Key materials must pass radiation resistance tests: under gamma-ray or neutron radiation (dose simulation of the cumulative radiation during the design life of a nuclear power plant), the attenuation rate of mechanical properties (such as tensile strength, elasticity) and filtration performance (efficiency, resistance) of the materials should be ≤10%, and no toxic substances (such as formaldehyde, heavy metals) should be released.

Iii. Performance Safety: Strict nuclear-level certification and testing standards
The non-woven high-efficiency filters used in nuclear power plants must meet the specific standards for the nuclear industry (rather than the ordinary industrial HEPA standards). The core performance tests and certifications include:
Testing of filtration efficiency and leakage rate
DOP/PAO scanning test (international standard ISO 14644-3) : Use 0.3μm DOP (dioctylphthalate) or PAO (polyalpha-olefin) aerosol, and conduct a full surface scan of the “filter material surface + sealing surface + frame seam” of the filter through a laser particle counter. Requirements:
The efficiency of the filter material area is ≥99.97% (for 0.3μm particles).
The leakage rate of the sealed area is ≤0.1% (i.e., the proportion of the leaked aerosol volume to the total intake volume is ≤0.1%), and the leakage concentration at a single leakage point is ≤0.01 mg/m³ (radioactive aerosol control threshold).
Temperature resistance, moisture resistance and chemical resistance tests
Temperature resistance test: Continuously operate at 150℃ (simulating the high temperature after a LOCA accident) for 24 hours. After cooling, the efficiency decline is ≤5%, and there is no structural damage.
Moisture resistance test: After continuous operation for 7 days in an environment of 95% relative humidity and 40℃, the resistance change rate is ≤15%, and there is no mold on the filter material or rust on the frame.
Chemical resistance test: It can withstand the chemical substances that may exist in nuclear power plants (such as boric acid solution droplets, ammonia vapor), and there is no significant decrease in the strength and efficiency of the filter material after immersion.
Nuclear-level certification
It must pass the nuclear-grade equipment certification of the National Nuclear Safety Administration (NNSA) (China HAF 604 standard) or the safety certification of the International Atomic Energy Agency (IAEA) to prove that it complies with the “defense in depth” principle of nuclear facilities and can serve as the “second barrier” for the spread of radioactive substances (the first barrier is the equipment casing/factory building wall).
Iv. Installation Safety: Precise Construction and Leakage Control
Even if the filter itself is qualified in performance, improper installation may still lead to “bypass leakage”. Therefore, the installation process must follow nuclear-grade construction standards (such as the relevant provisions in China’s GB 50235-2010 “Code for Construction of Industrial Metal Pipeline Engineering”). Core safeguard measures:
Pre-treatment before installation
The installation area (such as ventilation ducts, filter static pressure boxes) needs to undergo cleanliness testing: ensure there is no dust, welding slag, or metal debris (to prevent foreign objects from scratching the filter material after installation), and the concentration of air suspended particles should be ≤1000 particles /m³ (above 0.5μm).
Before opening the filter box, it is necessary to check the integrity of the packaging (to prevent damage to the filter material during transportation). After opening the box, a preliminary check of the filter material for holes or wrinkles should be conducted by the “light transmission method”.
Precise positioning and sealed installation
The installation is carried out by the “horizontal hoisting + guide rod positioning” method (to avoid uneven force on the filter material during manual handling), ensuring that the filter is completely aligned with the sealing surface of the installation frame, with a deviation of no more than 1mm.
After the sealing gasket is installed, a “pressure test” is required: Compressed air (at a pressure of 0.1MPa) is introduced into the gap between the filter and the frame. If no bubbles are produced by the bubble detector, it indicates that the seal is qualified.
Overall test after installation
Conduct an “aerosol challenge test” on the entire air purification system (including filters, air ducts, and fans) : Inject simulated radioactive aerosols (such as Eu-152 aerosol) into the system, and measure the aerosol concentration at the system outlet. The retention efficiency should be ≥99.99% to ensure no system-level leakage.
V. Operation and Maintenance Security: Performance Monitoring and Replacement throughout the entire lifecycle
The operation and maintenance of nuclear power plant filters need to establish a digital management system. Through real-time monitoring and regular inspection, it is ensured that they remain in a safe state throughout their life cycle.
Real-time performance monitoring
Differential pressure transmitters (with an accuracy of ±0.1Pa) are installed at the front and rear ends of the filter to monitor resistance changes in real time. When the resistance exceeds twice the initial resistance (or reaches the design upper limit, such as 500Pa), the system will automatically alarm and prompt that the filter needs to be replaced (to avoid excessive resistance causing fan overload or filter material damage).
Radioactive aerosol monitors (with a detection limit of ≤1×10⁻¹² Bq/m³) are installed at the filter outlets in key areas (such as the reactor containment vessel). If an abnormal increase in radioactive concentration is detected, emergency procedures (such as closing the air duct and activating the backup purification system) will be triggered immediately.
Regular inspection and maintenance
Conduct a “DOP scan retest” every six months: Focus on checking whether the sealing surface has leaked due to aging or vibration. If the leakage rate exceeds 0.1%, the sealing gasket or filter needs to be replaced.
Conduct a “filter material integrity check” once a year: Observe through an endoscope whether the filter material is damaged or wrinkled. If any defects are found, replace it immediately (to avoid radioactive leakage due to filter material damage).
Filter replacement and disposal safety
When replacing the filter, it should be carried out in a negative pressure glove box or “isolation area” (to prevent the operator from coming into contact with the filter that may be attached with radioactive substances). The replacement personnel need to wear nuclear-grade protective clothing (such as Tyvek protective clothing) and respiratory protective equipment.
The replaced waste filters need to be treated as radioactive solid waste: first, they should undergo “fixation treatment” (such as being placed in stainless steel containers and filled with concrete), then be transported to the radioactive waste storage facility of the nuclear power plant for temporary storage, and finally be disposed of in accordance with national regulations (such as deep burial disposal) to prevent the secondary diffusion of radioactive substances.
Vi. Emergency Safety: Redundant Assurance in Accident Conditions
In the design of nuclear power plants, the principle of “defense in depth” should be taken into account. The filter system needs to have emergency response capabilities to ensure that it can still play a safe role under accident conditions.
Redundant configuration
The key systems (such as the containment ventilation system) adopt a “one in use and one on standby” or “N+1” redundant design: during normal operation, one filter works while the rest are on standby. If the working filter fails, the backup filter can be automatically put into operation within 10 seconds to ensure that the purification system is not interrupted.
Adaptability to accident conditions
For LOCA incidents (water loss incidents), the filter must be able to withstand “high-temperature and high-pressure water vapor impact” : By installing a “steam-water separator” at the front end of the filter, most of the liquid water is removed first, and then “high-temperature resistant filter material” is used to retain aerosols, ensuring that the filtration efficiency can still be maintained at ≥99.97% within one hour after the incident.
For “fire accidents”, the filter must have flame-retardant performance: the oxygen index of the filter material and the frame material should be ≥32 (non-combustible material standard), to prevent the filter from burning and generating toxic gases or structural collapse during a fire, which could affect system safety.
Emergency response procedure
If severe leakage from the filter is detected (such as when the radioactive concentration exceeds the standard by more than ten times), immediately activate the “system isolation procedure” : close the electric air valves at the front and rear ends of the filter to cut off the airflow in this air duct, and simultaneously activate the “emergency purification system” (such as a mobile high-efficiency filtration device) to prevent the further spread of radioactive substances.
After an accident, a “comprehensive assessment” of the filter system is required: check the structural integrity, filtration efficiency and the degree of radioactive contamination of the filter, determine whether replacement or repair is needed, and ensure that there are no safety hazards before the system returns to normal operation.
Summary
The safety guarantee of the non-woven high-efficiency filter in nuclear power plants is a “multi-dimensional, full-chain” system. Its core logic is to build the filter into the “last line of defense” for air purification in nuclear facilities through design adaptability, material reliability, performance stability, installation accuracy, operation and maintenance controllability and emergency redundancy. This system not only needs to meet the strict standards of the nuclear industry, but also needs to further enhance its safety redundancy under extreme working conditions through continuous technological iterations (such as developing more radiation-resistant PTFE-coated filter materials and more intelligent digital monitoring systems), providing key support for the safe operation of nuclear power plants.