Monday, November 10, 2003
Copyright © Las Vegas Review-Journal
Study cites natural deposits in Yucca Mountain as barrier
But repository opponents counter absorbent zeolites won't outlast decaying nuclear waste
By KEITH ROGERS
REVIEW-JOURNAL
The same stuff in kitty litter that absorbs the ammonia smell of cat urine might serve as a natural safety net to trap some toxic remnants of nuclear waste should they escape from the planned Yucca Mountain repository.
That's one of the conclusions of a team of scientists led by Indiana University mineralogy professor David Bish, who reports in a respected scientific journal today that rock layers below the disposal site are rich in zeolites. The material is the fine, tan-colored grains used in many brands of cat litter.
Perhaps more importantly, zeolite deposits also could act like giant sponges to absorb and release large amounts of water moving through the mountain. This would in turn buffer heat from the repository, making it more conducive for safe storage of the waste.
Bish and colleagues at the Los Alamos, N.M., national laboratory formed their insights after analyzing more than 2,000 samples that were used to make a three-dimensional picture of the mountain, 100 miles northwest of Las Vegas.
Their federally funded study, he predicts, will be cited in upcoming discussions by the Nuclear Regulatory Commission when the Department of Energy applies for licenses to build and operate the repository and eventually receive spent fuel assemblies from nuclear power reactors across the nation.
Early on in the 20-year project to explore the geology of Yucca Mountain, scientists "were interested in zeolites because they would suck up radionuclides," Bish said. Radionuclides are radioactive particles in the spent fuel pellets that would be stored at Yucca Mountain.
"I realized the minerals at Yucca Mountain and particularly the zeolites were important in many different ways," he said last week in discussing the paper that appears in the November-December issue of American Mineralogist.
Among their properties, zeolites can attract the positively charged ions of radioactive cesium, barium and strontium from the 77,000 tons of spent fuel and highly radioactive waste that scientists intend to put there. The material would be sealed in metal canisters for storage beginning in 2010.
Should these man-made barriers fail from corrosion during the required 10,000-year period that the waste must be contained, zeolites beneath the repository floor could prevent some of the seeping radionuclides from contaminating the environment.
"The natural zeolites have acted as efficient retardation barriers to the migration of (strontium)" is confirmed by studies of the rock layers that make up Yucca Mountain, the paper states. The same holds true for radioactive barium and cesium particles, he said.
Bish is quick to note, however, that these particular contaminants will be around only for 300 years as they decay. A zeolite barrier then wouldn't prevent migration of some of the deadly, longer-lived byproducts in spent nuclear fuel that the claylike substance doesn't absorb.
The repository's design, nevertheless, should combine containment features of both natural and engineered barriers, Bish and his colleagues reported.
"I think it's safe to say that in the last five years there has been much greater reliance on the engineered barrier and less reliance on the geologic barrier," Bish said.
He said designers "should not lose sight" that over the long haul, geologic barriers will be around while questions remain about how long man-made barriers will hold up, particularly the alloy canisters.
Nevada Nuclear Projects Agency chief Bob Loux, a critic of the Department of Energy's repository plan, doesn't see zeolites as the cure-all to potential contamination issues at Yucca Mountain. But he does agree with some of Bish's conclusions.
"I think he's made a case that you can't rely on the canisters as DOE is doing," Loux said.
But much of what Bish's team reports in the journal already has been incorporated in DOE's assessment of how the repository will perform after it's loaded with spent fuel containers, Loux said.
Although zeolites might capture the shorter-lived radioactive materials, Loux noted that two dangerous ingredients will loom in the distant future as the waste continues to decay.
Technetium and radioactive iodine, he said, both have extremely long half-lives, 210,000 years and 16 million years, respectively. A half-life is the time it takes for half of the radioactive atoms in a substance to decay into a stable form.
"These are both very soluble in water, and neither is affected by zeolites. If the material does get out of the containers, the doses from those two alone could exceed the EPA (Environmental Protection Agency) standard within a few hundred years," Loux said.
