Abstract
Inadvertent withdrawal of a control rod cluster was observed during lifting of the upper core structure in the 15th outage of pressurized water reactor (PWR) Ikata Unit3 on January 12, 2020. This event would be caused by magnetite sludge in the head of control rod cluster, which was brought from corrosion of drive shaft in control rod drive mechanism (CRDM). The sludge adhered to the head of control rod cluster and led to incomplete separation between the cluster and the drive shaft. To prevent the events due to the corrosion products, we carried out some experiments and their consideration to accumulate the corrosion characteristics for type 410 stainless steel (SUS410), which was the main material of the drive shaft, in the PWR primary coolant circumstances, whereas less knowledge for the corrosion was available at the present time. As a result, it was elucidated that SUS410 material was corroded quickly under the conditions with weak acid and saturated dissolved oxygen (DO) concentration at 473 K, which was very similar to the circumstance of the drive shaft inside at start-up stage of Ikata Unit3, providing the corrosion product of magnetite. Countermeasures to reduce the magnetite formation were also suggested.
