It is necessary to consider the complexities of both natural and engineered components of a nuclear waste repository since fission products and minor actinides remain harmful to the environment for tens of thousands of years. In safety and performance assessments often used in decision-making about repository designs, the effect of uncertain initial guesses on the models’ output must be understood. As the necessary safe times and hence the simulated times are often in the order of magnitude of hundreds of thousands of years, uncertain initial values become increasingly important. To minimize the danger from high-level radioactive waste and to make informed decisions over designs, sensitivity analysis of the models used should be performed. The Simplified Total System Performance Assessment (STSPA) model developed by Golder Associates Inc., Booz-Allen Hamilton, Stone and Webster and the University of Nevada Reno and used in the Yucca Mountain nuclear waste repository performance assessment is analyzed for sensitivity by varying the activities of technetium-99 and iodine-129 by several orders of magnitude. The resultant dose to a maximally-exposed individual over time periods of 100,000 and 1,000,000 years is compared to the relevant regulatory limits. Incorrect estimates can be seen to have large effects on the behavior of the model while the method used allows conclusions to be drawn about the robustness of the model.

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