Temperature dependence of cobalt release rate from stainless steel in neutral water.

In order to evaluate the release source of Co from heated tubes of the feedwater line of BWR, Co release rate measurements were carried out by measuring 60Co released from irradiated stainless steel in contact with neutral water at an oxygen concentration of 20 ppb. The temperature- and time-dependences on Co release rate were studied for 670 h release experiments; the rate was found to decrease in proportion to t −frac12 (t −frac12 (t: exposure time) and to have its maximum value around 240°C. An empirical equation was obtained for the Co release rates in the temperature range of 150~240°C. RCo = 1.9 × 106 exp (-14,300/(RT)) t −frac12 where R co Co release rate (mg/(dm2·month)), R: Gas constant (cal/(mol·deg.)), T: Exposure temperature (K) and t: Exposure time (h). Decrease in the Co release rate with temperatures over 250°C, seemed to be ascribed to the formation of a protective oxide layer on the surface of stainless steel.