The unit was in maintenance. At 19:35 while performing the reactor cooldown effort coolant leak from a NB50 pipeline of the primary water makeup system into the sealed rooms occurred. At 21:03 the leak was stopped by isolating the affected pipeline. The leak from the pipeline was isolated under the normal design configuration in the Unit confinement. Radiation situation in the Unit attended rooms, on-site and off-site remained unchanged. Activity releases through the vent stack above limits prescribed for normal operation did not occur. Cause of the leak is a NB50 delivery pipeline rupture of makeup pumps between check valve and heat exchanger of reactor blowdown water in the area of weld seam close to the pipeline penetration.
BASIS FOR RATING:
This event is rated under the "degradation of defence-in-depth" criterion. Real initiator occurred - "small loss of coolant accident" (initiator frequency - medium/possible).
Safety function availability - full.
For this initiator the basic rating according to Table II (A2) of the INES User's Manual (1992 revision) is 1.
The event revealed additional factor that required uprating - Safety Culture deficiencies:
violation of the operating procedure (exceeding of permissible difference of coolant temperature in pressurizer and in reactor core outlet);
operator switched off the emergency makeup pumps without justification; deficiency of procedure on detection of the primary leaks.
Therefore level 2 has been chosen.
Event date: Thu, 03-03-1994