Abstract
The present communication identifies a previously unrecognized risk of helium-induced embrittlement in out-of-core PWR components exposed to tritium-contaminated coolant. While previous weldability concerns centered on 4He accumulation in neutron-irradiated alloys located at in-core or near-core regions, new measurements show that 3He generated by tritium decay can accumulate in out-of-core components. Because hydrogen isotopes readily diffuse to grain boundaries and become trapped there, significant 3He generation at grain boundaries may lead to cracking during weld repairs. Initial data from far-below-core flux thimble tubes confirm the presence of 3He levels above known cracking thresholds for repair welds. These findings indicate that out-of-core regions should be considered when defining safe weld repair windows in reactors operating to 60–100 years.
