FE simulation of rolling of pressure tube-end-fitting joint of Indian PHWR and evaluation of pull-out strength at different temperatures for severe accident research

In pressurized heavy water reactors, the Zr2.5Nb pressure tube is mechanically rolled to the SS403 end-fitting and the joint is called pressure-tube-end-fitting rolled joint. The pull-out strength of the joint in high temperature environment is an essential input for analysis in severe accident research. In this work, the pull out strength of the pressure-tube-end-fitting rolled joint assemblies of 220 MWe and 540 MWe PHWR has been evaluated using 3D finite element simulations. The pull out strength of the joint has been evaluated in the temperature range of 25 ℃ (room temperature) to 1100 ℃. The differences in the failure modes of the pressure tubes of 220 MWe and 540MWe PHWR during its pull-out process from the rolled joint have been highlighted. It was observed that the differences in the pull-out strength between 0.1 mm interference fit, zero clearance fit and 0.1 mm clearance fit joints are not significant at any test temperature for the case of 220 MWe PT-EF rolled-joint. Same was also observed for 540 MWe PT-EF rolled joints with 0.1 mm interference fit and zero clearance fit. However for the case of 0.1 mm clearance fit joint, the peak pull out strength was much smaller (up to 20% less at 298 K) compared to the strengths of other two type of joints at temperatures ranging from 298 K to 773 K. For zero and 0.1 mm clearance fit type rolled joints, the PT region detached from the EF grooves during the pull out operation, leading to the failure of the rolled joint. In the case of interference fit type rolled-joint for 220 MWe PHWR, tensile residual hoop and axial stresses were observed in the rolled region of the PT. However for the interference fit type rolled-joint of 540 MWe PHWR, these stresses were grossly compressive in the rolled region. The residual hoop stress near the end of the rolled region of the PT (at around 75 mm from PT end) were found to be tensile in nature having a magnitude of 145 MPa and 141 MPa at 298 K for 220 MWe and 540 MWe PHWR, respectively, in the case of 0.1 mm clearance fit type rolled-joint. For the other types of joints, the residual hoop stress at this region was not found to be compressive in nature. The residual stress data from FE simulations have been compared with experimental data (i.e., from hole-drilling technique) and these are within the range of experimental scatter. These results shall be useful for studies regarding coolant channel disassembly process in severe accident research. (author)