EFFECTS OF TEMPERATURE ON THE COMPOSITION OF SOLUBLE SALTS IN DUST AT YUCCA MOUNTAIN, NEVADA

Yucca Mountain in southern Nevada is the proposed site for a high-level nuclear waste repository. As part of the site studies, an underground heater test (Drift Scale Test or DST) was conducted in an alcove of the Exploratory Studies Facility (ESF) to assess the effect of temperature changes on the near-field environment of the proposed repository, including possible changes in composition of soluble salts in dust. Samples of dust collected from the DST alcove were leached with deionized water for 1 hour at a weight ratio of 20:1 water to dust. For comparison, samples of dust collected from the main tunnel of the ESF and atmospheric dust samples collected from a cyclone dust collector located near the south portal of the ESF also were leached using the same procedure. The water-soluble leachates were analyzed by ion chromatography and inductively coupled plasma mass spectrometry for major anions and cations. In addition to the general chemistry of the water-soluble salts, the weight ratio of nitrate (NO₃^-) to chloride (Cl^-) in the water-soluble salts is of interest because an excess of NO₃^- to Cl^- may inhibit the formation of potentially corrosive calcium chloride brines. ESF dust leachate compositions were variable but were predominately calcium-bicarbonate-sulfate types. NO₃^- to Cl^- ratios ranged from 0.82 to 5.17 with an average of 2.38±1.22 (1σ). DST dust leachate compositions were a calcium-sulfate-bicarbonate type. Compared to the ESF leachates, the DST leachates had higher Cl^- concentrations and lower NO₃^- concentrations with an average NO₃^- to Cl^- ratio of 0.11±0.02. In contrast to the dust leachates from the ESF and the DST, atmospheric dust leachates were a calcium-sodium-nitrate type with an average NO₃^- to Cl^- ratio of 11±3. To determine if the elevated temperature in the DST contributed to the low NO₃^- to Cl^- ratio, two samples of ESF dust were heated in an oven at 150ºC for 72 hours and then leached using the standard procedures. In both samples, the NO₃^- concentration in the leachates decreased by about 20% to 30% and the Cl^- concentration increased by about 240% relative to the unheated samples, indicating a possible relation between temperature and the composition of soluble salts in the dust samples from the DST.