On completion of unit preventive maintenance at Ntherm. - 655 MW and Nnom. 0 MW testing and adjustment of Main Safety Valves (MSVs) was in progress. Four MSVs have been tested in turn, and only one pilot valve (PV) functioned as designed during the test. The remaining PVs failed to completely close (remotely) and were giving oil steam. This led to pressure increase in the sealed rooms up to the setpoint of AZ-5 emergency protection actuation. A maximum design-basis accident signal was formed (MDBA signal) in the protection systems logic. Group 1 hydrovolumes of ECCS were activated. The mechanisms of ECCS subsystems 1,2 and 3 failed to adequately respond to the design algorithm. 7 mins. after the MDBA signal was formed in the 2nd set of protections logic the same signal was generated in the 1st set of protections logic. The mechanisms of ECCS subsystem 1 started and functioned according to the design algorithm. ECCS subsystems 2 and 3 failed to respond as was designed. Subsequently the primary circuit was being cooled down by personnel at a rate specified by procedure.
Basis for Event Rating:
Initiator - possible, real (coolant leak through the safety/relief valves system).
Safety function availability - adequate.
According to Table A2 level 3 (C-3R). The event reveals deficiency in the organization of operations, safety culture deficiencies. Level 3 has been chosen.
BASIS FOR RATING:
DIFFICULTIES IN RATING:
Event date: Mon, 22-07-1991