The World Nuclear Association’s annual report shows that the global nuclear power generation in 2018 exceeded 250 billion kWh, accounting for 10.5% of the global power supply. As of June 2019, 54 nuclear power plants are under construction worldwide. As a clean energy, nuclear power is a major contributor to global carbon emissions reduction, and it can play a greater role in the future. The cooling water system is an indispensable part of nuclear power plants and provides market opportunities for the application of duplex stainless steel pipes.
The key factor for the safe operation of nuclear power plants is the cooling water piping system. This complex system contains thousands of feet of pipes, ranging in diameter from large to small, and its function is to provide reliable water supply for the cooling of nuclear power plant equipment. There is usually a non-safety-level operating system and a safety-level emergency system. Non-safety-grade piping systems must provide sufficient cooling water for normal cooling of nuclear power equipment. The safety-level system must provide sufficient cooling water to keep the reactor under control and shut down safely in an emergency. The piping materials used in these systems must be resistant to corrosion by cooling water throughout the life of the equipment.
Depending on the location of the nuclear power plant, the types of cooling water range from relatively clean fresh water to polluted seawater. Therefore, the degree of corrosion of pipeline materials varies greatly. Experience has shown that as the system ages, various corrosion problems may occur. If not solved, it will cause damage to the system and prevent it from providing the required cooling water.
Materials and problems of cooling water system
When there is a problem with the cooling water pipeline, it usually involves the material and its interaction with the cooling water. System fouling (clogging) and leakage caused by corrosion are the most common problems. The following conditions often lead to fouling: accumulation of sediment, marine organisms (biofouling), accumulation of corrosion products, and clogging of foreign objects. Leakage problems are usually caused by microbiological corrosion (MIC). MIC is a very aggressive form of corrosion caused by certain microorganisms in the water. Carbon steel and low-alloy stainless steel often experience this form of corrosion.
Scaling/biofouling inside carbon steel pipes is a typical problem of water supply pipes
Most nuclear power plants operating in North America used carbon steel piping systems, sometimes coated or lined with cement or rubber. Due to reliability issues, continuous monitoring is necessary and maintenance costs are high. As a result, many units have replaced carbon steel pipes with more corrosion-resistant materials. In order to meet the design life of the new nuclear power station of more than 60 years, the use of better-performing materials will be considered.
Stainless steel has always been considered a viable option for constructing new water supply piping systems and repairing or replacing existing carbon steel systems. The commonly used stainless steel in the pipeline upgrade program is 304L, 316L or 6% Mo stainless steel. The performance and price gap between 316L and 6% Mo stainless steel is large. If the cooling medium is untreated water, which is highly corrosive and has the risk of microbial corrosion, 304L and 316L are not appropriate choices. Therefore, nuclear power plants have to upgrade to 6% Mo stainless steel or accept the high maintenance costs of carbon steel systems. Some nuclear power plants still use carbon steel lined pipes because of lower initial costs.
According to ASTM A240, the chemical composition range of stainless steel commonly used in industrial water supply piping systems (wt%)
2205 duplex stainless steel proved to be a good choice
Duke Power’s Catawba Nuclear Power Plant in South Carolina, USA is the first nuclear power plant to use 2205 (UNS S32205) duplex stainless steel in its system. This grade contains about 3.2% molybdenum, which improves corrosion resistance and is significantly better than 304L and 316L stainless steel. The piping system that transports make-up water to the cooling tower of the main condenser. The above-ground carbon steel lined pipes are replaced with 2205 duplex stainless steel pipes.
The newly replaced 2205 duplex stainless steel pipe was installed in 2002. The pipe length is 60 meters, the diameter is 76.2 cm and 91.4 cm, and the pipe wall thickness is 0.95 cm. The system is constructed in accordance with Case 153 of ASME B31.1 Power Piping Code. This code is one of the management codes for the safe use of power plant piping systems and is widely used in the world.
Large-diameter 2205 duplex stainless steel pipe installed at Catawba Nuclear Power Plant
After the pipeline was used for 500 days, the system was thoroughly inspected. During the inspection, no signs of scaling or corrosion were found, and the performance of 2205 duplex stainless steel was very good. For more than ten years since installation, 2205 stainless steel pipes have been performing well. Based on this experience, Duke Power has used 2205 duplex stainless steel piping in other parts of its system.
After 500 days of use, the inside of the 2205 pipeline. After removing the surface stains, no corrosion of the welds was found
Now, designers of nuclear power plant water supply systems have one more choice when choosing corrosion-resistant cooling water pipe materials. The successful application of 2205 duplex stainless steel can reduce maintenance costs, reduce downtime and ensure the safe operation of nuclear power plants.
