Japanese accident called a warning for Yucca Mountain

Friday, Oct. 1, 1999 | 10:45 a.m.

A UNLV nuclear engineer said the nuclear plant accident that spread radiation over workers in northeastern Japan holds a warning for a future repository at Yucca Mountain.

A major leak at a uranium processing plant about 70 miles northeast of Tokyo on Thursday sent radiation levels skyrocketing and contaminated dozens of workers, sending at least three with radiation poisoning to area hospitals.

"At Yucca Mountain the (nuclear waste) fuel itself is just as capable of going critical a million years from now as it is now," William Culbreth, associate professor in UNLV's mechanical engineering department, said Thursday hours after the processing plant in Japan spewed out 10,000 times its normal radiation.

Japanese officials today confirmed that eight times the proper amount of uranium was being processed at the time. Thirty-five pounds of uranium were involved.

A proposed repository at Yucca Mountain would hold 70,000 tons of nuclear waste, and all it would take to create a radioactive accident is water, Culbreth said.

The accident forced Japanese officials to evacuate about 150 people from the area around the plant.

A nuclear reaction begins when loose neutrons that occur naturally in uranium hit other particles in the uranium, causing atoms to split. That releases huge amounts of energy called a criticality, Culbreth said. It's the nuclear punch behind the atomic bomb. Water slows the neutrons down long enough to strike the other particles and start a reaction, he said.

The three reactor workers closest to the accident described a blue light and then they fell ill with nausea, diarrhea and high levels of white blood cells as the radiation depressed their immune systems.

The blue glow is called Cherenkov radiation, given off by radioactive particles from the uranium, Culbreth said.

What concerns Culbreth about Yucca Mountain is that Department of Energy officials cannot guarantee that the waste put into a repository would not be exposed to water within 10,000 years, the DOE's stated goal, let alone 1 million years.

Increasing the risk for such an accident at Yucca Mountain, Culbreth said, is the combination of uranium and plutonium proposed to be stored in the site. Plutonium remains radioactive far longer than uranium, and the danger of a serious accident could last a million years.

The Department of Energy is calculating ways to stop such a disastrous event at Yucca Mountain, he said.

DOE nuclear engineer Richard Spence said scientists on the Yucca Mountain project are studying the dangers.

"That is one of the occurrences you have to think about when you put uranium and plutonium there," Spence said of the mountain, 90 miles northwest of Las Vegas.

At nuclear power plants to unleash a nuclear force to create electricity, water is used to slow neutrons which strike the uranium, Spence said.

One of the first steps in a Yucca Mountain repository will be shielding the radioactive wastes from water seeping through the mountain, he said.

To poison the active neutrons, a substance known as hafnium will be added to the wastes. The material is similar to the boron used in Japan.

The wastes themselves will have very little uranium or plutonium that could start a nuclear reaction, Spence said.

Control rods placed with the buried wastes will include materials to absorb neutrons.

And, finally, nuclear waste containers will be placed far enough apart in the mountain to keep uranium from piling up and going nuclear, Spence said.

"The amount of radiation coming from the Japanese reactor is probably pretty close to the nuclear fallout from the Chinese atomic bomb tests of the 1970s," Spence said.

The questions the DOE still need to answer, Culbreth warned, is how long will water stay away from the wastes, how long will neutron poisons last and whether the control rods will last a million years.