Fracture mechanical analysis of a tungsten monoblock-type plasma-facing component without macroscopic interlayer for high-heat-flux divertor target

In the framework of the European DEMO divertor project, several novel design concepts of the plasma-facing components of vertical targets are being developed. One of those concepts is the tungsten monoblock design (similar to the ITER divertor) but with a very thin interlayer (roughly 25 mm thick only) at the armor/tube bond interface instead of a thick (1 mm) copper interlayer as has been the case in the conventional tungsten monoblock design developed for ITER divertor. The thin interlayer serves as bonding agent, but not as structural constituent. The reasoning for this novel design concept to omit the thick soft copper interlayer, which has been used as stress-relaxing buffer between the stiff armor block and tube, is to prevent plastic fatigue damage under cyclic high heat flux loads and irradiation embrittlement which the soft copper interlayer is predicted to undergo. On the other hand, the desirable stress relaxation effect on a global scale is abandoned. In this study, such trade-off effects are computationally investigated in a comparative assessment of structural impact, which the presence (or absence) of the thick copper interlayer is expected to bring forth, in terms of the fracture and fatigue behaviour of the armour block and cooling tube in two representative cases of tungsten monoblock plasma facing component design, namely, with and without a thick copper interlayer. Quantitative results of cyclic plastic strain history and crack tip fracture energy are presented for the armour surface, bond interface and tube of the respective plasma facing component models. The positive and negative implications of these impacts on the structural integrity are discussed.