Examination of backfill materials for high-level waste repository

Abstract

A popular material of choice for a backfill in a high-level waste (HLW) repository is bentonite. However, a concern has been expressed for the use of bentonite as backfill at the Yucca Mountain HLW repository due to the unsaturated nature of the site and the associated high-temperature conditions. In this article, the thermal and sorption properties of various minerals that might be used as backfill at Yucca Mountain were investigated. The minerals examined include bentonite, kaolinite, zeolite, apatite, calcite, and silica. Simulated thermal stability tests were performed on these materials to observe their individual dehydration properties. Batch sorption tests were conducted to examine the minerals' capacities to retard the migration of Sr 2+, Am 3+, Ra 2+, TcO 4 -, and I - by using nonradioactive surrogate tracers. Results indicated that silica, calcite, and kaolinite could provide good thermal properties for the Yucca Mountain backfill. Results of sorption studies showed zeolite, calcite, and apatite as good cation sorbers. No mineral was found to be a good anion sorber. A combination of these minerals (silica, kaolinite, calcite, apatite, and zeolite) is desirable to provide an effective backfill. More investigation is needed to add a good anion sorber.