LEAKAGE OF FEEDWATER THROUGH WELDED JOINT OF A NB 76 PIPELINE CONNECTING ECCS HEADER WITH DISTRIBUTION GROUP HEADER
Wednesday, March 2, 1994
On February 22, 1994, the unit was in normal operation at 50% of rated power (500 MWe). At 12:00 manual load reduction of the unit was performed to 200 MWe (within 20 mins.) due to appearance of steaming in the unmanned room with water pipelines to fuel channels in the primary. At 12:22 the reactor was manually tripped and the cooling down was started with normal rate of 10 degrees C. Designed system of controlled leaks, drain, their treatment as well as water makeup system maintained the normal cooling down of the primary during the whole process to 60 degrees C in the primary. Examination of equipment discovered the weld failure (50% of cross-sectional perimeter) in a NB76 pipeline connecting emergency core cooling system (SAOR) header with distribution group header (DGH) of the primary. The amount of leaked water was estimated to be 80 tons for all time of cooling. Safety systems' operation was not required. Radioactivity release to enviroment exceeding prescribed limits did not occur. Basis for rating: 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. Small leakage of feedwater occurred. But safety systems operation was not required because adequate cooling down was maintained by normal systems. All safety systems were fully operable. Additional factors are not applicable. Therefore level 0 has been preliminarily chosen.