HYDROLOGIC IMPACT OF EXPLORATORY SHAVFT EXTENSION INTO NONWELDED TUFF

The Calico Hills nonwelded unit of volcanic tuff at Yucca Mountain, Ne vada is considered a primary natural barrier to radionuclide migration between a potential high-level nuclear waste repository at Yucca Moun tain and the regional water table. Numerical simulations using a verif ied and benchmarked code of unsaturated flow in nonwelded volcanic tuf f were performed in support of a risk/benefit analysis to assess the h ydrologic impact of extending exploratory shafts into the Calico Hills nonwelded unit for characterization purposes. Additional characteriza tion of the Calico Hills nonwelded unit would improve the accuracy of model predictions of its performance as a natural barrier to radionucl ide migration, while the presence of shaft(s) required to characterize the unit may reduce the effectiveness of the barrier. Numerical simul ations were used to predict that the presence of a shaft and the surro unding modified permeability zone caused by shaft construction would r educe travel time for water in a radial zone with a diameter of approx imately 40 m by at least 58%, compared to undisturbed conditions. The results show that the choice of backfill materials used to seal the sh aft will be the most important factor in design to exploratory shafts with respect to hydrologic impacts. Heterogeneities, in the form of la yers of contrasting hydraulic properties, may also be important in the vicinity of a shaft where these differences may result in preferentia l paths for water flow.