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Nuclear plants are constructed such that they work for several decades. A major challenge faced in their maintenance is to identify the ways to assess two types of phenomena related to corrosion—stress corrosion cracking and activity build-up, or the build-up of activated corrosion products onto the reactor cooling system surfaces.
Vital Protective Layer
The temperature and pressure of the cooling water in nuclear power plants reach about 300 °C and 120 bar, respectively. The pressure-bearing components in contact with the cooling water are made of nickel-based alloys and stainless steel. A thin oxide layer forms upon the reaction of the outermost layers of the metals with oxygen in water, decreasing further corrosion. The addition of minor alloying elements to the steel could enhance the shielding of this layer.
When the cooling water flows, corrosion products are released from the thin metal oxide layer. These materials pass through the reactor’s core and become activated. Subsequently, they get deposited on the inner surfaces of the pipes. The resulting activity build-up makes the maintenance operations of nuclear plants more expensive in the long run.
Stress Corrosion Cracking
Stress corrosion cracking denotes the formation of cracks in metallic materials, caused by both corrosion and stress. Even in this phenomenon, the rate of propagation of the degradation is governed by the properties of the oxidized layer.
Studies performed in Finland, as well as international collaboration, have made it possible to develop more accurate methods for predicting the progress of phenomena related to corrosion and stress corrosion cracking.
Reducing Activity Build-Up Through Optimized Water Chemistry
The possible deposition of activated corrosion products onto the oxide films is a very slow process that could take a number of years or even decades. In general, this phenomenon is monitored through evaluations at the time of the annual refueling outage.
Several monitoring methods have been developed recently, which can be applied even during the power production stages of the plants. A few of these methods have been commercialized and provided to process industries and power plants in different countries. In Finland, these systems are being used at nuclear power plants to monitor tasks that will run for several years.
In laboratory analysis, the monitoring system is used to create water chemistry conditions that are best suited for a nuclear power plant. Similarly, the system is used to detect the effect of changes in water chemistry conditions on the materials of a plant at the time of the start-up and shutdown processes.
Keeping Cracking in Check
A global challenge faced recently was stress corrosion cracking of stainless steel welded pipes in boiling-water reactors. Thanks to international collaboration, this problem was identified very early in Finland, and the cost of the needed renovation work was relatively low. At present, the major challenge posed by stress corrosion cracking in the reactor is associated with parts that are affected by radiation. Stress corrosion cracking induced by irradiation is a very gradual process that takes several years or even decades to progress.
When indications of cracking are detected during the annual refueling outage, it is crucial to decide whether maintenance work will be carried instantly or whether it can be deferred and included in the scheduled repair work for the next year. It is vital for the decision to be made based on a sufficient understanding of the rate at which the crack progresses.
Factors governing the speed at which the crack advances are one of the main subjects of stress corrosion studies. These studies, which are carried out under simulated power plant conditions, allow decisions to be made with respect to the urgency of maintenance tasks at nuclear power plants.
Another problem related to corrosion, which mainly affects boiling-water reactors, is the mechanism known as transpassive corrosion. The coolant water at the core of such reactors exhibits strong oxidizing properties. When transpassive corrosion takes place, the chromium from the surface of stainless steel and nickel-based alloys tends to get dissolved. Transpassive corrosion is presumed to influence the initiation of irradiation-induced stress corrosion cracking.
At present, studies are underway to verify the consequences of transpassive corrosion at the core of the reactor, as well as to find out the water chemistry conditions that are most effective at curtailing the rate of progress of transpassive corrosion.