Scientists disagree on bomb fallout at Yucca

Thursday, Sept. 21, 2000 | 11:36 a.m.

Two Department of Energy laboratories cannot agree on whether 50-year-old radioactive fallout from bomb testing is present inside Yucca Mountain.

The answer could be crucial to determining whether the DOE can build the world's first high-level nuclear repository in the mountain 90 miles northwest of Las Vegas. Yucca Mountain is the only site being studied for the dump scheduled to hold 77,000 tons of nuclear waste.

The presence of the radioactivity, which travels with water, would indicate rain and snow melt from the desert surface seep more rapidly into the ground than scientists thought. Ground water could corrode containers holding radioactive waste, allowing dangerous leaks during the repository's 10,000-year life.

Scientists from DOE's Los Alamos National Laboratory in New Mexico in 1985 discovered what they believed to be fallout from 1940s and 1950s above-ground atomic bomb blasts in the Pacific Islands inside Yucca's earthquake faults.

But scientists from the Lawrence Livermore National Laboratory in California trying to find the same evidence have so far come up empty-handed, Mark Peters, a Yucca contractor from Los Alamos told an independent advisory committee on the project Wednesday.

Livermore scientists did find the radioactive element -- chlorine-36 -- inside the mountain, but in amounts small enough that that they believe it is naturally occurring, not from fallout.

However, Livermore may yet link the radioactivity to the bomb fallout, but not before next year, well after the DOE is expected to recommend Yucca as the world's first high-level nuclear waste repository, Peters said.

"Maybe this time next year we will have an answer for you," he told the Advisory Committee on Nuclear Waste, an independent panel overseeing regulatory work by the Nuclear Regulatory Commission. The NRC is responsible for licensing a repository if it passes scientific muster.

Meanwhile, other research into potential water problems at Yucca Mountain is ahead of schedule, the committee was told.

The Department of Energy, U.S. Geological Survey and a scientific team led by UNLV Associate Geoscience Professor Jean Cline expect to submit a report in March on when tiny air and water bubbles formed inside Yucca Mountain.

Nuclear regulators are waiting for the exact dates on when the bubbles, called fluid inclusions, were trapped in Yucca's volcanic ash. The bubbles are evidence of deep, hot ground water that existed inside the mountain at some time in its geologic history.

DOE scientists believe the bubbles are millions of years old, but if Cline's team find they are only thousands of years old, the ground water could pose another corrosion risk to the nuclear canisters.

"What we really want to do is absolutely date the fluid inclusions," Cline said.

The puzzle remains locked in complex pieces of calcite, a mineral deposited from water inside Yucca. The source of the water erupted into a scientific skirmish at the committee's Las Vegas meeting.

So far, Cline said about half of the 155 mineral samples taken from Yucca show a double bubble, with fluid surrounding an air pocket and formed in hotter temperatures. If the bubbles contain only liquid, that indicates it formed at a lower temperature, she said.

The U.S. Geological Survey believes that rainwater and snow melt deposited the minerals from Yucca's rocks inside the mountain's cracks and crannies, not geothermal water from deep within the mountain.

Opals formed with the calcite contain uranium, so scientists can guess at a date, USGS team leader Zel Peterman said. The USGS says the minerals are at least 12 million years old.

But Cline said the fluid inclusions point to a range of water temperatures, with microscopic bubbles trapped in older calcite from hotter water. How hot? From 122 degrees Fahrenheit to 176 degrees Fahrenheit, so far.

And younger-appearing layers of calcite contain magnesium not found in suspected older minerals where the fluid inclusions are found, she said.