Drought could shut down nuclear power plants

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Because adequate cooling was not available, the nuclear fuel overheated to the point at which the zirconium cladding (the long metal tubes which hold the nuclear fuel pellets) ruptured and the fuel pellets began to melt. It was later found that about one-half of the core melted during the early stages of the accident. Although the TMI-2 plant suffered a severe core meltdown, the most dangerous kind of nuclear power accident, it did not produce the worst-case consequences that reactor experts had long feared. In a worst-case accident, the melting of nuclear fuel would lead to a breach of the walls of the containment building and release massive quantities of radiation to the environment. But this did not occur as a result of the Three Mile Island accident.

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Within a short time, the presence of a large hydrogen bubble in the dome of the pressure vessel, the container that holds the reactor core, stirred new worries. The concern was that the hydrogen bubble might burn or even explode and rupture the pressure vessel. In that event, the core would fall into the containment building and perhaps cause a breach of containment. ...

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The second alarming development was the gas bubble containing hydrogen, 1,000 cubic feet in size, at the top of the reactor. The reactor had become so hot that the coolant water had decomposed into its primary elements: oxygen and hydrogen.

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Short of the meltdown, there was the possibility of an explosion, either in the containment building or in the reactor core. On the first day of the accident, there had been a small hydrogen explosion in the containment – an event Met Ed officials didn't tell state or federal authorities about. When NRC experts found out, they launched an immediate effort to analyze the physical chemistry of the bubble.

Thornburgh was told that the NRC's analysis showed that the hydrogen could become flammable or explosive in a matter of days.

A Princeton University scientist calculated that the energy in the bubble was enough to set off an explosion equal to three tons of TNT. Such a force could rip the top of the reactor dome right off, flooding the containment with radioactive debris. There were also fears that the hydrogen would escape to the containment and explode there. One engineer calculated that a hydrogen explosion three times the force of Wednesday's blast might break the four-foot- thick walls of the containment, releasing radioactive material into the air.

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Only the structural integrity of the containment building's walls prevented Three Mile Island from becoming a Chernobyl-style catastrophe. There was no rupture, meaning all the damage was contained within the facility.

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In the months following the accident, “the actual condition of the core was the subject of intense, often heated, speculation.” And yet, as this General Public Utilities engineer in charge of much of the cleanup acknowledged as the job was nearing completion, “the most severe prediction was short of the mark” (ref.23). The prevailing view among the reactor engineers was that “most of the core damage was to the cladding”—i.e., to the zirconium alloy tubes holding the uranium oxide fuel pellets—and “damage to the fuel pellets . . . was minimal at Three Mile Island” (ref.13, p. 33). Although the independent inquiry instituted by the Nuclear Regulatory Commission surmised that some fuel had melted and run down the channels between the fuel rods, it concluded, “Despite this amount of damage, a core meltdown, as normally considered, did not occur” (ref. 29, v.2, pp. 535-36).

To some few experts, one piece of evidence strongly suggested that a considerable part of the fuel in fact lay in the bottom of the reactor vessel, where none should be. A few hours into the accident the neutron detectors toward the bottom of the inner wall of the cylindrical concrete shield surrounding the reactor vessel (solid black in Figure 2.6) began reporting anomalously large numbers of neutrons. As only the uranium fuel itself, not any of its highly radioactive fission products, could be a significant source of neutrons, the onset and continuance of such high readings from detectors in the vicinity of the bottom of the reactor vessel pointed to a “relocation” of a considerable part of the fuel. But as no one could imagine that a considerable part of the fuel had melted, there was no plausible explanation for how such a “relocation” could have taken place.

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People & Events

Roger Mattson, Victor Stello and the Hydrogen Bubble

Roger Mattson and Victor Stello went back a long way. Together they had worked as part of the Atomic Energy Commission, when nuclear power was in its infancy. The two men, though different in demeanor and approach, held each other in high regard. In the early days of the accident they worked together at the Emergency Management Center, established by the NRC. Little did either man know that later a rift would develop between the two engineers that would send a shock wave through the entire nation.

Mattson, who was considered the nation's leading expert on emergency core cooling, came to the startling conclusion that a hydrogen gas bubble had formed above the reactor core. Speaking to NCR Chairman Dr. Joseph M. Hendrie, Mattson said, "They can't get rid of the bubble. They have tried cycling and pressurizing and depressurizing; they have tried natural convection a couple of days ago; they have been on forced circulation; they have steamed out the pressurizer; they have liquided out the pressurizer. The bubble stays." Mattson explained that in order to shut the reactor down, they must reduce the pressure. But lowering the pressure caused the bubble to get bigger. A bigger bubble could push all the water right out of the core and lead to a meltdown. Mattson told Hendrie they were involved in a "horse race," and he was quite unsure as to whether they could win it. Meanwhile, word of a possible meltdown begin to ripple through to the press. That night news anchorman Walter Cronkite, one of the most trusted men in America, updated the nation as to the gravity of events at Three Mile Island: "The world has never known a day quite like today. It faced the considerable uncertainties and dangers of the worst nuclear power plant accident of the atomic age. And the horror tonight is that it could get much worse. The potential is there for the ultimate risk of meltdown at Three Mile Island...."

Victor Stello traveled to Harrisburg with Harold Denton on Friday, March 30, the day of Cronkite's somber broadcast. According to Mike Gray, the screenwriter of the film "The China Syndrome," Mattson confronted Stello on Sunday as they waited for Carter to arrive and the two men engaged in a heated debate over the potential for the hydrogen bubble to explode. At the time President Carter entered the plant, the legitimacy of the hydrogen bubble risk was still undetermined.

On Sunday afternoon, while Carter was still there, Victor Stello found the proof he needed. They discovered that Mattson and his team of consultants had been using the wrong formula to determine the risk posed by the hydrogen bubble. Stello concluded that "hydrogen under pressure will prevent water from breaking apart into hydrogen and oxygen because it will tend to suppress the creation of more hydrogen. Without free oxygen, there can be no explosion." Plant operators began hooking devices to the containment building in order to slowly burn away the hydrogen, thereby bleeding away the bubble.