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Research on sealing materials for nuclear power valves and structural design of packings. Valves used in auxiliary equipment of nuclear power plants are widely employed as medium conveying and control devices during the design, construction, and operation of nuclear power plants. It is an important component in ensuring the safe operation of nuclear power plants. It handles all of the energy conversion and system control functions in a nuclear power plant, such as regulating the core temperature, controlling the flow rate in the circulation systems, managing the input and output of thermal energy, and enabling the isolation or connection of fluids in the pipelines. Compared to ordinary valves, nuclear power plant valves are required to meet specific performance standards and characteristics, in order to ensure safety in use, operational stability under high temperature and pressure conditions, and reliability for 24/7 operation. Due to the special conditions of their operating environment, nuclear power valves require a high level of technological sophistication in terms of sealing. The operation of nuclear power relies on safety as the top priority. Only when valves have reliable performance, responsive operation, excellent sealing capabilities, and a long service life can we ensure the safety of using nuclear energy. According to statistics, a second-generation pressurized water reactor nuclear power plant equipped with two 1 million kW reactors has more than 30,000 valves of various types. As for the third-generation technology of the AP1000, more than 10,000 units are also employed with a modular design. Based on the analysis of operational data from nuclear power plants, approximately 34% of valve failure rates in nuclear power plants are caused by packing leaks at the valve stem. Therefore, there is an urgent need for a high-performance, long-lasting packing to ensure the sealing of the valve stem. Among all natural materials in terms of physical properties, graphite is undoubtedly the ideal sealing material. Deeply processed flexible graphite is the best material for valve stem sealing after asbestos was banned. I. Properties of graphite materials: Graphite is a crystal with a hexagonal planar network structure. The three SP2 hybridized orbitals in each hexagonal plane are arranged at 120° to one another. The π electrons located above and below the plane overlap with each other, forming van der Waals bonds. As a result, graphite possesses good chemical stability and is not corroded by any strong acids, strong bases, or organic solvents. Due to its excellent resistance to irradiation by gamma rays, alpha rays, beta rays, etc., as well as its low cross-section for thermal neutrons, graphite is the only suitable moderator material in nuclear reactors. It can be used as a moderator in nuclear reactors and as a shield against nuclear radiation, making it an important material in the fields of national defense and the nuclear energy industry. Flexible graphite is the most widely used material in the sealing industry today, and it can be used safely within a temperature range of –200°C to 600°C. It features resistance to embrittlement at low temperatures, no aging due to aging effects, resistance to softening at high temperatures, no deformation, and no decomposition. It also possesses excellent plasticity that facilitates processing and shaping, as well as good lubricating properties. As a solid lubricant, it has a lower friction coefficient than lipid-based lubricants under any temperature conditions, making it highly suitable for the frequent movement of valve stems. It also exhibits good impermeability for the sealing of gases and liquids. II. Processing Technology of Graphite: The graphite used in nuclear-grade valve seals is obtained by processing natural minerals through various steps such as mineral processing, purification, chemical intercalation, swelling, rolling, and winding to produce flexible graphite sheets. These sheets are then cut and molded to create valve seals. The graphite particles used as raw materials must be natural, high-quality flake graphite; such materials contain few impurities and have a high purity. After expansion, the diameter of the resulting worm-like particles is large, which results in graphite sheets with high strength, low levels of harmful chemical elements, a stable production process, and excellent plasticity of the final products. A comparison of graphites with different mesh numbers is shown in Table 1; natural crystalline (flaky) graphite is shown in Figure 1, while expanded graphite and sheet products are shown in Figure 2. Requirements for graphite materials used in packing: Unless otherwise specified, the main chemical components, physical properties, and mechanical properties of the flexible graphite packing used in nuclear-grade valves shall meet the requirements specified in Table 2. For irradiation testing, flexible graphite sheet material is generally used as the sample, which is exposed to 1900 KGy of R60Co gamma radiation for 190 hours; no obvious abnormalities are observed on its surface, and it remains smooth in appearance. Similarly, for finished products containing such fillers, the loss in mechanical properties under the same irradiation conditions is ≤10%. III. Design of the packing structure: Most of the sealing packings used in current valves adopt a combined design, with two scrubbing rings at each end of the valve stem responsible for cleaning the surface of the stem, and also serving to protect the pure graphite ring in the middle from being damaged by the medium ； Several high-purity graphite rings are used as the main sealing elements for the entire system. Representative examples in terms of cross-sectional shape design include flat rings, conical rings, V-shaped rings, and butterfly rings. The design parameters for the packing used in nuclear-grade valves must meet the requirements of Table 3. (1) Scouring rings at both ends of the combined packing. It is to prevent erosion and damage to the sealing components when the medium pressure becomes too high ； Secondly, it also has the functions of uniform leveling and transmitting axial preload ； On the contact surfaces of the rings at both ends, a certain degree of roughness should be created, so as to allow for reciprocating friction as the valve stem moves, thereby keeping the surface of the valve stem smooth and clean. Commonly used materials at present include carbon fiber, as well as gasket rings woven from graphite reinforced with metal wires or graphite covered with metal wires. In practical applications, these end rings have certain defects, but they are still suitable for use in sealing ordinary valves. However, it struggles to handle the sealing requirements of high-end nuclear power valves. Here, a high-purity flexible graphite externally woven with nickel wire mesh gasket ring is discussed. Its design purpose is to utilize the softness and high compressibility of graphite, combined with the toughness and high elasticity of nickel wire mesh, in order to enhance the physical and mechanical properties of the product. The scraper rings produced do not contain any reinforcing fibers or adhesives for compounding, thereby ensuring high purity, high carbon content, and low loss on ignition for the graphite rings. Several types of scaling end rings are shown in Figure 3. The graphite scale-scraping ring woven with external nickel wire exhibits superior performance compared to carbon fiber-woven packing rings and graphite packing rings with internal reinforcing fibers; Table 4 shows a comparison of the physical properties of these three types of scale-scraping rings. (2) The middle part of the packing assembly is the main body of the seal. The number of graphite rings that can be used in this set can be calculated based on the design depth of the valve packing box. Since the packing seal is a contact-type fitting seal, it is required to be able to deform easily in order to fit against the inner wall of the stuffing box and the outer surface of the valve stem. The principle of packing sealing is to apply a preload in the axial direction, causing radial deformation that fills the interface between the valve stem and the valve packing box, thereby preventing the medium from leaking out and achieving sealing of the valve stem. When pursuing the most ideal radial deformation. A structural analysis was conducted on several packing sets shown in Figure 4. First, each type of packing was assembled into the valve’s packing box, and then a gland was placed over it with the same bolt pre-tightening torque applied. After maintaining the pressure for a certain period of time, the filler is removed; the positions of each ring are recorded and compared before and after. Measurements show that when a pre-tightening force is applied axially, the ends are the first to deform under stress. By the time the force reaches the middle part of the ring, some of its intensity has been reduced, resulting in an inability for the entire ring to deform radially. This reduces the effective contact area, thereby compromising the reliability of the seal. Figure 5 shows a schematic diagram of the valve packing. Tests revealed that only the packing assemblies with a V-shaped design experienced improved stress distribution; after axial pre-tensioning, the overall deformation was acceptable, but it was not possible to ensure uniform stress across each ring. If necessary, the density of the intermediate graphite ring must also be reduced to achieve good radial deformation. (3) The author studied a type of packing set applied to nuclear power valves (as shown in Figure 6). This design features an elastomeric butterfly shape, in which the junction between the graphite ring and the two scale-scraping rings, namely the areas marked L and 3 in the diagram, is designed to be concave on the top and convex on the bottom. When a pre-tightening force is applied to the packing assembly, the pressure acting on the two ends of the packing is in opposite directions, resulting in an N-shaped stress pattern across the entire packing assembly. During the pre-tightening process, the deformed area of the packing can accumulate some yield potential energy. When wear occurs as a result of the movement of the valve stem or when the pre-tightening force of the gland is insufficient, the previously accumulated energy is released to compensate for the wear of the packing during operation. When there are fluctuations in the medium pressure or thermal oscillations, the deformation of the packing group also changes accordingly. This achieves a self-sealing function for the valve packing, which becomes stronger in the face of strong forces and weaker in the face of weak forces. (4) The preload required for this N-shaped twisting deformation acts only on about 1/3 of the shaft surface of the packing, resulting in a doubling of the pressure transmission efficiency (side pressure coefficient). The pressure transmission efficiency between the packing groups is also at its highest, and accordingly the bolt preload torque decreases, leading to a significant reduction in the reciprocating resistance of the valve stem. The significance of this structural design lies in its ability to change the traditional approach to packing seals, which relies on axial compression and radial expansion deformation. Instead, it adopts an approach involving axial compression and radial insertion of the packing into the inside of the stuffing box, while also contracting around the outer wall of the valve stem to secure it; this represents an innovation in the design concept of packing seals. We analyzed the relationship between preload pressure and stem resistance for several filler pack types with different cross-sectional shapes under the same leakage equivalent, as shown in Figure 7. (5) From the perspective of cross-sectional design, cavities are also provided between the rings; this allows for selective control of the radial displacement of the packing under low-torque bolt preloading, ensuring that the preloading force applied to each ring in the packing set is distributed evenly, with each individual ring experiencing the same radial deformation. Efforts are made to ensure a perfect fit at the interface between the valve stem and the packing, in order to prevent the medium from leaking outward and achieve a reliable seal. During installation, the convex side of the graphite packing set is generally directed toward the medium. When the pressure of the medium inside the valve changes, it first acts on the protrusion at point L through the scale-scraping end ring. Since the convex top is located near the valve stem, the greater the internal pressure, the more the packing deforms, resulting in a more reliable seal. (6) Based on the manufacturing experience with packing rings, flexible graphite sheets with a thickness of 0.2–0.38 mm are relatively soft; they exhibit large deflection during molding and good processability. After slitting, the graphite sheet is rolled into a corrugated strip using toothed rollers, and then shaped using molds. This enables the cross-section of the graphite packing ring to exhibit a selective rolled shape at the corresponding product density. Labyrinthine W-fold wrinkles are formed. A lower axial pressure during valve pre-tightening results in the maximum radial deformation. Additionally, the wavy shape of the graphite strip facilitates easy radial deformation of the filler. The requirements regarding the compression height, specifications, and density of the packing ring can be calculated using the following formula: (7) The density of the composite packing not only determines the magnitude of the axial pressure but also affects the relationship between the compression rate and the rebound rate. In practice, at a constant pressure, both the compression rate and the rebound rate of a material are inversely proportional to each other; that is, the greater the compression, the smaller the rebound. In accordance with the standard JB/T6370 \"Test Methods for Physical and Mechanical Properties of Flexible Graphite Packing Rings\", by conducting tests on the compressibility and resilience of butterfly-shaped nuclear-grade packing assemblies as well as those with the structures mentioned earlier at different densities, it can be seen that the compressibility and resilience curves for butterfly-shaped nuclear-grade packing fall within a more reasonable range (see Figure 8). IV. Thermal cycling verification of butterfly-shaped nuclear-grade packing sets. In accordance with the verification requirements specified in “NB/T 20010.14-2010 Valves for pressurized water reactor power plants – Part 14: Technical specifications for flexible graphite packing rings”, a comparative experiment involving several types of packings was conducted using the “Thermal cycling simulation test device for valve packings (see Figure 9)”. The verification results are shown in Table 5. An analysis of the verification results in the table shows that the structural design of the packing has a significant impact on the leakage values, stem resistance, and preload force (number of gland tightening cycles) in the experiments. The more easily radial deformation can occur structurally, the less leakage of the medium there will be, and the resistance of the valve stem can also be reduced. In the case of flat-ring packing assemblies, their unreasonable structure makes radial deformation difficult. This not only requires more tightening operations during operation in order to increase the tightening force and achieve a sufficient degree of radial deformation, but it also leads to seal failure. During this testing process, it was also shown that the nuclear-grade packing assembly in butterfly shape is prone to deformation, resulting in optimal surface contact with both the radial direction and the valve stem, thereby achieving the most significant sealing effect. V. Conclusions (1) By verifying the physicochemical properties of graphite against relevant standards and technical specifications, it is clear that only large-flake, high-quality graphite is one of the few materials in nature capable of slowing down neutrons as well as other types of radiation; it is also the most ideal material for use in gasket seals for nuclear power valves ； (2) By using natural crystalline graphite with large flakes and high quality, the flexible graphite sheets produced can have very low levels of harmful trace elements remaining in them due to the advanced manufacturing techniques employed. The physical and mechanical properties of the flexible graphite filler are significantly improved, and all these properties meet the safety requirements for operation in nuclear power plants ； (3) The scale-scraping rings at both ends of the nuclear-grade valve packing are end rings molded from high-purity flexible graphite woven with nickel wire; compared to end rings made from other materials, they offer significant improvements in terms of wear resistance, pressure tolerance, resistance to high-temperature degradation, and safety factor ； (4) The novel butterfly-inspired structure design enables the valve stem to achieve a minimum travel torque at the same bolt preload during assembly. Under the pre-tightening force of the gland, the packing assembly undergoes an N-shaped deformation, which ensures a thorough fit between the inner wall of the packing box and the interface with the valve stem and graphite packing, thereby significantly increasing the effective radial sealing contact area ； (5) The concave-convex design on the axial surface structure enables the achievement of the goal of low leakage (LE) under conditions of low preload (LS), which requires only a relatively low bolt torque ； (6) The packing designed with an elastic butterfly cross-section is highly suitable for use in valves in nuclear power plants. It is appropriate for sealing media such as cooling water, steam, boric acid, air, nitrogen, and demineralized water during nuclear power operations. It possesses excellent properties such as high temperature resistance, low creep, and self-lubrication, and it does not experience any volume changes when subjected to temperature fluctuations.