Office of Civilian Radioactive Waste Management

TSPA circle with Yucca Mountain photo background. Caption: Total system performance assessment is an iterative process for analyzing long-term repository safety. It involves continutally cycling through these five major steps to improve our depth of knowledge and enhance the repository's design.

Why does the Department of Energy (DOE) use computer modeling?

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.

Augmenting field and laboratory science with advanced computer technology

"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."

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.

Total System Performance Assessments and a Yucca Mountain repository

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.

Yucca Mountain Project