A prefilming method called the alkaline process was successfully applied to a new Japanese boiling water reactor plant during the nuclear heatup phase during the plant preoperation phase. This method was developed to suppress deposition of radioactive corrosion products such as 60Co on ex-core components, which are primarily made of stainless steel. When stainless steel is exposed to actual reactor water that contains oxygen at pH of 8.0 to 8.5, at 288°C, for 410 h, corrosion-resistant oxide films can be formed. The results of the exposure tests indicate that an ∼30% reduction in the deposition of 60Co and 58Co was obtained by this process compared to ordinary operation procedures.