A small explosion occurred at 9:30 am on December 13, 1977, in the offgas system at the Millstone Nuclear Power Station Unit 1 in Connecticut. Unit 1 featured a boiling water reactor (BWR) that permanently closed in 1996 for reasons unrelated to this explosion.
In a BWR, the offgas system processes and filters the gases removed from the plant’s main condenser. The main condenser collects the steam exiting the turbines after it spins the turbine/generator to make electricity. The offgas system features a recombiner that puts hydrogen and oxygen molecules back together to form water. Any non-recombined hydrogen and oxygen, along with non-condensible gases (e.g, krypton and iodine), are routed through charcoal filters and then discharged through a tall stack for optimum dispersal of the radioactive gases.
In this case, hydrogen gas collected within the offgas system’s piping and then ignited. Damage was limited to breaking the glass faces on offgas system flow differential pressure gauges, causing the blowout of a rupture disk, and emptying the water-filled loop seals in the offgas system drain lines. The last is the one that mattered.
The offgas system had a number of drain lines to prevent water from collecting in piping and blocking the gas flow through the system. The drain lines routed this water to the sump, a large concrete pit, in the base of the stack building. To prevent air from leaking into the system and radioactive gases from leaking out of the system, these drain lines had water-filled seals much like the S-shaped bends in commode and sink piping.
The explosion blew the water out of the loop seals. Consequently, radioactive gases and hydrogen passed through the drain lines into the sump. The gases wafted out of the sump and filled the two-level building at the base of the stack.
The hydrogen in the stack building exploded at 1:00 pm. This explosion blew the stack building door into a warehouse nearly 200 feet away, breached the reinforced concrete ceiling between the stack chimney and the stack base space, extensively damaged the ceiling beams, dislodged the 2-ton concrete plugs in the floor above the offgas system particulate filters, and cracked the concrete stack.
This Millstone mishap was among nearly two dozen similar mishaps that occurred at U.S. boiling water reactors in the mid to late 1970s. It was not because every BWR owner had to try blowing stuff up with hydrogen because it was so fun. It really reflected the lack of information sharing among plant owners faced with similar problems – each had to learn hard lessons the hardest way, through their own trial and error experience.
The March 1979 accident at Three Mile Island expanded information sharing across the nuclear industry. The Nuclear Regulatory Commission broadened its generic communications program to warn plant owners about problems experienced at one plant. The Institute for Nuclear Power Operations, created by the nuclear industry after Three Mile Island, also warned plant owners about potentially common problems, but also shared best practices. Collectively, this information is called operating experience in the industry.
The tragedy at Fukushima Dai-Ichi has provided and will likely continue to provide ample operating experience for the NRC and the nuclear industry.
“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.