Geochemical alteration of backfill FY98 status report

Abstract

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 and chemical (e.g., sorptive) properties are also important. Our primary interest is in addressing concerns over possible changes in the magnitude of key hydrologic properties (i.e., porosity, permeability, and moisture retention characteristics) that could significantly affect the design performance of backfill in the EBS. We report on what we know from previous work about geochemical alteration of backfill material, focusing primarily on crushed tuff. In the second part of this report, we present our progress on geochemical studies on backfill materials. These include sub-boiling, two-phase column experiments, batch experiments at above boiling temperatures and associated numerical modeling.