- A Total System Performance Assessment (TSPA)
is a sophisticated computer-based tool for projecting how complex
natural and engineered systems are likely to work together for
10,000 years and beyond.
- Both the Environmental Protection Agency (EPA)
and the Nuclear Regulatory Commission (NRC) require DOE to use
a TSPA in assessing the likely future performance of a Yucca Mountain
repository.
- Modern computer technology applies the principles
of science to hard data to create “models” that mathematically
simulate and project how the systems will likely interact over
time.
- A TSPA is used to evaluate repository safety
by calculating radiation doses for tens of thousands of years
to someone living near the mountain.
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Science is about finding answers to questions. The most familiar way
of doing science is to conduct physical experiments to test theories
about how something works. By their very nature, experiments are focused
on very specific aspects of the way nature or materials behave.
Using advanced mathematics and computer technology, we now have a
way to pull together the results of many different types of experiments
to make broader predictions in space and time — using computerized
mathematical models.
With enough data, the right equations, and powerful enough digital
technology, scientists and engineers can simulate real-world behaviors
of complex systems based on a variety of observational and testing
results. In some cases, they can test theories about things that are
impossible to experiment on directly; thus, they can make predictions
about outcomes over time. This way of augmenting science is called
computer modeling.
Nowadays, for example, scientists use computers to predict the weather,
thereby saving lives and property. They even create new materials
and life-saving drugs with computer modeling. And they have unraveled
many of the mysteries of nature — from past ice ages to the
shape of the universe.
Using advanced software and high-powered computers, scientists and
engineers are also able to project how a Yucca Mountain repository
for spent nuclear fuel and high-level radioactive waste is likely
to behave.
A geologic repository at Yucca Mountain would involve both natural
and man-made (engineered) systems. These systems would interact with
each other, providing barriers to water, which is the major way radioactive
particles (radionuclides) could escape from a repository.
To find out how the systems would work together, both the Nuclear
Regulatory Commission and the Environmental Protection Agency require
a type of computer modeling called a total system performance assessment,
or TSPA.
Before performing such an overall assessment, scientists and engineers
have to understand the features, events, and processes that could
affect the natural and engineered systems of a deep underground repository
at the particular site.
Features are identified physical
characteristics of the total repository system — and how they
behave over time. The rock’s structure and hardness are examples
of repository features.
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, the corrosion of a metal barrier or water moving through
the mountain.
By doing TSPAs, scientists and engineers can identify which aspects
of the repository system work the best and how to improve the other
aspects. Every time they run the equations on the computer, they can
add more up-to-date data, which helps them to define new tests to
allow better support for, or even changes in, the modeling.
Adding the more up-to-date data to the equations also helps engineers
refine and improve the design of the proposed repository. This is
a process that is repeated, so that as time progresses, so does our
confidence in the modeling and the safety of the repository system.
The combined computer modeling and testing programs also allow scientists
to estimate the likely future radiation doses that a person living
near Yucca Mountain might receive from the repository. The TSPA projects
these doses out to 10,000 years and beyond for people living near
the mountain. During that period, and far beyond, the projected doses
are substantially below the standards set by the applicable regulations.1
In the very long term, at about a half million years into the future,
the highest potential doses are projected to occur. Even then, the
doses are forecast to be but a fraction of the annual background dose
received by the average American.
The total system, therefore, promises to still be protective even
after the engineered system has essentially lost its integrity (or
ability to function).2
The performance of a deep underground repository over 10,000 years
or more — longer than recorded human history — can never
be absolutely proven. But by performing total system performance assessments
in response to advances in scientific understanding and engineering,
scientists and engineers can increase confidence in how a repository
would likely work.3
1 For a detailed description of the Yucca Mountain
TSPA, see the Yucca Mountain Science and Engineering Report,
Rev. 1, or the Yucca Mountain Site Suitability Evaluation.
Both documents are available online at www.ocrwm.doe.gov.
2 See the Final Environmental
Impact Statement for a Repository at Yucca Mountain, Section
5.4.2.
3 One scientific activity used to check the modeling is studying locations
where activities similar to those expected in the repository have
occurred over very long periods of time in nature. Such locations,
called analogues, are typically places where either nature or industry
has placed radioactive materials into natural settings and processes
have modified those deposits in ways, and at rates, expected to be
analogous to Yucca Mountain. See the fact sheets “Oklo: A natural
nuclear repository” and “Scientists look to nature for
insights into how a repository would work,” both available at
www.ocrwm.doe.gov.
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