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The growth of the hydrogen bubble and its inherent threat of a meltdown alarmed the engineers at the Nuclear Regulatory Commission as much as any single incident after the onset of the accident.
"The principal problem we have right now is to work out a means of dealing with that gas bubble," NRC Chairman Joseph M. Hendrie had said in a weary voice at a crowded news conference Saturday afternoon at NRC headquarters in Bethesda. "We have to get that gas bubble out of the reactor."
The day before, early Friday afternoon, Sol Levine, head of the NRC's reactor safety division, called the Energy Department's Idaho National Engineering Laboratory 60 miles west of Idaho Falls and told it to crank up some schemes for getting rid of the bubble. Levine called EG & G in Idaho Falls, a major government contractor employing 4,000 nuclear technicians, to help come up with solutions to the troublesome bubble.
Engineers had told Levine there were four ways of dealing with the bubble, all involving some risk. The two simplest plans were favored. One called for raising the pressure inside the reactor in an attempt to collapse the bubble and dissolve it in the water flooding the reactor room. If the bubble dissolved, it could be pumped out with the coolant into waste water tanks outside the reactor.
Another plan was to lower the pressure, a safer procedure in itself but a riskier one in the long run because lowering the pressure would expand the bubble, exposing more of the reactor core. The attraction of this plan was that an attempt then could be made to siphon out the bubble with the coolant water.
Two other plans were discussed, both even riskier than the first two. One was to "sink" the bubble by dropping the water level, exposing most of the core, then flooding the reactor with fresh water. The fourth option was to restart the reactor and create so much heat that the water would flash to steam to saturate the bubble and break it.
This last plan was dropped almost as soon as it was proposed. Engineers pointed out that so many of the fuel rods had been bent and crumbled that there was no guarantee they could restart the reactor. The control rods that move in and out of the reactor to start or stop the chain reaction probably were also damaged.
"There's a chance the rods are damaged and won't fit back in and might not come back out when you wanted them to," one NRC official had said Friday night. "There's also the chance the rods might scrape on something and start a spark that could ignite the hydrogen bubble."
Back in Idaho Falls, technicians at EG & G flooded the NRC engineers in Middletown with questions. How big was the bubble? Where was it likely to be? How much hydrogen was in the bubble? Was there any helium in it? How much oxygen? Was the pressure fluctuating inside the reactor? By 9 o'clock Friday night, EG & G was reviving up its experimental replica of a nuclear plant to simulate the bubble and test a few solutions.
Five technicians went to work making the Idaho Falls replica look as much like Three Mile Island as they could. The Idaho Falls facility has a core heated with electricity instead of uranium, which makes it safe for technicians to simulate engineering problems on a small scale.
Modifications took five hours to complete. Sometime early Saturday morning, nitrogen gas was injected into the Idaho Falls reactor and quickly formed a bubble that approximated in size the best estimate of the bubble back at Three Mile Island.
At the same time, engineers at EG & G were working through the night in conference rooms, chalking on blackboards options for collapsing the bubble and venting the hydrogen. At 7 a.m. Saturday, crews began to run the first tests. A computer named Puff, the Magic Dragon, sent out 240 separate printouts of information on what was happening inside the replica of Three Mile Island.
Meanwhile, at Three Mile Island, encouraging news was developing concerning the reactor for the first time. The bubble had stopped growing and began to shrink. Early Sunday morning, the day President Carter decided to visit Three Mile Island, the bubble appeared to have shrunk from 1,000 cubic feet, down to 850 cubic feet and then to 650 cubic feet.
"When the president arrived," the NRC's Denton said, "we had seen a noticeable change."
Denton decided to treat what seemed like good news with caution. The only method technicians could use to measure the bubble's size was imprecise, involving as it did a possibility for error that ranged up to 200 cubic feet.
"I was afraid of being too optimistic," Denton said. "And when the data came down, I knew everyone wanted it to come down so much that I wanted to be sure they weren't forcing their views to come true."
By Sunday night, another piece of good news came from Three Mile Island. Engineers concluded that their early calculations on how much oxygen might be in the reactor were too high. And while a hydrogen bubble is bad, it is worse if a certain amount of oxygen joins the bubble and creates the spark for an explosion.
Despite the encouraging new signs, however, the NRC went ahead with preparations for what surely would have been a risky maneuver to get rid of the bubble later in the week. Plans still were being drawn up to evacuate the entire region around Three Mile Island. Privately, Denton was telling colleagues to pick a day, a time of day and a state of readiness: Operation Bubble was still a strong possibility.