Approaches to modeling coupled thermal, hydrological, and chemical processes in the drift scale heater test at Yucca Mountain

Abstract

Abstract A large-scale underground thermal test (Drift Scale Test–DST) in fractured volcanic tuff resulted in changes to water and gas chemistry as well as mineral precipitation and dissolution in fractures. Thermal, hydrological, and chemical (THC) processes in the DST were modeled by Lawrence Berkeley National Laboratory “LBNL” and Japan Nuclear Cycle Development Institute “JNC” as part of the international working group DECOVALEX. Predictions of THC processes in the DST for the 4-year heating and 4-year cooling periods were initially performed by the LBNL group, with the current model reflecting a revised heater operation history and model. JNC used primarily the original data from the prediction and created a new model to evaluate a selected set of data. The approaches taken by the groups differed in several ways and a comparison of the methodologies and results of the simulations allow for a better understanding of modeling coupled processes in unsaturated fractured rock. The LBNL model represented the fractures and rock matrix as a fully interacting dual-continuum (in terms of fluid, heat, and chemical transport) with the local mineral–water–gas reactions treated by kinetic and equilibrium reactions. The JNC model represented the fractures and matrix as a single effective continuum, with equilibrium mineral-water reactions controlling the chemical evolution. Both models considered aqueous species transport, with gas phase CO 2 transport only considered in the LBNL model. Comparisons to data collected from the DST illustrate the behavior of the models and their ability to capture the relevant THC processes. Overall, both models capture the temperature evolution in the rock quite closely, although the JNC model gave a closer match to the initial temperature rise in the rock, likely owing to the use of site-specific thermal data as opposed to average properties used for the LBNL model. Both models showed the contrasting solubility effects of increasing temperature on calcite and silica solubility; yet the dual-continuum approach better represented the effects of boiling and condensation on aqueous species chemistry and the distribution of mineral precipitation.