Hydrologic Calculations to Evaluate Backfilling Shafts and Drifts for a Prospective Nuclear Waste Repository in Unsaturated Tuff

Abstract

DISTRIBUTION CATEGORY UC-70Preliminary hydrologic calculations were performed to determine if choice of drift backfill could influence water flow past waste packages adjacent to a repository drift in unsaturated volcanic tuff. Additional calculations were performed to determine if water would flow into a shaft that penetrates a nonwelded-welded tuff interface. These hydrologic calculations consisted of numerical simulations using the computer code TRUST. Idealized configurations of a vertical shaft extending to the repository and a repository drift with horizontal and vertical emplacement of waste packages were evaluated. Both fine-grained and coarse-grained materials were considered as backfill to the drift and shaft. In the numerical simulations, coarse-grained backfill material drained more completely than fine-grained material and formed a more effective capillary barrier to water flow. Although the magnitude of flow in the modeled regions is small, selection of backfill was shown to influence flow through the repository drift and vertical shaft. However, it is concluded that selection of backfill does not significantly influence water flow past vertically or horizontally emplaced waste packages.

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

“…They concluded that mine water inflow is possible under conditions of unsaturated flow, and that the rate of mine water inflow is proportional to the magnitude of the recharge. These results are similar to those of Freshley, Dove, and Fernandez (1985), in that a high-recharge rate results in high saturations and that water flow into the drifts was predicted to occur. Rockhold, Sagar, and Connelly (1990) .…”

“…They did not evaluate the effects of MPZs surrounding drifts and shafts on their predicted flow fields. Freshley, Dove, and Fernandez (1985) found that coarse materials used for backfill, such as crushed tuff that was represented by hydraulic properties of coarse sand, performed more satisfactorily as a barrier to water flow through drifts and shafts than fine materials. Water flow into the drift was found to depend on the saturation level of the surrounding host rock.…”

“…Relatively early in the investigation of Yucca Mountain as a potential repository site, Fernandez and Freshley (1984) and Freshley, Dove, and Fernandez (1985) investigated the effects of shafts and drifts on flow in the Topopah Spring welded unit in their development of concepts for sealing the potential repository. They used a model, based on the TRUST computer code, to perform preliminary calculations for different backfill materials under consideration for sealing the potential repository in the Topopah Spring welded unit.…”

“…In addition to the analyses of the repository drift, Freshley, Dove, and Fernandez (1985) also evaluated the impacts of a vertical shaft that intersects a geologic contact between volcanic tuff units. Different combinations of welded and nonwelded tuff units were used to represent the geologic contact, which was inclined 8 degrees to the east.…”

“…The numerical model, based on the PORFL0-3C computer code and the conceptual model, Is three dimensional, radially symmetric, and centered on an exploratory shaft that extends into the CHn unit to a depth 50 m above the water table. The model builds on past work by Freshley, Dove, and Fernandez (1985); Fernandez, Hinkebein, and Case (1989);and Rockhold, Sagar, and Connelly (1990).…”