GEOCHEMISTRY OF DUST IN THE PROPOSED NUCLEAR WASTE REPOSITORY AT YUCCA MOUNTAIN, NEVADA

Dust eventually will contact waste canisters in the proposed repository at Yucca Mountain, and its potential for facilitating corrosion must be evaluated. Samples of construction dust were collected from the Exploratory Studies Facility, an 8-km-long tunnel adjacent to the repository block, and from the Enhanced Characterization of the Repository Block, a 2.7-km-long drift across the repository block. Bulk samples and size fractions of dust were analyzed by conventional rock-analysis methods for major and trace elements, and water leachates were analyzed for soluble anions and cations. About 95-98 percent of the dust is finely ground rock generated during tunnel construction. The water-soluble salts, which are only about 0.4 percent of the total mass, contain Ca²⁺, Mg²⁺, Na⁺, K⁺, NH₄⁺, Cl^-, Br^-, F^-, HCO3^-, NO₃^-, SO₄^2-, and PO₄^3-. These salts originate from evaporation of construction water and pore water at tunnel walls. Specific soluble salt minerals have not yet been identified, but normative minerals include calcite and dolomite; nitrates of potassium, sodium, and ammonia; and sulfates, such as glauberite and thenardite. The ratio of NO₃^- to Cl^- is important for determining if calcium chloride, a potentially corrosive compound, could form from solution and reprecipitation of these salts. A molar ratio greater than 0.5 is considered to be favorable for suppressing the formation of calcium chloride (Carl Steefel, Lawrence Berkeley National Laboratory, written commun., May 19, 2004). Mean NO₃^-/Cl^- ratios of soluble salts from various size fractions of dust range from 1.0±0.2 to 1.8±0.5 (uncertainties are standard errors of the mean.). After emplacement, atmospheric dust may enter the proposed repository and contact the waste canisters. The mean soluble-salt content of atmospheric dust samples around Yucca Mountain is13.3±1.0 weight percent (Marith Reheis’s USGS Open-File Report 03-138) with a mean NO₃^-/Cl^-ratio of 5.6±1.1 (unpublished data from John Izbicki, U.S. Geological Survey, April 23, 2004). Thus, both construction dust and atmospheric dust have benign NO₃^-/Cl^- ratios that may mediate the lower average ratio of 0.10±0.02 in salts derived from pore-water seepage.