The United States Department of Energy, Nevada Operations Office (DOE/NV) has submitted to the Nevada Division of Environmental Protection (NDEP) a Closure Plan (CLOSURE PLAN FOR CORRECTIVE ACTION UNIT 110: U-3AX/BL DISPOSAL UNIT, NEVADA TEST SITE, NEVADA) in August 2000 pursuant to the requirements of their Resource Conservation and Recovery Act (RCRA) Permit #HW009, Subpart G & K of Part 265 of the Code of Federal Regulations (40 CFR 265) and Nevada Administrative Code (NAC) 445A.227, for the closure of the U-3ax/bl Disposal Unit in Area 3 of the Nevada Test Site (NTS).

     Closure by installation of a vegetative cover has been proposed to isolate the radiological and hazardous waste in the disposal unit from the environment. The U-3ax/bl Disposal Unit is located in Area 3 of the Nevada Test Site, Nye County, Nevada. The disposal unit was created by excavating the soil separating two adjacent underground nuclear test craters, U-3ax formed in May 1962 and U-3bl formed in August 1962. The U-3ax/bl disposal unit has a surface area of 2.9 hectares (7.5 acres) with a length of 267 meters (868 feet) and a width of 135 meters (439 feet). The craters had a maximum depth of 14 meters (45 feet) below ground surface.

The U-3ax/bl Waste Unit was filled between July 1968 and December 1987 with 2.3 x 10⁵ cubic meters (8.12 x 10⁶ cubic feet) of waste. NTS nuclear weapons testing generated approximately 95 percent of the total volume disposed in U-3ax/bl, the majority of which came from the Waste Consolidation Project (80 percent of the total volume) consisting of construction material and debris generated as a result of above ground nuclear testing. After the U-3ax/bl Disposal Unit was officially closed in December 1987, a 3 meter (10 foot) operational cover was installed to fill the U-3ax/bl void space to surrounding surface grade elevation.

Based on waste acceptance records, a total radiological activity of 6.6 x 10³ Curies was disposed in the U-3ax/bl Disposal Unit. The remaining radiological activity has been estimated to be 1.3 X 10³ Curies. The activity was 90 percent tritium (3H), 5 percent cesium (137Cs), and 4 percent strontium (90Sr). The only documented hazardous constituents were lead and cadmium associated with shielding and neutron measuring instrumentation. Subsurface characterization samples collected from under the U-3ax/bl Disposal Unit did not detect any hazardous components. Radiological constituents detected are associated with the underground weapons tests that formed the U-3ax and U-3bl craters.

A vegetative cover meets the criteria of a RCRA equivalent cover. A vegetative cover protects the waste unit by using natural processes (plant transpiration) to remove moisture from the cover. Evaporation and plant root uptake removes moisture from the cover before it can migrate into the waste zone. With no moisture entering the waste zone, there is no mechanism to transport waste outside of the waste cell.

Closure activities will include the installation of four Time-Domain Reflectometry (TDR) profile arrays in the operational cover to measure the soil moisture content; installation of an additional 0.3 meters (1 foot) of soil on top of the operational cover to facilitate plant rooting; installation of subsidence monitoring stations; planting native vegetation typical to Area 3, across the U-3ax/bl Disposal Unit cover; and installation of fencing and warning signs around the perimeter of the U-3ax/bl site.

Upon completion of the installation of the U-3ax/bl vegetative cover, post-closure monitoring activities will be initiated. The following post-closure monitoring activities will be conducted: evaluating vegetation growth, establishing background cover moisture levels (using the TDR monitoring system), and conducting elevation surveys to detect cover subsidence.

The Closure Plan will be available for public review and comment for a period of thirty (30) days beginning September 6, 2000 and ending October 6, 2000 at the following areas:

A public hearing will be held should the Administrator determine there is significant public interest. Should a public hearing be scheduled, the NDEP will provide thirty days (30 days) notice before the hearing.

For further information, please contact Matthew DeBurle at the Nevada Division of Environmental Protection (NDEP) at 775-687-4670 ext. 3031. Written comments or requests for more information may also be sent to the NDEP's Carson City address.

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