Research of the Extended Layup on the Secondary Side in Japanese PWR Plants

摘要

All PWR plants in Japan have been forced to suspend operations after the Fukushima accident. No NPPs were working when this report was written, and many plants are to stay suspended for a while. Japanese utilities have several experiences of extended layup after some defects or failures. In those cases, layup methods were determined to fit each situation, based on the vender's recommendation and their experiences. However, such an extensive suspension without any mechanical malfunction is beyond the scope of assumption for the utilities and the vender. There was no definitive standard or guideline for extended layup in Japan. Before the Fukushima accident, only the steam generator (SG) layup method for ordinary outage was confirmed and described in the vendor's standard. In addition, layup methods for the secondary system were simply listed in general. However these methods do not clearly correspond to each type of equipment and piping. To establish reliable and rational methods for the extended layup, the Japanese utilities and the vendor examined the layup conditions and those results in the plants, such as initial hydrazine concentration and its decreasing rate. Then, new recommendations for the extended layup started to be made. In this paper, the significant examination results of this extraordinary situation are presented.