Research activities for optimizing pipe wall thinning management

Yoneda K., Morita R., Fujiwara K., Inada F.

Flow accelerated corrosion (FAC) and liquid droplet impingement erosion (LDI) are the main pipe wall thinning phenomena in piping system of power plants in Japan. Authors have promoted the development of prediction method to evaluate local thinning trend by FAC/LDI. To apply the method to pipe wall thinning management in power plants, it is required to be transformed into practical tools for easy usage. In Japan, discussion is being made to considerate the introduction of prediction tools into wall thinning management based on wall thickness measurement at present. Authors have simplified their FAC/LDI models to predict wall thinning trend one-dimensionally along piping layout, and applied to actual thinning data of power plants. With PWR’s FAC data and BWR’s LDI data, maximum thinning rate for each pipe elements were roughly predictable with considerable accuracy. Especially for high thinning rate data, which is important in plant management, the model was able to evaluate within the factor of 2. By installing this model, prediction software “FALSET” was developed, equipped with practical functions for the management. With the further verification and improvement of each function, there are prospects for this software to be utilized as a management tool in power plants.

Fujiwara K., Domae M., Yoneda K., Inada F., Ohira T., Hisamune K.

Flow accelerated corrosion (FAC) of the carbon steel is one of the most important subjects in the coolant systems of the power plants. FAC is influenced by the composition of the material, the flow condition, temperature, and the water chemistry conditions. It is considered that the solubility of iron (Fe) is the most important factor in the water chemistry parameters affecting FAC. In the present study, the effects of temperature and pH on the Fe solubility were evaluated in consideration of the hydrolysis reactions of the ferrous iron, the dissolution equilibria of Fe3O4, FeO, and Fe(OH)2, and the charge balance. The correlation between the Fe solubility and the FAC behavior was discussed. It has been suggested that the product of the Fe solubility equilibrated with Fe3O4 and the mass transfer coefficient can explain the temperature and pH dependence of FAC. These results indicate the presence of the magnetite on the surface of the carbon steel. Diffusion of the Fe from the saturated layer to the bulk solution determines the FAC rate from the water chemistry aspect.

Zander A., Janning C.

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.