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 testing can help focus characterization, modeling, and testing activities required to support model validation and build robust site suitability and licensing arguments. In situ heater tests can address the following hypotheses: (a) repository-driven heat flow is dominated by heat conduction; (b) a region of above-boiling temperatures surrounding the repository corresponds to the absence of liquid water at the waste package environment; (c) fracture density and connectivity are sufficient to promote rock dryout due to boiling and condensate shedding; (d) rewetting of the dryout zone lags significantly behind the end of the boiling period, and (e) large-scale, buoyant, gas-phase convection may eventually 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 applicable to repository performance during the entire thermal loading cycle are examined, including heating (boiling and dryout) and cooldown (rewetting). For in situ heater tests to be applicable to actual repository conditions, a minimum heater test duration of 6 to 7 yr (including 4 yr of full-power heating) is required. The parallel use of highly accelerated, shorter duration tests may also provide timely information for the license application.
