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Nuclear storage
Slim chance of volcanic disaster equals notable risk

The nation’s planned nuclear waste storage facility at Yucca Mountain in Nevada sits across the border from California’s Death Valley in a relatively quiet yet recently active volcanic field. Cinder cones dot the ancient basaltic lava flows. At the southern tip of Yucca Mountain, the Lathrop Wells cinder cone stands as a reminder of the most recent eruption, which struck about 80,000 years ago.

Now, a new disaster scenario involving the uncertain consequences of a volcanic eruption under the proposed nuclear waste repository is under investigation. Past studies questioned the damages of a magma dike intersecting the site along a straight line to the surface. An international group of researchers under contract from the Nuclear Regulatory Commission (NRC) has modeled the possibility of magma from a dike hitting a tunnel or drift, where the waste would be stored, and then changing course. This change in direction would place more canisters of wastewithin the path of the magma and could potentially cause a more serious risk to environmental and public health.

The Lathrop Wells cinder cone, which erupted 80,000 years ago, stands as a sentry before Yucca Mountain. Photo courtesy of the Department of Energy.

The model, published in the August issue of Geophysical Research Letters, demonstrates that the magma is likely to travel for some distance along the lower pressure gradient within the tunnel and either continue along the conduit until it reaches the surface or, if the tunnel is blocked, build up enough pressure to break through to the outside. The researchers also report that it is likely that water and gas within the magma could cause the solution to rapidly expand as it moves from high pressure underground to atmospheric pressure within the tunnel. This expansion could fragment the magma into a frothy pyroclastic eruption, accelerating it to speeds of tens to hundreds of meters per second, according to the article. The eruption would displace the air within the tunnel and push it ahead of the traveling flow, like the rush of air ahead of an oncoming subway train. This shockwave would have the speed of several hundred meters per second, the researchers say.

If the tunnels of waste are blocked within 200 to 300 meters of the dike intrusion, the researchers add, this shock of air would rebound back into the magma flow. In 10 seconds the pressures within the tunnel would be as high as within the original dike. Under such pressures the ceiling would presumably fracture and create a new escape route for the magma.

This simplified fluid dynamic model, while riddled with assumptions, provides the first published attempt to address what would happen if magma from a dike detoured into a tunnel. “We’ve highlighted the uncertainties involved in understanding the complex processes,” says Brittain Hill of the Center for Nuclear Waste Regulatory Analysis at the Southwest Research Institute in San Antonio, Texas, and co-author of the report. “This is just the beginning.” Indeed, the model did not address the level of radioactive risk such consequences of a detour might incur.

For scientists to quantify the risk a volcanic eruption mixed with nuclear waste might pose to the public, they multiply the chance of occurrence by the consequences of magma disrupting canisters at the site. Currently, scientists on all sides agree that the chance of a magma intrusion into the repository is slim, but is legally enough of a chance to warrant further study. The odds are sometimes couched on a per year basis as a chance of one in about 10 million to 100 million. But the repository must be safe for the entire 10,000 year period. The probability of an eruption happening at all during that time is more likely: one in about 1,000 to 10,000. Those odds put the scenario within the legal responsibility of the Department of Energy (DOE) to identify the consequences of such an event and any risk to the public.

“This is right on the fence of regulatory screening,” says Eric Smistad, Yucca Mountain scientific integration leader for DOE. “If it was 10-9 [or one in 1 billion] we would not even have to look at it.” The next step is a better understanding of the consequences.

Recently NRC, in charge of licensing the Nevada site, requested that DOE investigate the possibility that a magma intrusion might dogleg through the tunnels if a dike intersected the repository. Both NRC and scientists working at DOE’s Los Alamos National Laboratory are in the process of following up on the study with robust 3-D models investigating the issue, Smistad says.

DOE plans to submit its application for a license to begin construction in December of 2004. NRC at that point has three or perhaps four years, if needed, to determine whether the application deserves the construction license. After construction begins, DOE will need another license before storing waste in the site. The earliest anticipated date for waste disposal at Yucca Mountain is 2010, if NRC approves both licenses.

Christina Reed


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