Providing a Scientific and Technical Basis for Repository Decisions Page: 2 of 3
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" Features are physical, chemical, and thermal
characteristics of the total repository system
- and how they work over time. The
rock's structure and hardness, and the way
they could respond to heat given off by
radionuclide decay, are examples of
" Events are occurrences that have a specific
starting time and are usually of short
duration - an earthquake, for example.
" Processes are activities that have gradual but
continuous interactions with the overall
repository system - for instance, water
moving through the mountain.
as water moving through the mountain, the lifetime
of the waste packages, the release of radionuclides
from the engineered barrier system, and their
transport through the natural barrier system.
Using specialized computer software, scientists and
engineers build detailed mathematical
representations or models, of the features, events,
and processes that could affect the performance of
the repository. They then incorporate the results of
each of these individual, detailed models into one
overall mathematical model of the repository
system. This overall model of the relevant features,
events, and processes - both natural and
engineered - is called the "total system
performance assessment model." The TSPA model
allows scientists to project how a repository
designed for the conditions of Yucca Mountain is
likely to work over thousands of years.
What the TSPAs tell us
Using these assessments, scientists and engineers
can identify which aspects of the repository system
require additional study. They can also identify
those elements of the repository system that are
most important to their understanding of how well
the repository is likely to work. Finally, the
assessments tell scientists and engineers where they
need to improve either their own scientific
understanding or the performance of the repository
The mathematical computer models for the decision on
site recommendation will address the following
" What happens to water as it moves through the system.
This process includes groundwater flow, water seepage
that may occur in the tunnels that will hold the waste
packages, and the effects of groundwater movement
due to changes in temperature.
" How the waste packages and drip shields will be
affected by chemistry, heat, water, and the physical and
mechanical environment inside the emplacement
" How radioactive material that could be released from
the waste packages would move through the nearly one
thousand feet of rock beneath the repository level and
reach the water table.
" How radioactive material that could be released to the
water table would move south to southeast of the
repository and eventually reach a well used for a small
farming and residential community.
The TSPA will forecast out to 10,000 years. As required
by federal regulations, the projections will cover several
situations: expected, disruptive events, and human
intrusion. In the expected situation, the repository is not
significantly disturbed by either human or natural forces
and performs according to design. The second situation
that must be modeled covers "disruptive events." The
probability of a disruptive event such as a volcano
disturbing a repository at Yucca Mountain is extremely
low; however, should such a disruption occur, there
could be significant consequences. So the TSPA will
project the probable behavior and effects of these high-
consequence, but unlikely, events. Finally, a separate
TSPA must assess what would happen if a human
drilled completely through a waste package while
exploring for water 100 years after the repository is
The performance of a deep underground repository over
10,000 years or more - longer than recorded human
history - can never be proven. Scientists can, however,
observe and analyze the results of laboratory and field
experiments. They can then compare those observed
results with the results projected by the computer
calculations and, through the TSPA, gain confidence
about the repository's likely performance.
U.S. Department of Energy, Office of Civilian Radioactive Waste Management
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Providing a Scientific and Technical Basis for Repository Decisions, report, February 28, 2001; Las Vegas, Nevada. (https://digital.library.unt.edu/ark:/67531/metadc722861/m1/2/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.