Thermal, Chemical, and Mass-Transport Processes Induced in Abyssal Sediments by the Emplacement of Nuclear Waste: Experimental and Modeling Results

摘要

This paper discusses heat and mass transport studies of marine red clay sediments being considered as a nuclear waste isolation medium. Numerical models indicate that for a maximum allowable sediment/canister interface temperature of 200 to 250 exp 0 C, the sediment can absorb about 1.5 kW initial power from waste in a 3 m long by 0.3 m dia canister buried 30 m in the sediment. Fluid displacement due to convection is found to be less than 1 m. Laboratory studies of the geochemical effects induced by heating sediment/seawater mixtures indicate that the canister and waste form must be designed to resist a hot, acid (pH 3 to 4) oxidizing environment. Since the thermally altered sediment volume of about 5.5 m exp 3 is small relative to the sediment volume overlying the canister, the acid and oxidizing conditions are not anticipated to effect the properties of the far field. Using sorption coefficient correlations, the migration of four nuclides exp 239 Pu, exp 137 Cs, exp 129 I, and exp 99 Tc were computer for a canister buried 30 m deep in a 60 m thick red clay sediment layer. It was found that the exp 239 Pu and exp 137 Cs are essentially completely contained in the sediments, while exp 129 I and exp 99 Tc broke through the 30 m of sediment in about 5000 years. The resultant peak injection rates of 4.6 x 10 exp -5 mu Ci/year-m exp 2 for exp 129 I and 1.6 x 10 exp -2 mu Ci/year-m exp 2 for exp 99 Tc were less than the natural radioactive flux of exp 226 Ra (3.5 to 8.8 x 10 exp -4 mu Ci/year-m exp 2 ) and exp 222 Rn (0.26 to 0.88 mu Ci/year-m exp 2 ). (ERA citation 05:020185)