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DISTRIBUTION flr THIS D'f;C'oi.'.r!i IS MJ&I.

This report uses the Advanced Conceptual Design (ACD) from TRW, 1996, as the Reference Design for the repository The geometry of the ACD has a spacing between drift centerlines roughly the same as axial spacing between WPs The drift spacing is 22 5 m, and the area1 mass loadiig AML is 83 4 MTU/acre

The radioactive decay heat from nuclear waste packages may, depending on the thermal load, create coupled thermal-mechanical-hydrological-chemical (TMHC) processes in the nearfield environment of a repository. A group of tests on a large block (LBT) are planned to provide a timely opportunity to test and calibrate some of the TMHC model concepts. The LBT is advantageous for testing and verifying model concepts because the boundary conditions are coi.uolled, and the block can be characterized before and after the experiment. A block of Topopah Spring tuff of about 3 x 3 x 4.5 m will be sawed and isolated at Fran Ridge, Nevada Test Site. Small blocks of the rock adjacent to the large block will be collected for laboratory testing of some individual thermal-mechanical, hydrological, and chemical processes. A constant load of about 4 MPa will be applied to the top and sides of the large block. The sides will be sealed with moisture and thermal barriers. The large block will be heated by heaters within and guard heaters on the sides so that a dry-out zone and a condensate zone will exist simultaneously. Temperature, moisture content, pore pressure, chemical composition, stress, and displacement will be measured throughout the block during the heating and cool-down phases. The results from the experiments on small blocks and the tests on the large block will provide a better understanding of some concepts of the coupled TMHC processes. The progress of the project is presented in this paper. IntroductionA major concern for the disposal of high level nuclear wastes in deep geological formations is the quantity and quality of water that may contact waste packages. The Yucca Mountain Site Characterization Project (YMP) is investigating the Topopah Spring tuff at Yucca Mountain, Nevada, for its suitability as a host rock for the disposal of high level nuclear wastes. The host rock at the potential repository horizon is a partially saturated, fractured, densely welded, nonlithophysal tuff. Work to date suggests that the host rock at the potential repository horizon has a mean matrix porosity of 14% and a mean water saturation of 65% W. Therefore, the pores of the host rock are filled with both air and liquid water.The expected development of the near-field environment in a repository is discussed in the next two paragraphs. The radioactive decay heat from waste packages will increase the temperature in the rock mass in the near field of the repository. The temperature in the rock mass depends on the thermal load in the repository. Results from a heater test in G-Tunnel, Nevadi Test

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