Fluid flow and reactive transport around potential nuclear waste emplacement tunnels at Yucca Mountain, Nevada

Abstract

The evolution of fluid chemistry and mineral alteration around a potential waste emplacement tunnel (drift) is evaluated using numerical modeling. The model considers the flow of water, gas, and heat, plus reactions between minerals, CO 2 gas, and aqueous species, and porositypermeability-capillary pressure coupling for a dual permeability (fractures and matrix) medium.Two possible operating temperature modes are investigated: a "high-temperature" case with temperatures exceeding the boiling point of water for several hundred years, and a "low-temperature" case with temperatures remaining below boiling for the entire life of the repository. In both cases, possible seepage waters are characterized by dilute to moderate salinities and mildly alkaline pH values. These trends in fluid composition and mineral alteration are controlled by various coupled mechanisms. For example, upon heating and boiling, CO 2 exsolution from pore waters raises pH and causes calcite precipitation. In condensation zones, this CO 2 redissolves, resulting in a decrease in pH that causes calcite dissolution and enhances feldspar alteration to clays. Heat also enhances dissolution of wallrock minerals leading to elevated silica concentrations. Amorphous silica precipitates through evaporative concentration caused by boiling in the high-temperature case, but does not precipitate in the low-temperature case. Some alteration of feldspars to clays and zeolites is predicted in the high-temperature case. In both cases, calcite precipitates when percolating waters are heated near the drift. The predicted porosity decrease 2 around drifts in the high-temperature case (several percent of the fracture volume) is larger by at least one order of magnitude than in the low temperature case. Although there are important differences between the two investigated temperature modes in the predicted evolution of fluid compositions and mineral alteration around drifts, these differences are small relative to the model uncertainty and the variability of water compositions at Yucca Mountain.

TOUGHREACT User's Guide: A Simulation Program for Non-isothermal Multiphase Reactive Geochemical Transport in Variably Saturated Geologic Media, V1.2.1

“…TOUGHREACT was further enhanced with the addition of (1) treatment of mineral-water-gas reactive-transport under boiling conditions, (2) an improved HKF activity model for aqueous species, (3) gas species diffusion coefficients calculated as a function of pressure, temperature, and molecular properties, (4) mineral reactive surface area formulations for fractured and porous media, and (5) porosity, permeability, and capillary pressure changes owing to mineral precipitation/dissolution (Sonnenthal et al, , 2000(Sonnenthal et al, , 2001Spycher et al, 2003a).…”

A Novel Approach to Experimental Studies of Mineral Dissolution Kinetics

“…Establishing reliable rates for low-temperature feldspar dissolution is essential to quantify many basic geological and environmental processes. Among these are the functional relationship between silicate weathering and the global climate over geologic time (Berner and Berner, 1997), controls on surface and groundwater quality, global elemental cycling (Lasaga et al, 1994), the availability of inorganic nutrients in soils (Federer et al, 1989;Likens et al, 1998), impacts of acid mine drainage, neutralization of acid precipitation in watersheds , safety of nuclear waste repositories (Spycher et al, 2003), and geological carbon sequestration .…”

A bio-chemo-hydro-mechanical model for microbially induced calcite precipitation in soils

“…The first reactive transport studies were published in the mid-eighties by Lichtner [35]. A number of reactive transport models were later proposed [36][37][38][39].…”