Why is duplex stainless steel used in nuclear power plant cooling water systems?
As a clean energy source, nuclear power is a major contributor to reducing carbon emissions worldwide. The cooling water piping system is the key to the safe operation of a nuclear power plant. It consists of thousands of feet of pipes of various diameters and sizes. It provides a reliable water supply for the cooling of plant equipment. The non-safety piping system must provide enough cooling water to cool the plant, while the safety system must provide enough cooling water to bring the reactor under control and safely shut it down in case of an emergency.
These pipe materials must be resistant to cooling water corrosion throughout the service life of the equipment. Depending on the plant’s location, the type of cooling water can range from relatively clean fresh water to contaminated seawater. Experience has shown that as systems age, a variety of corrosion problems and varying degrees of corrosion can occur, damaging the system and preventing it from providing the required cooling water.
Problems with cooling water piping often involve materials and their interactions with cooling water. Leakage from fouling (plugging) and corrosion of the system are the most common problems, including sediment accumulation, Marine biological attachment (biofouling), accumulation of corrosion products, and blockage of foreign matter. Leakage is usually caused by microbial corrosion (MIC), which is very corrosive corrosion caused by certain microorganisms in water. This form of corrosion occurs frequently in carbon steel and low-alloyed stainless steel.
Stainless steel has long been considered a viable option for building new water supply piping systems and for repairing or replacing existing carbon steel systems. The stainless steel commonly used in piping upgrade solutions is 304L, 316L, or 6%-Mo stainless steel. 316L and 6% Mo stainless steel yo big differences in performance and price. If the cooling medium is untreated water, which is highly corrosive and carries a risk of microbial corrosion, 304L and 316L are not suitable choices. As a result, nuclear plants have had 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 lining pipes because of the lower initial cost. According to ASTM A240，Industrial water supply piping systems are often made of stainless steel below:
| Grades | UNS | 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 | 23.0-25.0 | 6.0-7.0 | 0.75 |
| 2205 | S32205 | 0.03 | 0.14-0.2 | 22.0-23.0 | 4.5-6.5 | 3.0-3.5 | / |
The 2205 duplex stainless steel proved to be an excellent choice. Duke Power’s Catawba nuclear power plant in South Carolina is the first nuclear power plant to use 2205 (UNS S32205) dual-phase stainless steel in its systems. This grade contains approximately 3.2% molybdenum and has improved corrosion resistance and significantly better microbial corrosion resistance than 304L and 316L stainless steels.
The carbon steel lining piping on the overground portion of the piping system conveying the supply water to the cooling tower of the main condenser was replaced with 2205 duplex stainless steel piping.
The new replacement 2205 duplex stainless steel pipe was installed in 2002. The pipe is 60 meters long, 76.2 cm and 91.4 cm in diameter, and the wall thickness of the pipe is 0.95 cm. The system specified in accordance with ASME B31.1 Power piping, which is one of the management codes for the safe use of power plant piping systems and is widely used in the world. After 500 days of service, the system was thoroughly inspected. No scaling or corrosion was found during the inspection. 2205 duplex stainless steel performed very well. 2205 stainless steel piping has been performing well for more than a decade since its installation. Based on this experience, Duke Power has used 2205 duplex stainless steel pipes in other parts of its system.
Internal of 2205 pipe after 500 days use.
Designers of nuclear power plant water systems now have one more option when it comes to choosing piping materials for corrosion-resistant cooling water. The successful application of 2205 duplex stainless steel can reduce maintenance costs, reduce downtime and ensure the operation safety of nuclear power plants.
