High-level nuclear waste at government's proposed Yucca Mt. nuclear waste repository unlikely to explode, UC Berkeley team concludes

by Robert Sanders

Berkeley -- Plans to build a federal repository for nuclear waste in Nevada's Yucca Mountain were dealt a serious blow last year when two scientists warned that surplus, weapons-grade plutonium stored at the site could conceivably explode like a nuclear bomb.

Though many dismissed the suggestion as improbable, a nine-member team of engineers and scientists at the University of California at Berkeley launched a systematic and more quantitative analysis of the scenario.

The eight-month, $125,000 project -- the first and only independent analysis of the scenario first proposed by two scientists at Los Alamos National Laboratory -- indicates that while weapons-grade plutonium or highly enriched uranium could theoretically go supercritical, these scenarios are highly unlikely given the hydrology, geology and geochemistry of the Yucca Mountain site.

Furthermore, the UC Berkeley team said, the repository and stored waste could be engineered to reduce the likelihood to virtually nil.

The group will report their findings at a half-day symposium on the UC Berkeley campus Wednesday, March 13, as part of the annual Industrial Liaison Program conference hosted by the Colleges of Engineering and Chemistry. They plan to submit a complete report to Los Alamos, which funded the study, and have submitted their findings for publication in the journal Nuclear Technology.

The New York Times first publicized the warning by Los Alamos scientists Charles Bowman and Francesco Venneri on March 5 last year, stimulating doomsday headlines around the nation.

The two researchers suggested that over the lifespan of the repository groundwater would corrode the steel containers and glass encasing the waste, carrying plutonium and uranium into the surrounding rock. Should a critical mass accumulate, they argued, it would trigger an uncontrolled chain reaction leading to a nuclear explosion.

"The crux of the matter is, by the time you accumulate the necessary plutonium for an explosion -- about 250 kilograms -- most of it has decayed," said William Kastenberg, project organizer and professor and chair of nuclear engineering at UC Berkeley. "And uranium will be flushed out of the system into the groundwater without accumulating a critical mass."

"Our conclusion was that at the Yucca Mountain site there don't appear to be any geochemical or geophysical mechanisms for these supercritical scenarios to happen," he said.

Yucca Mountain was chosen in 1987 by the Department of Energy as the sole national repository for spent nuclear fuel from commercial power plants. Opposition quickly arose, primarily from residents of Nevada and from environmentalists and antinuclear groups.

Two years ago, with the end of the Cold War, the government proposed also to stash 100,000 pounds of excess weapons-grade plutonium at the site, plus highly enriched uranium derived from the spent fuel of naval and research reactors. The opposition grew.

After Bowman and Venneri issued their warning in an internal Los Alamos paper, their lab colleagues conducted an internal review of the proposed scenario, which scientists at Lawrence Livermore National Laboratory also subsequently reviewed. The consensus was that the scenario had serious flaws, though Los Alamos scientists could not entirely rule out an explosion.

Kastenberg proposed an entirely independent analysis of the scenario by UC Berkeley faculty. Given the go-ahead by Los Alamos director Siegfried Hecker, Kastenberg gathered "a remarkable team from diverse areas such as neutron physics, nuclear dynamics, nuclear chemistry, geochemistry, geophysics and geology," he said.

The principal members of the team were Assistant Professor Joonhong Ahn, Professor Paul Chambre, Professor-In-Residence Ehud Greenspan, Professor Donald R. Olander, Associate Professor Per F. Peterson and Assistant Professor Jasmina L. Vujic from the department of nuclear engineering; and associate professor Fiona M. Doyle and professor Neville G. W. Cook of the department of materials science and mineral engineering.

Meeting weekly from May through December of last year, the team created its own computer model of the site and asked two primary questions. Are there configurations in the Yucca Mountain environment that could lead to a critical mass of fissionable material and a subsequent explosion? And in the geologic setting of Yucca Mountain, could such configurations occur within a reasonable timescale?

The model they constructed accounted for the geologic structure of Yucca Mountain, which is composed of volcanic tuff (hardened ash) riddled with long cracks and fissures ranging from under a millimeter to a half-centimeter or more in width, Kastenberg said. They then simulated how groundwater would dissolve plutonium and uranium and deposit it in the cracks.

They found that it was theoretically possible for plutonium to accumulate to a critical mass of 250 kilograms (550 pounds), but it would take 100,000 years for enough of it to move through the fissure system. This is because plutonium is virtually insoluble in water and would move only as large particles or colloids, depositing close to the original emplacement. Because the half life of plutonium is only 25,000 years, most of the plutonium would have decayed by that time to uranium.

Uranium, on the other hand, is much more soluble in water and would move readily through the system of fissures. However, to precipitate out of the water and deposit in the cracks, uranium needs "reducing" agents such as iron or organic matter, which preliminary measurements suggest are not available at Yucca Mountain.

Therefore most of the uranium would remain in the groundwater and eventually drain out of the mountain, Kastenberg said.

"While most of the neutronics calculations made by the Los Alamos scientists were correct, the real flaws were with their assumptions about which configurations could be generated at the site," said Per Peterson. "That's what we looked at closely, and we couldn't come up with any scenarios that can't be eliminated by modest changes to the repository design."

The group came up with three engineering suggestions that would further insure that the chance of a catastrophic explosion at Yucca Mountain is vanishingly small. They are:

  • Add to the waste depleted uranium, which the government already has in excess, so that the ratio of fissionable to nonfissionable uranium is reduced and the criticality problem goes away.

  • Encase the wasteform in a capillary barrier that would keep water out. Backfilling with gravel and then an outer layer of fine sandy material would keep the water in the sand and out of the waste. Natural analogs of this have worked for hundreds of thousands of years, Peterson said.

  • To prevent criticality within the original waste canister -- a possibility the Berkeley group considered but judged to be even less likely than autocriticality in surrounding cracks and crevices -- it could be packed with nuclear "poisons" that absorb neutrons and would prevent a chain reaction leading to an explosion.

    "It's very important that the potential for autocriticality be considered in the design of the repository and the waste packages," Peterson said. "But I also think it will be relatively easy to deal with this without adding much to the cost and difficulty of the project."


    The members of the UC Berkeley team can be reached at:

    William Kastenberg‹(510) 643-0574 or

    Joonhong Ahn‹(510) 642-5107 or

    Paul Chambre‹(510) 642-2152 or

    Neville G. W. Cook‹(510) 642-5609

    Fiona M. Doyle‹(510) 642-2846 or

    Ehud Greenspan‹(510) 643-9983 or

    Donald R. Olander‹(510) 642-7055

    Per F. Peterson‹(510) 643-7749 or

    Jasmina L. Vujic‹(510) 643-8085 or

    A graphic is available at, showing a cross section through the proposed Yucca Mountain repository and the three scenarios considered in the UC Berkeley autocriticality study.

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