The boiling water reactor (BWR) at the Cooper Nuclear Station in Nebraska dusted off history (Fission Stories #66) the hard way on September 17, 1999. Shortly after lunch, operators declared both of the standby gas treatment systems at the reactor inoperable. They didn’t know what caused both systems to fail, but immediately started shutting down the plant because the plant’s operating license did not permit the reactor to operate without at least one standby gas treatment system available.
Each of the two independent, redundant standby gas treatment systems has a fan and a series of filters. The fan pulls air from within the reactor building and refueling floor space and sends it through the filters. The standby gas treatment system is an essential part of a BWR’s containment – it reduces the amount of radiation released to the atmosphere following an accident by nearly a factor of 100.
A funny thing happened on the way to shut down. While working on the standby gas treatment system problem, workers discovered evidence of a hydrogen explosion in the offgas system. The offgas system handles radioactive gases released from the plant during normal operation, while the standby gas treatment system handles radioactive gases released from the plant following accidents. Both systems route the gases through a tall stack to disperse them (i.e., the solution to pollution is dilution). The hydrogen explosion in the offgas system damaged equipment of the standby gas treatment system in the base of the stack, thereby causing the problem that led to the shutdown.
The Cooperites don’t know exactly when the explosion occurred. They guessed it happened about six or seven hours before they figured out the standby gas treatment system was broken. They don’t know why the hydrogen gas exploded, but they think it was caused by a procedure error that allowed a valve in the offgas system to be throttled. This partially closed valve, according to the Cooperites, caused hydrogen gas inside the offgas system to build up until it reached an explosive mixture.
As reported in Fission Stories #66, the offgas system at boiling water reactors like Cooper experienced nearly two dozen hydrogen explosions in the mid to late 1970s. Yet more than two decades later, Cooper’s offgas system apparently experienced a hydrogen detonation severe enough to disable the standby gas treatment system but subtle enough to do undetected.
Was the monitoring capability truly so limited as not to detect the pressure, temperature, flow, and noise consequences from this detonation? Or were workers so blaise as to overlook the readily available indications? Actually, neither explanation is proper justification. Nuclear power reactors are billion dollar assets that can have accidents causing billions of dollars in damages. It should not be possible to have a significant hydrogen detonation at a nuclear plant remain undetected for several hours, perhaps longer. That’s unacceptable and inexcusable.
In addition, two independent and redundant safety systems were disabled by a single cause. Such common mode or common cause failures played a role in the Fukushima Daiichi disaster when the tsunami knocked out all the backup power and significantly impaired the ability to recover from that condition by disabling many of the vital electrical panels. Defense-in-depth becomes too shallow when a single act can defeat so much safety equipment.
“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.