THERMAL-HYDROLOGIC MODELING OF A DEEP BOREHOLE DISPOSAL SYSTEM.

Research on deep borehole disposal of high-level radioactive waste has indicated that the deep borehole disposal concept is a viable potential alternative to a mined repository system. Previous modeling indicates that thermally induced fluid flow associated with heat from the waste has the greatest potential for the upward transport of dissolved radionuclides. An updated thermalhydrologic model of a deep borehole disposal system is constructed using a recently developed reference design, incorporating a more realistic heterogeneous representation of the hydrogeological system with depthvarying permeability and thermal conductivity, coupled stratification of salinity and fluid density, and arrays of up to 81 disposal boreholes. Results show that peak temperatures near the borehole occur within 20 years of disposal, with an extended period of elevated temperatures beyond 10,000 years and a second, lower peak in temperature near the center of larger borehole arrays. Simulated vertical upward groundwater flux in the borehole and disturbed rock zone occurs in the waste disposal zone and overlying seal zone at greatest magnitude for about 100 years, but persists for an extended period of time. The persistence of simulated vertical groundwater flow beyond 1,000 years increases with the number of disposal boreholes in the array. This persistence in flow may have important implications for the maximum number of disposal boreholes that could be safely emplaced at a single site.