Deposition of 60Co contained in reactor water results in contamination of boiling water reactor out-of-core components. To elucidate the deposition mechanism, the deposition kinetics of radionuclides on carbon steel were evaluated through exposure tests in actual reactor water. On the basis of the data, a radiation buildup model was developed. Cationic 60Co and 58Co were difficult to transmit through the oxide film, which was probably charged positively, and incorporate into the inner oxide layer; consequently these amounts on the steel did not increase even after 1000 h. However, anionic 51Cr was easily captured by the growing oxide film, and the amount showed an increasing trend up to 500 h.