Applications of seals in nuclear power plants

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With the development of industrial technology, high-performance seals have been widely used in nuclear power plants, and they also play an important role in ensuring the safe and stable operation of such plants. Taking the units with the largest power generation capacity in China at present as examples, this text explains the application and role of seals in nuclear power plants. Nuclear power plants require higher levels of safety and technological maturity compared to civil thermal power plants, hydroelectric stations, and other types of power generation facilities. Therefore, the seals installed in nuclear power plants also need to possess high safety, high reliability, and uniformity for easy replacement. Seals are widely used in the design of the structures, systems, and components (SSC) of nuclear power plants. They include, among other things, sealing rings on doors that are used to meet the sealing requirements of the structural elements; seals for pipeline flanges in process fluid systems; seals for flanges on various pressure-bearing equipment; and especially seals for the roof of the pressure vessel that constitutes the core of a nuclear reactor, one of the key pieces of equipment in nuclear power plants. 1: Application of seals in structure design. Taking the seals used in the underwater sealing doors installed between the fuel pool and the fuel transfer channels in nuclear power plants as an example, this section discusses the design characteristics, structural features, and sealing performance requirements of seals that are used to meet the functional demands of such structures. In nuclear power plant design, underwater seal doors are items related to nuclear safety. The seals of these doors are key components for ensuring their sealing performance; when the door is in the closed position, inflation of the cavity inside the seal causes it to expand and press against the surrounding plates, thereby achieving sealing. The medium surrounding these seal doors is boron-containing water, with a boron content of 2000 to 2500 ppm, and the pH value of this medium at 250°C ranges from 4.7 to 5.5. When the sealed door is closed, there is a significant pressure difference on either side of the door; when one side has no water while the other side has normal water levels, the difference in water level can reach 6.5 meters. Therefore, the material used for the seals must possess both good elasticity and good toughness, and butyl rubber is typically chosen for this purpose. II: Application of seals in system design. In nuclear power plant design, seals are most widely used in process fluid systems. The pipe flanges in these fluid systems are generally designed in accordance with standards, with the ASME series being the main standard used, such as ASME B16.5 and ASME B16.47. The gaskets used to seal between flanges are designed to match the structure of those flanges; selecting appropriate seals is crucial for ensuring the proper and safe operation of the process fluid systems. The selection of gaskets for pipeline flanges is closely related to the medium in the fluid system to which the flange belongs, as well as the design temperature and design pressure. In current nuclear power plants, the size range of pipe flanges is 1/2” to 38” (DN15 to DN950), and the flange class range is Class150 to 2500 ； The design temperature range for the process systems in the nuclear island of nuclear power plants is: 15°C to 343°C℃ ； The design pressure range is: 0.1 MPa to 20.3 MPa ； The main fluid media include: reactor coolant, boron-containing water, steam, cooling water, deionized water, 30% sodium hydroxide solution, 7% nitric acid solution, oxygen, nitrogen, diesel, etc. The main types of gaskets used for pipeline flanges in process systems include: metal-graphite gaskets, flexible wound gaskets with inner and outer reinforcement rings, and non-metallic flat gaskets. 1. Applications of metal-graphite gaskets: In critical pipeline systems for nuclear safety, metal-graphite gaskets are primarily used. The main feature of these gaskets is the application of a \"metal-to-metal\" sealing concept. They consist of inner and outer metal rings along with a graphite ring; the inner and outer metal rings play different roles in ensuring the seal, while the graphite ring serves as the sealing element of the gasket. This sealing structure ensures that the gasket remains in a sealed state, maintaining full contact between the metal ring of the gasket and the flange’s sealing surface, thereby ensuring constant working stress on the graphite sealing ring ； Furthermore, a metal-to-metal sealing structure can also effectively reduce the adverse effects of applied bending moments, as well as temperature and pressure fluctuations, on the sealing surface. The sealing ring in the metal-graphite gasket is formed by compressing high-purity expanded graphite sheets; high-purity graphite rings possess excellent radiation resistance. The metal retaining ring is made of austenitic stainless steel with good corrosion resistance, with the main material grade being 316L. The current size range of metal graphite gaskets is 1/2” to 24” (DN15~DN600), and they are suitable for flanges of various grades manufactured in accordance with ASME B16.5. Metal-graphite gaskets require raised face flanges for the flange sealing surfaces; to ensure the sealing performance of the gasket, the roughness of the flange sealing surfaces should not exceed 3.2 μm. The good sealing performance of metal-graphite gaskets is shown in Table 1. 2. Flexible wound gaskets with inner and outer reinforcing rings are also widely used in the process fluid systems of nuclear power plants. The structural dimensions of these flexible wound gaskets comply with the requirements of standard ASME B16.20; the inner and outer rings as well as the metal strips are made of austenitic stainless steel, while the filler strip is made of pure graphite. Flexible wound gaskets are widely used in industries such as machinery and chemicals. The flexible wound gaskets used in nuclear power plants have requirements for graphite that differ from those in other industrial sectors; details are provided in Table 2. 3. Non-metallic flat gaskets: In the water systems of nuclear power plants, especially those using seawater, the diameter of the system pipes is large, and the flanges used in these pipelines are of large diameter with flat face sealing surfaces. To ensure good sealing of the system pipes, 100% expanded polytetrafluoroethylene (e-PTFE) non-metallic flat gaskets are employed in the design. Due to the material properties of PTFE gaskets – namely 100% multidirectionally expanded polytetrafluoroethylene – these gaskets possess excellent resistance to chemical corrosion. The main performance characteristics of these gaskets are shown in Table 3. III: Applications of seals in equipment components. Flange seals used in pressure-bearing equipment are typically custom-made or come in non-standard dimensions, as per the design drawings. The materials used for these gaskets include EPDM rubber, asbestos-free materials, polyethylene, and other similar substances. Gaskets used in pressure-bearing equipment are mainly applied at flange connections such as those found on equipment manholes. The metal C-ring seals used on the reactor pressure vessel, which is a core component of nuclear power plants, possess the following characteristics as metal seals: a linear sealing structure, high gasket strength, and low leakage rates. To ensure the sealing performance of these metal seals, it is necessary that the sealing surfaces have high precision in fabrication, with the surface roughness of those surfaces controlled within the range of 0.8 to 1.6 μm. In summary, seals, as accessories for pipelines in process systems, are widely used in the primary and secondary circuits of nuclear power plants as well as in auxiliary process systems. The design and selection of seals ensure high sealing performance for the structures, systems, and components in nuclear power plants. Since seals are single-use replacement parts, their manufacturing must exhibit high consistency. As the weak links in pressure-bearing boundaries, seals need to maintain safe and reliable performance under all circumstances. The aforementioned seals are widely used in the nuclear power industry, where they have demonstrated excellent performance and positive feedback from users. The concept of efficient and reliable sealing design is widely applied in nuclear power plant design.