Stainless Steel Pipe Fittings

Application of high performance stainless steel tube for nuclear power

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In recent years, the quality, performance and greening level of China’s key basic steel new materials have been significantly improved. The development of new materials industry has also provided a new opportunity for the transformation and upgrading of iron and steel enterprises. At the same time, steel enterprises must also establish a sound upstream and downstream industry. Cooperation mechanism to accelerate the industrial application of new steel materials. The future high-performance, high-tech, high-quality “three high” steel new materials will become the main direction of high-quality development of the steel industry. In order to accelerate the research and development of new steel materials in China, this newspaper specially organized a special report on this new material, with a view to promoting the iterative upgrading of new steel material manufacturing technology from the aspects of production and application.
In recent years, China has accelerated the pace of development of nuclear power. The development of nuclear power will not only improve China’s energy supply structure, but also help protect national energy security and economic security. According to the National Medium- and Long-Term Development Plan for Nuclear Power, by 2020, China’s nuclear power installed capacity will reach 58 million kilowatts. However, as of June 2018, there were 39 power stations in China, with an installed capacity of about 38 million kilowatts, accounting for about 4% of the nation’s electricity supply; 18 units under construction, with an installed capacity of about 20 million kilowatts. It can be seen that the current construction progress of nuclear power is far from the planned target. In order to complete the planning target, at least 6-8 units will be built every year by 2020 to reach the nuclear power ratio of western developed countries.
At the end of April 2018, AP1000’s world’s first three-door No. 1 nuclear power unit and Taishan EPR nuclear power unit were respectively approved by the National Nuclear Safety Administration. At the end of June, Sanmen No. 1 and Taishan EPR units were connected to the grid for power generation, which means China’s nuclear power. Construction will usher in a new period of development.

1 Development of nuclear power technology in the world

The current development of nuclear power technology in the world can be roughly divided into four generations. The first generation technology is mainly a prototype reactor nuclear power plant developed between 1950 and 1960. The second generation technology is mainly a large commercial nuclear power plant developed and built between 1960 and 1990. The vast majority of nuclear power plants are second-generation technologies; the third-generation technologies are mainly AP1000, EPR, CAP1400 and Hualong No.1 developed since 1990-2010, of which AP1000 and EPR units were successfully connected to the grid for the first time in June 2018. Near full power will be connected to the grid for power generation. Domestic third-generation nuclear power units such as CAP1400 and Hualong No. 1 demonstration power station are also in full swing. In addition, the world is actively researching and developing fourth-generation nuclear power technologies, such as gas-cooled fast reactors, sodium-cooled fast reactors, lead-cooled fast reactors, ultra-high temperature gas-cooled reactors, supercritical water reactors and molten salt reactors; Technologies such as nuclear fusion reactors are also actively developing in the process of multinational alliances. Among these advanced nuclear power technologies, especially domestic nuclear power technology, many key equipments and key materials, such as high-performance steel pipes and forgings, need to be localized to narrow the gap between China and the world’s advanced countries in the field of nuclear power, and for large commercial nuclear power plants. Localization lays the foundation.
Therefore, it is expected that stainless steel pipes for nuclear power equipment will usher in good development opportunities in the next five years. In the materials used in pressurized water reactor nuclear power plants, a large number of high-performance steel pipes, forgings, pipe fittings and other materials are required, and many types of materials are involved, such as alloy steel, stainless steel, zirconium alloy, titanium alloy, and nickel-based alloy.

2 Various types of stainless steel tubes for nuclear power

The nuclear island is a general term for nuclear reactors and reactor-related systems in the containment of nuclear power plants, including nuclear steam supply systems, containment sprinkler systems, and auxiliary systems. A large number of high performance stainless steel tubes are required in the nuclear island system.

2.1 steam generator U-shaped heat transfer tube

The steam generator heat transfer tube is an important component of the primary circuit pressure boundary of the nuclear power plant, and is also the main barrier to prevent the leakage of radioactive fission products. The steam generator heat transfer tube must withstand high temperature and high pressure and medium corrosion wear during service. Quality is the key to ensuring safe and reliable operation of the steam generator. According to statistics, about 30% of the pressurized water reactors are unplanned and shut down due to corrosion damage of the steam generator heat transfer tubes.
At present, the steam generator heat transfer tube is mainly selected from 800 alloy and 690 alloy. Due to the high requirements on surface quality, size control, heat treatment system and organization control, steam generator heat transfer tubes have high technical barriers, and there are few domestic and foreign manufacturers. Currently, foreign countries are mainly Valluic, France, Sumitomo Metals, and The three companies of Sandvik, Sweden, are mainly Zhejiang Jiuli and Baoyin.
The third-generation nuclear power technology steam generator heat transfer tube is mainly selected from 690 alloy. A million-kilowatt unit requires more than 10,000 heat transfer tubes, with a total weight of more than 200 tons. The main specifications are Ф19.05mm×1.09. Mm and so on. The main production process of steam generator heat transfer tubes includes slab smelting and forging, hot extrusion forming, cold forming, solution heat treatment and TT heat treatment, non-destructive testing and tube forming.
At present, Jiuli has completed the manufacture and installation of the 690 alloy steam generator heat transfer tubes required for Hualong No.1, CAP1400 and export overseas projects, and has been well received by customers.

2.2 Other stainless steel tubes for nuclear islands

The primary circuit of the primary circuit is an important barrier to prevent nuclear fission products from leaking out to the containment under normal, abnormal, accidental and experimental conditions. Main pipes are usually required to withstand high temperatures, high pressures and corrosion. Early primary pipes generally used 18-8 austenitic stainless steel, but there were problems such as insufficient strength and intergranular corrosion. Subsequent development of the casting duplex stainless steel main pipeline due to aging phenomena and other deficiencies, the current third-generation nuclear power AP1000 and other primary circuit uses the overall forging shape of 316LN austenitic stainless steel, with high strength, good plastic toughness and other characteristics, currently domestic The manufacturers have mastered the key technologies of large-scale steel ingot smelting, large-scale forging of large stainless steel materials, and overall processing of pipe sections and achieved localization.
The passive safety principle is a prominent feature of the third generation nuclear power technology. The passive residual heat removal heat exchanger is the core equipment of the core cooling system and plays an important role in the safe operation of the reactor. Among them, the non-dynamic residual heat discharge heat exchanger is connected between the upper and lower heads through a C-shaped heat exchanger tube. The material of the heat exchange tube mainly includes TP304L and 690 alloy, etc. The specifications are mainly Ф19.05mm×1.24mm, Ф19.05mm ×1.65mm, etc., each million-kilowatt unit requires a C-shaped heat transfer tube of about 1t.
The reactor pressure vessel is an important equipment in the main circuit coolant pressure boundary barrier. It is a nuclear safety primary equipment and cannot be replaced during the service life of the nuclear power plant. Its service life determines the service life of the entire nuclear power plant. Since the container contains high temperature, high pressure and radioactive working medium, it is required that the structure can be kept intact under various working conditions without cracking or leakage. Therefore, the sealing design and material selection of the reactor pressure vessel are safe for the equipment. Operation is crucial.
At present, in the reactor pressure vessel seal design, the double-channel metal O-ring is sealed between the cylinder and the top cover, that is, the double-ring groove is opened on the upper flange sealing surface to form inner, middle and outer three islands, arranged inside, The outer two metal O-rings form a two-channel metal O-ring seal. In the third-generation nuclear power technology, the O-ring seal ring mainly adopts the 718 alloy seamless pipe, and the specifications mainly include Ф30mm×1.2mm, Ф12.7mm×1.27mm, and the like.
O-ring seal ring with 718 alloy seamless tube, due to high requirements on surface quality, size control, heat treatment system, resilience performance, etc., there are high technical barriers, mainly monopolized by American companies, there is a long product supply cycle, the price Expensive and other features. Since 2014, Jiuli has successfully developed the 718 alloy seamless tube for the sealing ring, which has the characteristics of good surface quality, high dimensional accuracy and excellent resilience performance. In addition, there is only one solder joint when soldering the O-ring, which reduces the difficulty of heat treatment and inspection. At present, the product has been successfully used in domestic nuclear power plants.

2.3 Conventional island stainless steel tube

Most of the nuclear power plants are built along the coast, and the loop uses a large amount of seawater as a cooling medium. Due to the inherent properties of titanium, titanium welded pipe is particularly suitable for use as a condenser tube for seawater or heavily polluted fresh water as a cooling medium due to its excellent corrosion resistance, erosion resistance, thermal conductivity and mechanical properties. In a million-kilowatt nuclear power unit condenser, the titanium tube used in each unit is about 200t. However, due to the difficulty in manufacturing titanium tubes, it is mainly difficult to weld and heat-treat. Foreign manufacturers mainly produce manufacturing enterprises such as the United States and Japan. At present, most of the titanium tubes used in nuclear power plant condensers rely on imports, and the domestic titanium welded pipe market has a large gap.
Therefore, it is necessary to study the production process of high-quality thin-walled titanium welded pipe to meet the growing domestic construction demand. The current condenser tube specifications are mainly mm22mm×0.5mm, and the titanium material is mainly TA2 (Gr.2).
The high-pressure feed water heater is an important feed water heating device in the regenerative system, which plays an important role in the economics of the second-circuit operation of the unit. The heat exchanger usually adopts a U-shaped heat exchange tube. TP439 ferritic stainless steel has high strength, good erosion resistance and wear resistance, low price and good resistance to chloride ion corrosion. It is often used as the main material for U-shaped heat exchanger tubes and is widely used in foreign nuclear power plants. At present, the TP439 welding U-shaped tube of a million kilowatt nuclear power unit is about 150t, and the main specifications of the required products are Ф16mm×1.5mm.

2.4 Advanced stacking stainless steel tube

While vigorously building the third-generation commercial nuclear power plants, many research institutions at home and abroad are vigorously researching and constructing the test reactors and demonstration reactors of the fourth and fifth generation nuclear power technologies, mainly including bismuth-based molten salt reactors and sodium-cooled fast reactors. , lead-type fast reactor, high temperature gas cooled reactor, nuclear fusion reactor and other pile types. In the construction process of the test reactor and the demonstration reactor, various high-performance stainless steel pipes are also required, and the materials involve stainless steel and nickel-based alloys.
The bismuth-based molten salt reactor is one of the six fourth-generation nuclear power alternative reactor types. It has atmospheric pressure, no water cooling, no need to make fuel components, and can realize on-line fueling and online post-processing. However, it has problems such as high service temperature and strong corrosion of molten salt. High requirements are placed on the main pipe and heat exchange pipe materials. At present, the main material selection is nickel-chromium-molybdenum alloy, such as GH3535 alloy. The manufacture of GH3535 alloy heat exchange tube has problems such as smelting difficulty, easy segregation, and hot processing. There are few domestic and foreign manufacturers.
The sodium-cooled fast reactor is also one of the six fourth-generation nuclear power alternative reactor types. It is the second step of China’s nuclear energy three-step development plan and an important part of China’s nuclear energy closed cycle. Compared with pressurized water reactors, sodium-cooled fast reactors have higher temperature (above 500 °C) and higher irradiation dose. Therefore, higher requirements are placed on the structural components of the core assembly, such as stainless steel cladding tubes. In addition, sodium cooled fast reactor steam generators, heat exchangers and other pipelines also require a large number of high-performance austenitic stainless steel pipes, such as 316Ti, 316H and so on. The specifications of high-performance stainless steel pipes involved in sodium-cooled fast reactors are mainly mm6.0mm×0.4mm, Ф22 mm×1.4mm, etc.

3 Conclusions and prospects

Zhejiang Yaang Pipe Industry Co., Limited has long been committed to the research and development and production of high-performance stainless steel tubes for nuclear power. Its related products have been widely used in domestic and foreign nuclear power plants, such as 690 alloy, 800 alloy steam generator U-shaped The heat transfer tube, the O-shaped seal ring 718 alloy tube, the TP439 high heat exchanger tube, the condenser titanium welded tube, the internal component tube, the nuclear grade instrument tube, etc., have been recognized by the majority of users.
Nuclear energy is an important direction for future energy development. With the research and development of third- and fourth-generation advanced nuclear power technologies, nuclear islands and conventional islands require a large number of high-performance stainless steel pipes, especially with the “Belt and Road Initiative” initiative. China’s nuclear power will go global and require localized steel products and technologies. Therefore, steel companies must strengthen the research and development and application of advanced materials and advanced manufacturing processes, and develop a series of high-performance stainless steel pipes required for advanced nuclear power technology.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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