 which is widely used for PWR fuel cladding. In addition, clear understanding of secondary hydriding mechanism has not been established yet so far. In this study, thorough experimental investigations were carried out in order to determine the critical value for commercial Zircaloy-4 and give a mechanistic understanding of the secondary hydriding degradation