FRACTURE OF IRRADIATED ZIRCALOY-2

This paper summarizes the results of a series of investigations to evaluate the fracture behavior of Zircaloy-2 as influenced by BWR and PWR conditions. The results show that the response of the fracture toughness of Zircaloy-2 to various combinations of cold work, hydrogen content and neutron fluence in hot pressurized water is characterized by embrittlement to a point where saturation in the fracture toughness is attained. Further in-reactor exposure beyond this saturation point appears to have no effect on toughness although other mechanical properties such as flow stress continue to change. In addition, anisotropy in the toughness of rolled plate material, evident in the unirradiated condition, is retained during in-reactor exposure and after increases in hydrogen content. Several processes are thought to be contributing to the toughness of Zircaloy-2 during irradiation. The reduction in toughness at low exposures must result from defect interactions with the deformation modes and the formation of the brittle hydride phase. However, the occurrence of saturation is not explained by these mechanisms in view of data on other mechanical properties and corrosion rates. It is suggested that the difference in the conditions for initiation of slip and twinning would indicate that the twinning component of deformation is not reduced by irradiation damage as much as the slip component. Saturation is, therefore, interprétable on the basis that twinning plays a major role in the crack tip plastic zone after irradiation. Additional study of the importance of twinning in determining the toughness of Zircaloy-2 was attempted by examining the relationship between texture and the anisotropy in fracture toughness. A correlation is shown to exist between the crack tip shear stresses resolved on the 1121 < 1126 > twin system and the toughness anisotropy. © 1969.