The hydrogen explosions that shattered the tops of two reactor buildings at the Fukushima 1 nuclear complex followed the venting of hydrogen and steam by plant operators desperate to prevent a far greater disaster: a high-pressure explosion of the primary reactor containment shell and radioactivity release, a former senior U.S. nuclear official concludes.
In an analysis shared with other U.S. nuclear experts Saturday, Lake Barrett, who led the Nuclear Regulatory Commission's investigation of the Three Mile Island accident, describes the burning of zirconium cladding on fuel rods in the reactor cores after normal cooling operations failed because of a lack of electric power. A perilous buildup of hydrogen and steam followed within the concrete primary containment shell surrounding the reactor pressure vessel in units 1 and 3, and then in 2.
"To prevent a catastrophic primary containment system failure the operators vented the primary containment through the safety venting system trying to reject heat and excess gases up the 100 meter tall stacks at the plants," he wrote. "Normally there are operable fans and filters to control this dangerous mixture, but there was no electrical power for the fans. So most, if not all, of this dangerous mix of hydrogen gas seeped into the reactor building in Units 1 and 3. The hydrogen, being lighter than air, mixed with air in the upper large refueling floor area."
Some source ignited the explosive mixture, blasting away the sheet metal roofs and sides of the top section of the outer secondary containment building in units 1 and 3, he said. Braving dangerous conditions, workers had time to remove a wall panel at the top of the unit 2 reactor building providing an exit for hydrogen, avoiding a similar roof-level explosion, he said. The damage to the buildings 1 and 3 and the opening in 2 created an exit route for radioactive releases from the spent fuel pools at the top of the reactor pools.
Barrett's analysis is shared by many officials and experts. A series of slides created by Matthias Braun, with the German office of Areva SA, the French nuclear reactor builder, depicts the sequence of events Barrett describes. It is also being widely shared among U.S. nuclear experts.
Switching from seawater to fresh water cooling
Another former senior NRC official, speaking on background, said he believed -- based on the still incomplete evidence from Fukushima -- that hydrogen was vented from the primary containment through duct work that allowed the hydrogen to collect in units 1 and 3 in the refueling floor in the secondary containment building. Normally, an exhaust system could have filtered and removed the hydrogen, but it was not working because of the loss of outside electric power. When the hydrogen accumulated above the 8 percent detonation limit in air in units 1 and 3, it exploded.
Tokyo Electric Power Co. said today it intends to switch from pumping seawater to pumping fresh water into the three reactors, to prevent corrosion and salt buildup on the remaining fuel units and the molten residue in the core bottoms. Salt deposits could interfere with the flow of cooling water as operators try to bring down temperatures inside the units.
The focus on operators' actions during the peak of the crisis to relieve high pressures inside the containment structures points toward one of the central unanswered question in the accident: How did hydrogen escape from the venting system into the top area of the reactor buildings?
Almost immediately after the onset of the crisis, U.S. industry officials and regulators stressed that U.S. versions of the General Electric Mark I reactor were modified at the direction of the Nuclear Regulatory Commission in the late 1980s, after the Three Mile Island reactor accident. Reactor owners were required to strengthen the venting system to prevent a leakage of hydrogen into the secondary containment building.
In the Mark I design, the reactor pressure vessel is fitted within a concrete primary containment shell that resembles an inverted light bulb. At the bottom of the shell is doughnut-shaped water pool, the pressure suppression system, or torus. During the accident, when pressures rose inside the primary reactor containment structure, the exit path was into the torus. The venting system at the Fukushima units, leading from the torus into the secondary reactor building, evidently could not contain the pressure.
At the Tennessee Valley Authority's Browns Ferry plant near Athens, Ala., the Mark I reactors were fitted with 14-inch diameter carbon steel piping that leads from the primary containment and then out of the secondary containment building, said Ray Golden, senior manager of nuclear communications at TVA. The pipes go to a gas scrubbing unit and then to tall stacks near the reactor buildings.
The original venting system would not have been able to contain high pressure gas at the levels reported from the Fukushima reactors, he said.
Possible leak from the top of the reactor
Bill Borchardt, NRC executive director for operations, was asked at an NRC meeting Monday whether the Fukushima Mark I units had hardened venting systems. "That we're not clear on. I'm not sure. I can't really answer that question."
David Lochbaum, chief nuclear safety official of the Union of Concerned Scientists, has another possible explanation. As steam and hydrogen built up within the primary containment shell, high pressure may have forced an opening between the primary containment shell, called the drywell, and the metal cap that is bolted onto the shell. A pressure test decades ago at the Brunswick Nuclear Plant in North Carolina demonstrated that such a high-pressure leak could occur in Mark I reactors built at the Fukushima plant, Lochbaum said.
"This tragedy will be closely examined for its causes, what happened and why," Lochbaum said. "That scrutiny must determine how hydrogen got into the reactor buildings to cause the catastrophic explosions. The drywell head pathway may be that answer. We need to stress that we're not putting this forward as the only answer for this question, but it's the most plausible explanation that we've heard to date."
In his email, Barrett apologized to colleagues for a delay in his unofficial assessments and adds, "based on the imperfect information publicly available, I believe I have a preliminary speculative understanding of the basics of the situation. ... It is based on very incomplete information and I am filling many blank spots with what I think happened," he said.
"In summary I believe we are on the downside of the first phase of the accident: getting thermal control," he wrote in the message Saturday. Although he expressed concern about the condition of Unit 2, he said, "the worst should be over now."
Barrett predicted that the melted reactor cores at the three Fukushima units resemble those of Three Mile Island's reactor 1: "a bed of rubble with localized melting of composite materials of steel, zirconium, and uranium. Sort of like a highly radioactive steel mill slag-like material. These cores are likely still in the reactor vessels, and are being cooled by seawater injection using highly pressurized fire engine pumps," he said. Venting continues to contain pressures in the primary containment.
"This current 'feed and bleed' method of cooling with salt water is not a sustainable long term cooling method. Salt deposits are likely building up in locations in the thermally heterogeneous core rubble pile. This configuration is completely unknown. But the Fukushima reactors, I believe, are much more damaged and contaminated than TMI was and there are three of them in this state."
Once the reactors have been sufficiently cooled and stabilized, weeks of environmental assessments lie ahead, with decisions on how to cope with radioactive contamination, he said. The cleanup will take years.