Fatigue Risk Evaluation of a Pressure Vessel Plug Subject to Flow Induced Vibration

The steam generator (SG) channel head forms part of the reactor coolant pressure boundary and is of high nuclear safety duty. The channel head failure is considered intolerable and there are no reasonably practicable physical provisions available to prevent its failure. Therefore the channel head is classified as either an Incredibility of Failure (IoF) or High Integrity (HI) component and it requires additional analyses and assessments beyond the design code requirements to achieve and demonstrate its structural integrity. The hydrodynamic effects of the primary coolant in the annulus around the drain plug of the SG channel head are not very well understood, but are exacerbated by high flow rates in its immediate vicinity due to typical design details. Vibration of the drain plug due to coolant flow may result in fatigue induced failure of the channel head drain penetration weld. In the study presented here, random vibration analyses and a fatigue assessment have been carried out for a submerged drain plug in a pressurized water reactor (PWR) SG channel head. A finite element (FE) model of the drain plug submerged in water coolant has been developed. Modal analyses confirmed that the natural frequency of the submerged drain plug is significantly reduced by the large hydrodynamic added mass from the surrounding fluid. The fatigue evaluation undertaken using ASME III fatigue curve concluded that the fatigue life usage due to the vibration of the drain plug is negligible even after an extended plant life. Therefore the coolant flow-induced drain plug vibration is not a threat to the channel head integrity.