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SAFETY

Fire protection

Evolution of nuclear plant fire protection
Nuclear power plants are built mainly of concrete and steel, which are relatively impervious to fire damage. Yet fire is an important safety concern at nuclear plants because, depending on its severity and where it occurs, a fire could threaten systems and components that are needed to shut down the plant and provide coolant for the fuel. The ultimate goal of fire protection programs at nuclear power plants is to protect public health and safety by ensuring that plants can be shut down and maintained in a safe condition.

Plant design and regulation have always considered fire safety to some extent, but in the early years of commercial nuclear energy in the United States—the 1960s and early 1970s—the requirements and standards related to fire protection at nuclear power plants were not well developed. Nuclear plants had to meet applicable building codes-the same codes used for office complexes and other types of industrial facilities. In addition, the NRC's general design criteria for nuclear power plants, issued in the 1970s, included general requirements for fire prevention, detection and response. It was not until 1981 that the NRC issued detailed requirements and guidance on how to meet those requirements.

A serious fire at a nuclear power plant in the mid-1970s revealed shortcomings in fire protection requirements. The plant's fuel was not affected and no one was seriously hurt, but the incident prompted a detailed examination of fire protection practices and led to sweeping changes in the NRC's fire protection regulations.

Lessons learned from Browns Ferry fire
On March 22, 1975, workers at the Browns Ferry nuclear plant in northern Alabama were using a candle to check for air leaks at openings in a wall where trays of electrical cables pass from one room to another. As a worker held a candle near one of the openings, the flow of air through it drew the flame toward the polyurethane foam that had recently been installed to seal the openings. The foam ignited and fire spread along the cables, shorting out backup safety systems as the cable insulation burned off.

Workers immediately tried to extinguish the fire, without success, and also notified the control room. Shortly afterward, a municipal fire department was called to the scene. Operators manually shut down both units and were able to keep the fuel covered with water and the plant adequately cooled throughout the incident. However, fire fighting efforts were hampered by the limited availability of breathing apparatus and reluctance to use water in electrical equipment areas. The fire was officially declared "out" more than seven hours after it started.

Four days later, the NRC appointed a special review group to study the lessons learned and make recommendations. The review group issued a report in February 1976 that identified a number of needed improvements in fire protection requirements and programs.

The report noted that public health and safety were protected during the incident at Browns Ferry despite significant shortcomings in fire protection:

"The question naturally arises: How can a serious fire that involved inoperability of so many important systems result in no adverse effect on the public health and safety? The answer is to be found in the defense-in-depth used to provide safety in nuclear power plants today."

The same principle should be applied to fire protection, suggested the review group:

"This defense-in-depth principle would be aimed at achieving safety through an adequate balance in:

1.	Preventing fires from getting started.
2.	Detecting and extinguishing quickly such fires as do get started and limiting their damage.
3.	Designing the plant to minimize the effect of fires on essential functions."

Designing the plant to minimize the effect of fires on essential functions."

NRC issues fire protection rule
The NRC issued fire protection guidance to utilities shortly after the Browns Ferry fire and, over the next several years, developed a rule that sets detailed requirements related to fire prevention and detection, fire brigade training, and other areas of fire protection. The requirements were codified in 1981 in 10 CFR 50.48 and Appendix R to 10 CFR 50.

Among other things, the rule requires fire protection equipment to limit damage done to structures and equipment that are "important to safe shutdown." Nuclear plant operating companies use various means—fire detectors, heat sensors, remote cameras, automatic sprinklers and so on—depending on which area of the plant is involved. These help ensure that a fire will be quickly detected and extinguished. The inaccessibility of some plant areas posed problems for firefighters battling the Browns Ferry fire. Some burning cable trays, for example, were as much as 30 feet above the floor and difficult to reach with fire hoses. Built-in fire suppression systems are intended to solve this type of problem.

But the rule's requirement for "automatic" suppression systems creates a different problem: If something triggers the system falsely, there is a potential for electrical equipment to be damaged by the suppression system—when no fire threat exists. So some companies have asked the NRC's permission to use manually activated suppression in areas where electrical equipment is located. The NRC has granted a number of such requests, which require an exemption from the rule's requirements.

The rule also requires that one set of systems needed to safely shut down the plant be kept free of fire damage—one of the major lessons from the Browns Ferry fire. The NRC changed its regulations to require that designs for new plants—those that started operating after Jan. 1, 1979—provide at least 20 feet of separation between certain systems and components. But for plants already built, the difficulty was how to provide the necessary separation between systems.

In some cases, electrical circuits needed for safe shutdown can be rerouted to separate them from a backup system. Where this is not feasible or desirable, the rule provides another option: fire barriers. A utility can meet the requirement with either a fire barrier rated to survive a standard fire for one hour, plus automatic suppression and detection equipment, or with a three-hour rated barrier.

These fire rating requirements are somewhat arbitrary. They apply equally to all areas where fire barriers are used, regardless of the actual fire hazard in a given area. In practice, however, the NRC has granted limited exemptions for plant areas where the fire hazard is low and where features of the plant would make it extremely difficult to install a fire barrier.

The rule also includes detailed requirements pertaining to fire brigades—their formation, equipment and training—as well as administrative controls governing the use and storage of combustible materials, the use of ignition sources (welding or grinding, for example), and periodic "housekeeping" inspections to ensure that employees comply with these controls. If a company wants to take a different approach to accomplish the same objective-for example, make changes in the way it trains fire brigades—it must apply to the NRC for an exemption to the rule.

The NRC has granted more than 1,200 exemptions to specific requirements of the fire protection rule since it was issued in 1981. Each exemption request involves a case-by-case safety review by NRC staff.

Fire protection: Applying defense-in-depth
Nuclear power plant fire protection programs integrate plant design features, management resources, training, equipment and procedures to provide defense-in-depth protection of public health and safety. Fire protection programs include:

Administrative controls—The introduction of combustible materials into safety-related areas of the plant is limited, and their handling, storage and use are strictly controlled. For example, some inside walls are painted with easy-to-clean epoxy paints that are combustible when they are being applied. To minimize the possibility of fueling an accidental fire, bulk quantities of the paint are stored outside the building and used only as needed. Other administrative controls minimize ignition sources. Permits are required, for example, before any "hot work" such as welding can be performed. In addition, fire-watch guards are posted during the performance of such welding operations.

Training—Utilities typically provide five types of fire protection training for nuclear plant employees, depending on their responsibilities: general, fire watch, fire department and brigade, maintenance, and operations. All employees receive training about the importance of fire prevention, how to report a fire and any administrative controls that govern their work activities.

Fire watches—Fire watches are used when normal fire protection equipment (including barriers) is not functional—while it is being serviced, for example. Employees who participate in fire watches receive special training that includes the location and use of fire extinguishers and other plant fire protection equipment. Either fixed or roving fire watches are used, depending on the situation.

Maintenance—All fire protection equipment used in nuclear power plants is subject to regularly scheduled preventive maintenance and testing.

Sharing industry experience—The nuclear industry places great emphasis on sharing lessons learned from experience. The Institute of Nuclear Power Operations regularly collects, analyzes and distributes information on numerous aspects of plant operation, including fire protection. Plants routinely receive the latest data on potential fire hazards and take necessary actions where appropriate.

Fire protection regulations: A need for update and enhancement
The NRC and the industry agree that, in general, regulations should become more risk-informed and performance-based. A performance-based approach to fire safety in a nuclear power plant would include an assessment of the actual risks in various areas-the amount of combustible material, potential ignition sources, whether fire suppression systems have been installed and so on. It also would consider the relative importance of the systems and components in that area to achieving and maintaining safe shutdown of the plant. Fire protection measures would then be based on a more realistic assessment of the actual fire hazard than is assumed in existing requirements.

Since 1995, the NRC and the industry have attempted to transition fire protection to a more risk-informed, performance-based structure. One of the industry's key objectives is to ensure that plant fire protection programs are not affected adversely by any new rule. Industry and NRC goals for risk-informed, performance-based fire protection regulation can be achieved within the existing regulations.

In June 1998, the commission directed the NRC staff to work with the National Fire Protection Association committee that is developing NFPA Standard 805, a risk-informed, performance-based fire protection standard for nuclear power plants. The NRC has initiated rulemaking to allow nuclear plant operating companies to adopt this standard as an alternative to existing regulations, if they choose, when the standard is complete in late 2000. Companies also will be able to use risk techniques from NFPA 805 if they choose to maintain their current licensing bases. This achieves the industry goal of risk-informing fire protection programs without making this approach mandatory.

NRC is also risk-informing the inspection process. NRC has developed baseline inspection guidance that involves routine resident inspections of fire protection programs and triennial regional team inspections of safe shutdown programs. Safety insights from probabilistic safety studies are used on the front end of these inspections to focus the inspection on safety-significant areas and at the end of the inspection to assess the safety significance of the findings.

The NRC is developing a comprehensive regulatory guide to consolidate existing regulatory guidance for plants choosing to maintain compliance with current regulations. This guidance is intended by NRC to provide best practices for future plant fire protection program changes.