Technical summary of the Performance Assessment Calculational Exercises for 1990 (PACE-90)

A Performance Assessment Calculational Exercise for 1990 (PACE-90) was coordinated by the Yucca Mountain Site Characterization Project Office for a total-system performance-assessment problem. The primary objectives of the exercise were to develop performance-assessment computational capabilities of the Yucca Mountain Project participates and to aid in identifying critical elements and processes associated with the calculation. The problem defined for PACE-90 was simulation of a ''nominal case'' groundwater flow and transport of a selected group of radionuclides through a portion of Yucca Mountain. Both 1-D and 2-D calculations were run for a modeling period of 100,000 years. The nuclides used, 99Tc, 135Cs, 129I, and 237Np, were representative of ''classes'' of long-lived nuclides expected to be present in the waste inventory. Movement of the radionuclides was simulated through a detailed hydrostratigraphy developed from Yucca Mountain data specifically for this exercise. The results showed that, for the specified conditions with the conceptual models used in the problem, no radioactive contamination reached the water table, 230 m below the repository. However, due to the unavailability of sufficient site-specific data, the results of this exercise cannot be considered a comprehensive total-system- performance assessment of the Yucca Mountain site as a high-level- waste repository. 46 refs., 94 figs., 19 tabs