The Japanese prime candidate alloy (JPCA). a titanium-modified austenitic stainless steel, has shown good performance after irradiation in the high flux isotope reactor (HFIR) up to 34 dpa at 300 to 600°C, but considerable void swelling develops in solution annealed (SA) JPCA after irradiation to 57 dpa at 500°C. However, cold worked (CW) or cold worked and aged (CW + A) JPCA still demonstrates good performance after similar irradiation. Swelling resistance appears to depend strongly on the behavior of fine titanium-rich MC precipitates. This paper describes the microstructural evolution process observed in the JPCA steel during HFIR irradiation. The onset of rapid void swelling was related to MC precipitate dissolution, and the instability of the MC was interpreted in terms of a model involving the build and subsequent effects of a solute segregation zone in the matrix surrounding the precipitate particle.