Why is duplex stainless steel used in cooling water piping system of nuclear power plant?
According to the annual report of the World Nuclear Association, global nuclear power generation exceeded 250 billion kwh in 2018, accounting for 10.5% of the global power supply. As of June 2019, there are 54 nuclear power units under construction in the world. As a clean energy, nuclear power is the main contributor to reduce carbon emissions in the world, and it can play a greater role in the future. Cooling water piping system is an essential part of nuclear power plant, which provides a market opportunity for the application of duplex stainless steel piping.
The key factor for safe operation of nuclear power plant is cooling water piping system. This complex system consists of thousands of feet of pipe, ranging in diameter from large to small, and its function is to provide reliable water supply for cooling nuclear power plant equipment. There is usually a non security level operating system and a security level emergency system. The non safety piping system must provide enough cooling water for normal cooling of nuclear power equipment. The safety level system must provide enough cooling water to keep the reactor under control and shut it down safely in case of emergency. The piping materials used in these systems must be resistant to the corrosion of cooling water throughout the service life of the equipment.
Depending on the location of the plant, the types of cooling water range from relatively clean fresh water to contaminated seawater. Therefore, the corrosion degree of pipeline materials varies greatly. Experience has shown that with the aging of the system, various corrosion problems may occur, which, if not solved, will cause damage to the system and prevent it from providing the required cooling water.
Materials and problems of cooling water piping system
When problems occur in the cooling water piping, the material and its interaction with the cooling water are usually involved. The most common problem is the leakage caused by scaling (blocking) and corrosion of the system. Scaling is often caused by sediment accumulation, marine biofouling (biofouling), accumulation of corrosion products, and blockage of foreign materials. Leakage problems are usually caused by microbial corrosion (MIC). Mic is a very aggressive form of corrosion caused by some microorganisms in water. This form of corrosion often occurs in carbon steel and low alloy stainless steel.
Scaling / biofouling in carbon steel pipes is a typical problem in water supply pipelines
Most nuclear power plants operating in North America used to use carbon steel piping systems, sometimes coated or lined with cement or rubber. Due to the reliability problems, continuous monitoring and maintenance cost are very high. As a result, many units have replaced carbon steel piping with more corrosion-resistant materials. In order to meet the design life of the new nuclear power plant for more than 60 years, materials with better performance will be considered.
Stainless steel has always been considered a viable option for the construction of new water supply piping systems and the maintenance or replacement of existing carbon steel systems. 304L, 316L or 6% Mo stainless steel are commonly used in pipeline upgrading scheme. However, 316L stainless steel and 6% Mo stainless steel have a big difference in performance and price. If the cooling medium is untreated water with strong corrosivity and the risk of microbial corrosion, 304L and 316L are not suitable choices. As a result, nuclear power plants have to upgrade to 6% Mo stainless steel or accept high maintenance costs for carbon steel systems. Carbon steel lined piping is still used in some nuclear power plants because of the lower initial cost.
According to ASTM a240, the chemical composition range of stainless steel commonly used in industrial water supply piping system (%）
| UNS Number | C | N | Cr | Ni | Mo | Cu | |
| 304L | S30403 | 0.03 | – | 18.0-20.0 | 8.0-12.0 | – | – |
| 316L | S31603 | 0.03 | – | 16.0-18.0 | 10.0-14.0 | 2.0-3.0 | – |
| 6% Mo | N08367 | 0.03 | 0.18-0.25 | 20.0-22.0 | 23.5-25.5 | 6.0-7.0 | 0.75 |
| 2205 | S32205 | 0.03 | 0.14-0.20 | 22.0-23.0 | 4.5-6.5 | 3.0-3.5 | – |
2205 duplex stainless steel proved to be a good choice
Duke power’s katoba nuclear power plant in South Carolina is the first nuclear power plant to use 2205 (UNS S32205) duplex stainless steel in its system. The grade contains about 3.2% molybdenum, and its corrosion resistance is improved, and its microbial corrosion resistance is significantly better than that of 304L and 316L stainless steel. The above ground carbon steel lined piping of the piping system to convey makeup water to the main condenser cooling tower was replaced with 2205 duplex stainless steel piping.
The newly replaced 2205 duplex stainless steel pipe was installed in 2002. The length of the pipe is 60 meters, the diameter is 76.2 cm and 91.4 cm, and the wall thickness of the pipe is 0.95 cm. The system was built in accordance with ASME b31.1 power piping code case 153. This code is one of the management standards for safe use of power plant piping system, which is widely used in the world.
Large diameter 2205 duplex stainless steel pipe installed in katoba nuclear power station
After 500 days of service, the system was thoroughly inspected. No scaling or corrosion was found during the inspection, and 2205 duplex stainless steel performed very well. For more than ten years since installation, 2205 stainless steel pipe has been performing well. Based on this experience, Duke power has used 2205 duplex stainless steel piping in other parts of its system.
2205 days after use, inside the pipe. After removing the surface stains, no corrosion of the weld was found
Now, designers of nuclear power plant water supply system have another choice when choosing the pipe material of corrosion-resistant cooling water. The successful application of 2205 duplex stainless steel can reduce maintenance cost, reduce downtime and ensure the safety of nuclear power plant.
Source: China Duplex Stainless Steel Piping System Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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