Terminal Solid Solubility of Hydrogen in Unalloyed Zirconium by Differential Scanning Calorimetry

Zircaloy-2 cladding lined with pure Zr (Zr liner cladding) was developed as a candidate for pellet-clad interaction/ stress-corrosion cracking (PCI/SCC) failure resistant BWR cladding, and its good performance has been proved in many power ramp tests, up to burnups of about 50 GWd/t. However, at higher burnups above 50-60 GWd/t, another type of PCI failure, which apparently possesses an outside-in type cracking, has been recently reported. Though a hydride precipitation assisted-failure mechanism has been proposed for this type of fuel failure, there is no established theory. When considering the fuel performance, two phenomenological hydride behaviors seem to be important: (1) a dynamic relationship between hydride precipitation at crack tips and crack propagation in Zircaloy, and (2) a heterogeneous accumulation of hydrides in the Zr liner region adjacent to the Zr liner/Zircaloy-2 interface, which was observed in high burnup BWR claddings after base irradiation. The latter phenomenon may influence on hydride re-precipitation in the cladding outside region at power ramp conditions. In order to clarify both phenomena, basic understanding of dissolution and precipitation behavior of hydrides not only in Zircaloy-2 but also in Zr liner is essential. The terminal solid solubility during the dissolution of hydrides (TSSD) at heatup and during the precipitation of hydrides (TSSP) at cooldown have been the subjects of many reports for Zr and its alloys, using various measuring techniques. However, there is considerable scatter in the published data, especially in TSSP data. Moreover, there are no systematic data sets of TSSD and TSSP solvi for Zircaloy-2 and unalloyed Zr using the same technique.