When selecting a material for ventilation and aspiration systems at facilities using nuclear energy, the decision must be based on the combined assessment of regulatory requirements, operating conditions, and radiation-safety requirements. This article presents the areas where stainless steel should be used instead of galvanized sheet, the selection criteria for various NPP zones, and the regulatory framework for ventilation-system design. It also provides recommendations for organizing installation, quality control, and improving the service life of air ducts.
Regulatory Requirements for Ventilation Systems at Facilities Using Nuclear Energy
Requirements for nuclear power plant ventilation systems are governed by a set of federal rules and regulations (NP-001-15, NP-041-22) and industry standards (NP-509-21, NP-511-21).
Ventilation-system materials must have the following properties:
• corrosion resistance during long-term operation in humid environments;
• resistance to temperature fluctuations;
• resistance to alkaline and acidic decontamination solutions;
• resistance to ionizing radiation within the dose range defined for the relevant NPP zone.
• For controlled access zones (CAZ) and strict-regime zones (SRZ), additional restrictions are imposed on material porosity (to prevent the accumulation of radioactive contamination) and weldability (to ensure the integrity of sealed joints).
Quality control of metal during the manufacture and installation of ventilation systems is regulated by NP-105-18, Rules for Metal Inspection of Equipment and Pipelines of Nuclear Power Installations during Manufacture and Installation.
Galvanized Steel Air Ducts
In administrative and amenity buildings, auxiliary buildings, and non-classified rooms (class 3A/4 under the Russian NGO classification), where there is no direct exposure to aggressive substances, galvanized steel air ducts may be used. Their service life is 15-25 years, provided they are operated in a dry or moderately humid environment without chemical reagents.
Regulatory Requirements for Ventilation Systems at Facilities Using Nuclear Energy
Requirements for nuclear power plant ventilation systems are governed by a set of federal rules and regulations (NP-001-15, NP-041-22) and industry standards (NP-509-21, NP-511-21).
Ventilation-system materials must have the following properties:
• corrosion resistance during long-term operation in humid environments;
• resistance to temperature fluctuations;
• resistance to alkaline and acidic decontamination solutions;
• resistance to ionizing radiation within the dose range defined for the relevant NPP zone.
• For controlled access zones (CAZ) and strict-regime zones (SRZ), additional restrictions are imposed on material porosity (to prevent the accumulation of radioactive contamination) and weldability (to ensure the integrity of sealed joints).
Quality control of metal during the manufacture and installation of ventilation systems is regulated by NP-105-18, Rules for Metal Inspection of Equipment and Pipelines of Nuclear Power Installations during Manufacture and Installation.
Galvanized Steel Air Ducts
In administrative and amenity buildings, auxiliary buildings, and non-classified rooms (class 3A/4 under the Russian NGO classification), where there is no direct exposure to aggressive substances, galvanized steel air ducts may be used. Their service life is 15-25 years, provided they are operated in a dry or moderately humid environment without chemical reagents.
Critical limitation of galvanized steel: when it comes into contact with acids, including acidic condensate, the protective zinc layer dissolves and the steel begins to rust intensively. This rules out the use of galvanized steel in areas where chemical reagents are handled and in chemical-control rooms.
Stainless Steel Air Ducts
Ventilation systems in areas with chemically aggressive media (acids, alkalis, decontamination solutions), high humidity, and areas where contaminated air that must be decontaminated may be present should use stainless steel air ducts.
The 300-series austenitic steel grades are the most widely used in the nuclear industry: AISI 304, AISI 316, and AISI 321.
Key advantages of stainless-steel air ducts:
Corrosion resistance and chemical inertness. The high chromium content (12% or more) and molybdenum content (in AISI 316) provide exceptional corrosion resistance: the surface does not oxidize when exposed to aggressive media and does not require an additional anti-corrosion coating that could impair decontaminability.
Radiation resistance. Stainless steels retain their mechanical properties over a wide range of irradiation doses. For CAZ and SRZ areas, resistance to irradiation without changes in material structure has been confirmed, which cannot be guaranteed for galvanized-sheet coatings.
Decontaminability (the property of a smooth, non-porous surface). The absence of pores and cracks on the surface of stainless steel makes it possible to remove radioactive particles easily using standard decontamination solutions in accordance with GOST R 51102-97 (decontamination factor >0.9).
Heat resistance (up to 600-900°C depending on grade). In emergency ventilation and smoke-extraction systems, ventilation ducts must remain intact during a fire within the required fire-resistance limit. Stainless steel retains its strength when heated to temperatures at which galvanizing completely loses its properties.
Long-term maintenance-free service life. High mechanical strength and rigidity (600 N/mm²) guarantee a service life of at least 50 years, provided the design sheet thickness is maintained (1.0-8.0 mm for industrial facilities). This significantly reduces the need for wall-thickness measurements, unlike degrading galvanized coatings.
Rejection of Galvanized Air Ducts at Nuclear Power Facilities
The main reasons why galvanized steel is not permitted or is not recommended for ventilation systems in safety zones 1-3A under NP-041-22 are as follows.
Limited corrosion resistance in humid and chemically aggressive environments
Decontamination solutions widely used for cleaning controlled-access rooms are acidic or alkaline. When the zinc coating comes into contact with aggressive reagents, it quickly breaks down, exposing the carbon steel, which then begins to rust actively.
Absence or instability of radiation resistance
The coating on galvanized sheet is not designed for long-term operation under ionizing radiation, so preservation of its properties throughout the entire service life cannot be guaranteed.
Low decontaminability
Even if the zinc layer remains intact, the roughness of galvanized sheet promotes the retention of radioactive particles. This may lead to elevated radiation background levels in specified rooms and require more frequent repairs.
Low heat resistance and risk of gas release during fire
The melting point of zinc (420°C) does not allow galvanized steel to be used in emergency ventilation systems, because an abrupt increase in air temperature is possible during a beyond-design-basis accident. In addition, zinc may release harmful aerosols when heated.
Recommendations for Selecting the Design of Ventilation Systems
When designing air ducts for nuclear power plants, the following principles should be followed.
Stainless Steel Air Ducts
Ventilation systems in areas with chemically aggressive media (acids, alkalis, decontamination solutions), high humidity, and areas where contaminated air that must be decontaminated may be present should use stainless steel air ducts.
The 300-series austenitic steel grades are the most widely used in the nuclear industry: AISI 304, AISI 316, and AISI 321.
Key advantages of stainless-steel air ducts:
Corrosion resistance and chemical inertness. The high chromium content (12% or more) and molybdenum content (in AISI 316) provide exceptional corrosion resistance: the surface does not oxidize when exposed to aggressive media and does not require an additional anti-corrosion coating that could impair decontaminability.
Radiation resistance. Stainless steels retain their mechanical properties over a wide range of irradiation doses. For CAZ and SRZ areas, resistance to irradiation without changes in material structure has been confirmed, which cannot be guaranteed for galvanized-sheet coatings.
Decontaminability (the property of a smooth, non-porous surface). The absence of pores and cracks on the surface of stainless steel makes it possible to remove radioactive particles easily using standard decontamination solutions in accordance with GOST R 51102-97 (decontamination factor >0.9).
Heat resistance (up to 600-900°C depending on grade). In emergency ventilation and smoke-extraction systems, ventilation ducts must remain intact during a fire within the required fire-resistance limit. Stainless steel retains its strength when heated to temperatures at which galvanizing completely loses its properties.
Long-term maintenance-free service life. High mechanical strength and rigidity (600 N/mm²) guarantee a service life of at least 50 years, provided the design sheet thickness is maintained (1.0-8.0 mm for industrial facilities). This significantly reduces the need for wall-thickness measurements, unlike degrading galvanized coatings.
Rejection of Galvanized Air Ducts at Nuclear Power Facilities
The main reasons why galvanized steel is not permitted or is not recommended for ventilation systems in safety zones 1-3A under NP-041-22 are as follows.
Limited corrosion resistance in humid and chemically aggressive environments
Decontamination solutions widely used for cleaning controlled-access rooms are acidic or alkaline. When the zinc coating comes into contact with aggressive reagents, it quickly breaks down, exposing the carbon steel, which then begins to rust actively.
Absence or instability of radiation resistance
The coating on galvanized sheet is not designed for long-term operation under ionizing radiation, so preservation of its properties throughout the entire service life cannot be guaranteed.
Low decontaminability
Even if the zinc layer remains intact, the roughness of galvanized sheet promotes the retention of radioactive particles. This may lead to elevated radiation background levels in specified rooms and require more frequent repairs.
Low heat resistance and risk of gas release during fire
The melting point of zinc (420°C) does not allow galvanized steel to be used in emergency ventilation systems, because an abrupt increase in air temperature is possible during a beyond-design-basis accident. In addition, zinc may release harmful aerosols when heated.
Recommendations for Selecting the Design of Ventilation Systems
When designing air ducts for nuclear power plants, the following principles should be followed.
Conclusions
The use of stainless steel for air ducts at nuclear power plants is not a matter of redundancy; it is a direct safety requirement.
In zones where radioactive aerosols may be released, corrosion resistance alone is not enough. The surface must be decontaminable and resistant to acids and alkalis, which is achieved only by using austenitic stainless steel (AISI 304, AISI 316L, or AISI 321) with controlled surface cleanliness. The high material cost is repaid many times over during the entire 60-year service life, because it eliminates the need to replace air ducts after only 15-20 years, as happens with galvanized systems in a moderate climate, let alone in the aggressive environment of an NPP.
The design of ventilation systems using stainless steels must be carried out in accordance with STO 95 12009-2017 (rules for concurrent works) and must include:
• radiation control of materials;
• development of a Work Execution Plan (WEP) with a section on quality control of welded joints and leak tightness;
• preparation of a ventilation-system passport with control test reports;
• admission and qualification of welders under PNAE G-7-010-89.
The metalworking process cycle (cutting, bending, and welding of austenitic stainless steels) must prevent carburization and corrosion damage to welded seams. This requires using only stainless-steel-based tools (brushes, cutting wheels) and cleaning weld areas to a bright finish, while avoiding contact with carbon steel.
A standard ventilation design must contain a schedule of stainless-steel grades indicating sheet thickness (from 0.8 to 2.0 mm for air ducts and from 2.0 to 8.0 mm for industrial assemblies) and surface-cleanliness requirements, such as a 2B finish or mirror polishing for especially clean zones.
*Compliance with industry design standards (NP-041-22, NP-509-21) and quality control at the stages of stainless rolled-stock supply, welding, and ventilation installation is the only way to guarantee the service life of the ventilation system for the entire operating life of the power unit (60 years).*
To receive a commercial proposal for the design, supply, and installation of stainless-steel ventilation systems for your facility, send a technical specification indicating the room safety class, types of aggressive media, and design service life to the commercial department of TechAtomStroy LLC via the feedback form on the website. A cost estimate, work schedule, and list of measuring and control materials under NP-105-18 will be prepared.
*This material was prepared on the basis of NP-041-22, NP-509-21, NP-511-21, NP-105-18, GOST R 51102-97, and industry requirements for ventilation systems at facilities using nuclear energy.*
The use of stainless steel for air ducts at nuclear power plants is not a matter of redundancy; it is a direct safety requirement.
In zones where radioactive aerosols may be released, corrosion resistance alone is not enough. The surface must be decontaminable and resistant to acids and alkalis, which is achieved only by using austenitic stainless steel (AISI 304, AISI 316L, or AISI 321) with controlled surface cleanliness. The high material cost is repaid many times over during the entire 60-year service life, because it eliminates the need to replace air ducts after only 15-20 years, as happens with galvanized systems in a moderate climate, let alone in the aggressive environment of an NPP.
The design of ventilation systems using stainless steels must be carried out in accordance with STO 95 12009-2017 (rules for concurrent works) and must include:
• radiation control of materials;
• development of a Work Execution Plan (WEP) with a section on quality control of welded joints and leak tightness;
• preparation of a ventilation-system passport with control test reports;
• admission and qualification of welders under PNAE G-7-010-89.
The metalworking process cycle (cutting, bending, and welding of austenitic stainless steels) must prevent carburization and corrosion damage to welded seams. This requires using only stainless-steel-based tools (brushes, cutting wheels) and cleaning weld areas to a bright finish, while avoiding contact with carbon steel.
A standard ventilation design must contain a schedule of stainless-steel grades indicating sheet thickness (from 0.8 to 2.0 mm for air ducts and from 2.0 to 8.0 mm for industrial assemblies) and surface-cleanliness requirements, such as a 2B finish or mirror polishing for especially clean zones.
*Compliance with industry design standards (NP-041-22, NP-509-21) and quality control at the stages of stainless rolled-stock supply, welding, and ventilation installation is the only way to guarantee the service life of the ventilation system for the entire operating life of the power unit (60 years).*
To receive a commercial proposal for the design, supply, and installation of stainless-steel ventilation systems for your facility, send a technical specification indicating the room safety class, types of aggressive media, and design service life to the commercial department of TechAtomStroy LLC via the feedback form on the website. A cost estimate, work schedule, and list of measuring and control materials under NP-105-18 will be prepared.
*This material was prepared on the basis of NP-041-22, NP-509-21, NP-511-21, NP-105-18, GOST R 51102-97, and industry requirements for ventilation systems at facilities using nuclear energy.*