HYROGEOLOGIC CHARACTERIZATION OF THE U-3at COLLAPSE ZONE

NEVADA, BECHTEL

BECHTEL NEVADA

关键词: Low-Level Radioactive Wastes;    Sensitivity;    Tritium;    Boreholes;    Hydraulics;   

DOI  :  10.2172/859949
RP-ID  :  DOE/NV/11718--199-REV 1
RP-ID  :  AC03-96NV11718
RP-ID  :  859949

美国|英语

来源: UNT Digital Library

【 摘 要 】

Hydrogeologic characteristics and properties of the U-3at collapse zone were determined from cores obtained from two angled boreholes drilled in 1996 under the subsidence crater formed by an underground nuclear test in 1963. This crater, used for disposal of bulk low-level radioactive waste since 1988, is located within the Area 3 Radioactive Waste Management Site in Yucca Flat on the Nevada Test Site in Nye County, Nevada. The purpose of this characterization effort was to provide data required to develop a conceptual hydrogeologic model and to complete flow and transport modeling required for the Area 3 performance assessment and composite analysis. To minimize disturbance of the core samples, drilling was done by a method that used only air as the drilling fluid. No evidence of preferential pathways for water flow was detected via visual geologic descriptions of the core samples. Laboratory analyses showed physical and hydraulic properties that are typical of alluvial valleys at the Nevada Test Site. Water content values ranged from 0.05 to 0.33 cubic meters per cubic meter, with water content tending to increase with depth. Water potential values ranged from -10.7 MegaPascals at a depth of 15.3 meters to greater than -0.40 MegaPascals at 78.9 meters. Tritium concentrations in pore water extracted from the cores were below the detection limits from the surface to a depth of 118.9 meters. Below this depth, concentrations generally increased to a maximum of 2,200 picoCuries per liter in the deepest sample at 146.9 meters. Unusually wet alluvium recovered from the drill bit during drilling between 50.9 and 71.6 meters in the first borehole raised concerns that water condensed in the compressed air system or excess oil introduced into the borehole may have penetrated core segments intended for analyses. The drilling process was modified to preclude introduction of fluids and the borehole was completed without further evidence of excess water. Using the same drilling method, the second borehole adjacent to the first was drilled and sampled over the interval from 47.5 to 78.3 meters. Analyses of samples from the second borehole showed water content values, water potential values, and stable isotope concentrations consistent with those of samples from the first borehole. This demonstrated that fluids had not been introduced into core segments intended for analyses during drilling of the first borehole, and thus the elevated water values are considered to be natural.