Abstract Six microstructural variants of Prime Candidate Alloy (PCA) were evaluated for swelling resistance during HFIR irradiation, together with several heats of type 316 stainless steel (316). Swelling was negligible in all the steels at 300°C after ~44 dpa. At 500 to 600°C 25%-cold-worked PCA showed better void swelling resistance than type 316 at ~44 dpa. There was less swelling variability among alloys at 400°C, but again 25%-cold-worked PCA was the best. Microstructurally, swelling resistance correlated with development of fine, stable bubbles whereas high swelling was due to coarser distributions of bubbles becoming unstable and converting to voids (bias-driven cavities).