ON THE ADEQUACY OF SITE CHARACTERIZATION AND OTHER DATA TO CONSTRAIN REACTIVE TRANSPORT MODELS OF THE UNSATURATED ZONE AT YUCCA MOUNTAIN, NEVADA

Reactive transport models are commonly used to constrain predictions of the long-term performance of Yucca Mountain as a possible high level waste repository. Some of these models evaluate coupled thermal-hydrological-chemical processes over 10,000 years to predict the flux and chemistry of water that might seep into a repository drift and initiate aqueous corrosion of the drip shields and waste packages. In principle, reactive transport models can provide robust predictions of the evolution of the system because they are theoretically based. However, these models have substantial uncertainties, which have proven difficult to quantify. In addition to questions regarding the uniqueness of the conceptual model, a typical reactive transport model may require hundreds of uncertain parameters. Thus, confidence in the models must be established by successful applications. An essential primary test of a reactive transport model of Yucca Mountain is that it can reasonably represent the ambient hydrogeochemical system. A vast amount of site characterization data has been collected at Yucca Mountain. Yet, comparison of reactive transport model results to ambient natural system characteristics is challenging, because of the slow hydrologic and geochemical evolution of natural water-rock systems, which requires consideration of subtle differences in the rates of chemical reactions over long periods of time. A primary focus of this paper is to discuss development of a calibrated, one-dimensional dual permeability reactive transport model for the ambient system at Yucca Mountain. This work illustrates some of the difficulties of developing an ambient system model for Yucca Mountain, and raises cautions about the reliability of predictions by more complex reactive transport models that superimpose effects of transient thermal perturbations on ambient system conditions.

This abstract is an independent product of the CNWRA and does not necessarily reflect the views or regulatory position of the NRC.