The NPP Leibstadt Program for the Prevention of Piping Degradation due to Flow-Accelerated Corrosion Phenomena

Safety-related aspects and the targeted high availability of the power plant as well as the requirements stipulated by Swiss regulatory authorities prompted the operator of NPP Leibstadt (KKL) to introduce a procedure for the early identification of susceptible piping sections and components due to flow-accelerated material degradation mechanism. Flow-accelerated corrosion (FAC) is a degradation process resulting in wall thinning of piping, vessels, heat exchanger and further equipment made of carbon and low alloy steel. The FAC degradation mechanism occurs only locally under specific condition of flow, water chemistry, temperature and materials applied. In order to deal with the flow-accelerated corrosion issue, the NPP Leibstadt (KKL) adopted the computer code COMSY for the preparation of targeted wall thickness inspection programs. In a first step a systematic screening procedure was applied with the intension to reliably identify system areas which may be subject to a flow-accelerated corrosion attack. For system areas sensitive to degradation a detailed lifetime analysis was performed for individual piping elements. Based on the predicted service life, elements were prioritized for inspection programs. The results of examination were fed back to further optimization of the lifetime predictions. Due to systematic approach applied, a significant gain of knowledge be achieved regarding the impact of FAC on a plant-wide basis. The combination of predictions, the measured data and other inspection results (e.g. visual examinations) are used to prepare a catalog of weak points. The weak-point catalog is thus a tool for as-is information and inspection of piping systems / components. It serves to optimize the condition-oriented planning of future measures and inspections and will be revised and updated on an annual basis.