Abstract
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 ir-radiated 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-1/2 (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 150240°C.
Rco =1.9 x 106 exp (- 14, 300/ (RT))t-1/2
where Rco Co release rate (mg/ (dm2•month)), R: Gas constant (cal/ (mol•deg.) ), T: Ex-posure 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.
