Study of Anisotropy in High Temperature Tensile and Shear Deformation Behavior of Zirconium-2.5Nb Alloy and Evaluation of Parameters of Hill’s Material Model

Pressure tubes (channels containing fuel in CANDU type nuclear reactors) of Indian pressurized heavy water reactors are made from quadruple-melted Zr-2.5Nb alloy. Owing to the texture and crystal structure, these tubes exhibit anisotropy in mechanical properties. During postulated severe accident scenario such as loss of coolant accident, the temperature of the pressure tube may rise rapidly due to disruption in the heat removal porcess from the fuel bundles. The deformation behavior of the pressure tube under such high temperature shall effect the integrity of the coolant channel. Hence, it is crucial to model the high temperature deformation behavior of pressure tube under these conditions. For design and safety analysis of pressure tube, the high temperature properties are required. In this work, tensile tests were carried out using the specimens cut from quadruple melted Zr2.5%Nb pressure tube along longitudinal, transverse and radial directions at temperatures, i.e., from 25˚C to 800˚C. Shear properties were also evaluated by using the specimen machined from longitudinal-circumferential orientation of tube. It was observed that the specimen oriented along the circumferential direction has highest strength, while the radial specimen has the lowest strength as compared to other direction specimens at all temperature conditions. With the increase in temperature above 600 °C, the material undergoes superplastic deformation with strain values reaches above 400% at 800˚C. In addition, an algorithm has been developed to determine the anisotropic parameters of Hill’s yield function as a function of temperature and equivalent plastic strain using the experimental data. The equivalent stress-strain curves considering anisotropy have also been evaluated as a function of temperature. These data shall be useful for design and safety analysis of the pressure tube for different types of postulated loading and accidental conditions.