In September, 1997, scientists from the U.S. Department of Energy's Lawrence Livermore and Los Alamos National Laboratories reported that plutonium from an underground nuclear weapons test at Pahute Mesa on the Nevada Test Site had migrated almost a mile from the where the test took place. This finding contradicts DOE predictions about how fast plutonium can move through the underground rock. Until now, DOE and its scientists had contended that plutonium movement would be very slow - several inches or feet over hundreds of years. The discovery that plutonium has moved almost a mile in less than 30 years has major implications for DOE's plans to isolate spent nuclear fuel and high-level radioactive waste at Yucca Mountain, since such wastes contain nearly 1,000 tons of plutonium that remains extremely dangerous for tens of thousands of years.

The Discovery of Fast Groundwater Pathways at Yucca Mountain

In 1994, DOE scientists began reporting evidence of radioactive tritium in water samples in drill holes hundreds of feet below the proposed repository horizon at Yucca Mountain. Since tritium is an element that is produced by nuclear weapons testing, it could only have come from the deposition of fallout during the atmospheric testing programs of the early 1950's to early 1960's [1951 - 1963]. The discovery of tritium at a depth of more than 3,000 feet below the surface had to mean that it was carried there by the percolation of rain water in no more than 50 years.

In 1996 - 1997, DOE researchers reported finding radioactive Chlorine-36, another element created by the atmospheric bomb tests that were carried out in the Pacific Ocean in the 1950's, in faults and fractures within the actual host rock where DOE proposes to build the repository at Yucca Mountain. (See related article on Chlorine-36 fast water pathways.) This discovery, which was made in the course of excavating the 5 mile loop through the mountain that serves as an exploratory studies facility, could only mean that the Chlorine 36 was deposited by water from the surface moving through the layers of rock, and that it had to have done so in the less than 50 years.

The discovery of these "markers" at and below the level of the proposed repository cast into doubt DOE's entire hydrologic model for Yucca Mountain, which had assumed that water traveled extraordinarily slowly through the subsurface, principally by moving through the pores or matrix of the rocks. Such movement was postulated to occur just a few feet per thousand years. (It is important to note that, as early as 1984, State of Nevada scientists had warned DOE that water moving rapidly through fractures - not matrix flow - was predominant at Yucca Mountain.) Since water is considered the most likely transport mechanism for leeching radioactive materials out of a repository and into the accessible environment (i.e. the underground aquifer system below the facility), a very slow groundwater travel time was considered essential to demonstrating that Yucca Mountain could isolate extremely long-lived radionuclides (like plutonium)for the thousands of years necessary to meet NRC and EPA requirements.

Even with mounting evidence of more-rapid-than-expected water movement through and around the repository block, DOE steadfastly held to its contention that the tritium and Chlorine 36 findings were anomalies and, in any case, the natural sorptive properties of zeolites (asbestos-like minerals) associated with volcanic tuff would adequately retard any radioactive materials that might escape in water passing through the facility after closure. The discovery, announced in September, 1997 by Lawrence Livermore and Los Alamos researchers (see "Colloid Movement at the Nevada Test Site," by J.L. Thompson, A.B. Kersting, and D.L. Finnegan), that plutonium migrated almost one mile from the site of an underground nuclear test at Pahute Mesa on the Nevada Test Site in less than 30 years appears to confirm the State of Nevada's position about (1)the existence of fast water pathways through the fractured volcanic tuff in the area and (2) the inability of zeolite materials to absorb and retard such radionuclide migration.

Implications for Waste Isolation

A repository for the permanent disposal of spent nuclear fuel and high-level radioactive waste will need to isolate over 70,000 metric tons of radioactive materials from the environment and do so over the period of at least 10,000 and perhaps as many as 100,000 years. Of the radionuclides in a repository, there are nearly 1,000 tons of plutonium, which has a half life of 24,400 years. That means plutonium decays very slowly and remains extremely toxic for hundreds of thousands of years - hence the need for long term geologic isolation.

In assessing the Yucca Mountain site's ability to safety and adequately isolate radionuclides from the environment, DOE must assume that the canisters holding the spent fuel and HLW will deteriorate and release their contents before the longer lived isotopes (like plutonium) have decayed to anywhere near safe levels. The speed at which water can disburse these materials into the underground aquifer system becomes critical in determining whether the site, in fact, meets the requirements for "permanent" disposal. The new finding by the Lawrence Livermore and Los Alamos researchers suggests that the geologic and hydrologic environment at and around Yucca Mountain will not be able to isolate materials like plutonium from people and the environment for the time necessary.

Findings at NTS Confirm Other Studies

This is not the first time that government scientists have been surprised by the migration of plutonium and other radioactive elements. In 1974, the Kentucky Department of Human Resources and subsequently the U.S Environmental Protection Agency (EPA) found that plutonium deposited by the federal government at a burial facility near Maxey Flats had migrated hundreds of feet in less than 10 years. This despite Atomic Energy Commission assurance that plutonium was so immobile that it would move "only half an inch in 24,000 years."

In December 1983, test wells showed that plutonium from the commercial reprocessing facility at West Valley, New York had moved more than 50 feet in less than 25 years. Likewise, in the 1980's, Ohio state and EPA scientists found radionuclide contamination in groundwater adjacent to DOE's Fernald facility.

In 1989, scientists from Lawrence Livermore, Los Alamos, the U.S. Geological Survey, and Nevada's Desert Research Institute discovered that minute particles or colloids carried radiation up to 1,000 feet from an underground bomb cavity that DOE experts thought had sealed itself.

Researchers also found plutonium-laced liquids which escaped into a ditch at Los Alamos, New Mexico between 1945 and 1952 had infiltrated more than 1,000 feet below the surface by 1978. Again, DOE experts had predicted that the plutonium would remain immobile for thousands of years.

DOE's Guidelines for Repository Siting and the Fast Pathway Issue

DOE's own guidelines for determining whether a site is suitable for use as a deep geologic repository address the groundwater travel time issue directly. The guidelines, developed by DOE in 1984 with formal concurrence by the Nuclear Regulatory Commission, specify that a potential repository site must be disqualified if the time it takes for water to travel from the repository horizon to the accessible environment is less than 1,000 years. The discovery of the extremely rapid water pathways at and around Yucca Mountain appears, of itself, to require the disqualification of the Nevada site. When combined with the graphic evidence of rapid plutonium movement through the subsurface, the prospects for Yucca Mountain passing licensing and regulatory muster as a permanent repository for spent fuel and high-level radioactive waste appear dim indeed.

Related Articles

Plutonium Movement through Groundwater Triggers Questions (Las Vegas Sun)
Radioactive Rock Movements Yield Yucca Questions (Las Vegas Sun)

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