REPOSITORY-HEAT-DRIVEN HYDROTHERMAL FLOW AT YUCCA MOUNTAIN .2. LARGE-SCALE IN-SITU HEATER TESTS

In situ heater tests are needed to provide an understanding of coupled geomechanical-hydrothermal-geochemical behavior in the engineered and natural barriers under repository thermal loadings and to support the validation of related numerical and conceptual models. Hypothesis tes ting can help focus characterization, modeling, and testing activities required to support model validation and build robust site suitabilit y and licensing arguments. In situ heater tests can address the follow ing hypotheses: (a) repository-driven heat flow is dominated by heat c onduction; (b) a region of above-boiling temperatures surrounding the repository corresponds to the absence of liquid water at the waste pac kage environment; (c) fracture density and connectivity are sufficient to promote rock dryout due to boiling and condensate shedding; (d) re wetting of the dryout zone lags significantly behind the end of the bo iling period, and (e) large-scale, buoyant, gas-phase convection may e ventually dominate moisture movement in the unsaturated zone. Because of limited time, some of the in situ tests will have to be accelerated relative to actual thermal loading conditions. The trade-offs between the limited test duration and generating hydrothermal conditions appl icable to repository performance during the entire thermal loading cyc le are examined, including heating (boiling and dryout) and cooldown ( rewetting). For in situ heater tests to be applicable to actual reposi tory conditions, a minimum heater test duration of 6 to 7 yr (includin g 4 yr of full-power heating) is required. The parallel use of highly accelerated, shorter duration tests may also provide timely informatio n for the license application.