In-reactor creep and swelling of Type 348 stainless steel from ATR SN-5 and ETR H-10 in-pile tube measurements were investigated to identify and characterize their mechanistic relationships at temperatures less than 723 K. The principal creep mechanism appears to be irradiation-enhanced-diffusion-accommodated flow along high conductivity paths near the grain boundaries. The irradiation creep is a function of temperature and is presented as an empirical equation. The swelling in the ATR in-pile tubes is also presented as an empirical equation.