Assessment of hydrologic impact of extending exploratory shafts into the Calico Hills nonwelded tuff unit at Yucca Mountain, Nevada

The US Department of Energy (DOE) is performing analyses to address an objection by the US Nuclear Regulatory Commission to plans in the Consultation Draft of the Site Characterization Plan for direct excavation of the Calico Hills nonwelded (CHn) unit within the repository exploration block at Yucca Mountain, Nevada. The excavation was planned as part of site characterization activities for the potential high-level nuclear waste repository at Yucca Mountain. This characterization activities for the potential high-level nuclear waste repository at Yucca Mountain. This characterization activity has been deferred, pending the results of a risk/benefit analysis of alternative methods for obtaining needed characterization data from CHn unit. The benefits from characterizing the CHn unit are generally related to obtaining information leading to improved confidence in predictions of site performance. The risks are generally associated with potential adverse impacts to site performance that result from excavation or other intrusion into the CHn unit. The purpose of the risk/benefit analysis is to produce a recommendation to the Director, Regulatory and Site Evaluation Division. DOE/Yucca Mountain Site Characterization Project Office for a strategy for characterizing the CHn unit. The recommendation will describe characterization activities that are expected to provide the needed information while limiting adverse impacts to site performance to the extent practical. The risk/benefit analysis was supported with scoping calculations to provide a quantitative evaluation of the impacts associated with different strategies. The working group responsible for the risk/benefit analysis requested that these scoping calculations to be supported with more detailed performance assessments for evaluating impacts of different characterization activities. This report summarizes the results of these performance assessment analyses. 9 refs., 30 figs., 1 tab

Sensitivity and uncertainty analyses of unsaturated flow travel time in the CHnz unit of Yucca Mountain, Nevada

This report documents the results of sensitivity and uncertainty analyses conducted to improve understanding of unsaturated zone ground-water travel time distribution at Yucca Mountain, Nevada. The US Department of Energy (DOE) is currently performing detailed studies at Yucca Mountain to determine its suitability as a host for a geologic repository for the containment of high-level nuclear wastes. As part of these studies, DOE is conducting a series of Performance Assessment Calculational Exercises, referred to as the PACE problems. The work documented in this report represents a part of the PACE-90 problems that addresses the effects of natural barriers of the site that will stop or impede the long-term movement of radionuclides from the potential repository to the accessible environment. In particular, analyses described in this report were designed to investigate the sensitivity of the ground-water travel time distribution to different input parameters and the impact of uncertainty associated with those input parameters. Five input parameters were investigated in this study: recharge rate, saturated hydraulic conductivity, matrix porosity, and two curve-fitting parameters used for the van Genuchten relations to quantify the unsaturated moisture-retention and hydraulic characteristics of the matrix. 23 refs., 20 figs., 10 tabs

Development of a three-dimensional ground-water model of the Hanford Site unconfined aquifer system: FY 1995 status report

A three-dimensional numerical model of ground-water flow was developed for the uppermost unconfined aquifer at the Hanford Site in south-central Washington. Development of the model is supported by the Hanford Site Ground-Water Surveillance Project, managed by the Pacific Northwest National Laboratory, which is responsible for monitoring the sitewide movement of contaminants in ground water beneath the Hanford Site. Two objectives of the Ground-Water Surveillance Project are to (1) identify and quantify existing, emerging, or potential ground-water quality problems, and (2) assess the potential for contaminants to migrate from the Hanford Site through the ground-water pathway. Numerical models of the ground-water flow system are important tools for estimating future aquifer conditions and predicting the movement of contaminants through ground water. The Ground-Water Surveillance Project has supported development and maintenance of a two-dimensional model of the unconfined aquifer. This report describes upgrade of the two-dimensional model to a three-dimensional model. The numerical model is based on a three-dimensional conceptual model that will be continually refined and updated as additional information becomes available. This report presents a description of the three-dimensional conceptual model of ground-water flow in the unconfined aquifer system and then discusses the cur-rent state of the three-dimensional numerical model