On August 02, 2017, plant was in shutdown state when bonnet gasket of moderator drain tank outlet valve was replaced as per preventive maintenance program. After the maintenance work, moderator drain tank was slowly filled with heavy water on August 04, 2017 while its outlet valve was kept close as per procedure. After about 2 hours of moderator drain tank filling, drop-wise leak of heavy water started from its outlet valve bonnet gasket that gradually kept on increasing. To save heavy water from leaking, tank outlet valve was manually opened and heavy water was transferred to dump space. Boiler room was on purging with fresh air before this leak to carry out maintenance jobs. Purging was terminated and spilled heavy water was collected.
In response to the requirement of the regulatory body, K-1 performed Root Cause Analysis (RCA) of this event and submitted the report on October 02, 2017. According to this report, about 1589 kg spilled heavy water of moderator system was collected from reactor building floor. Among the workers involved in valve isolation and heavy water collection, four individuals exceeded the annual regulatory dose limit of 20 mSv/year. The doses received by the overexposed workers were 20.8 mSv, 24.2 mSv, 30.9 mSv and 36.2 mSv respectively.
The investigation reveals that the causes of bonnet gasket failure of moderator drain tank outlet valve are less than adequate procedure adherence and absence of supervision during work. The overexposure of four workers resulted due to inadequate preparedness for collection of spilled heavy water. Protective clothing of the personnel involved in leak isolation became wet with heavy water that eventually gave exposure in excess of annual regulatory limit.