ISO 26802:2010 — Nuclear Facilities — Design Criteria for Containment and Ventilation Systems

1. Containment and Ventilation System Architecture for Nuclear Reactors

ISO 26802:2010 specifies criteria for the design and operation of containment and ventilation systems for nuclear power plants (NPPs) and research reactors, prepared by ISO/TC 85/SC 2 (Radiological protection). It addresses primary containment envelopes, secondary confinement volumes, and ancillary areas within the reactor building. The standard classifies ventilation zones according to radiological contamination hazard and defines the functional requirements for each zone, including air exchange rates, pressure cascading, and filtration efficiency. It is complementary to ISO 17873, which covers non-reactor nuclear installations. The standard draws on operating experience from commercial nuclear plants worldwide and incorporates lessons learned from major events.

Three main ventilation categories are defined: ventilation of volumes within the primary containment envelope, within the secondary confinement, and outside the secondary confinement. Each category has distinct requirements for leaktightness, filtration, and monitoring. The primary containment envelope must be designed to withstand design basis accident pressures and temperatures while maintaining leaktightness. Primary containment typically comprises a steel-lined concrete structure designed for pressures of 400-600 kPa(g) and temperatures up to 150 degree C during accident conditions, with leakage rates below 0.1-0.5% of containment volume per day at design pressure.

2. Safety Classification and Risk Assessment

The standard adopts a comprehensive risk-based approach. Design basis accidents (DBAs) are those against which the facility is designed with established criteria, ensuring fuel damage and radioactive release remain within authorised limits. Beyond-design basis accidents (BDBA) and severe accidents involving core degradation require additional consideration, particularly for combustible gas control (hydrogen) in the reactor building following a loss-of-coolant accident. Hydrogen can be generated by zirconium-water reaction during core degradation, and its concentration must be kept below 4% by volume (the flammability limit in air). The standard addresses hydrogen recombination strategies including passive autocatalytic recombiners (PARs) that operate without external power.

Ventilation system safety functions include: confinement of radioactive material by maintaining pressure cascades from low-contamination to high-contamination areas; filtration of exhaust air through HEPA and iodine filters; control of airborne contamination within prescribed limits; and management of ambient conditions to protect equipment and personnel. The standard defines key performance metrics including decontamination factor (DF), air exchange rates, and pressure differentials between zones. The pressure cascade typically maintains a gradient of -50 to -200 Pa between adjacent zones, with the most contaminated areas at the lowest pressure relative to surrounding volumes.

3. Engineering Design Insights for Nuclear Ventilation

Several key design principles emerge from this standard. First, the concept of dynamic confinement — maintaining preferential airflow from areas of lower to higher contamination potential — is fundamental. This is achieved through controlled pressure gradients using balancing dampers and variable-speed fans. Second, fire hazards are intrinsically linked to contamination spread: fire dampers must be coordinated with containment isolation valves, and fire compartments must be designed to also function as containment compartments. Clause 8 addresses management of specific risks including combustible gases, heat releases, toxic vapours, and external hazards.

The standard requires comprehensive monitoring of ventilation system parameters including airflow rates, pressure differentials, filter differential pressure, radiation levels in exhaust ducts, and temperature/humidity in critical areas. Alarms must alert operators to deviations that could compromise containment integrity. Instrumentation requirements are specified in Clause 10, covering control systems, monitoring instrumentation, and alarm thresholds for each safety-classified ventilation function. The monitoring architecture typically involves redundant sensors arranged in a 2-out-of-3 voting configuration for safety-critical parameters, ensuring continued functionality even with single instrument failures.

From a maintenance perspective, HEPA filters must be tested for penetration at installation and periodically throughout their service life. Iodine adsorbers require laboratory analysis of activated charcoal samples to verify retention efficiency. The standard provides guidance on testing intervals, typically annual for HEPA filters and semi-annual for iodine adsorbers. Filter change-out procedures must account for radiation exposure of maintenance personnel, with pre-shielding and remote handling provisions incorporated into the system design.

The design principles outlined in this standard have been validated through decades of nuclear power plant operation and have been continuously refined based on operating experience feedback from the global nuclear industry.

4. Frequently Asked Questions