Isotope-Geochmical Evidence For Uranium Retardation in Zeolitized Tuffs at Yucca Mountain, Nevada, USA
Retardation of radionuclides by sorption on minerals in the rocks along downgradient groundwater flow paths is a positive attribute of the natural barrier at Yucca Mountain, Nevada, the site of a proposed high-level nuclear waste repository. Alteration of volcanic glass in nonwelded tuffs beneath the proposed repository horizon produced thick, widespread zones of zeolite- and clay-rich rocks with high sorptive capacities. The high sorptive capacity of these rocks is enhanced by the large surface area of tabular to fibrous mineral forms, which is about 10 times larger in zeolitic tuffs than in devitrified tuffs and about 30 times larger than in vitric tuffs. The alteration of glass to zeolites, however, was accompanied by expansion that reduced the matrix porosity and permeability. Because water would then flow mainly through fractures, the overall effectiveness of radionuclide retardation in the zeolitized matrix actually may be decreased relative to unaltered vitric tuff. Isotope ratios in the decay chain of {sup 238}U are sensitive indicators of long-term water-rock interaction. In systems older than about 1 m.y. that remain closed to mass transfer, decay products of {sup 238}U are in secular radioactive equilibrium where {sup 234}U/{sup 238}U activity ratios (AR) are unity. However, water-rock interaction along flow paths may result in radioactive disequilibrium in both the water and the rock, the degree of which depends on water flux, rock dissolution rates, {alpha}-recoil processes, adsorption and desorption, and the precipitation of secondary minerals. The effects of long-term water-rock interaction that may cause radionuclide retardation were measured in samples of Miocene-age subrepository zeolitized tuffs of the Calico Hills Formation (Tac) and the Prow Pass Tuff (Tcp) from borehole USW SD-9 near the northern part of the proposed repository area (sampled depth interval from 451.1 to 633.7 m; Engstrom and Rautman, 1996). Mineral abundances and whole-rock chemical and U-series isotopic compositions were measured in unfractured core samples representing rock matrix, in rubble (about 1 cm) rock fragments representing zones of higher permeability (assuming that the rubble core indicates a broken zone in the rock mass rather than an artifact of drilling), and in samples from surfaces of natural fractures representing potential fracture pathways. U concentrations and isotopic compositions also were measured in samples of pore water obtained by ultracentrifugation or by leaching rock samples with deionized H{sub 2}O. The concentrations and isotopic compositions of loosely bound U adsorbed on reactive mineral surfaces were obtained by analyzing 1 M sodium acetate (NaOAc) leachates of whole-rock samples.
- Research Organization:
- Yucca Mountain Project, Las Vegas, NV (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- NA
- OSTI ID:
- 899953
- Report Number(s):
- NA; MOL.20070214.0087, DC# 47631; TRN: US0702333
- Country of Publication:
- United States
- Language:
- English
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