Friday, April 26, 1996 | 11:59 a.m.
Researchers have detected higher than expected levels of radioactive chlorine in rocks collected at Yucca Mountain, raising questions about the site's suitability for nuclear waste storage.
The elevated levels of chlorine-36, found at various depths up to 600 feet below the surface, suggest that rainwater carried the radioactive material into the tunnel area within the past 50 years, Los Alamos National Laboratory scientists said in an 80-page preliminary report released Thursday.
The chemical could have come from above-ground nuclear testing at the Nevada Test Site and in the South Pacific in the 1950s and early 1960s. Its seepage to a depth of 600 feet below the surface may indicate that a leak in the proposed nuclear waste dump could contaminate groundwater.
Hydrologist Carl Johnson of the state Nuclear Waste Project Office said the findings are not surprising, but show that rainwater can penetrate deep into the mountain, 90 miles northwest of Las Vegas. Yucca Mountain is the only site under study for a national nuclear waste dump.
"This is another piece of evidence that says we do have fast pathways at Yucca Mountain," Johnson said. "It came from bomb tests, not cosmic rays."
Evidence that groundwater travels through Yucca Mountain faster than 1,000 years could disqualify the old volcano from becoming a nuclear waste dump, according to the Department of Energy's own guidelines, Johnson said.
In 1987, DOE scientists discovered radioactive tritium and chlorine-36 in samples taken below the dump site, located 1,000 feet beneath the surface of Yucca Mountain.
The most likely source of the chlorine found in rock samples is atmospheric atomic weapons tests in the South Pacific less than 50 years ago, Johnson said.
While airborne nuclear weapons experiments at the Test Site produced little chlorine-36, because of low humidity, the bomb pulses blasted in the Pacific produced the radioactive chemical from ocean water and it fell to earth worldwide.
From the surface fallout, the radioactivity then washed into the mountain after rainstorms.
What the Los Alamos scientists don't know is how fast the radioactive chlorine traveled in Yucca Mountain, whether it streamed along the cracks in less than one year or more than 50 years. They said they would continue studying the findings and produce a final report in August.
"The chlorine-36 was found clustered at five locations along the first 3,400 meters (2.11 miles) of the ESF (Exploratory Study Facility) tunnel," the report said.
The scientists took 52 rock samples for the draft report.
Four of the five locations where the radioactive traces were discovered were along observed faults or fractures. The fifth sample was found in an area "resembling a fault."
There are at least 33 major earthquake faults at Yucca Mountain. The Ghost Dance fault slices through the block of rock that cold be selected for keeping nuclear waste out of the environment for at least 10,000 years.
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