Fission Stories #184
Nuclear industry and NRC representatives often talk and write about their Open Door policies. For example, Doug Coe and June Cai, leaders of a task force formed to examine the NRC’s internal safety culture, briefed their Chairman and Commissioners on the agency’s Open Door policy for concerns raised by NRC staffers. And the owner of the Wolf Creek nuclear plant in Kansas recently informed the NRC about steps it had taken and planned to cultivate and maintain a safety conscious work environment in response to NRC’s concerns about reasons some workers had been terminated.
But not every Open Door is a good thing.
The owner of the Monticello nuclear plant in Minnesota informed the NRC that a fire door failed to properly close and latch during a semi-annual inspection. In event of a fire, this door needed to close securely to prevent the smoke and flames from spreading and damaging more equipment. Otherwise, a single fire might be able to disable primary safety systems and their backups.
The owner of the Quad Cities nuclear plant in Illinois informed the NRC about two doors being opened at the same time (see Fig. 1). The doors were between the turbine building and the reactor building for Unit 1. Two doors had been installed with an interlock intended to allow only one door to be opened at a time. The reactor building forms part of the secondary containment at this plant and the two-door access configuration is designed to limit the amount of radioactivity released to the environment during an accident. A bent locking bolt on one of the doors defeated the single-door open interlock measure.
The owner of the Duane Arnold nuclear plant in Iowa informed the NRC about two reactor building doors being opened at the same time. These doors performed the same function as the doors at Quad Cities with the same consequence for being opened at the same time. In this case, an overly strong magnet caused the problem. The doors were equipped with magnets that helped the electrical interlock circuit allow only one door to open at a time. The magnets told the circuit when a door was open. But the magnet mounted on one door was so strong it fooled the circuit into thinking the other door was closed when it was actually opened.
The owner of the Limerick nuclear plant in Pennsylvania informed the NRC about two reactor building doors being opened at the same time. Like at Quad Cities, the doors were between the turbine building and the reactor building. Unlike at Quad Cities, these pair of doors did not have an interlock designed to allow only one door to be opened. Instead, these doors had sets of red and green indicating lights. When either door was opened, the red lights illuminated as a signal for workers not to open the other door. When both doors were closed, the green lights illuminated as a sign that workers could open either door. In this case, the red lights lit up but workers ignored them and opened the second door anyway.
The owner of the LaSalle nuclear plant in Illinois informed the NRC about two reactor building doors being opened at the same time. These doors were between the reactor building and a Chemistry Lab where samples were analyzed. The cause this time was gravity. The single mounting bolt holding the armature plate to the door worked itself free and fell to the floor, disabling the electromagnetic lock.
The owner of the Dresden nuclear plant in Illinois informed the NRC about two reactor building doors being opened at the same time. These doors were between the reactor building and one of the emergency diesel generator buildings. An electrical maintenance supervisor opened the door in the emergency diesel generator building and stepped into the space between the doors. At the same time, two equipment operators opened the door in the reactor building and joined the electrical maintenance supervisor in the small passageway between the two opened doors. The company’s report to the NRC stated that the workers recognized that both doors were opened and “immediately secured both doors in the closed position.” The company’s report wasn’t clear whether the three workers remain enclosed between the closed doors, or have since wandered out. These doors were equipped with an interlock designed to prevent both from being opened at the same time. Workers did some troubleshooting on the doors and their interlock and the doors were cycled about 300 times over the next 24 hours without the problem from being identified or recurring. Because the magnet on the limit switch informing the interlock circuit whether the door was opened or closed had caused problems long ago, it was blamed this time, too.
These examples all occurred within the past year and form only a subset of similar reports to the NRC during this period.
Open Doors that facilitate healthy, productive communications between workers and managers promote nuclear safety.
Open Doors that facilitate the spread of fires or radioactivity degrade nuclear safety.
Pairs of doors are installed at access points to the reactor buildings of boiling water reactors because these building provide a barrier—the last barrier—preventing radioactive material from reaching the environment. Each pair comes with either physical interlocks or administrative controls intended to ensure that one door is always closed to preserve the integrity of this key protective barrier.
The reports reveal that the physical interlocks and administrative controls sometimes fail, creating large holes in the key protective barriers.
The flurry of reports over the past year—considerably higher volume than reported in prior years—might suggest that the reliability of the physical interlocks and administrative controls is declining over time.
But it might also suggest heightened focus on and attention to this area than in the past. In other words, the number of problems this past year is comparable to the number of problems in past years but the tolerance of such problems has been lowered.
In either case, the relatively high number of reported Open Door problems over the past year bodes well for the future. The first step in any recovery/remedial program involves indentifying the problem. That step seems to have been taken many times at many plants, taking us one step closer to having the problems resolved.
“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.