Exceeding allowed maximum primary coolant temperature during planned outage

Unit N1 was in unplanned outage from 20.04.96, the reactor in cold shutdown condition. Residual heat was removed routinely by second channel of core cooling system. The first channel of this system was taken out of service for maintenance, the third one was in standby mode of operation. At 18:17 30.04.96 running pump of second channel switched off due to pressure drop at the pump inlet. The third channel's pump started up and switched off shortly afterwards due to the same reason. Feed and bleed was arranged (i.e. injections of cold make-up water and drainage into the primary coolant purification system), but before personnel managed to restart core cooling system, temperature of primary coolant had reached 82 deg. C, that was a violation of operational limits (reactor outlet water temperature should not exceed 70 deg. C). The event investigation is in progress, but the incident root cause has not been revealed yet.
Off-site and on-site impact criteria are not appropriate for the event rating. The event is rated under the degradation of defence in depth criterion. According to para: III-3.1.1. "degradation of reactor core cooling" of the user's manual, the event is provisionally rated at level 1. The final rating and additional factors will be considered after investigations have been completed.

Event date: Tue, 30-04-1996
Nuclear event report
Legenda & explanation