UC San Diego

The use of tungsten as a plasma-facing material necessitates a transition joint to the structural material of the primary coolant loop at some location in order to transport the coolant to the heat exchanger. A critical issue in transition joints is the thermal expansion mismatch between materials, which can lead to unacceptably high thermal stresses. Detailed 2D and 3D analyses were performed to study the behavior of a transition from tungsten to ferritic steel (FS) with an intermediate layer of tantalum, located outside of the high heat flux region. This thesis describes the results of FEM analyses including primary and secondary stresses under various time-dependent loading conditions such as warm and cold shutdown, and allowing for inelastic behaviors leading to stress relaxation and ratcheting. In previous FEM analyses at UCLA, it was concluded the residual stress in Ta exceeds the ultimate and tensile strength. In such configuration that is under high pressure and temperature loading conditions, plastic deformation has to be considered during the analysis. It also causes stress relaxation on the configuration. The results exhibited that the original transition joint did not satisfy the design requirement on maximum accumulated plastic principal strain due to stress concentration. Some modifications were made on the transition joint. Fabrication steps were analyzed in 2D models utilizing ANSYS. The results of the modified design exhibited less plastic deformation in Ta alloy and ODS steel as well as no ratcheting in cold shutdown. Both 2D and 3D cold and warm shutdown were analyzed as well