The Impact of Preferential Flow on the Vaporization Barrier Above Waste Emplacement Drifts at Yucca Mountain, Nevada

Predicting the amount of water that may seep into waste emplacement drifts is important for assessing the performance of the proposed geologic repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. The repository would be located in thick, partially saturated, fractured volcanic tuff that will be heated to above-boiling temperatures as a result of heat generation from the decay of nuclear waste. Since infiltrating water will be subject to vigorous boiling for a significant time period, the superheated rock zone (i.e., rock temperature above the boiling point of water) can form an effective vaporization barrier that reduces the possibility of water arrival at emplacement drifts. This paper analyzes the behavior of episodic preferential flow events that penetrate the hot fractured rock, evaluate the impact of such flow behavior on the effectiveness of the vaporization barrier, and discuss the implications for the performance assessment of the repository. Our analysis demonstrates that no liquid water is expected to arrive at emplacement drifts during the first several hundred years after waste emplacement, when the rock temperature is high in the drift vicinity and boiling conditions exist in a sufficiently large region above the drifts.