The Browns Ferry nuclear plant comprises three 1100MW boiling water reactors, which began construction in 1967 and 1968. Operating licences were issued to the Tennessee Valley Authority (TVA) for unit 1 in 1973 and for unit 2 in 1974. Unit 3 was under construction when the fire occurred, and units 1&2 were operating at full power.
On March 22, workers were testing the seal of a 5in by 18in sleeve in a 4ft by 4ft penetration through the wall between the reactor building (RB) and the cable spreading room (CSR) to ensure that the ventilation system was maintaining the appropriate pressure differential between the CSR and the RB. The penetration was designed to allow cables to pass through a reinforced concrete wall. The design of the seal that protected the penetration consisted of foamed-in-place polyurethane with a fire retardant material applied to the face of the combustible polyurethane. However, at the time of the fire, the fire retardant material was not in place.
Workers were using a candle to determine if there were leaks through the seal. When the workers discovered a strong draft, they placed foamed-polyurethane sheeting material into the opening to reduce the airflow. When they tested this seal again, the candle flame started the polyurethane on fire. The fire ignited at about 12:20pm.
The workers tried to extinguish the fire using various methods: beating it with a flashlight; stuffing rags into the penetration; discharging carbon dioxide fire extinguishers and dry chemical fire extinguishers at the penetration. But none of this worked. At 12:34pm, the shift engineer was notified that there was a fire in the reactor building.
The Athens, Alabama Fire Department was notified and arrived on scene at 1:45pm. But it was not allowed to use water to extinguish the fire because it was considered to be too risky by plant personnel. A fixed carbon dioxide fire extinguishing system was manually discharged in the CSR three times, and the fire was reported extinguished at 4:20pm. But the fire continued to burn in the reactor building. It was located about 20 feet above the floor, making access difficult. Finally, at about 7:00pm, the fire department was authorised to use water to extinguish the reactor-building fire, and it was declared out at 7:45pm.
The fire involved a total area of about 1200 sq ft. About 1600 cables were damaged; 628 of these cables were "safety related." Commercial nuclear power plants in the US typically have two redundant trains of safety- related equipment, referred to as Division I and Division II. During the fire, safety-related equipment,including the emergency core cooling systems, failed to operate properly because of damage to control cabling serving the equipment. The fact that safety-related equipment in both safety trains was affected by the fire was unexpected and represented a serious failure of the plant’s safety programme.
Control room operators reported that control panel lights were giving incorrect indications, and that various circuits serving both of the Division I and Division II redundant safety trains were failing. During the fire, the plant operators depressurised the reactor so that a makeshift arrangement using a condensate booster pump could provide the necessary cooling for the reactor core. The water level in the Browns Ferry 1 dropped from about 16ft above the top of the fuel rods to about 4ft above the fuel rods. If the water level had continued to drop, the reactor could have reached a "melt-down" condition, similar to the melt-down that
occurred at the Fukushimi Daiichi plant in Japan after the March 2011 earthquake and Tsunami.
The Browns Ferry fire caused a direct loss estimated at $10 million and an indirect loss estimated at about $300-500 million (in 1976 dollars), plus 1000 man-years of effort during the 18-month recovery process. Much of this loss was due to the need to purchase power from alternate suppliers and the loss of a return on the TVA’s $1 billion investment in the power plant for a period of 18 months.
However, the overall impact on the nuclear power industry was far greater. All of the commercial nuclear power plants in the US were required to modify their fire protection programmes. They were required to separate redundant safety-related cables and equipment by at least 20ft of clear space, or provide passive fire barriers and active fire protection to portions of the plant that are deemed to be critical to safely shut down the plant.
As a result of the Browns Ferry fire, the Nuclear Regulatory Commission (NRC) issued Branch Technical Position Paper (BTP) 9.5-1 and Appendix A to BTP 9.5-1 giving guidance to the licensees about the required fire protection programmes at commercial nuclear power plants. The NRC also required each licensee to conduct a Fire Hazards Analysis (FHA) at each power plant and to respond to the requirements of the BTP. In 1980 the NRC issued Appendix R, which further codified the minimum levels of fire protection that must be provided by the licensees. Appendix R is a highly prescriptive regulation with which existing nuclear power plants must comply or from which they must request exemptions.
In 2004, the NRC ruled that licensees could utilise the National Fire Protection Association (NFPA) Standard 805, Performance-Based Standard for Fire Protection for Light Water Reactor Electric Generating Plants, 2001 edition to use tools such as fire modelling and risk analysis to develop a fire protection programme to provide the level of fire protection necessary to provide adequate safety at nuclear power plants. The NRC has indicated that NFPA 805 can be used as a performance-based method to demonstrate that fire protection safety requirements have been met, subject to certain conditions, rather than compliance with prescriptive fire protection requirements.
Two nuclear power plants (Shearon Harris and Oconee) have utilised NFPA 805 to develop performance-based fire protection programmes under a pilotprogramme. The results of this pilot programme indicated that the use of NFPA 805 in lieu of prescriptive fire protection methods was significantly more complex and required significantly more resources than had previously been recognised.
Among the other impacts on the fire protection industry:
- all utilities that operate nuclear plants are required to have a robust fire protection programme that utilises a "defence-in- depth" concept to ensure that programmes meet fire protection objectives;
- the utility industry formed the Edison Electric Institute (EEI) Fire Protection Committee to deal with fire protection issues; and
- the Electric Power Research Institute was directed to develop research programmes for fire protection-related topics.
The cost of retrofitting US nuclear power plants was estimated in 1976 by one observer to be between $7 billion and $12 billion in 1976 dollars. We do not know the actual cost of retrofitting the plants. Forty years after the fire, the nuclear licensees are still retrofitting the plants or designing alternative fire protection schemes to demonstrate an equivalent level of safety to that required by Appendix R. This process will probably continue until these nuclear power plants are decommissioned, which could potentially be 40 years away.
About the author
Joseph H. Talbert, PE, ARM, is a senior consultant at Aon Fire Protection Engineering, 4 Overlook Point, Lincolnshire, IL 60069, USA.
