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Nuclear plants are built to function for a number of decades. One major difficulty in their maintenance is to find ways to evaluate two types of phenomena associated with corrosion: stress corrosion cracking and activity build-up, or the accumulation of activated corrosion products onto the surfaces of the reactor cooling system.
Vital Protective Layer
In nuclear power plants the cooling water temperature reaches nearly 300 °C and its pressure up to 120 bar. Stainless steel or nickel base alloys are used to fabricate the pressure bearing components in contact with the cooling water. When oxygen in water reacts with the outermost layers of the metals a thin oxide layer is formed, slowing down further corrosion. The shielding of this layer could be improved by adding minor alloying elements to the steel.
The flow of the cooling water leads to the release of corrosion products from the thin metal oxide layer. When these materials pass through the core of the reactor, they become activated and are deposited on the inner surfaces of the pipes. Due to the ensuing activity build-up, the maintenance operations of nuclear plants become more expensive in the long term.
Stress Corrosion Cracking
Stress corrosion cracking is the development of cracks in metallic materials, promoted by both stress and corrosion. Even in this phenomenon the rate at which the degradation propagates is affected by the properties of the oxidized layer.
Through international collaboration and studies performed in Finland, it has been feasible to create more accurate techniques to predict the advancement of phenomena associated with corrosion and stress corrosion cracking.
Reducing Activity Build-Up Through Optimized Water Chemistry
Potential deposition of activated corrosion products into the oxide films is a gradual process that may take several years or even decades. Typically it is monitored through measurements during the annual refuelling outage.
Recently, a number of monitoring methods have been devised that can be applied even when the plant produces power. Some of these techniques have been commercialized and offered to power plants and process industries in various countries. At nuclear power plants in Finland these systems are being employed to monitor tasks that will last a number of years. The monitoring system is used in laboratory analysis to design optimal water chemistry conditions for a nuclear power plant. In the same manner, the system is employed to identify the impact of variations in water chemistry conditions on the materials of a plant during the start-up and shutdown processes.
Keeping Cracking in Check
In the recent past, stress corrosion cracking of stainless steel welded pipes in boiling-water reactors was a global challenge. In Finland, this problem was detected very early due to international collaboration and the cost of the necessary renovation work was comparatively low. Currently, the major stress corrosion cracking challenge is related to parts that are affected by radiation in the reactor. Irradiation-induced stress corrosion cracking is a process that progresses very gradually, taking years or even decades.
On detecting signs of cracking at the time of the annual refuelling outage, it is necessary to make a decision whether maintenance work will be performed immediately or if it can be delayed and included in the scheduled repair work for the following year. It is essential for the decision to be based on adequate knowledge of the rate at which the crack advances.
Factors that determine the cracking speed are one of the principal subjects of stress corrosion studies. These studies are performed under simulated power plant conditions and enable decisions to be made related to the urgency of maintenance tasks at nuclear power plants.
The mechanism called transpassive corrosion is another corrosion-related problem that specifically affects boiling-water reactors. In such reactors, the coolant water at the reactor core has powerful oxidizing properties. During transpassive corrosion, the chromium from the surface of nickel-base alloys and stainless steel gets dissolved. It is presumed that transpassive corrosion has an impact on the initiation of irradiation-induced stress corrosion cracking.
Currently, research work is ongoing to check the implications of transpassive corrosion at the reactor core and to verify the water chemistry conditions that are most effective in bringing the transpassive corrosion rate down.