Defense-In-Depth

The NRC regulations in 10 CFR 50.48, Fire Protection and 10 CFR 50, Appendix R, apply the concept of defense-in-depth to protecting the health and safety of the public from fires at nuclear power plants, with the following objectives:

• To prevent fires from starting;

• To detect rapidly, control, and extinguish promptly those fires that do occur;

• To protect the reactor so that a fire that is not promptly extinguished by the fire suppression activities will not prevent the safe shutdown of the plant.

The NRC utilizes national codes and standards such as the National Fire Protection Association (NFPA) standards (NFPA Website), national standards and testing laboratories (for example, Underwriters Laboratory Website), and NRC developed guidance (for example, Regulatory Guide 1.189, "Fire Protection for Nuclear Power Plants") to lay out expectations on licensee actions to achieve defense-in-depth.

• Prevention – Involves the limiting of combustibles and ignition sources throughout the plant, such that the likelihood of fires occurring is reduced. Prevention involves, but is not limited to, the following plant features and activities:

  • Fire Marshall – Controls and monitors fire hazards – Nuclear power plants have staff responsible for monitoring fire hazards including the storage of combustible materials and ignition sources such as cutting and welding.

  • NRC Inspections – Resident Inspector, Regional and Headquarters Staff – The NRC resident inspectors' duty station is on site at the nuclear power plant and monitor daily plant activities. These inspectors perform quarterly and annual inspections of fire safety (Inspection Procedure 71111.05AQ, Fire Protection Annual/Quarterly). The regional inspectors perform team inspections every three years and perform in depth inspections assessing prevention, suppression and safe shutdown (Inspection Procedures 71111.05T Fire Protection (Triennial) and 71111.05XT Fire Protection – NFPA 805 (Triennial)). Headquarters staff reviews many types of plant changes to ensure that fire safety is not reduced unacceptably and support the resident and regional inspectors.

  • Training and Fire Drills – Nuclear power plant staff is trained on fire safety and performs periodic fire drills to practice responding to possible fires. Plants prepare fire "preplans," which describe the types of fire hazards at the plant and the location of firefighting equipment in the event of a fire.

• Suppression – Involves plant features and staff prepared to control and extinguish fires that are not prevented. Suppression involves, but is not limited to, the following plant features and activities:

  • Automatic Fixed Suppression – Fire Sprinklers, Foam – Plants have installed firefighting systems that operate automatically to control and extinguish plant fires. General area protection such as fire sprinklers and special hazard protection for oil based hazards, such as foam, are installed.

  • Detection
    • Fire Detectors – Plants use fire detectors, including some very high sensitivity detectors, to identify fires in their early stages before significant plant damage can occur. The detectors alarm to a constantly staffed location where plant staff can investigate the alarm and call the onsite fire brigade to respond with fire hoses and fire extinguishers.
    • Video Surveillance – Plants may temporarily rely on constantly monitored video surveillance to detect fires in areas where there are high levels of radioactivity.
  • Fire Protection Loop
    • Fire Pump Building – Nuclear power plants have redundant and dedicated pumps to ensure that there is sufficient fire protection water to extinguish plant fires. These pumps are capable of producing water flows of thousands of gallons of water per minute.
    • Natural Water Supply (Lake, River, or Ocean) – Some nuclear plants are situated to have access to an essentially unlimited water supply in the form of lakes, rivers or the ocean for their fire water systems. Plants take precautions to ensure that the raw water from natural water supplies does not damage fire protection systems through corrosion or due to obstructions.
    • Peripheral Fire Hydrant – The plant fire pumps and water supplies feed both the fire suppression systems within the structures and fire hydrants located strategically around the plant. Plant fire brigades and offsite fire departments use these hydrants to supply water to extinguish fires outside of the plant.
    • Water Tank – Many plants rely on large water tanks to provide fire water. Hundreds of thousands of gallons water are dedicated to fire fighter use.

  • Offsite Support – Nuclear operators have agreements with offsite fire departments to support firefighting operations if needed.

  • Onsite Fire Brigade – Each nuclear plant has a fire brigade or fire department of staff trained and equipped to extinguish plant fires. Training for these firefighters includes understanding the plant layout and hazards located in and around the plant site.

• Safe Shutdown – Safe shutdown involves installed equipment and procedures that provide reasonable assurance that he reactor can be made safe even if a plant fire is not prevented or rapidly suppressed. Safe shutdown includes, but is not limited to, the following features:

  • Backup Power Supply – In the event of a loss of grid power, nuclear plants have reliable alternative power sources to power equipment necessary to safely shutdown the plant. Plants have diesel driven fire pumps or fire pumps powered from these reliable onsite electrical systems that supply water for firefighting even when grid power is unavailable.

  • Redundant Safety Pumps – Cooling the nuclear fuel with water can be done with numerous pumps at a nuclear power station. The fire protection program ensures that these pumps are separated from being damaged by a single fire. Information regarding separating redundant controls and cables from the same fire are included in NUREG-1778, "Knowledge Base for Post-Fire Safe-Shutdown Analysis."

  • Separation – Fire Walls – Fire walls are used to separate redundant safety equipment including pumps, into areas that would be impacted by a single fire. The walls are typically concrete or concrete block with fire resistant sealant protecting the openings through which pipes and cables go through. For information regarding methods of sealing holes in fire walls see Barriers Historical Web page and NUREG-1552, "Fire Barrier Penetration Seals in Nuclear Power Plants."

  • Wrapped Cabling – When a cable for a redundant safe shutdown component can't be run in a separate fire area, the cable can be wrapped in fire resistant material. This material will have been tested to last one or three hours, depending on the application. More information on cable wraps is included in NUREG-1924, Electrical Raceway Fire Barrier Systems in U.S. Nuclear Power Plants."

Another aspect of defense-in-depth is the use on operator manual actions to safely shutdown the reactor. Information on operator manual actions is located on this Manual Actions Historical Web page. Information regarding prescriptive review of operator manual actions is included in NUREG-1852, "Demonstrating the Feasibility and Reliably of Operator Manual Actions in Response to Fire," and for risk-informed applications staff uses NUREG-1921, "EPRI/NRC-RES Fire Human Reliability Analysis Guidelines."