Hydrogeologic Characterization of the U-3bl Collapse Zone

Services, NSTec Geotechnical

National Security Technologies, LLC

关键词: Low-Level Radioactive Wastes;    Radioactive Waste Management;    Testing;    Alluvium;    Tritium;   

DOI  :  10.2172/917719
RP-ID  :  DOE/NV/25946--042
RP-ID  :  DE-AC52-06NA25946
RP-ID  :  917719

美国|英语

来源: UNT Digital Library

【 摘 要 】

The U-3bl collapse crater was formed by an underground nuclear test in August 1962. This crater and the adjoining U-3ax crater were subsequently developed and used as a bulk low-level radioactive waste disposal cell (U-3ax/bl), which is part of the Area 3 Radioactive Waste Management Site at the Nevada Test Site (NTS). Various investigations have been conducted to assess the hydrogeologic characteristics and properties in the vicinity of the U-3ax/bl waste disposal cell. This report presents data from one of these investigations, conducted in 1996. Also included in this report is a review of pertinent nuclear testing records, which shows that the testing operations and hydrogeologic setting of the U-3ax/bl site were typical for the period and location of testing. Borehole U-3bl-D2 is a 45-degree-angle hole drilled from the edge of the crater under the waste cell to intercept the U-3bl collapse zone, the disturbed alluvium between the crater (surface collapse sink) and the nuclear test cavity. A casing-advance system with an air percussion hammer was used to drill the borehole, and air was used as the drilling fluid. Properties of the U-3bl crater collapse zone were determined from cores collected within the interval, 42.1 to 96.6 meters (138 to 317 feet) below the ground surface. Selected core samples were analyzed for particle density, particle size, bulk density, water retention, hydraulic conductivity, water content, water potential, chloride, carbonate, stable isotopes, and tritium. Physical and hydraulic properties were typical of alluvial valley sediments at the NTS. No visual evidence of preferential pathways for water transport was observed in the core samples. Soil parameters showed no trends with depth. Volumetric water content values ranged from 0.08 to 0.20 cubic meters per cubic meter, and tended to increase with depth. Water-retention relations were typical for soils of similar texture. Water potentials ranged from -1.9 MegaPascals at a depth of 42.10 meters (138 feet) to -0.4 MegaPascals at 94.58 meters (310 feet), generally increasing with depth. Relationships between hydraulic conductivity and water content were typical of sandy soil, with hydraulic conductivity decreasing rapidly as the soil dried. Variability of hydraulic conductivity reflected layering and showed no trend with depth. Stable isotope compositions were typical of water that had infiltrated during cooler past climate conditions. Uniformity of concentrations versus depth indicated that evaporation was not occurring at the sampled interval. Tritium concentrations in pore water ranged from 2.68 x10{sup 3} to 1.22 x 10{sup 4} picoCuries per liter, which are greater than expected from atmospheric deposition of tritium, but not at a level to raise environmental concerns. The tritium is most likely a product of nuclear testing.