Study on creep-fatigue life of ODS-Cu as heat sink in divertor PFU

Abstract

In fusion devices, the actively cooled W-monoblocks of the PFUs (Plasma Facing Units) are installed in the divertor to protect the vacuum vessel and magnets against neutron flux in the bottom region of the vessel. Copper alloy is considered to be the ideal heat sink material due to its high thermal conductivity and sufficient mechanical strength, in which ODS (oxide dispersion-strengthened)-Cu is selected as a candidate. With the increasing of operating parameters, such as higher heat flux (∼20 MW/m2, H-mode) and longer pulse (up to 2.00 h) in EU-DEMO [1], the heat sink structure will undergo thermal creep induced by high temperature, stress and long holding time, which accelerates fatigue failure and could potentially be a very critical issue for the long-term safe operation of the fusion reactor. In this paper, an ITER-like monoblock with ODS-Cu as heat sink was studied for its creep fatigue life under DEMO operation conditions. The temperature distribution and stress/strain results of ODS-Cu were determined using finite element analysis. The investigation shows that high temperature creep occurs at the heat sink. Based on a damage accumulation model, the predicted life of the PFUs for the divertor are investigated at different operating modes. The analysis provides theoretical feasibility for selecting ODS-Cu as a heat sink material. The effect of stress relaxation on creep behavior during thermal stress cycle was also demonstrated and this was compared with the same for CuCrZr.