Yucca Mountain was formed twelve to fifteen million years ago by multiple eruptions of a composite volcano type known as a caldera. The events that formed Yucca Mountain created four major layers of ash-flow tuff interspersed with minor ash-fall layers. These events produced more than 600 cubic miles of silic magma alternating in layers of relatively porous and nonporous rock. The volcanic tuff at Yucca Mountain is at least 6,000 feet thick (see Figure 12).
It is the unique properties of this welded tuff combined with the deep dispersal of groundwater and lack of rainfall that make Yucca Mountain a promising site for a nuclear waste repository. Among these critical reasons are:
1) Deep groundwater, 1300 feet below the top of Yucca Mountain and 800 feet below the repository level.
2) The absence of mineral resources. Although there are gold and silver mines in nearby mountains, the volcanic tuff of the caldera is not commercially interesting.
3) The rock has a high yield strength - 25,000 psi - important for carrying the load of heavy nuclear waste containers.
4) The presence of zeolites, a class of minerals that retards the diffusion of radioactive substances through the rock.
5) A closed drainage system. Groundwater from Yucca Mountain drains eventually into Death Valley, below sea level, and not into aquifers which drain towards the ocean.
About 1,000 feet below the surface of Yucca Mountain is a densely welded rock formation known as the Topopah Spring welded tuff. This ancient layer is being studied as the possible site for the nuclear waste repository and the research is by no means trivial.
One factor affecting both the geology and chemistry of the repository is the temperature at which the site is designed. At full capacity, there will be an energy load of approximately 50 to 100 megawatts on the site which can have some environmental impact at the surface by raising the ground temperature by approximately a degree Centigrade.
A critical question is whether to design a "hot" or "cold" repository, where hot is defined as temperatures high enough to drive away water from the area as vapor. Unfortunately, even a hot repository may not ensure dryness because vapor driven from the site will tend to condense at a distance and may flow back towards the drifts through fractures.
High temperatures will also have other impacts. The mechanical structure of the rock can be degraded by long term exposure to heat, perhaps necessitating rock-bolting of tunnels and backfilling of drifts to prevent caveins. Corrosive chemical reactions are also accelerated by higher temperatures.
Finally, the higher the repository temperature, the more hostile the working environment for human beings. Air conditioning the drifts may be problematic because of the volumes of air to be moved and the need to control humidity, velocity, local temperature and other factors in an efficient manor.
The Nevada Nuclear Waste Project Office claims a number of geologic features disqualify the repository site, including a fault through the repository (the Sundance fault), the presence of earthquakes and the nearness of volcanos. They have at times suggested granite would be a better repository material, though they have been reluctant to discuss a specific site. One NWPO claim is that there are valuable minerals on the site that would invite exploration and intrusion by future generations.
Yucca Mountain is situated within a world-class precious metal mining district. Millions of dollars of gold and silver may be located in the area. ["Why Nevada Opposes Yucca Mtn.", NWPO, June 1993]
This is a half-truth. Yucca Mountain is composed of volcanic tuff left over from massive volcanic activity from millions of years ago. The area around this calderas is loaded with precious metals, but the tuff from which Yucca Mountain is formed is mineralogically worthless. The nearest bedrock that might conceivably contain precious minerals is six thousand feet below the surface of Yucca Mountain and five to ten kilometers away on the surface.
LONG TERM RADIOLOGIC QUALITY OF WASTE
One question that arises is what the spent fuel will look like after various periods of time and when when residual dangers will become trivial. The first thing to note is that the canisters holding the waste are likely to break down after as little as three hundred years even under the best of conditions. This will becaused by corrosive chemistry present even in a dry environment and from the effects of radiation bombardment of the container materials.
Long term, it was expected that the spent fuel would tend to degrade towards a form in some manner equivalent in radiologic risk (though not chemical or isotopic composition) to naturally occurring uranium ore. Recent long term criticality studies by Dr. Bill Culbreth of the University of Nevada Las Vegas and Doctoral candidate Paige Zelinsky suggest the Actinide series elements may decay more towards uranium 235, a radiologically active isotope. The Culbreth studies also suggest possible complications from the use of boron as a moderator because of its solubility.
Zeolites are a mineral that composes much of the welded tuff that underlies Yucca Mountain. Zeolites are a family of hydrated silicate minerals which are found in sedimentary and volcanic rocks. The most common zeolite minerals at Yucca Mountain are Clinoptilolite and Mordenite.
The interesting thing about zeolites is their ability to absorb the metallic compounds that radioactive substances represent. What this means is that radioactive substances being transported by water within the rock will travel at a substantially slower speed than the water. This occurs because the radioactive particles are alternately bound and unbound to the rock minerals, retarding their movement.
While this is an added protective safeguard at Yucca Mountain, it is not totally foolproof. Tecnetium and some other isotopes are able to move through zeolites nearly unimpeded.ÿ