During the morning shift on 07/07/2008, two workers, involved in the maintenance of safety valves, were instructed to repair a leakage at a safety valve in the cooling water system of the recirculation fans used for cooling the containment air space of Unit 1. The entry to the room is possible through an air-lock. The air-locking operations were carried out by the two workers, having entrance permit to the area, by disregarding the relevant regulations. After opening the external door of the air-lock chamber, the magnetically operated latch of the door lock moved to closed position and became stuck in failed position. The door could no longer be locked as a result of this failure. The workers failed to report this event to the Health Physics service, and proceed with opening the internal door of the lock chamber. Inconsistently with the regulations, they did not request the Health Physics service for releasing the magnetically operated latch of the internal door, but they manually moved it into released position and then they operated the mechanical lock to allow the opening of the door. Following this, they removed the leakage of the cooling water system by tightening the bolts of the leaking flange. The Health Physics personnel, who issued the permit for the entrance, seeing the position indicators of the doors in the Health Physics Control Room, detected that both door of the lock chamber was open and tried to contact the two workers by phone, but he could not manage. The deterioration of the containment depression was also detected in the Unit Control Room and the Unit Shift Supervisor gave instruction to start the stand-by exhaust fans of the containment, and again, he instructed the primary circuit chief engineer to go to the event scene to inspect and close the air lock. By the time the chief engineer arrived at the air lock, the two workers had locked the air lock doors. As a consequence of the event, the value of the depression in the containment was smaller, during the period between 09.10 and 09.19 am, than the 1.5 mbar as specified in the Technical Specification. The “smallest” value of the depression was 0.3 mbar.
Reasons for the event rating: There was no initiating event. The safety functions affected were available within the operational limits and conditions. Therefore, the event was initially rated to INES 0 as specified by Table 4 in Section 220.127.116.11.3.b of the INES Manual.
Consideration of aggravation factors: During the event, the rules of accessing the containment were serially violated by the maintenance personnel. In addition, this type of human error is considered recurring due to an event of similar cause that occurred two years ago. Furthermore, human errors committed during the maintenance work refer to the lack of basic knowledge, which resulted in the violation of the Work Site Radiation Protection Rules and this fact points to safety deficiencies. Consequently, the reclassification of the event to one higher INES category is justifiable, as specified in Section 18.104.22.168.3. c) of the INES Manual.
The final INES rating of the event is: 1
Event date: Mon, 07-07-2008