LAS VEGAS - Studying microscopic bubbles of water and gas trapped in minerals inside Yucca Mountain could be crucial in determining whether the site is approved for a nuclear waste repository.
Scientists studying the bubbles haven't found evidence of deposits younger than 2 million years old.
If they find bubbles, called fluid inclusions, younger than 1 million years old, the ability of the repository to keep radioactive elements out of the environment and away from people would be called into question.
Almost one-third of the way through research, the scientific team, led by UNLV geoscience professor Jean Cline, gave an update Wednesday on its work studying the possibility of water seeping into Yucca Mountain.
If the minerals are older, their ages would help support the Department of Energy's view that Yucca can safely store the waste for thousands of years.
''Dating the minerals is the last step,'' Cline said after the daylong meeting at the university.
Whether the studies clinch the argument of how quickly water travels inside Yucca Mountain is months away, Cline said.
With only 44 of 155 rock samples examined after they were removed from the mountain, 90 miles northwest of Las Vegas, the scientists said they cannot draw a conclusion on when water invaded Yucca.
And deciding if the mountain is suitable as a repository is out of the scientists' hands, she said.
''Some engineer or policy person will determine that,'' Cline said, adding that decision may not come for years.
The Royal Ontario Museum Laboratory in Canada has agreed to date a few samples at a time of Yucca's rock over the coming months, Cline said. By measuring uranium's radioactive decay and dating uranium and lead content of the minerals, the laboratory could answer the question of when the bubbles formed.
The scientific team including university, U.S. Geological Survey and independent experts expect to publish study results about April 2001, she said.
There are more questions than answers raised in the past year by the team, which received a $1.4 million grant from the DOE to put the bubbles to the test.
The U.S. Geological Survey believes the bubbles formed 12.7 million years ago, when volcanic ash formed the mountain. USGS Project Manager Zel Peterman said the nearby volcanic activity created extremely hot rock, eight times hotter than the boiling point. It could have taken tens of thousands of years to cool down, he said.
The bubbles could have formed from rainwater invading the hot rock, Peterman said.
But Yuri Dublyansky, a Russia geologist who was hired by the Nevada Agency for Nuclear Projects, said water from deep within the earth flooded the repository site within thousands of years.
Dublyansky said he has studied such bubbles in unsaturated rock all over the world. The fluid inclusions found in drier rock and in Yucca do not shrink, remaining the same size because they do not undergo cooling, he said.
''At this time we cannot conclude water moved up or down,'' Cline said. ''The data can be interpreted a number of ways.''
