The performance of mild steel was excellent during the three-year operation of the Heavy Water Components Test Reactor (HWCTR). Surfaces were covered uniformly with an adherent film of black oxide, and no significant pitting occurred. The total corrosion of mild steel test specimens was 3.1 mg/cm2 after 6 months, and 5.5 mg/cm2 after 33 months. The rate of iron release to the D2O moderator-coolant was 0.16 mg/cm2 per month during the first 6 months and approximately 0.04 mg/cm2 per month, thereafter. No problems occurred because of particulate matter in the D2O. The concentration of particulates in D2O was approximately 80 parts per billion (ppb) during the first two years of operation and was approximately 6 ppb during the last year. The purification system operated satisfactorily without a prefilter. The radiation intensity from typical primary circuit piping was approximately 30 mR/h at 3 in. when reactor operation was terminated.