The advantages of a salt/bentonite backfill for Waste Isolation Pilot Plant disposal rooms

This paper concludes that a 70/30 wt % salt/bentonite mixture is preferable to pure crushed salt as backfill for disposal rooms in the Waste Isolation Pilot Plant. The Waste Isolation Pilot Plant, near Carlsbad, NM, is designed to be the first mined geologic repository for the safe disposal of transuranic (TRU) radioactive waste generated by DOE defense programs since 1970. The repository is located about 655 m below the land surface in an extensive bedded salt formation. This report examines the performance of two backfill materials with regard to various selection criteria, such as the need for low permeability after closure, chemical stability, strength, ease of emplacement, and sorption potential for brine and radionuclides. Both salt and salt/bentonite are expected to consolidate to a state of permeability ≤ 10-18 m2 that is adequate for satisfying regulations for nuclear repositories. The results of finite-element calculations that were used to arrive at this conclusion will be described. The real advantage of the salt/bentonite. backfill depends, therefore, on bentonite's potential for sorbing brine and radionuclides. Estimates of the impact of these properties on backfill performance are presented