Modeling coupled thermal–hydrological–chemical processes in the unsaturated fractured rock of Yucca Mountain, Nevada: Heterogeneity and seepage

Abstract

An understanding of processes affecting seepage into emplacement tunnels is needed for correctly predicting the performance of underground radioactive waste repositories. It has been previously estimated that the capillary and vaporization barriers in the unsaturated fractured rock of Yucca Mountain are enough to prevent seepage under present day infiltration conditions. It has also been thought that a substantially elevated infiltration flux will be required to cause seepage after the thermal period is over. While coupled thermal-hydrological-chemical (THC) changes in Yucca Mountain host rock due to repository heating has been previously investigated, those THC models did not incorporate elements of the seepage model. In this paper, we combine the THC processes in unsaturated fractured rock with the processes affecting seepage.We observe that the THC processes alter the hydrological properties of the fractured rock through mineral precipitation and dissolution. We show that such alteration in the hydrological properties of the rock often leads to local flow channeling. We conclude that such local flow channeling may result in seepage under certain conditions, even with nonelevated infiltration fluxes.

Disposal of High-Level Nuclear Waste in Deep Horizontal Drillholes

“…It also generates a pressure buildup that induces vapor flow and condensation, which are effective moisture and heat transfer mechanisms. Such coupled thermal-hydrological-geochemical processes have been studied for repository conditions during the thermal period, both experimentally and numerically [85][86][87]. Such analyses are very complicated, specifically because of the two-phase conditions induced by boiling, which make the resulting predictions of repository performance uncertain.…”

Modeling of thermally driven hydrological processes in partially saturated fractured rock

“…Research on thermally driven coupled processes in the rock has historically centered on the TH processes because they constitute the most prominent and direct impact on flow and transport. More recently, however, investigators have begun incorporating chemical [ Xu et al , 2001; Spycher et al , 2003; Xu et al , 2004; Sonnenthal et al , 2005; Mukhopadhyay et al , 2006; Xu et al , 2006] and mechanical [e.g., Rutqvist et al , 2002; Rutqvist and Tsang , 2003; Rutqvist et al , 2004, 2005] processes into the thermally driven hydrological processes, within a coupled modeling framework.…”

“…In short, the coupled thermal‐hydrologic‐chemical processes dynamically alter the hydrological properties of the rock. The feedback of the thermal‐chemical processes on the hydrologic properties of the unsaturated fractured host rock has been considered in recent studies [ Mukhopadhyay et al , 2006, 2007a, 2007b]. These studies show that the feedback of thermal‐chemical processes on fracture hydraulic properties can significantly impact the local flow channeling and amount of seepage.…”

“…The large set of data generated from the field thermal tests provided the unique opportunity for testing of coupled‐process models. Modeling that is not the subject of this review paper, incorporating coupled chemical [ Xu et al , 2001; Spycher et al , 2003; Sonnenthal et al , 2005; Mukhopadhyay et al , 2006] and mechanical [ Rutqvist et al , 2004, 2005] processes, was initiated in the late 1990s and progressed in earnest with the carrying out of the 8‐year‐long drift scale test.…”