The unit has been operating at nominal power since 12.02.91 after the scheduled maintenance outage. 26.02.91 at 03.47 routine operability test of safety system train 1 was in progress. Operating personnel has identified the failure to control the valve at the discharge side of HPSI pump. In-site inspection has revealed disconnected power cable of the above valve. The deficiency was eleminated and valve operability check was performed. 27.02.91 at 11.40 inoperability of HPSI system trains eliminated without reducing the unit power.
Disconnection of power cables from HPSI pump valve motors was performed on 04.02.91 after unit shutdown in the course of cooldown process to avoid reactor vessel water hammer effect in cold condition at possible unauthorized start of these pumps. However, due to deficiencies in the work and surveillance program and in the design as regards valve position monitoring from the control room, the power supply was incompletely restored and this ocurrence was not discovered during the prestart testing.
Breach of unit safe operation condition occurred as regards the number of operable HPSI system trains.
BASIS FOR RATING: Unit operational occurrence - failure of three trains of HPSI system without initiating event. Criterion - degradation of defence-in-depth. Event - postulated, possible. Safety function availability: inadequate, which corresponds to level 2/3 (D-2P). Since the failure existed for a long time and the event cause shows safety culture deficiencies, the level /according to 4.4 a and 4.4 b is adopted as maximum.
DIFFICULTIES IN RATING:
Event date: Tue, 26-02-1991