Newspaper: The Patriot
Date: April 3, 1979
Title: Bubble Shrinks-Maybe
Author: Richard Roberts, Staff Writer
A federal Nuclear Regulatory Commission official Monday expressed cautious optimism that a hazardous gas bubble inside the Unit 2 reactor vessel at Three Mile Island Nuclear Generating Station may be decreasing in size and may be less potentially explosive than previously feared.
Harold Denton, director of the NRC’s Office of Reactor Regulation, said at a news conference at Middletown Borough Hall that initial mathematical computations show a “dramatic decrease in bubble size.”
But he declined to say the situation is no longer serious. He also declined to say whether an evacuation of area residents still might be required.
The bubble, composed mostly of hydrogen gas, was estimated to be 80 cubic feet on Monday, compared to 850 cubic feet “a few days back,” he said. The bubble is considered a major stumbling block to cooling down the damaged reactor core.
“The equation used to calculate the bubble size is sort of a first-order approximation. It’s shown a dramatic decrease in bubble size,” he said.
The new figure “is certainly reason for optimism,” he said. “It is certainly going the direction that I’d like to see.”
But Denton cautioned that the figure may not be accurate because some influencing factors, or “effects” were not taken into consideration. He said the apparent dramatic change in the size of the gas bubble “has caused me some skepticism.”
“I don’t want to stampeded into concurring that the bubble is actually this small,” he said. “We’re trying to do more sophisticated analysis to be sure that the equations that are used to calculate the bubble size properly include all effects.
“We didn’t focus on the accuracy of that calculation as long as it wasn’t changing. I’m having the staff right now look into the details of that number, and I hope to be able to agree with it or not in the near future.”
Denton could not explain the reason for the apparent change in the bubble size, adding that there was no change or acceleration in efforts to remove dissolved gas from the primary coolant.
“I didn’t expect such a rapid change, and that’s one reason I want to carefully look at it,” he said.
Denton said new calculations show that the rate of oxygen production inside the reactor vessel is less than he reported on Sunday. A mixture of oxygen and hydrogen in certain proportions can be highly explosive.
Both hydrogen and oxygen are being produced continually in the reactor core by the natural process of radiolysis-the breaking down of water into its component elements, hydrogen and oxygen.
Denton warned Sunday that because on the danger of explosion, a decision on whether to take extraordinary steps to force the bubble out of the reactor vessel would have to be made “within five days or less.”
It was feared that an explosion (illegible) reactor vessel could cause a (illegible) of coolant and lead to a melt-down, in which tremendous amounts of radioactive material would be released.
Denton said the revised rate of (illegible) generation is based upon (illegible) consensus of technical (illegible) that “for situations such as this where there’s high oxygen overpressure in a vessel, that the oxygen evolution rate is very low.”
Based upon the revised calculations, Denton said a decision on whether to force the bubble out no longer must be made within five days. Asked when the final cooling-off process of “cold shutdown” would occur, he said: “there is no time-frame pressure for cold shutdown.
“The core is being adequately cooled in this pressure mode, and before we go to cold shutdown, we want to check out the heat-removal system.”
Asked if he thought an evacuation still might be needed, Denton said: “I think the evacuation plans are controlled by the state. My own view is that my own concerns with regard to the potential for a hydrogen explosion to the bubble are diminishing.
“I briefed the governor last night. I briefed him this morning on the events as I see them. The decision regarding evacuation is the governor’s responsibility.”
Denton said he did not advise Gov. Dick Thornburgh to order an evacuation.
The temperature of fuel rod assemblies in the reactor core are declining, Denton said. But he said two of the assemblies registered more than 500 degrees and that four of the assemblies registered more than 400 degrees.
The levels of radiation in the area surrounding Three Mile Island are declining, Denton said. Dosimeter readings from 37 locations around the plant during a 24-hour period ending late Sunday night showed a high of 1.1 millirems per hour and a low of .04 millirems per hour.
The NRC is investigating two reported findings of iodine 131, a radioisotope that accumulates on grass, is ingested by cows and eventually contaminates their milk. “We’re still checking these out,” Denton said.
Efforts to remove hydrogen gas from the reactor containment building from using “recombiners” to convert hydrogen and air into water were not initiated early Monday morning.
Scientists are concerned that the continual generation of hydrogen and oxygen by radiolysis in the containment building might create a potentially explosive mixture.
Denying that there is any “significance” to the delay, Denton said he wanted to be “firmly convinced” that the recombiners are properly installed, and tested for leaks.