Uranium Leak at Tennessee Laboratory Brings Fears of an Accidental Chain Reaction
By MATTHEW L. WALD
Published: November 25, 1994
Uranium leaking from an old experimental reactor at the Oak Ridge National Laboratory in Tennessee has lodged in a pipe outside the reactor building, raising fears of an accidental nuclear chain reaction.
Energy Department officials said they had not decided what to do with the 4.4 pounds of bomb-grade uranium, which may still be accumulating. They discovered the leak earlier this year and later evacuated an office building next to the reactor.
Engineers have gingerly drained the cooling water around the 6-inch-wide filter pipe to reduce the chance that the water will leak into the pipe. That would have made a nuclear reaction, which physicists call a "criticality accident," much more likely.
Such a reaction would not have the force of a nuclear explosion, but it would probably spread radioactive contaminants.
Over the years, there have been scattered accidents at bomb plants where nuclear reactions have caused small blasts and showers of radiation.
A local group, the Oak Ridge Environmental Peace Alliance, which obtained documents from the Energy Department describing the leak, called it "an accident in progress" and said that removing the water had raised the temperature to levels at which the chemicals inside the pipe would attack the pipe itself more aggressively and possibly cause a leak.
But Energy Department officials said that pumping out the water stabilized the problem and that the temperature was within acceptable limits.
The plant, the Molten Salt Reactor Experiment, is one of a number of places in the Energy Department's weapons complex where operations were stopped with no long-term plan to store or dispose of the radioactive materials. The department has tons of used uranium reactor fuel that is leaking or rusting.
The Molten Salt project, which ran from 1965 to 1969, was built by the old Atomic Energy Commission to test the feasibility of nuclear power to fly airplanes.
"We would never have anticipated this to happen when it was shut down," said Bryan D. Walker, the acting assistant manager for environmental management at the Energy Department's Oak Ridge operations office. "That's why it has taken us a while to figure out what to do."
His office is developing a schedule for removing about 68 pounds of the uranium-233, an artificial isotope used in bombs or reactors, most of which is still in drainage tanks. But there are no plans yet, he said.
The engineers would like to remove the material from the drainage tanks as a liquid, so it could be pumped, but they were not sure they could do so. It is now in a compound with other chemicals in a solid form. The radiation field around the tanks is so high that all work will have to be done with remote-controlled equipment.
The escaped uranium in the pipe illustrates the difficulty of keeping track of weapons-grade material. About one pound of the uranium would be enough for a bomb under ideal circumstances. Engineers think that more than three times that amount has escaped.
The material spread through a complex, unanticipated series of chemical transformations in the fuel that were partly the result of continuing radioactive decay.
The uranium that is now in the filter pipe came from the drainage tanks, where molten salt was sent to solidify when the reactor was shut down. The filter, originally meant to capture radioactive gases produced when the reactor ran, takes the form of a 19-foot-high U-shaped pipe, made of stainless steel and filled with charcoal. The charcoal has caught the uranium in a section of pipe about one foot in length, the Energy Department said.
Energy Department officials said they had traced the problem to the outside pipe after technicians found elevated radiation in part of the reactor building. Measuring radiation fields outside the pipe, they concluded that about 4.4 pounds of uranium in a fluoride compound had been transformed into a gas and flowed into the pipe, and then resolidified as it cooled.
If the plant were newer, the transformation into a gas would not have mattered because all the valves would still be working. But the gas was able to travel because a valve was stuck open.
"We don't know when it was last closed," said Clayton S. Gist, branch chief of decontamination and decommissioning at Oak Ridge, in a telephone interview.
What exactly would happen if there were a nuclear reaction at Oak Ridge is not clear.
James E. Phelps, a spokesman for the local Oak Ridge environmental group and a former radiation instrument designer at Oak Ridge, theorized that the charcoal could burn and slide down to the bottom of the U-shaped tube "like a pile of ashes," in a way that would concentrate the uranium, allowing a nuclear reaction to occur. He said this reaction could also blow water from the filter building into the reactor building, where the remaining salt mixture is stored, allowing another chain reaction.
"They've got a nuclear pipe bomb in their backyard there," he said.
But Mr. Gist of the Energy Department said that there was no longer water present in the silo that holds the pipe and that the charcoal would not burn.
The Molten Salt Reactor Experiment has an unusual design. The molten salt mixture is both fuel and coolant: in the reactor it sustains a chain reaction and heats up; then it is pumped out of the reactor where it gives off its energy in a heat exchanger. The heat can be used to generate steam for electricity. The idea was that with fuel being continuously pumped through the reactor and out again, small quantities of the fuel could be diverted and the nuclear waste, in the form of fission fragments, could be filtered out. More uranium could be added, and then the fuel would be reinjected.
Thus, it would never need to shut for refueling.
The fuel is not supposed to be able to sustain a chain reaction outside the reactor. Pumping out the water surrounding the clogged pipe was a crucial step. If water leaked into the pipe, it would slow down the subatomic particles called neutrons. When slowed down, they can split other atoms, causing a chain reaction.
The Energy Department and its critics agree that the underlying problem at the Molten Salt plant is neglect.
Mr. Walker said, "I'm certain that when it was shut down, it was not shut down with the thought, 'Now we're going to walk away and leave it for 20 years.' "
Diagram: "THE SCENE: How Uranium Escaped From a Defunct Reactor" 1. The fuel from the Molten Salt Reactor Experiment was a liquor at the reactor's operating temperatures. 2. After the reactor was shut down 25 years ago, the fuel was drained into storage tanks and allowed to solidify. 3. Over the years, some of the uranium converted to gaseous form and flowed int a gas treatment system, where the temperature was lower, and it re-solidified. 4. About 4.4 pounds of the ukranium has accumulated in a six-inch wide pipe. Under certain circumstances - including mixing with water - this amount could be enough to sustain a chain reaction, which would cause a small explosion and a shower of radiation. To reduce the chance of a nuclear reaction, engineers have pumped out the cooling water. (Source: Energy Department)