UNIT SCRAM DUE TO INADVERTENT ACTUATION OF PRESSURIZER PILOT RELIEF VALVE
Wednesday, November 3, 1993
The unit was operating at 330 MWe power. Due to inadvertent opening of one out of two pressurizer pilot relief valves (PRV) primary circuit pressure drop occurred. On primary pressure drop to 115 kgf/cm2 AZ-1 protection actuated and the reactor was scrammed. All safety systems functioned as designed. While safety system operation during the 1 hr 10 mins. period several PRV actuations occurrred; primary pressure varied in the range of 128-72 kgf/cm2; as a result of pressurizer PRV and AZ-1 protection actuation the temperature droppped from 283 C to 254 C during 7 mins. The cause of pressurizer PRV actuation was short-circuit between cable conductors from electric contact manometer (ECM) in the sealed penetration which led to the formation of permitting signal at the input of the pressurizer PRV control logic unit. Since insulation resistance varied from 0 to 1.5 mOhm, the signal at the logic unit input reoccurred for short periods of time. This has led to the appearance of oscillating voltage on the opening solenoid causing repeated opening/closing of pressurizer PRV. The reactor was brought to subcriticality and normal shutdown cooling was maintained.
Basis for rating: 1. No off-site impacts. 2. No on-site impacts. 3. This event is rated under the "degradation of defence-in-depth" criterion. Two real initiators were observed: a) primary pressure drop as a result of inadvertent actuation of active components (initiator frequency - high expected). b) reactor scram (initiator frequency - high expected). Safety function availability - full. For each of the intiators in question the level according to Table II of the INES user's Manual (1992 revision) is 0. This event is possibly related to safety culture deficiencies, namely, deficiency in the process of quality assurance (sealed penetration inspection prior to and during operation, absence of thermal shrinking tube on the cable conductors passing through the stopper device of the sealed penetration). Therefore Level 1 has been chosen.