Examining Repository Loading Options to Expand Yucca Mountain Repository Capacity
- Department of Nuclear Engineering, North Carolina State University (United States)
Siting a high level nuclear waste repository entails high economic, social, and political costs. Given the difficulty in siting the Yucca Mountain repository and the already identified need for additional capacity, the concept of expanding the capacity of the Yucca Mountain repository is of significant interest to the nuclear industry and the Department of Energy (DOE). As the capacity of the repository is limited by the decay heat inventory of the spent nuclear fuel in relation to the thermal design limits, expanding the capacity requires appropriate schemes for decay heat and spent fuel loading management. The current Yucca Mountain repository is based on a single level, fixed drift spacing design for a fixed area or footprint. Studies performed to date investigating the capacity of Yucca Mountain often assume that the loading of spent fuel is uniform throughout the repository and use the concept of a linear loading or areal power density (APD). However, use of linear loading or APD can be problematic with the various cooling times involved. The temperature within the repository at any point in time is controlled by the integral of the heat deposited in the repository. The integral of the decay heat varies as a function of pre-loading cooling periods even for a fixed linear loading. A meaningful repository capacity analysis requires the use of a computer model that describes the time-dependent temperature distributions of the rock from the dissipation of the heat through the repository system. If variations from the current Yucca Mountain repository design were to be considered, expanding the capacity of the repository would be pursued in several ways including: (1) increase the footprint size; (2) implement multiple-levels in the repository for the given footprint; (3) allow the drift distance to vary within thermal limits; and, (4) allow non-uniform loading of wastes into the drifts within thermal limits. Options (1) and (2) have been investigated by other researchers. This paper investigates options (3) and (4) for possible expansion of the Yucca Mountain repository capacity. To support the work, a thermal analysis model was needed to describe the temperature changes in the rock around the waste packages against the thermal design limits as a function of spent fuel characteristics and composition. Under the high temperature operating mode (HTOM), the relevant thermal design limits are: (1) the rock temperature midway between adjacent drifts must remain below the local boiling point (96 deg. C); and (2) the rock temperature at drift walls must remain below 200 deg. C. As the work involves a large number of calculations, examining the compliance within thermal design limits, the capability to perform efficient mountain-scale heat-transfer analyses was necessary. A related topic of importance in this investigation was also the effect of uncertainty. As the modeling exercise relies on the use of computational models, uncertainties are unavoidable and understanding the uncertainty in the interpretation of the results is important. The concept of variable drift spacing and variable drift thermal loading was investigated with respect to possible capacity expansion of the Yucca Mountain repository. Also, a computer model was developed for efficient repository heat transfer calculations and sensitivity and uncertainty analyses were performed to identify key parameters and to estimate the uncertainty in the results and understand how the repository capacity estimation would be affected by the uncertainty. (authors)
- Research Organization:
- American Nuclear Society, 555 North Kensington Avenue, La Grange Park, IL 60526 (United States)
- OSTI ID:
- 20979588
- Country of Publication:
- United States
- Language:
- English
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