Excerpt from Draft Environmental Impact Statement, Vol. 2 of 2: Waste Isolation Pilot Plant; Appendices, April 1979

Crystalline rocks do not dissipate heat as well as salt does; the thermal conductivities of crystalline rocks are about one-fourth that of salt. Each repository in crystalline rocks will be designed with heat loads adjusted to the thermal conductivity prevailing at the site. For some time heat transfer through crystalline rock has been considered a potential problem because the effects of cracks on thermal conductivity are not well known; heat dissipation in a medium with a random pattern of cracks is presently difficult to analyze. Experiments measuring heat conduction in granite are under way in Sweden and at the Nevada Test Site (nts). The test at nts showed that the cracks in nts rocks affected the thermal conductivity by less than Tests conducted at both locations confirm that temperature distributions in hard rock can be cal culated with a high level of accuracy.

Although large formations of salt, while soluble in water, are impervious to the flow of water, large formations of crystalline rocks are full of frac tures that would provide convenient paths for water flow. In a backfilled, sealed repository built below the water table in crystalline rock, the cracks and void spaces may eventually fill with water. Because the cracks throughout the formation are mostly small, the ratio of water volume to rock volume is small. Nevertheless, a major drawback is that it is not yet possible to cal culate the total flow and mass transport under the fracture-flow conditions. In addition, it is not yet possible to identify the effects that thermal loading will exert on the flow of water into or out of a sealed repository Techniques for making these calculations are being developed.

Because the water in crystalline rocks is more mobile than the water in salt, it may contribute to slow leaching of the radioactive nuclides from the waste. Although this condition might appear to be a problem, the magnitude of the problem is diminished because granite and basalt have sorptive properties that cause the radioactive elements in the water to be removed by chemically reacting with the rock. Water found in these formations typically has low ionic strengths that reduce the possible adverse effects on these sorptive properties. Because of these favorable natural conditions, it appears that the corrosion of waste canisters stored in a crystalline-rock repository will be slow; the canister may maintain its integrity over many hundreds of years.

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