A LARGE-SCALE MODIFICATION WORK OF SUPPRESSION CHAMBER AND INTERNAL STRUCTURES AT ONAGAWA NPP

Abstract

The suppression chamber and its internal structures, that is a vent system consisted of vent pipe, vent header, and downcomer at Onagawa Nuclear Power Plant Unit 2 of Tohoku Electric Power Co., Inc., were subjected to a significant increase in the acceleration response level applying to the seismic design due to the enforcement of the new regulatory standards in 2013. In order to ensure seismic resistance to the acceleration response level, a large-scale modification work of the components was required in addition to the upgrading of the evaluation method.

The main features of this modification work were followings. First, it was a large-scale modification work that our company or other companies had never done before. Second, the feasibility of the modification was carefully examined by using full-scale mock-up models. Third, the work plan was elaborated in consideration of the specific characteristics of the on-site work environment, and the work was conducted systematically under thorough management in preparation for the plant's restart.

Based on these efforts, the modification work is currently underway with the cooperation of the plant manufacturer (Toshiba Energy Systems & Solutions Corporation), who is in charge of the design and work, to complete the work as planned, while paying attention to safety work.