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Yucca Mountain is a mountain in the Nellis Military Operations Area in Nevadamarker approximately 80 miles northwest of the Las Vegasmarker metropolitan area. It was the proposed site for the Yucca Mountain nuclear waste repositorymarker from 1987 to 2009.

Location

Yucca Mountain is part of the Nellis Military Operations Area in Nevadamarker. It neighbors the Nellis Air Force Basemarker bombing range and the Nevada Test and Training Rangemarker. The Nevada Test and Training Range was home to 904 atomic bomb tests between 1945 and 1992. Although the Department of Energy is planning to consider other options to Yucca Mountain, it remains the only legal site in the United States for development as a deep geologic repository. The Yucca Mountain Development Act was passed by the Congress and signed by the President in 2002 making development of Yucca Mountain the Law. Until Congress amends or changes the law, the Secretary of Energy is charged with pursuing development of Yucca Mountain as the Nation's geologic repository.

Geology

The formation that makes up Yucca Mountain was created by several large eruptions from a caldera volcano and is composed of alternating layers of ignimbrite (welded tuff), non-welded tuff, and semi-welded tuff. Tuff has special physical, chemical and thermal characteristics that some experts believe make it a suitable material to entomb radioactive waste for the hundreds of thousands of years required for the waste to become safe through radioactive decay.

The volcanic units have been tilted along fault lines, thus forming the current ridge line called Yucca Mountain. In addition to these faults, Yucca Mountain is criss-crossed by fractures, many of which formed when the volcanic units cooled. While the fractures are usually confined to individual layers of tuff, the faults extend from the planned storage area all the way to the water table below.

The volcanic tuff at Yucca Mountain is appreciably fractured and movement of water through an aquifer below the waste repository is primarily through fractures. Future water transport from the surface to waste containers is likely to be dominated by fractures. There is evidence that surface water has been transported down through the of overburden to the exploratory tunnel at Yucca Mountain in less than 50 years.Norris, A.E., H.W. Bentley, S. Cheng, P.W. Kubik, P. Sharma, and H.E. Gove. 1990. “36Cl studies of water movements deep within unsaturated tuffs,” Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 52 (2 December): 455-460.

Some site opponents assert that, after the predicted containment failure of the waste containers tens of thousands of years from now, these cracks may provide a route for movement of radioactive waste that dissolves in the water flowing downward from the desert surface.

Officials state that the waste containers will be stored in such a way as to minimize or even nearly eliminate this possibility. Even without faults and fractures, tuff is slightly permeable to water, but due to the depth of the water table it is estimated that by the time the waste enters the water supply it will be safe.

Map of the Location of the Mountain
The area around Yucca Mountain received much more rain in the geologic past and the water table was consequently much higher than it is today, though well below the level of the repository. Critics contend that future climate cannot be predicted to 10,000 years so it is optimistic to assume that the area will always be as arid as it is today. Most geologists who have worked at the site still maintain that the geology will adequately slow the rate of waste seepage to protect water supplies even if the local climate becomes much wetter.

Earthquakes

Nevada ranks third in the nation for current seismic activity. Earthquake databases (the Council of the National Seismic System Composite Catalogue and the Southern Great Basin Seismic Network) provide current and historical earthquake information.Analysis of the available data in 1996 indicates that, since 1976, there have been 621 seismic events of magnitude greater than 2.5 within a 50-mile radius of Yucca Mountain. Reported underground nuclear weapons tests at the Nevada Test Site have been excluded from this count.

DOE has stated that seismic and tectonic effects on the natural systems at Yucca Mountain will not significantly affect repository performance. Yucca Mountain lies in a region of ongoing tectonic deformation, but the deformation rates are too slow to significantly affect the mountain during the 10,000-year regulatory compliance period. Rises in the water table caused by seismic activity would be, at most, a few tens of meters and would not reach the repository. The fractured and faulted volcanic tuff that Yucca Mountain comprises reflects the occurrence of many earthquake-faulting and strong ground motion events during the last several million years, and the hydrological characteristics of the rock would not be changed significantly by seismic events that may occur in the next 10,000 years. The engineered barrier system components will reportedly provide substantial protection of the waste form from seepage water, even under severe seismic loading.

In September 2007, it was discovered that the Bow Ridge fault line ran underneath the facility, hundreds of feet east of where it was originally thought to be located, beneath a storage pad where spent radioactive fuel canisters would be cooled before being sealed in a maze of tunnels. The discovery required several structures to be moved several hundred feet further to the east, and drew criticism from Robert R. Loux, then head of the Nevada Agency for Nuclear Projects, who argues that Yucca administrators should have known about the fault line's location years prior, and called the movement of the structures “just-in-time engineering.”In June 2008, a major nuclear equipment supplier, Holtec International, criticized the Department of Energy's safety plan for handling containers of radioactive waste before they are buried at the proposed Yucca Mountain dump. The concern is that, in an earthquake, the unanchored casks of nuclear waste material awaiting burial at Yucca Mountain could be sent into a "chaotic melee of bouncing and rolling juggernauts". Loux resigned from office in October 2008 amid discontent that he had increased salaries for himself and other employees beyond authorized levels after one employee left.

Volcanic history

A series of large explosive volcanic eruptions occurred to the north of Yucca Mountain millions of years ago, producing dense clouds of volcanic ash and rock fragments which melted or compressed together to create layers of rock called tuff, forming the mountains and hills of the region. The volcanic eruptions that produced Yucca Mountain ended about 12 million years ago. This explosive volcanism produced almost all (more than 99 percent) of the volcanic material in the Yucca Mountain region.

Several million years ago, a different type of eruption began in the area. These eruptions were smaller and much less explosive. These small eruptions were marked by lava and cinders seeping and sputtering from cones or fissures. The last such small eruption occurred about 80,000 years ago. The remaining volcanic material (less than 1 percent) in the Yucca Mountain region is a result of these smaller eruptions.

The size of the Yucca Mountain volcanic field is not presently well known. Eight Quaternary basalt volcanoes erupted within 50 km of the proposed repository in the past million years, and higher than previously predicted recurrence rates for Yucca Mountain volcanism are possible in the future.

References

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