The Yucca Mountain Project is considering some type of backfill, possibly emplaced as a capillary barrier, for inclusion in the Engineering Barrier System (EBS) at the potential Yucca Mountain nuclear waste repository site. The performance of capillary barriers in isothermal, low- temperature, environments has been studied extensively (e.g., Ho and Webb, 1998; OZdenburg and Pruess, 1993; Ross, 1990). The performance of capillary barriers in an non-isothermal, high temperature environment, such as during the thermal pulse phase of a nuclear waste repository, has received much less attention. One concern is that the backfill materials may be altered from that of the as-placed material by the hydrothermal regime imposed by the emplacement of waste in the repository, changing hydrologic properties in a way that degrades the performance of the EBS system. This report is a status report on our efforts to address this concern. The work was initiated by SCR #98-76-041 and was authorized to begin at LLNL in summer 1998. This report is organized as follows. In the first part, we discuss our understanding of the relevant issues of backfill performance based on thermal hydrology. We focus here on changes to hydrologic properties, but we recognize that changes to thermal, mechanical …