A design study with the COMSY - code for validating new EPR design concerning piping degradation due to Flow-accelerated corrosion

For the save and long-term operation the integrity of the mechanical component and structure is one of the key objectives. For older plants (nuclear as fossil) the aging and plant life management showed a lot of piping to be changed due to degradation caused by Flow-accelerated corrosion (FAC). Replacing large bore piping such as extractions, condensate piping etc. is very cost intensive and leads in some cases to additional shutdown time for the plants. For this and to avoid this scenario for the future, a design study for the water-steam cycle was in progress to investigate the new design of the advanced EPR (European Pressurized Reactor) respective to FAC degradation mechanism. 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 most cases, only a small percentage of mechanical components are subject to significant degradation, e.g. flow accelerated corrosion which may affect the integrity or the function of components. For this purpose AREVA NP GmbH has developed the computer code COMSY, which utilities more than 30 years of experience resulting from operational experience and research activities. The COMSY code provides the capability to establish a program guided technical documentation by utilizing a virtual plant model which includes information regarding thermal hydraulic operation, water chemical conditions and materials applied for mechanical components. It provides the option to perform a plant-wide screening for identifying system areas, which are sensitive for degradation mechanisms like FAC. This software is also to be used for analyzing new designs or changes in system operation conditions (e.g. power uprate) for the effect on FAC degradation rates before implementation and appropriate precautions are initiated. In a first step a systematic screening procedure is applied with the intension to reliably identify system areas which may be subject to a flow-accelerated corrosion attack. If a system area is identified as being susceptible to degradation, a detailed analysis function enables the condition oriented service life evaluation of vessels and piping systems in order to localize and conservatively quantify the effect of degradation. Based on these forecasts with COMSY, specific strategies can be developed to mitigate the effect of degradation and design changes can be focused on degradation sensitive areas. Due to systematic approach applied, a significant gain of knowledge be achieved regarding design variants of FAC on a plant-wide basis. This comparison of design variants are used to discuss the optimum solution for piping design respective to FAC.