Problems and Solutions in Nuclear Power Valves

In each core system of a nuclear power plant, valve equipment functions like an artery, responsible for the transmission and control of key media, and many of these valves are crucial for nuclear safety. With the rapid progress of nuclear power technology, the demand for and scale of nuclear power valves have shown a significant growth trend. Nuclear power, as an internationally recognized form of clean energy, provides strong support for energy supply while ensuring environmental sustainability. As a result, more countries are actively choosing nuclear power as a key focus for energy development.
 

In the operation of nuclear power plants, valves are crucial for ensuring safe and stable operation. Valves are widely used in nuclear power plants, covering almost all systems and ensuring the normal operation of the plant. For example, in the CPR1000 project, there are more than 28,400 valves in two million-kilowatt units, and the investment in these valves accounts for 4% of the total equipment cost. Throughout the 40 to 60-year operational cycle of a nuclear power plant, valve maintenance is essential, with its costs accounting for a significant proportion of overall operating expenses. Generally, the annual maintenance costs of nuclear power valves often exceed half of the total cost of operating the entire nuclear power plant.

The importance of key valves, such as the main steam isolation valve, regulator safety valve, and main steam safety valve, is self-evident. These valves are not only core components of the power plant but are also critical to ensuring the safety and stability of the nuclear power plant. Similarly, various types of nuclear-grade valves in the primary circuit system bear significant responsibilities, and any failure could threaten the normal operation of the power plant. Historically, the Three Mile Island nuclear leakage accident in 1979 and the Fukushima Daiichi Nuclear Power Plant nuclear leakage accident in Japan in 2011 highlighted the critical impact of proper valve operation and accurate fault handling on nuclear power plant safety. These incidents demonstrate that proper valve operation and troubleshooting are crucial to ensuring the safe operation of nuclear power plants.
 

Gate Valve: A hydraulically actuated gate valve typically uses its pressurized water to open or close the piston. The working pressure of the gate valve is PN17.5MPa; the nominal diameter is DN350-400mm, and the working temperature is 315°C. Fully enclosed electric gate valves typically use a special screen-type motor, relying on an inner planetary reducer that operates in water to ensure normal opening and closing of the valve plate. The working pressure of this valve ranges from PN2.5 to 45.0MPa; the nominal diameter is DN100-800mm, and the working temperature is 200-500°C.
Globe Valve: Globe valves are divided into bellows globe valves, packing globe valves, and metal diaphragm globe valves—commonly used in auxiliary pipelines. A glove valve typically handles medium-temperature, medium-pressure water and steam, with a nominal diameter of DN10-150mm.
Butterfly Valve: It is widely used in the air transport and cooling systems within the containment. It primarily includes three types: eccentric metal-seated butterfly valves, coaxial direct-connected rubber-lined butterfly valves, and double-acting metal-seated butterfly valves. The working pressure of this valve is less than PN4.0MPa; the nominal diameter is up to DN2500mm, and the working temperature is 100-150°C.
Check Valve-Type Isolation Valve: It is widely used in the steam systems of nuclear power plants. Its structure is similar to that of a lifting check valve. The working pressure of this valve ranges from PN1.0 to 42.0MPa; the nominal diameter is DN64-800mm, and the working temperature is -29 to 1050°C.
Main Steam Isolation Valve: This includes the main water supply valve and the main steam isolation valve for both conventional and nuclear islands. The working pressure of this valve is PN40.0MPa; the nominal diameter is DN800mm, and the working temperature is 700°C.
Additionally, during the nuclear fuel extraction process in nuclear power plants, safety valves that meet seismic requirements, top-mounted nuclear ballistic valves, and soft- and hard-closing high-vacuum valves are also utilized.
 

The causes may include natural factors, such as the wear of the sealing element over time, and human factors, such as poor grinding of the sealing surface, damage caused by over-tightening the packing, using non-corrosion-resistant materials, foreign matter obstructing the sealing surface, or operational errors.

The causes can be attributed to several factors. First, poor practices during the welding process, such as slag inclusion, incomplete welding, or welding cracks, are significant causes of valve performance issues. Second, quality issues in casting materials, such as blowholes, slag inclusions, and other defects, can directly affect the quality of the valve body and valve bonnet. Finally, gasket failure in the valve body and bonnet is also a key factor leading to reduced valve performance. These issues collectively impact the normal operation and safety of the valve.

Mechanical component failures on-site are another major cause of valve malfunctions. Nuclear power plant units usually operate under a base load, with the main valve of the high-pressure cylinder and the regulating valve of the low-pressure cylinder remaining fully open. As a result, the pilot valve spool in the valve positioner remains in a fixed position. These valves only require a smaller opening when the turbine load drops to a certain level.
 

In engineering practice, improper assembly has led to serious consequences. For example, during the debugging of the RRI system in a nuclear power plant, valve screws fell off, leading to significant financial and time costs for troubleshooting. Additionally, when the valve body is connected to the flange of the transmission box, the insufficient length of the stud bolts leaves too little of the bolt exposed after tightening the nuts, creating potential safety hazards and maintenance issues.

The main causes include insufficient or expired corrosion resistance of the packing material, installation defects such as improper packing size or poor screw joints, bending wear from low valve stem accuracy, leakage due to an insufficient number of packing rings or improper compression of the gland, and improper installation or handling during operation.

Valve testing and inspection are crucial to ensuring that the valve design and manufacturing are flawless, as well as safe and reliable for use. After final assembly, performance testing must be conducted to verify whether it meets the design and national standards. Routine tests include shell strength, sealing, low-pressure sealing, and action tests. Each item must pass qualification before proceeding to the next. Testing can effectively expose defects in materials, blanks, heat treatments, machining, and assembly.


 

No.1 Measures to Reduce Valve Body or Valve Bonnet Leakage
a. Follow proper welding procedures and strengthen post-weld inspections.
b. Select high-quality and appropriate casting materials.
c. Gasket selection must align with the working medium conditions.
d. Evaluate frequently leaking valves and replace them when necessary.
No.2 Measures to Solve Packing Leakage
a. Select the appropriate packing material and type based on the working conditions.
b. Install the packing correctly; tighten it gradually, ensuring the joint is at 30° or 45°.
c. Replace aged or damaged packing promptly.
d. Straighten or repair bent or worn valve stems, and promptly replace severely damaged ones.
e. Install the packing using the specified number of turns, and tighten the gland evenly and symmetrically.
f. Repair or replace damaged glands, bolts, and other components promptly.
g. Maintain a constant speed and steady force during operation, and properly adjust the gap between the gland and valve stem to ensure it is neither too large nor too small.
No.3 Measures to Prevent Internal Sealing Failure
a. Regularly inspect internal components and promptly replace aged or worn parts.
b. Maintain strict control over the grinding quality of the valve sealing surface to ensure effective sealing.
c. Analyze failed components, optimize material selection, and prevent failures due to material issues.
d. Strengthen foreign matter control during maintenance and clean pipes prone to impurity deposits to prevent internal leakage.
 

To fundamentally resolve the technical and quality issues of nuclear power valves and meet the urgent needs of industry development, we need to take effective measures in several areas:
Improve technical standards: Accelerate the development and enhancement of relevant standards for nuclear power valves, ensure alignment with international standards, and improve the standardization of design, material selection, manufacturing, and inspection.
Resource sharing and experimental capability building: Integrate industry resources, establish a shared platform for experimentation, testing, and identification, reduce redundant investments, and improve the research and development efficiency and product quality of nuclear power valves.
Talent and technology introduction: Strengthen the training of nuclear power professionals, actively introduce advanced international technology, and improve the design capabilities and production capacity of nuclear power valves.
Software and hardware upgrades: Increase investment in software and hardware to ensure the advancement of design and manufacturing methods, and guarantee the processing and assembly quality of components and complete systems.
Special process personnel training: Strengthen the training and assessment of personnel involved in special processes such as welding, non-destructive testing, and heat treatment to ensure that the manufacturing quality of components meets design requirements.
Nuclear safety awareness and quality management: Strengthen nuclear safety awareness education, improve the quality management of all employees, and ensure that the entire design and manufacturing process for nuclear power valves adheres to high standards and strict requirements.
 


Nuclear power valve technology began to develop in the 1980s and has achieved remarkable progress worldwide. In countries with leading nuclear power valve technology, production capacity and management levels far exceed those of in China. This technological gap has become a key factor limiting the development of China's nuclear power valve industry and enterprises, while also hindering the overall progress of the nuclear power industry. Faced with this challenge, manufacturers in China's nuclear power valve industry must respond quickly and accelerate innovation. They should actively introduce internationally advanced production equipment, continuously invest in research and development, learn from and absorb advanced international experience, and strive for optimization and improvement in all aspects. Only in this way can China's R&D and production capabilities for nuclear power valves be fundamentally improved, ensuring the steady development of the nuclear power industry.