Research on the Evolution Model of Grain Boundary Helium Bubbles in the Weld Heat-Affected Zone of Austenitic Stainless Steel After Irradiation

The weldability of neutron-irradiated austenitic stainless steels in pressurized water reactors deteriorates significantly due to helium accumulation at grain boundaries 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), temperature, 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 accelerates cooling-stage expansion with saturation at ~200 MPa. Excessive welding heat input prematurely terminates coalescence, increasing final bubble density/diameter and promoting He-induced cracking, as validated by SEM/TEM observations of GB void chains. 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.