Abstract
The weldability of neutron-irradiated austenitic stainless steels in pressurized water reactors (PWRs) deteriorates significantly due to helium accumulation at grain boundaries (GB) originating from nuclear transmutation reactions. This study develops a multistage model to elucidate helium bubble evolution during welding, including nucleation, coalescence, high-temperature growth, and cooling-induced tensile stages, by quantitatively correlating bubble density/diameter with initial He concentration (0.2–20 appm (atomic parts per million)), peak temperature (1450 K, 1566 K, 1641 K, 1657 K), and stress (0–200 MPa). Key results demonstrate that He concentration nonlinearly affects bubble growth (threefold diameter increase at 20 appm versus 0.2 appm during cooling), while temperature governs bubble kinetics with critical threshold behavior (>900 K) and tensile stress promotes cooling-stage expansion over the investigated stress range. Excessive welding heat input prematurely terminates coalescence, increasing final bubble density/diameter and promoting He-induced cracking, consistent with the reported morphology and order of magnitude of grain-boundary cavities. The model provides a theoretical framework for predicting He embrittlement and optimizing stress-modified welding techniques (e.g., auxiliary beam laser welding) for nuclear component repair.