Nuclear energy facilities take a defense-in-depth approach to protect against fires by creating multiple independent and redundant layers of protection to compensate for potential human and mechanical failures. Nuclear energy facilities have comprehensive fire protection systems, equipment and procedures to ensure safety as well as programs to manage combustible materials and ignition sources. No fires have significantly challenged safety systems at these facilities in nearly 40 years.
All nuclear energy facilities are subject to stringent fire protection requirements established by the independent U.S. Nuclear Regulatory Commission to protect the critical systems needed to maintain the reactor and to shut it down safely in an emergency.
NRC regulations offer two alternatives for meeting fire protection requirements. The original 1981 rule requires specific measures to protect safety functions against a predetermined set of hypothetical fires. In 2004, aided by advances in probabilistic risk assessment, the NRC issued an alternative rule that takes into account the relative likelihood of various fire scenarios and manages the risk. The NRC made adoption of the rule voluntary because existing fire protection programs are effective, and transitioning to a new approach is resource-intensive.
The voluntary rule is based on a consensus fire protection standard published by the National Fire Protection Association (NFPA), called “NFPA 805.” One reactor has implemented the new approach. Three reactors have NRC approval to do so and are transitioning to NFPA 805. The NRC expects a total of 46 reactors to pursue the new approach.
NRC regulatory guidance provides enhanced fire protection criteria for new reactors. Among other improvements, new designs offer improved separation of safety-related electrical cabling and power supplies, which ensures availability of systems required to shut down the plant safely.
NRC Enforces Strict Requirements
Through the early 1970s, nuclear power plants generally followed the same local fire protection codes that governed other industrial facilities. However, a 1975 fire at a nuclear power plant seriously challenged safety systems. The NRC subsequently issued detailed requirements (10 CFR 50.48 and 10 CFR 50, Appendix R) that address fire prevention and detection, fire brigade training, and other aspects of fire protection. These regulations require plants to protect critical safety structures and reactor equipment “important to safe shutdown” in the unlikely event of a fire.
Nuclear power plants use various systems and features, including fire protection barriers, physical separation, and fire detection and suppression equipment to meet these requirements. The overall defense-in-depth approach to fire protection combines three major elements:
rapidly detecting, controlling and promptly extinguishing any fires that do occur, thereby limiting fire damage
providing sufficient fire protection for structures, systems and components important to safety so that, in the event a fire is not extinguished promptly, it will not prevent essential plant safety functions from being performed.
The NRC’s original fire protection regulations are prescriptive, directing licensees to take specific measures in various areas of the plant. Since they were issued before sophisticated risk assessment techniques became available, these actions are not necessarily focused on the most important areas. Generally, they go well beyond what is needed for safety, but in some instances, the prescriptive requirements could be enhanced. Nuclear power plant operators are developing probabilistic assessments of fire risk to help ensure that safety measures are appropriate for the actual risk in a given area.
Problems With Prescriptive, ‘One-Size-Fits-All’ Rules
During the past 40 years, the NRC and the industry have found that prescriptive requirements like the original fire protection rule often need fine-tuning to achieve their intended purpose in real-world applications. Given the differences among U.S. reactors, a one-size-fits-all approach to regulation has inevitable limitations.
A further difficulty is that the NRC’s prescriptive fire protection requirements were issued after many plants had been built. While licensees building new plants can factor prescriptive requirements into their designs, those with older plants faced a significant challenge implementing the requirements on systems, structures and components already in place.
The NRC has allowed nuclear energy facilities to use alternatives to some of its prescriptive requirements as long as they provide equivalent protection. Because this involves a deviation from a requirement, the licensee must submit a proposed approach to the NRC for review. If the NRC approves the request, it grants an exemption from the specific requirement and instead requires compliance with the approved alternative.
A regulatory exemption is not a waiver. It is the exchange of one binding requirement for another that is as effective or more effective in meeting the NRC’s safety objectives and more practical for the plant to implement. Once approved, that alternative becomes binding and enforceable.
NRC Develops Risk-Informed Approach to Fire Protection
In 2004, the NRC issued 10 CFR 50.48(c), a voluntary approach—also known as NFPA 805—allowing risk-informed approaches that model expected fire sizes and effects. This approach involves assessing plant design and actual fire risks in each area of a facility, taking into account such factors as the amount of combustible material, potential ignition sources and fire suppression systems. Plant owners that implement the risk-informed approach may need to install additional equipment or take other measures if the analysis calls for them.
The NRC has endorsed guidance that the industry developed to aid plants in implementing risk-informed fire protection under the 2004 rule.
Currently, fire protection programs at most plants follow the NRC’s original fire protection requirements. Two plants—Progress Energy’s Shearon Harris and Duke Energy’s Oconee—participated in a pilot program to evaluate the new approach and subsequently received NRC approval to adopt it. Harris has completed implementation, and the transition is in progress at the three-reactor Oconee site. The NRC expects a total of 46 reactors to adopt the NFPA 805 approach.
Nuclear power plants are developing probabilistic assessments of fire risk—fire “PRAs”—to support implementation of risk-informed approaches in a variety of plant activities, in addition to a possible change to the NFPA 805 approach to overall plant fire protection. In addition to NRC research under way, the industry is working with the Electric Power Research Institute to achieve better realism in fire PRA.
The NRC also is making the fire protection inspection process more risk-informed. It has developed baseline inspection guidance that involves frequent inspections of fire protection programs and triennial regional team inspections of safe shutdown programs. Probabilistic safety studies and the NRC’s process for determining safety significance focus inspections on the most significant plant areas.