The plant was in 60% power operation. During activities to identify the cause of an anomaly in the turbine oil system the operators actuated a turbine trip with the turbine bypass in manual mode and closed. As to be expected by design the reactor was tripped by high main steam pressure and the pressure in the main steam lines was automatically controlled via the main steam safety valves and the main steam relief valves. After about 30 sec the turbine bypass was brought back to operation . This terminated the event.
When performing the routine post trip check the operators recognized that the main steam safety valve and the main steam relief valve of 1 out of 4 steam generators had not opened during the transient. The investigation revealed that the pilot lines of the main steam safety and relief valve station of steam generator 3 were isolated thus preventing operation of the main steam safety and relief valves as well as of the main steam isolation valve which was not demanded during the event. The pilot lines had been isolated for some steam generator maintenance during a preceding short plant shutdown and had not been re-opened. this was not discovered when starting up the plant two days ago and by the routine surveillance before the event.
INES rating explanation: Basic rating level 1 (table 2, B1) because of the amount of available redundancy and diversity of the safety function affected. The rating was upgraded by one level due to deficiencies in administrative procedures and control of safety equipment availability.
Event date: Sat, 06-06-1998