Backgrounder on Fire Protection for Nuclear Power Plants
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On March 22, 1975, a fire at the Browns Ferry Nuclear Power Plant, operating near Decatur, Ala., fundamentally changed how the NRC dealt with fire protection at U.S. nuclear power plants. The fire started when plant workers were testing fire barriers for leaks in the room where electric cables enter the reactor building. The workers used a candle flame to check for air leaks. An air leak caused the candle flame to ignite both a seal and the cables passing through it.
By the time firefighters extinguished the fire, it had burned for almost seven hours. More than 1,600 electrical cables were affected, 628 of which were important to plant safety. The fire damaged cables for power, control systems and instrumentation, affecting reactor safety systems. The fire damaged enough cables that operators could not monitor the plant normally and had to perform emergency repairs on systems needed to shut the reactor down safely.
Investigations after the fire revealed shortcomings in both fire protection design at nuclear power plants and in plant procedures for responding to a fire. The event demonstrated that a fire in certain locations at a nuclear plant could damage redundant safety systems and components, making it difficult to shut the reactor down safely.
Fire Protection Regulations
After the Browns Ferry fire, the NRC revised its fire protection regulations to reasonably ensure a reactor maintains the ability to shut down safely in the event of a fire by:
- Minimizing the potential for fires and explosions;
- Rapidly detecting, controlling and extinguishing fires that do occur; and
- nsuring that operators can shut down the reactor safely despite a fire to minimize the risk of significant radioactive releases to the environment.
Nuclear power plants today use multiple layers of fire protection features to protect plant safety systems. Some of these features include fire barriers (such as insulation), fire detection systems and fire suppression systems (such as sprinklers). Every plant must have a fire protection plan outlining the fire protection program, installed fire protection systems, and the means to assure the reactor can be safely shutdown in the event of a fire. The NRC lists these requirements in 10 CFR 50.48(a). The NRC regularly inspects how plants achieve and maintain the reactor’s safe shutdown capability in the event of a fire.
Today, plants can choose between two approaches for managing their fire safety:
- Deterministic fire protection establishes fire safety by ensuring systems needed to shut down the reactor will survive a fire. This approach, based on an assumed serious fire, was developed when the staff and the industry had system-based tools for considering fire risk, rather than today’s more detailed, component-based information. The NRC lists deterministic requirements in 10 CFR 50.48(b) and Appendix R of 10 CFR 50. Regulatory Guide 1.189 provides guidance to plants regarding meeting these requirements.
- Risk-informed, performance-based fire protection considers risk insights as well as other factors to better focus attention and resources on design and operational issues according to their importance in safety. This performance-based approach relies on a required outcome rather than requiring a specific process or technique to achieve that outcome. The NRC lists these requirements in 10 CFR 50.48(c).
The NRC approved the risk-informed and performance-based fire protection rule in July 2004, allowing licensees to focus their fire protection activities on the areas of greatest risk. The rule incorporates the requirements contained in the National Fire Protection Association (NFPA) Standard 805, with some exceptions. In order to help those licensees moving their fire protection program to one based on NFPA 805, NRC staff issued Regulatory Guide 1.205. The NRC guide also endorses the Nuclear Energy Institute’s NEI 04-02 document, because its methods acceptably implement NFPA 805 and comply with the regulatory guide. The staff issued Revision 1 to RG 1.205 in December 2009.
The NRC’s first approval for a plant moving to risk-informed fire protection was for the Shearon Harris plant in North Carolina, in June 2010. The agency approved the second transition, for the Oconee plant in South Carolina, in December 2010. The NRC currently expects 47 other reactors will transition to the NFPA 805 standard.
Recent Fire Protection Activities for Operating Reactors
The NRC oversees fire protection at nuclear power plants through the Reactor Oversight Process, which includes systematic fire protection inspections once a quarter, once a year and once every three years. The NRC analyzes fire protection inspection findings to determine how they could prevent a plant from meeting its fire protection objectives.
Fire Barriers: Fire barriers are fire-resistant materials that separate redundant series of fire safety equipment located within a fire area. Nuclear plant fire barriers include: Thermo-lag, Hemyc/MT, Kaowool and FP-60. The NRC tested these materials in full-scale fires and found that materials used to protect cables did not perform as designed. In one test for Hemyc, it was found the outer covering of the barrier could shrink during a fire, opening joints in the material and potentially allowing the fire to damage cables inside. The NRC contacted the licensees of the 11 plants using Hemyc so the licensees could take appropriate compensatory actions. The NRC then issued Generic Letter 2006-03 in April 2006, asking several questions so that the affected plants would evaluate all their fire barriers and take permanent corrective actions. All the plants responded to the generic letter, describing the adequacy of their Hemyc and other fire barriers installed at their plants. The NRC has reviewed the responses, closed out the generic letter for all plants and concludes these fire barrier issues have been accounted for.
Post-Fire Operator Manual Actions: Plants must protect at least one set of plant safe shutdown equipment from fires using a combination of physical separation, barriers, and methods to detect and control or extinguish fires. Operator manual actions, documented in plant procedures, can be used to recover other safe shutdown equipment that could be damaged during a fire. In some cases, however, plants inappropriately used manual actions in place of fire protection features for one set of safe shutdown equipment. The agency NRC clarified its expectations on the topic in Regulatory Issue Summary RIS-06-10.
Fire Protection for New Reactors
The NRC is applying fire protection lessons from currently licensed reactors to ensure fire safety at new reactors. The “Fire Protection for New Reactors” section in Regulatory Guide 1.189 includes enhanced fire protection criteria for new reactor designs to achieve safe shutdown by assuming that a fire in any one area will disable all the equipment there. This exceeds the deterministic standard for the current fleet of nuclear power plants.
More information on fire protection is available on the NRC’s Web site. The NRC’s regulations for nuclear power plants can be found in Title 10 of the Code of Federal Regulations (10 CFR). Fire protection regulations are detailed in Section 50.48 of 10 CFR and Appendices A and R to Part 50.
Involved Industry Organizations