Valve maintenance guideline to prevent thermal fatigue damage of the reactor pressure boundary piping

Low temperature water flow through leaking stop valves into reactor pressure boundary mixes with hot cavity flow in the branch piping, and causes thermal cycles at the mixing portion. The safety injection piping of Farley Unit 2 suffered fatigue damage by the thermal cycles. To prevent this type of damage, it is important to understand the dependency of the thermal cycle characteristics on the leak flow rate through valves, and to maintain the flow rate enough below the critical value in view of the fatigue limit of the material. These were experimentally assessed, and the maintenance guideline of the valves to prevent the pipings damage was discussed.