Creep--Fatigue Lifetime Assessment of the European DEMO Helium-Cooled Pebble Bed Breeding Blanket

Within the European DEMO program, the helium-cooled pebble bed (HCPB) concept is a key breeding blanket (BB) design being evaluated for its potential to enable tritium breeding, effective thermal energy extraction, and shielding against irradiation. Due to the high operating temperature range (~ 350~∘C–550~∘C) of Eurofer97 steel, a primary structural material, it is critical to assess failure modes such as creep–fatigue, which can significantly affect component lifetime under fusion-relevant conditions. In this work, we evaluated the creep–fatigue lifetime of the HCPB using finite element (FE) analyses based on RCC-MRx elastic design-by-analysis (DBA) rules. Predicted lifetimes range from 1 million cycles to as few as five cycles, depending on local stress and temperature distributions within the blanket structure. Although certain regions exhibit long fatigue lives, the combined creep–fatigue effect drastically reduces lifetime in areas with high thermal stresses, where creep becomes the dominant limiting factor. These results emphasize the importance of including creep–fatigue interaction in BB structural assessments. They also highlight the need to refine design rules and support the development of structural materials with enhanced creep resistance, ultimately contributing to more robust and reliable in-vessel components for future fusion reactors.