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Title: Testing the Concept of Drift Shadow at Yucca Mountain, Nevada

If proven, the concept of drift shadow, a zone of reduced water content and slower ground-water travel time beneath openings in fractured rock of the unsaturated zone, may increase performance of a proposed geologic repository for high-level radioactive waste at Yucca Mountain. To test this concept under natural-flow conditions present in the proposed repository horizon, isotopes within the uranium-series decay chain (uranium-238, uranium-234, and thorium-230, or {sup 238}U-{sup 234}U-{sup 230}Th) have been analyzed in samples of rock from beneath four naturally occurring lithophysal cavities. All samples show {sup 234}U depletion relative to parent {sup 238}U, indicating varying degrees of water-rock interaction over the past million years. Variations in {sup 234}U/{sup 238}U activity ratios indicate that depletion of {sup 234}U relative to {sup 238}U can be either smaller or greater in rock beneath cavity floors relative to rock near cavity margins. These results are consistent with the concept of drift shadow and with numerical simulations of meter-scale spherical cavities in fractured tuff. Differences in distribution patterns of {sup 234}U/{sup 238}U activity ratios in rock beneath the cavity floors are interpreted to reflect differences in the amount of past seepage into lithophysal cavities, as indicated by the abundance of secondary mineral depositsmore » present on the cavity floors.« less
Authors:
; ; ;
Publication Date:
OSTI Identifier:
893814
Report Number(s):
NA
MOL.20060425.0176, DC# 47398; TRN: US0606128
DOE Contract Number:
NA
Resource Type:
Technical Report
Research Org:
Yucca Mountain Project, Las Vegas, Nevada
Sponsoring Org:
USDOE
Country of Publication:
United States
Language:
English
Subject:
12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; ABUNDANCE; CAVITIES; DECAY; DISTRIBUTION; FLOORS; GROUND WATER; HIGH-LEVEL RADIOACTIVE WASTES; OPENINGS; PERFORMANCE; TESTING; THORIUM 230; TUFF; URANIUM 234; URANIUM 238; YUCCA MOUNTAIN