Comparative Creep–Fatigue Assessment of a Representative Intermediate Heat Exchanger in a High-Temperature Gas-Cooled Reactor System

The high temperature gas-cooled reactor is a generation IV reactor design, operating at helium outlet temperatures of 700–950 °C and pressures typically between 4 and 7 MPa. Under such conditions, creep-fatigue becomes a critical failure mode, driven by high-temperature exposure and thermo-mechanical load fluctuations. This study applies the creep-fatigue damage evaluation rules of ASME BPVC Section III Division 5, and Code Cases N-862 and N-898 to an Intermediate Heat Exchanger vessel, modelled on the Japanese High Temperature Test Reactor. A finite element model was developed, and a coupled thermo-mechanical analysis was performed for a representative loading sequence. Three code-permitted procedures — (i) elastic, (ii) inelastic, and (iii) elastic-perfectly plastic — were applied to the same loading sequence to quantify creep-fatigue damage and the corresponding allowable lifetimes. The elastic, inelastic, and elastic-perfectly plastic methodologies predicted lifetimes of 21, 80, and 6,005 cycles, respectively. Possible reasons for the discrepancy between the inelastic and elastic-perfectly plastic predictions are discussed, including the effects of cyclic hardening and the factor . Overall, the study clarifies the differences among ASME BPVC creep-fatigue evaluation methodologies and indicates that some approaches may yield overly conservative lifetime predictions.