[en] Prediction of the amount of water that may seep into the waste emplacement drifts is an important aspect of assessing the performance of the proposed geologic nuclear waste repository at Yucca Mountain, Nevada. The repository is to be located in thick, partially saturated fractured tuff that will be heated to above-boiling temperatures as a result of heat generation from the decay of nuclear waste. Since water percolating down towards the repository 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. In this paper, we analyze the behavior of episodic preferential flow events that penetrate the hot fractured rock, and we evaluate the impact of such flow behavior on the effectiveness of the vaporization barrier