Causes of Corrosion Holes in Hamaoka Nuclear Power Station Reactor No. 5 Condensate Storage Tank Lining and Measures Adopted to Prevent Recurrence
May 28, 2012
Chubu Electric Power Co.,Inc.
During the process of shutting down Hamaoka Nuclear Power Station Unit No. 5 (advanced boiling water reactor; rated output 1,380 MW) on May 14, 2011, damage to the main condenser tubes resulted in an inflow of seawater into the main condenser, and part of this seawater also infiltrated the reactor facilities and other areas.
The extent of infiltration of the seawater was determined, and the effects on the facilities were investigated. During this process, corrosion holes were discovered in and around the welded areas of the condensate storage tank lining. (Reported March 30, 2012)
Today, a report concerning the causes of the corrosion holes in the condensate storage tank lining and measures adopted to prevent a recurrence of the problem was compiled and submitted to the Nuclear and Industrial Safety Agency (NISA).
1. Conjectured cause
It is believed that the infiltration of seawater into the condensate storage tank while the tank had a build-up of crud(*) resulted in the formation of an environment that promoted corrosion in the crevices between the crud and the lining. Corrosion proceeded in these crevices until holes penetrated the lining.
2. Measures to prevent recurrence
The cause of the present incident was the infiltration of seawater into the condensate storage tank as a result of an inflow of seawater due to damage to the tubes of the main condenser in a number of areas. Given this, the following measures will be implemented in order to prevent a recurrence of the incident.
(1) Clarification of response procedures during infiltration of seawater
Based on the present incident, we have added procedures to prevent, to the greatest possible extent, infiltration of seawater into the reactor facilities, and to prevent the spread of the problem, to our previously existing operating procedures for responding to miniscule leaks of seawater from the tubes. We will also be conducting continuous education of operational staff.
(2) Prevention of infiltration of seawater into main condenser
The damage to the main condenser tubes was caused by the rupture of a pipe. We intend to modify the structure of these pipes to prevent a recurrence of the incident.
(*) Formed by rust from the inner walls of the pipes, etc. being carried into the tank and settling on the bottom. Crud is normally discovered during the periodic inspection of the tanks, and is not unique to this incident.