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The Total System Performance Assessment projects how the repository will perform over tens of thousands of years. Click to enlarge.
Assessment of Engineering Processes and Procedures [pdf]
Total System Performance Assessment- Draft Supplemental EIS [pdf]
National Laboratories

Total System Performance Assessment
Federal regulations require us to evaluate the safety of the entire repository system using performance assessment. Our tool to do this is a computer model called a Total System Performance Assessment, or TSPA.

Using data from our site characterization studies we have developed hundreds of computer models that simulate the different geologic, hydrologic, physical, and chemical processes of the repository. These models are used to analyze how the different parts of the repository work.

Experts then incorporate the results of these analysis models into more comprehensive models for analyzing primary repository processes, which include:

  • climate change
  • water infiltration into the mountain
  • water movement above the repository level
  • water seepage in the repository tunnels
  • the tunnel environment
  • the effects on the engineered features
  • the effects on the waste itself
  • the transport of radioactive particles through the tunnel floor
  • the transport of particles through the rock below the repository
  • the transport of particles in the groundwater
  • the effects on people and the environment

In addition, process models were used to examine how different disruptive events such as earthquakes or volcanoes could affect repository safety.


The Total System Performance Assessment calculates the radiation dose that people living at the repository boundary (about 11 miles from the repository) could receive over the next 10,000 to 1 million years. The dose calculations consider the conditions we expect to happen, which include minor earthquakes and climate change. They also consider unlikely damaging events, such as major earthquakes and volcanoes.

This sum is compared to the Environmental Protection Agency’s dose and groundwater standards for Yucca Mountain. The radiation doses projected by the TSPA are substantially lower than the EPA's proposed standards.

In fact, for the first 10,000 years after closure of the repository, for all the scenarios added together, the mean annual dose received by the most exposed person would be less than one additional millirem per year.

What is a millirem?

A millirem is a measurement of radiation dose absorbed by the human body. The average American receives 360 millirem per year from all radiation sources, both natural and man-made. In some areas of the U.S. this number is much higher.

Natural radiation comes from radon gas coming up from the Earth’s crust, cosmic rays from outer space, food, and many other sources.

Man-made doses come from medical and dental procedures, color television sets, computer monitors, and similar sources.

  • A typical chest X-ray gives the patient about 10 millirem of radiation.
  • Watching television for three hours per day for a year gives the typical viewer about 1 millirem.
  • Those who smoke cigarettes add considerably to the national per-person average. Smoking 1.5 packs of cigarettes a day gives the smoker an annual additional radiation dose of 1,300 millirem.

Moreover, for the post-10,000-year period, within the period of geologic stability (up to 1 million years), the mean peak (highest) calculated dose received by a person living at the site boundary would be less than one millirem.


The U.S. Department of Energy is confident in its assessments of future repository safety. We are still collecting and analyzing additional information. If we receive a license to construct and operate a repository, we will continue scientific studies during the entire time the repository is operational. Our plans for collecting new information will be guided by internal evaluations and oversight groups and will reflect the current state of knowledge and the best methods for continuing to reduce uncertainties.

The nation can have confidence in Yucca Mountain's safety because the entire repository development process will bring in the formal licensing requirements of the Nuclear Regulatory Commission. The Commission will thoroughly review, question, and oversee every scientific and engineering aspect of the repository for many decades to come.

Last reviewed: 03/08


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This page last modified on: March 20, 2008  
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