Technical Requirements for Nuclear Power Valves

Technical Requirements for Nuclear Power Valves


Compared with the valves used in conventional large-scale thermal power plants, although the pressure and temperature of PWR nuclear power plant valves are lower, the valves used in nuclear power plants have better technical characteristics and higher requirements.
 
1.1 Strength and stiffness
In the design, the main parts of the valve should first be able to withstand the force of various loads under persistent or instantaneous pressure and temperature changes, and there should be no obvious elastic-plastic deformation. In addition to conventional strength calculations, methods such as finite element stress analysis and seismic calculation analysis should be used to ensure the reliability of valves. 
 
1.2 Sealing
Since the conveying medium of the primary circuit system of a nuclear reactor is mostly radioactive, no external leakage is allowed. Strict and effective measures should be taken to ensure the structural design of the valve, the selection of seals such as bellows, diaphragms, packing and gaskets, and quality control of materials and finished products. The connection between the valve body and the pipeline is mostly butt welding or socket welding.
 
1.3 Service life
The economic loss caused by a one-day shutdown is about tens of millions of RMB due to the great investment in the construction of nuclear power plants and the long payback period. Therefore, the valve meets the design specifications to minimize the number of repairs and shorten the repair cycle. General regulations require a service life of 30 to 40 years. For this reason, the valve should be verified to have the ability to complete the specified duty cycle according to the given probability.
 
1.4 Cleanliness
The medium conveyed in the primary loop system of the nuclear reactor is high-purity borated water. Therefore, not only the cleanliness required for the relevant parts in contact with the medium, but also the surface of the parts in the inner cavity that are in contact with the medium must be smooth, and the metal surface roughness Ra should be less than 6.3μm. All human bodies, objects, equipment tools and media in contact with the valve must meet the requirements of relevant cleaning procedures.
 
1.5 Safety and reliability
Valves need to be used reliably under various design conditions, including normal, abnormal and critical situations and accidents, and even maintain the integrity and operability of valves under earthquake disasters or loss of coolant accidents. Therefore, the prototype must be tested in various simulated working conditions. 
 
1.6 Properties of materials 
The selected materials must have good corrosion resistance, radiation resistance, impact resistance and intergranular corrosion resistance. Therefore, low-carbon or even ultra-low-carbon austenitic stainless steel is used as the main material in some main systems and choose some alloys with superior performance such as strength, toughness, temperature resistance, pressure resistance, erosion resistance, and abrasion resistance to make valve stems or sealing surfaces and other parts. Various physical and chemical properties and non-destructive testing shall be carried out as required. The content of chlorine, fluorine and sulfur ions in non-metallic sealing materials such as packings and gaskets should be strictly controlled, and all data should be lower than the indicators specified in the specifications to ensure that no damage is caused to the metal substrate due to corrosion.
 
1.7 Control devices
The nuclear power plant valves used in the accident state should play a role in the protection of the system and the emergency response to the accident. Timely and accurate action is very important for these valves, whether it is timely but not accurate or accurate but not timely can not meet the needs of protecting nuclear power plants. If the main steam isolation valve is only opened for a few seconds as required, if the action is not timely or malfunctioned, it will lead to serious consequences. Therefore, the performance and quality of valve actuators are very important and critical. 
 
1.8 Related specifications
Personnel engaged in the design and manufacture of nuclear power plant valves must first be familiar with and master the relevant provisions of a set of nuclear safety regulations and regulations issued by the country, and apply to the competent authority and the National Nuclear Safety Administration at the same time. The work can only be carried out after the license of relevant activities is issued after examination and approval by the superior party concerned. In practice, it is necessary to implement some recognized regulations and standards in China and the world, such as nuclear safety regulations HAF, nuclear safety guidelines HAD, nuclear industry standards EJ, American Society of Mechanical Engineers standard ASME, American Institute of Electrical and Electronics Engineers standards IEEE and French pressurized water reactor nuclear island mechanical equipment design and construction rules RCC to M and other normative documents. Enterprise standards must be based on the premise of ensuring that the above-mentioned various regulations and specifications are met to ensure that the nuclear-grade products produced meet the recognized requirements. 
 
1.9 Quality Control
Manufacturers of nuclear power plant valves must establish a complete quality assurance system, prepare corresponding quality assurance programs and related procedures and documents for quality control. In daily activities, strictly and conscientiously follow the outline, procedures and requirements of the document, and each step should follow the rules; accept the supervision and inspection of activities by the National Nuclear Safety Administration and relevant parties at any time. When major quality problems or non-conformities occur, the activity party has no right to deal with them privately, and must report to relevant parties for determination.
 
1.10 Documentation
From the beginning of the project approval to the factory acceptance process, a complete file should be established, including technical specifications, design drawings, calculation books, including stress and seismic analysis, process specifications, key process evaluation, quality plans, tracking records, the physical and chemical properties of various materials and non-destructive testing reports, functional test reports, use and maintenance manuals and manufacturing completion reports and other documents should be provided for archiving.
 
1.11 Special requirements for valves of nuclear power plants
Since the media, technical parameters, and environmental conditions of the primary circuit have different requirements for the safety and reliability of valves from conventional power stations, in addition to the general requirements, there are special requirements as follows: 
 
1) Requirements for radiation resistance 
The cumulative radiation of valves in nuclear power plants is 1200000GY. Therefore, in addition to meeting the working medium, the materials selected for the valves should also be resistant to radiation, that is, the performance of the material will not change after being irradiated; it will not be activated, that is, the half-life of the material should be short. Materials such as copper and cobalt should be used as little as possible, or even not used. Nickel-based alloys are ready to be used abroad, and currently, China is also ready to use them.
 
2) Requirements for sealing
Since the medium is boron-containing water with radioactivity, it is required that the valve has good sealing performance and no leakage. The flange seal should be safe and reliable. To ensure that the valve stem has no leakage, bellows and intermediate leakage should be used in the structure. To facilitate cleaning, the inner and outer surfaces of the valve must have a certain surface roughness, and dead angles should be avoided in the valve body to prevent the deposition of radioactive particles. 
 
3) Requirements for packing and gasket
due to the requirements for water quality of the primary circuit, stainless steel equipment is required. Especially thin-walled equipment such as fuel element cladding, evaporation pipes, bellows, water quality damaged by chloride ions will cause pitting corrosion of stainless steel equipment. Therefore, the chloride ion content of the gasket should be less than 100ppm, and not contain halogen for the packing selected for the valve. 
 
4) Requirements for banned materials
NaOH and boron are contained in the safety spray liquid in the containment caused by the accident, so the use of aluminum, zinc, or galvanized valves is generally strictly prohibited, and the overall design must be approved if it is used. Low melting points materials such as tin and lead must not be used for valves. Electroplating and nitriding are prohibited on the surface in contact with the medium. 
 
5) Requirements for LOCA 
The valves installed in the containment should meet the requirements for LOCA, that is, the valves can still operate in the state of loss of coolant accidents, which requires that the electric devices, instruments and other accessories selected for the valves should pass the LOCA test. 
 
6) Requirements for the structure
Because the medium of the valve is radioactive, and the environment where it is located has a certain radiation dose, it is required to disassemble and maintain the valve quickly. Therefore, the valve is required to have the characteristics of a simple structure, easy assembly, disassembly and maintenance. The design of the flow path of the valve body should be smooth, and the dead angle should be reduced or avoided as much as possible. The surface is smooth, and easy to clean and rinse. 
 
7) Requirements for safety and reliability
Valves play a very important role in the operation of nuclear power plants. The performance of valves will directly affect the safety of nuclear power plants. Therefore, nuclear power plants require that the performance of valves must be safe and reliable. To achieve this goal, the manufacturer is required to select typical and representative valves as prototypes before formally providing products for nuclear power plants. The prototype must undergo a series of tests, such as cold state performance tests, hot state simulation tests, life tests, earthquake tests and LOCA tests to verify the safety and reliability of valve performance.
 
8) Requirements for seismic resistance
Nuclear power plants require nuclear-grade valves to continue to maintain the integrity of the valve structure and pressure-bearing boundaries, as well as good working characteristics, during or after an earthquake in the power plant.