G. RUSKAUFF and B. CROWE, Navarro-Intera, LLC, USA and S. DRELLACK, National Security

Technologies, LLC, USA

DOI: 10.1533/9780857097446.3.801

Abstract: This chapter outlines the hydrogeological setting of the Nevada National Security Site (NNSS) and the expected pathways of groundwater flow and radionuclide transport. It describes the evolving strategy developed cooperatively between the National Nuclear Security Administration Nevada Site Office (NNSA/NSO) and the Nevada Division of Environment Protection (NDEP) to assess groundwater contamination from underground testing of nuclear weapons and to protect the health and safety of the public. The modeling challenges and progress in the Underground Test Area Project (UGTA) are also discussed.

Key words: radionuclide contamination, groundwater, flow and transport model, regulatory strategy.

26.1 Introduction

The Underground Test Area Project (UGTA) of the US Department of Energy (DOE), National Nuclear Security Administration Nevada Site Office (NNSA/NSO) is implementing remediation strategies for protecting the health and safety of the public and the environment from radioactive contamination of groundwater produced during past underground testing of nuclear weapons at the Nevada National Security Site (NNSS; formerly called the Nevada Test Site). The NNSS was chosen as the continental site for testing nuclear weapons in 1950 because of the sparse population in the arid southwest region of the United States, the availability of nearby facili­ties for operational support, and to reduce the cost and logistical difficulties of testing in the western Pacific (US Department of Energy (DOE), 2000a). The first atmospheric tests were conducted in 1951 and the NNSS subse­quently became the primary site for testing nuclear weapons. Following the Limited Test Ban Treaty of 1963, atmospheric testing ceased, and nearly 90 percent of the underground weapons tests by the United States were deto­nated at the NNSS (USDOE, 2000a). Congress imposed a moratorium on

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testing of nuclear weapons, and in September of 1992, underground testing ceased.

The NNSS continues to be used for national defense activities and is a major remediation site for the DOE Environmental Management mission of cleanup of the environmental legacy from nuclear weapons and nuclear energy research. The Environmental Restoration Project was established in 1989 for evaluating and remediating contaminated sites on the NNSS and other areas of the state of Nevada. The UGTA under the Environmental Restoration Project is tasked with assessing contaminated groundwater from underground testing. The NNSA/NSO also operates and maintains two facilities located in alluvial basins of the NNSS that dispose of low-level radioactive waste (RAW) and mixed low-level radioactive waste. The RAW is from cleanup activities on the NNSS and from cleanup activities at multiple remediation sites across the DOE complex (nationwide). The RAW is buried in shallow trenches, pits, subsidence craters created by underground testing of nuclear weapons and large-diameter boreholes (greater confinement boreholes) (Shott et al., 1998, 2000; Crowe et al., 2002,2005; USDOE, 2005).

The UGTA is evaluating 907 underground nuclear detonations that were conducted at the NNSS; all underground tests are listed in a compendium of weapons tests conducted by the United States from July 1945 through September 1992 (USDOE, 2000b). The NNSS tests were conducted above, near and below the groundwater table in alluvial basins, in volcanic high­lands, in shafts and tunnels of zeolitized volcanic rocks, and in tunnels mined in granitic rock.

The phenomenology of underground nuclear explosions is summarized in Borg et al. (1976), US Congress Office of Technology Assessment (1989), and the International Atomic Energy Agency (IAEA, 1998). An under­ground test produces a spherical cavity from combined vaporization, melting and shock compression of the host rock. As the detonation pressure subsides, the rocks above the cavity typically collapse (timeframe of seconds to days after the test) and the cavity is filled with rubble consisting of col­lapsed rock, and solidified rock melt (melt glass). The collapse void can propagate upward variable distances forming a chimney that may or may not extend to the surface forming a subsidence crater. The temperature and pressure history of an explosion and response of the surrounding host rock control the distribution of radionuclides around the test. Radionuclides produced underground include tritium, fission products, actinides and acti­vation products. Refractory radionuclides (higher boiling points) are trapped primarily in the melt glass, and in cavity rubble and compressed rock around the cavity (up to 1.5 cavity radii from the test point); volatile species circulate outward and condense in cracks and void spaces for dis­tances of 1-3 cavity radii from the test point (Tompson et al., 1999;Tompson, 2008 ; Pawloski et al., 2008 ).

The radionuclides deposited underground from detonation of a nuclear device are referred to as the radiological source term; the portion of the inventory that is migrating in groundwater is the hydrological source term, a subset of the radiological source term. Underground testing on the NNSS deposited an estimated 132 million curies of radioactivity below ground, decay corrected to 1992 (the radiological source of Bowen et al, 2001). Unclassified estimates of this radiological inventory are apportioned among 43 radionuclides and these radionuclides define the source term used in the modeling studies.

Important features of the NNSS with respect to radionuclide contamina­tion of groundwater are the considerable depth from the surface to ground­water throughout most of the site and the absence of natural springs or surface areas of groundwater discharge on the NNSS which would allow radioactive contaminants to be released in the environment. Accordingly, there are no immediate hazards to workers or the public from exposure to contaminated groundwater. The challenges facing the UGTA are to under­stand the physical and chemical processes of migration of radionuclides within and adjacent to the NNSS, to forecast migration of radionuclides over 1,000 years, and to support regulatory decisions to protect the public. The approach used to address these challenges is a combination of data collection and development of numerical models of groundwater flow and radionuclide transport, model evaluation to test and build confidence in model results sufficient to design a long-term monitoring network, and identification of institutional control policies to restrict public access to contaminated groundwater.

The goals for this chapter are:

• to describe the hydrogeological setting of the NNSS and the expected pathways of groundwater flow and radionuclide transport,

• to describe the evolving strategy developed cooperatively between the NNSA/NSO and the Nevada Division of Environment Protection (NDEP) to assess groundwater contamination from underground testing of nuclear weapons and to protect the health and safety of the public,

• to describe the modeling challenges and progress in UGTA.