Low-Temperature Irradiation Damage in SMR Welds 

Small modular reactors (SMRs) introduce a fundamental materials challenge that has not been adequately addressed within existing design frameworks at VTT. The combination of lower operating temperatures and extended in-service lifetimes creates a radiation environment markedly different from that of conventional light-water reactors. Under such conditions, defect mobility is severely restricted, and recovery processes that normally mitigate radiation damage become inefficient. The result is a regime of enhanced defect accumulation, leading to a disproportionate increase in hardening and loss of fracture toughness per unit dose.
The issue is particularly critical for welded joints in the reactor pressure vessel, where microstructural inhomogeneity, residual stresses, and segregated alloying elements act as preferred sites for defect clustering and radiation-induced segregation. These characteristics make weld regions intrinsically more vulnerable to low-temperature embrittlement than the base metal. Nevertheless, present embrittlement correlations and surveillance practices are largely derived from data at higher temperatures, and they do not capture the kinetics of damage evolution in this low-temperature regime.