Anthony Sarrack, one of the two engineers, notified staff members at the Nuclear Regulatory Commission that the design of venting systems was seriously flawed at his reactor and others in the United States similar to the ones in Japan. He later left the industry in frustration because managers and regulators did not agree.
Mr. Sarrack said that the vents, which are supposed to relieve pressure at crippled plants and keep containment structures intact, should not be dependent on electric power and workers’ ability to operate critical valves because power might be cut in an emergency and workers might be incapacitated. Part of the reason the venting system in Japan failed — allowing disastrous hydrogen explosions — is that power to the plant was knocked out by a tsunami that followed a major earthquake.
Copies of Mr. Sarrack’s correspondence with the N.R.C. were supplied by David Lochbaum, a boiling-water-reactor expert who works for the Union of Concerned Scientists, a nonprofit group based in Cambridge, Mass., that is generally hostile to nuclear power.
“The Nuclear Regulatory Commission cannot claim ignorance about this one,” he said.
Plant managers and nuclear regulators are warned about far more problems each year than actually occur, but in this case, the cautionary note was eerily prescient and could rekindle debate over whether automatic venting systems are safer alternatives.
While staff members at the Nuclear Regulatory Commission considered Mr. Sarrack’s warning, they decided against changes.
On Wednesday, a commission spokesman, Scott Burnell, said the commission still believed that existing venting systems were a “reasonable and appropriate means” of dealing with a rise in pressure after an accident. But he has also said that the commission’s staff members are studying the events at Fukushima Daiichi for “lessons learned,” and that they had identified means of “reducing risk even further” by making the vents “more passive.” He said the staff had not yet chosen a way to do that.
One way would be using rupture disks, relatively thin sheets of steel that break and allow venting without any operator command or moving parts when the pressure reaches a specified level. But many in the industry argue that using such a disk requires that there be a way to close the vent once pressure is relieved in order to hold in radioactive materials.
The accident at the Fukushima Daiichi plant was the first time the venting systems were put to the test.
Pressure began to build in three reactors soon after the tsunami hit because the plant’s cooling system stopped operating when the electricity went off. Without an adequate flow of cool water in the reactors, the fuel rods began to overheat and produce explosive hydrogen gas.
Managers were worried about venting because it would release significant amounts of radioactive materials, but when they finally gave the order to do so — after being told to by the government — the workers found the venting system inoperable. With the power out, their commands from the control room did not open the valves. They then discovered that that radiation levels at one reactor were so high they could not attempt to manually open the valve. And at two other reactors, their attempts to open the valves failed, possibly because the equipment itself was damaged in the earthquake.
In Units No. 1 and No. 3, the gas leaked from primary containment structures and fueled explosions that ripped apart the reactor buildings, spewing radioactive material into the air. Unit No. 2 suffered a hydrogen explosion inside the primary containment.
Mr. Sarrack, reached by telephone, said that his proposal was opposed by the operations department officials at his company, who wanted direct control over the reactor rather than employing automatic systems. He was working at the time at the nuclear plant in Monticello, on the Mississippi River near Minneapolis.
He said he continued to believe that a passive system, like one using a rupture disk, would work better and could be set to rupture at a pressure just slightly less than the pressure at which the containment would rupture. In those cases, he said, venting is always preferable; the releases of radioactive materials during deliberate venting are expected to be lower than those resulting from explosions.
But the consensus in the nuclear industry supports the existing systems. Douglas E. True, the president of ERIN Engineering and Research of Walnut Creek, Calif., said: “In some cases you can argue it might be better to have a rupture disk. In other cases, it would certainly be better to have a manually controlled system.” For example, he said, the disk is backed up by a valve that is normally in the open position. If the disk ruptured and there was no electricity, it might be impossible to close the valve, and the venting would be permanent.
The Fukushima plant was designed by General Electric, and the venting systems that failed in Japan exists at similar plants designed by G.E. in the United States.
In a statement, James Klapproth, the nuclear energy chief consulting engineer at GE Hitachi, said that his company believed that the venting system would have operated in an accident within the “design basis” of the plant,” but that the Fukushima disaster was worse than what the plant was designed for. He said that the industry in this country had considered passive systems “at one time.”
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