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A Mechanism for Corrosion Product Deposition on the Carbon Steel Piping in the Residual Heat Removal System of BWRs

The dose rate of the residual heat removal (RHR) piping has been considered to be caused by accumulation of insoluble (crud) radioactive corrosion products on carbon steel surfaces. Soft shutdown procedures (i.e., plant shutdown with moderate coolant temperature reduction rate) used to be applied to reduce crud radioactivity release from the fuel surface, but these are no longer used because of the need for shorter plant shutdown times. In order to apply other suitable countermeasures to reduce RHR dose rate, assessment of plant data, experiments on deposition of crud and ion species on carbon steel, and mass balance evaluation of radioactive corrosion products based on plant and laboratory data were carried out and the following findings were made. 1. Deposits of ion species on carbon steel surfaces of the RHR piping was much more numerous than for crud. 2. Ion species accumulation behavior on RHR piping, which is temperature dependent, can be evaluated with the calculation model used for the dehydration reaction of corrosion products generated during the wet lay-up period. 3. Deposition amounts could be reduced to 1/2.5 when the starting RHR system operation temperature was lowered from 155°C to 120°C.