Background

Nuclear plants are designed for decades of operation. One of the challenges in their maintenance is how to predict two types of phenomena related to corrosion: stress corrosion cracking and activity build-up, i.e. deposition of activated corrosion products onto the surfaces of the reactor cooling system.

Vital Protective Layer

In nuclear power plants the temperature of the cooling water reaches about 300oC and the pressure up to 120 bar. The pressure bearing components in contact with the cooling water are made of stainless steel or nickel base alloys. Oxygen in water reacts with the outermost layers of these metals, forming a thin oxide layer that slows down further corrosion. Minor alloying elements added to the steel may enhance the protectiveness of this layer. Corrosion products are released from the thin metal oxide layer by the flow of the cooling water. These particles become activated as they pass through the core of the reactor, and are deposited on the inner surfaces of the pipes. The resulting activity build-up tends to make the maintenance operations of nuclear plants more costly in the long run. Stress Corrosion Cracking Stress corrosion cracking is the growth of cracks in metallic materials, enhanced by both stress and corrosion. In this phenomenon, too, it is the properties of the oxidised layer that affect the pace at which the degradation proceeds. As a result of international collaboration and research carried out in Finland it has been possible to develop increasingly accurate methods for predicting the progress of phenomena related both to corrosion and stress corrosion cracking. Reducing Activity Build-up By Optimised Water Chemistry Possible accumulation of activated corrosion products into the oxide films is a slow process that may take years or even decades. It is usually monitored by measurements carried out during the annual refuelling outage. Monitoring techniques have been developed recently that can be used also while the plant is producing power. Some of these methods have been commercialised and delivered to power plants and process industries in several countries. At Finnish nuclear power plants these systems are being used in monitoring tasks that will last for several years. In laboratory studies the monitoring system is used in designing optimal water chemistry conditions for a plant. Similarly, the system is used to determine how changing water chemistry conditions affect a plant’s materials during the shutdown and startup processes. Keeping Cracking in Check A few years ago stress corrosion cracking of welded pipes made of stainless steel was a worldwide problem in boiling-water reactors. As a result of international collaboration this problem was detected at an early stage in Finland, and the cost of the required renovation work was relatively low. The greatest stress corrosion cracking challenge of today concerns parts that are affected by radiation in the reactor. Irradiation assisted stress corrosion cracking is a process that advances very slowly, taking years or even decades. If indications of cracking are found during the annual refuelling outage, a decision has to be made whether maintenance work will be carried out right away or whether it can be postponed so that it can be included in the planned repair work for the following year. The decision has to be based on sufficient knowledge of the rate at which the crack grows. Factors determining the speed of cracking are one of the main subjects of stress corrosion research. This research is carried out in simulated power plant conditions and facilitates the making of decisions about the urgency of maintenance tasks at nuclear power plants. Transpassive Corrosion Another corrosion-related problem is a mechanism known as transpassive corrosion, which affects in particular boiling-water reactors. In these reactors the coolant water at the reactor core is strongly oxidising. Transpassive corrosion involves the dissolution of chromium from the surface of stainless steel and nickel-base alloys. It is suspected that transpassive corrosion affects the initiation of irradiation assisted stress corrosion cracking. At the moment studies are being carried out to see what the significance of transpassive corrosion at the reactor core is and which water chemistry conditions are most effective in reducing transpassive corrosion rate.