On March 22, 1975, Tennessee Valley Authority’s (TVA) Browns Ferry nuclear power station, at full power, experienced a near catastrophic fire near Athens, Alabama. Workers were using a candle flame to check for airflow leaks and plug holes along an extensive network of electrical conduits and cable trays under the Unit 1 control room. The flame was instead sucked into a cable tray accidentally setting fire to the combustible polyurethane foam insulation wrapping the electrical circuits inside a cable tray. The ensuing fire quickly spread inside the cable spreading room like a blow torch and through wall penetrations into the reactor building. More than 1,600 electrical cables routed in 117 conduits and 26 cable trays were destroyed within the first hour including 628 “safety-related” cables vital to safely shutting down the reactors. The fire knocked out more than a dozen reactor control systems including all of the emergency core cooling systems for Unit 1 and most of the cooling systems for Unit 2. Like most equipment, the automated fire suppression systems failed when the fire destroyed power cables. Smoke and fumes entered the control room. The fire burned out of control for seven and a half hours with temperatures reaching 1500 degrees Fahrenheit before it was finally extinguished by the local fire department. A catastrophic nuclear meltdown was narrowly averted only by “sheer luck” according to one official.
The harrowing experience prompted a major regulatory overhaul of fire protection at US nuclear power stations. After five years of wrangling with a nuclear power industry, in 1980, the U.S. Nuclear Regulatory Commission passed a compromised version of new safety requirements of the nuclear industry to comply with an upgraded federal fire code. The new regulations were made law to assure that no single fire can ever again simultaneously knock out both the primary and the backup equipment and associated electrical circuits needed to achieve safe shutdown and maintain cooling of the reactor.
In specific, where both the primary and backup electrical circuits are routed into the same fire zone; 1) one set of associated circuits must be wrapped in a three hour protective fire barrier system; 2) alternately, one train wrapped in a one hour rated fire resistant barrier with no intervening combustible material and seperated from the backup associated circuits by automated fire detection and suppression systems or; 3) a minimum of 20 feet separating the two associated circuit systems.
US nuclear power plant operators including TVA were to comply with the required federal fire safety upgrades. Meanwhile, TVA had to shut down five nuclear power plants, including Browns Ferry Unit 1, 2 and 3, in 1985 and place them on “administrative hold” after discovering that the plants' construction did not match the approved design blueprints. Beginning in 1992, there was an industry-wide scandal involving widely deployed but fraudulently tested fire barrier materials that turned out to be combustible and drastically failed to meet time requirements for protecting the electric cables.
Browns Ferry Unit 1 remained at zero power for more than two decades while TVA spent $1.9 billion to bring the reactor’s as-built configuration into compliance with design specifications; all except for the NRC fire protection code requirements. In May 2007, the NRC allowed Browns Ferry Unit 1 to restart the reactor exempted from the very fire code its 1975 fire promulgated. Instead, unable to enforce compliance with the 1980 fire code, NRC provided TVA, along with almost half of the remaining US reactor fleet, with a still unapproved, complex alternative safe shutdown fire protection plan through computer modeling basically arguing that fires of duration similar to the 1975 fire are too remote to worry about any longer. Industry complains today that the alternative "fire modelling" plan is too expensive to implement. The other half of the fleet is essentially operating on nearly one thousand exemptions granted by NRC from the 1980 fire code.
As a result, US nuclear reactors remain in a nether world dangerously straddled between two fire protection regulations and not fully compliant with either.
The Browns Ferry Unit 1, 2 and 3 are GE Mark I boiling water reactors identical to Japan's Fukushima Daiichi units that melted down on March 11, 2011.
March 30, 2015
