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If a repository were built at Yucca Mountain, it would rely on two
different systems to prevent radioactive materials from escaping into
the environment. These systems act as barriers to the movement of
radionuclides (radioactive atoms).
The first system involves natural barriers — characteristics of the
rocks and the groundwater at Yucca Mountain.
The second system includes man-made, or engineered, barriers that
give the repository defense-in-depth and added safety margins.
The two systems would work together to protect public health and safety
and the environment.
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The precipitation averages about 7.5 inches (190 mm) per year, most
of which (more than 95%) either runs off, evaporates, or is taken
up by the desert vegetation.
The mountain’s water table is unusually far beneath the surface
— on average, about 2,000 feet (600 m) underground.
Yucca Mountain is located in the Death Valley hydrologic basin. Water
in this basin does not flow into any rivers or oceans and is isolated
from the aquifer systems of Las Vegas and Pahrump, the major nearby
town, located about 40 miles (70 km) from the mountain.
Yucca Mountain has several natural characteristics that would work
together to contain and isolate spent nuclear fuel and high-level
radioactive waste. The most important natural barriers include the
following:
- The surface soils and the natural physical shape and configuration
of the mountain and its geologic environs (i.e., topography) —
which limit the ability of water to infiltrate the surface
- Unsaturated rock layers above the repository level —
which limit the ability of water to move down into the repository’s
emplacement tunnels
- Unsaturated rock layers below the repository level —
which limit transport of radionuclides that might escape from
repository tunnels
- Volcanic rocks and water-deposited clay, silt, and sands (alluvial
deposits) below the water table — which limit radionuclide
transport in the saturated zone
With very little water available to start with, the surface soils
and topography of Yucca Mountain and its region limit the amount of
water that can infiltrate the mountain’s surface.
Perhaps the most important natural barrier, however, can be found
in the rock layers and minerals of Yucca Mountain. The repository
would be located about 1,000 feet (300 m) below the mountain’s
surface and, on average, about 1,000 feet above the water table —
in the unsaturated zone of rock.
The unsaturated zone is the expanse of rock in which the microscopic
pores are not completely filled with water. Water tends to move very
slowly through such rock.
At most locations within the mountain, it takes thousands of years
for the small amounts of water that can infiltrate the surface to
reach the level of the repository. It would then take thousands of
additional years for the water to move through the next approximately
1,000 feet (300 m) of unsaturated rock to reach the water table.
In addition, certain minerals within the rock actually strain radioactive
particles from contaminated water, holding them in place in the rock.
Of any particles that do reach the water table, the silts, rocks,
and clays would slow, or capture, them.
From there, any radioactive particles must then be transported more
than 11 miles (18 km) through the rock in the saturated zone before
reaching a location where the water is likely to be pumped to the
surface and used by anyone. It would take many thousands of years
for these processes to occur.
By itself, the mountain would provide a high degree of protection
to the public. To enhance the mountain’s natural barriers, scientists
and engineers have devised a series of man-made, or engineered, barriers
to augment the natural system. The major engineered barriers include
the following:
- Drip shields — which limit the ability of water to contact
the waste package
- Waste packages — which limit the water contacting the
actual waste forms inside
- Cladding (corrosion-resistant metal tubes that contain the
ceramic fuel pellets) — which limits the water contacting
the commercial spent nuclear fuel portion of the waste
- Solid waste forms — which limit the rate of radionuclides
picked up by any water that does contact the waste
- Inverts (the floors of stainless steel and crushed volcanic
rock added to the emplacement tunnels) — which limit the
rate of release of radionuclides to the natural barriers.
The repository tunnels themselves also would serve as important engineered
barriers to potential radioactive releases. The tunnels would be constructed
away from large fractures in the rock because in unsaturated rock
water moves fastest in large fractures. However, because of capillary
forces, any water in small fractures near a larger opening, such as
a tunnel, tends to stay in the fractures. The emplacement tunnels
would also be designed so that any water that does enter them can
drain, by gravity, out of the tunnels and away from any others.
When designing disposal systems intended to last longer than recorded
human history, scientists and engineers must consider the possibility
that one or more barriers, natural or engineered, could fail to perform
as expected. Waste packages may fail earlier than expected because
of undetected defects. Unforeseen circumstances could cause more water
than anticipated to seep into the tunnels.
Fortunately, the repository’s ability to contain and isolate
its contents would not depend on any single barrier, natural or man-made.
Having a combination of barriers is called defense-in-depth, meaning
if one barrier fails to perform as expected, other barriers will continue
to function in a way that compensates for the unexpected failure.
Each of the barriers would work with the others to support a system
designed to protect the public’s health and safety and safeguard
the environment.
In fact, considering all the barriers working together, sophisticated
computer calculations project that for at least 10,000 years after
the repository is closed, the radiation a person could receive from
the repository would be far below the radiation protection standards
for public health and safety.
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