During 1991 refueling outage the two pressurizer valves of the Stade NPP have been replaced with valves of a new design qualified for feed and bleed. Going back to power operation on June 5, 1991 minor leakage of coolant to pressurizer relief tank has been observed. The leakage was caused by seat leakage of 5 out of 6 pilot valves of the pressurizer safety valves. The licensing authority allowed power operation to continue unless the limit of leakage specified in the operation manual would be exceeded. After it became evident that the specified limit of leakage would be exceeded in a short time, the plant was brought to cold shutdown by the operator on August 14, 1991 in agreement with the authority. In depth investigations identified thermal stresses due to temperature differences in the valve assemblies as the root causes of the leakage.
BASIS FOR RATING: The event did not cause any radiological consequences off-site or on-site. The limit for leakage specified in the operation manual has not been reached during the event. Thus, the plant was operated within the operational limits and conditions and level zero has been chosen.
DIFFICULTIES IN RATING: The event indicates a common mode effect. But none of the pilot valves affected actually failed. The operation manual specifies limits for pilot valve leakage and actions to be taken if these limits are exceeded. The plant was operated within this specification. Therefore, uprating does not seem to be justified.

Location: STADE
Event date: Wed, 14-08-1991
Nuclear event report
Legenda & explanation