A strategy to seal exploratory boreholes in unsaturated tuff; Yucca Mountain Site Characterization Project

Abstract

This report presents a strategy for sealing exploratory boreholes associated with the Yucca Mountain Site Characterization Project. Over 500existing and proposed boreholes have been considered in the development of this strategy, ranging from shallow (penetrating into alluvium only) to deep (penetrating into the groundwater table). Among the comprehensive list of recommendations are the following: Those boreholes within the potential repository boundary and penetrating through the potential repository horizon are the most significant boreholes from a perfonnance standpoint and should be sealed. Shallow boreholes are comparatively insignificant ,andrequire only nominal sealing. The primary areas in which to place seals are away from high-temperature zones at a distance from the potential repository horizon in the Paintbrush nonwelded tuff and the upper portion of the Topopah Spring Member and in the tuffaceous beds of the Calico Hills Unit. Seals should be placed prior to waste emplacement.Performance goals for borehole seals both above and below the potential repository are proposed. Detailed construction information on the boreholes that could be used for future design specifications is provided along with a description of the environmental setting, i.e., the geology, hydrology, and the in situ and thermal stress states. A borehole classification scheme based on the condition of the borehole wall in different tuffaceous units is also proposed. In addition, calculations are presented to assess the significance of the boreholes acting ,as preferential pathways for the release of radionuclides.Design calculations are presented to answer the concerns of when, where, and how to seal. As part of the strategy development, available technologies to seal exploratory boreholes (including casing removal, borehole wall reconditioning, and seal emplacement) are reviewed. It is recommended that the surface-based site characterization program maintain exploration boreholes with casing so that seals can be placed at selected sealing locations, that grout not be introduced at sealing locations, and that work plans for drilling additional boreholes be developed that address sealing issues. Borehole-specific sealing plans should also be developed. _3__TIONOF THIS DOOU ' IMIT,

Detailed design report for an operational phase panel-closure system

“…Bench-scale tests done by Terra-Tek showed the importance of interface zone permeability (Fernandez, et al, 1994). For sealed boreholes, flow occurs either through the seal matrix or through the interface zone.…”

Site characterization progress report: Yucca Mountain, Nevada, October 1, 1993--March 31, 1994

“…Plans were developed for conducting laboratory tests and limited field demonstrations. Test concepts were based on those identified in "A Strategy to Seal Exploratory Boreholes in Unsaturated Tuff' (Fernandez et al, 1993). The laboratory tests are necessary prior to full-scale tests to establish confidence in material fluid flow and mechanical performance.…”