Flow Accelerated Corrosion in Nuclear Power Plants

Corrosion erosion is a chemical degradation that corresponds to a special case of generalized corrosion. In nuclear power plants, it particularly affects unalloyed or low-alloyed steel components subjected to single-phase liquid water or two-phase wet steam flow. It results in a loss of thickness that can ultimately lead to component failure. This phenomenon occurs on many components in the water-steam circuit of nuclear and thermal power plants. It has already been the cause of fatal accidents in the USA and Japan. In addition to the loss of component thickness, this phenomenon generates corrosion products which are carried to the steam generators (SG), where they become a source of fouling and clogging. In the long term, this can lead to a loss of thermal performance in power plants, necessitating costly maintenance operations. Corrosion erosion is influenced by temperature and by chemical, hydraulic and material-related parameters. For nuclear power plants, it is monitored and controlled in most cases, particularly in France, by calculation codes that predict the loss of thickness of piping components likely to be affected. Calculation results also take into account non-destructive testing (NDT) carried out in the plant to optimize maintenance operations and avoid critical thickness losses. These codes are developed on the basis of research and development studies and feedback.

The aim of this article is to present :

• the corrosion-erosion mechanism and the influencing parameters;

• potentially affected components ;

• the impact of erosion-corrosion on power plant operation ;

• monitoring issues for operators and regulations ;

• the methods and tools used to study the phenomenon ;

• the calculation codes used to predict deterioration linked to the phenomenon ;

• dimensional and chemical composition checks on the materials used in the plants;

• parades ;

• other generalized degradation modes ;

• information sharing at the heart of prevention.