Numerical experiments on the probability of seepage into underground openings in heterogeneous fractured rock

An important issue for the performance of underground nuclear waste repositories is the rate of seepage into the waste emplacement drifts. A prediction of this rate is particularly complicated for the potential repository site at Yucca Mountain, Nevada, because it is located in thick, unsaturated, fractured tuff formations. Underground openings in unsaturated media might act as capillary barriers, diverting water around them. In the present work, we study the potential rate of seepage into drifts as a function of the percolation flux at Yucca Mountain, based on a stochastic model of the fractured rock mass in the drift vicinity. A variety of flow scenarios are considered, assuming present-day and possible future climate conditions. We show that the heterogeneity in the flow domain is a key factor controlling seepage rates, since it causes channelized flow and local pending in the unsaturated flow field.