Nuclear-waste-package program for high-level isolation in Nevada tuff

The objective of the waste package program is to insure that a package is designed suitable for a repository in tuff that meets performance requirements of the NRC. In brief, the current (draft) regulation requires that the radionuclides be contained in the engineered system for 1000 years, and that, thereafter, no more than one part in 105 of the nuclides per year leave the boundary of the system. Studies completed as of this writing are thermal modeling of waste packages in a tuff repository and analysis of sodium bentonite as a potential backfill material. Both studies will be presented. Thermal calculations coupled with analysis of the geochemical literature on bentonite indicate that extensive chemical and physical alteration of bentonite would result at the high power densities proposed (ca. 2 kW/package and an area density of 25 W/m2), in part due to compacted bentonite's relatively low thermal conductivity when dehydrated (approx. 0.6 +- 0.2 W/m0C). Because our groundwater contains K+, an upper hydrothermal temperature limit appears to be 120 to 1500C. At much lower power densities (less than 1 kW per package and an areal density of 12 W/m2), bentonite may be suitable