This memo report summarizes our current knowledge of the appearance of melt glass formed and subsequently deposited in the subsurface after an underground nuclear test. We have collected archived pictures and melt glass samples from a variety of underground nuclear tests that were conducted at the Nevada Test Site (NTS) during the U.S. nuclear testing program. The purpose of our work is to better determine the actual variation in texture and surface area of the melt glass material. This study is motivated by our need to better determine the rate at which the radionuclides incorporated in the melt glass are released into the subsurface under saturated and partially saturated conditions. The rate at which radionuclides are released from the glass is controlled by the dissolution rate of the glass. Glass dissolution, in turn, is a strong function of surface area, glass composition, water temperature and water chemistry (Bourcier, 1994). This work feeds into an ongoing experimental effort to measure the change in surface area of analog glasses as a function of dissolution rate. The conclusions drawn from this study help bound the variation in the textures of analog glass samples needed for the experimental studies. The experimental work is a collaboration between Desert Research Institute (DRI) and Earth and Environmental Sciences-Lawrence Livermore National Laboratory (EES-LLNL). On March 4, 1999 we hosted a meeting at LLNL to present and discuss our findings. The names of the attendees appear at the end of this memo. This memo report further serves to outline and summarize the conclusions drawn from our meeting. The United States detonated over 800 underground nuclear tests at the NTS between 1951 and 1992. In an effort to evaluate the performance of the nuclear tests, drill-back operations were carried out to retrieve samples of rock in the vicinity of the nuclear test. Drill-back samples were sent to Los Alamos National Laboratory (LANL) and Lawrence Livermore National Laboratory (LLNL) and analyzed for diagnostic purposes. As a result of these activities, a body of knowledge consisting of personal accounts, photos, reports and archived solid samples was gained regarding the physical nature of the melt glass that formed during an underground nuclear test. In this memo report, we summarize previously published reports, compile archived photos, document and describe melt glass samples and summarized discussions from former field engineers and radiochemists who had direct knowledge of drill-back samples. All the information presented in the report was gathered from unclassified sources. We have included as wide a variation of samples as we could document. Unfortunately, as part of the drill-back and diagnostic efforts, it was not common practice to photograph or physically describe the material returned to the surface.

For the last several years, the Underground Test Area (UGTA) program has funded a series of studies carried out by scientists to investigate the role of colloids in facilitating the transport of low-solubility radionuclides in groundwater, specifically plutonium (Pu). Although the studies were carried out independently, the overarching goals of these studies has been to determine if colloids in groundwater at the NTS can and will transport low-solubility radionuclides such as Pu, define the geochemical mechanisms under which this may or may not occur, determine the hydrologic parameters that may or may not enhance transport through fractures and provide recommendations for incorporating this information into future modeling efforts. The initial motivation for this work came from the observation in 1997 and 1998 by scientists from Lawrence Livermore National Laboratory (LLNL) and Los Alamos National Laboratory (LANL) that low levels of Pu originally from the Benham underground nuclear test were detected in groundwater from two different aquifers collected from wells 1.3 km downgradient (Kersting et al., 1999). Greater than 90% of the Pu and other radionuclides were associated with the naturally occurring colloidal fraction (

In the Yucca Flat basin of the Nevada Test Site (NTS), 747 shaft and tunnel nuclear detonations were conducted primarily within the tuff confining unit (TCU) or the overlying alluvium. The TCU in the Yucca Flat basin is hypothesized to reduce radionuclide migration to the regional carbonate aquifer (lower carbonate aquifer) due to its wide-spread aerial extent and chemical reactivity. However, shortcuts through the TCU by way of fractures may provide a migration path for radionuclides to the lower carbonate aquifer (LCA). It is, therefore, imperative to understand how radionuclides migrate or are retarded in TCU fractures. Furthermore, understanding the migration behavior of radionuclides once they reach the fractured LCA is important for predicting contaminant transport within the regional aquifer. The work presented in this report includes: (1) information on the radionuclide reactive transport through Yucca Flat TCU fractures (likely to be the primary conduit to the LCA), (2) information on the reactive transport of radionuclides through LCA fractures and (3) data needed to calibrate the fracture flow conceptualization of predictive models. The predictive models are used to define the extent of contamination for the Underground Test Area (UGTA) project. Because of the complex nature of reactive transport in fractures, a stepwise approach to identifying mechanisms controlling radionuclide transport was used. In the first set of TCU experiments, radionuclide transport through simple synthetic parallel-plate fractured tuff cores was examined. In the second, naturally fractured TCU cores were used. For the fractured LCA experiments, both parallel-plate and rough-walled fracture transport experiments were conducted to evaluate how fracture topography affects radionuclide transport. Tuff cores were prepared from archived UE-7az and UE-7ba core obtained from the USGS core library, Mercury, Nevada. Carbonate cores were prepared from archived ER-6-1 core, also obtained from the USGS core library, Mercury, Nevada.

Colloids are small particulates (ranging in size from 1 to 0.001 micron) composed of inorganic and organic material and found in all natural water. Due to their small size, they have the ability to remain suspended in water and transported. Small amounts of plutonium (Pu) and americium (Am) can adsorb (attach) to colloids, and/or form colloidal-sized polymers and migrate in water. At Rocky Flats Environmental Technology Site (RFETS) sedimentation and resuspension of particulates and colloids in surface waters represent the dominant process for Pu and Am migration. The amount of Pu and Am that can be transported at RFETS has been quantified in the Pathway Analysis Report. The Pathway Analysis Report shows that the two dominant pathways for Pu and Am transport at RFETS are air and surface water. Shallow groundwater and biological pathways are minor.

This report contains highlights of FY 2001 and 2002 technical studies conducted by the Analytical and Nuclear Chemistry Division (ANCD) at Lawrence Livermore National Laboratory (LLNL) in support of the Hydrologic Resources Management Program (HRMP) and the Underground Test Area (UGTA) Project. These programs are administered by the U.S. Department of Energy, National Nuclear Security Administration, Nevada Site Office (NNSA/NSO) through the Defense Programs and Environmental Restoration Divisions, respectively. HRMP-sponsored work emphasizes the Defense Programs goal of responsible management of natural resources at the NTS, while UGTA-funded work focuses on defining the extent of radionuclide contamination in NTS groundwater resulting from underground nuclear testing. The report is organized on a topical basis, and contains eight chapters that reflect the range of technical work performed by LLNL-ANCD in support of HRMP and UGTA. Chapter 1 describes recent hot well sampling efforts at the NTS, and presents the results of chemical and isotopic analyses of groundwater samples from six near-field wells. These include the Cambric (UE-5n), Bilby (U-3cn PS No. 2), Bourbon (UE-7nS), Nash (UE-2ce), Tybo/Benham (ER-20-5 No. 3), and Almendro (U-19v PS No. 1ds) sites. The data generated by the hot well program is vital to the development and validation of contaminant transport models at the NTS. Chapter 2 discusses the results of xenon isotope measurements of groundwater samples from the six near-field wells described in Chapter 1. This work demonstrates that fission xenon is present in the water at levels that are readily measurable and highlights the significant differences in xenon concentrations and isotopic abundances at different sites. These differences provide insight into the early cooling history of nuclear test cavities, and may assist in predicting the distribution of the source term in the near-field environment. Chapter 3 is an investigation of the distribution and abundance of actinides in a nuclear test cavity and chimney. This work demonstrates that early-time processes can widely disperse actinides at low concentrations outside the melt glass, implying that melt glass dissolution may not be the sole mechanism for the release of actinides to groundwater. The study also provides evidence for the isotopic fractionation of plutonium under the extreme conditions accompanying nuclear explosions. In Chapter 4, X-ray absorption spectroscopy measurements were used to determine the redox state of Fe and U in nuclear melt glass samples from the NTS. Both elements were found to occur in mixed valence states (Fe{sup 2+}/Fe{sup 3+} and U{sup 5+}/U{sup 6+}) in all samples. Comparison of the Fe and U redox states with published redox studies of synthetic glasses suggests that plutonium is predominantly in the Pu{sup 4+} oxidation state in the melt glasses. In Chapter 5, alpha autoradiography is used in a NTS field study to investigate the spatial distribution and transport of actinides in soils, and to help identify the size distribution and morphology of the actinide particles. It was found that {alpha}-emitting radionuclides have moved to at least 39 cm depth in the soil profile, far deeper than expected. The methodology that was developed could easily be applied to other field locations where actinides are dispersed in the soil zone. Chapter 6 summarizes the development of a method for measuring environmental levels of {sup 241}Am on the multi-collector inductively coupled plasma mass spectrometer. The method detection limit of 0.017 pCi/L is about two times lower than the best analyses possible by alpha spectrometry. Chapter 7 describes a chlorine-36 study of vertical groundwater transport processes in Frenchman Flat. Mass balance calculations developed from a {sup 36}Cl mixing model at well ER-5-3 No. 2 are used to estimate vertical transport fluxes and average vertical flow velocities through the thick volcanic section underlying the basin. The study also documents the variations in {sup 36}Cl/Cl ratios within the three principal hydrostratigraphic units in Frenchman Flat. Chapter 8 discusses an ongoing stable isotope investigation of precipitation and recharge processes in central Nevada. Precipitation, spring water, and shallow infiltration samples have been collected at four locations on a biannual basis since 1999. The results show that winter precipitation accounts for>90% of the recharge at these sites. Lysimeter data suggest that most of the evaporation occurring during recharge is due to water vapor loss through the soil zone during periods of slow infiltration. In addition to the topical investigations described above, LLNL-ANCD contributed to several other major collaborative technical products during FY 2001 and 2002.

The purpose of this report is to assess the decay and in-growth of radionuclides from the radionuclide source term (RST) deposited by underground nuclear weapons tests conducted at the NTS from 1951 through 1992. A priority of the Underground Test Area (UGTA) project, administered by the Environmental Restoration Division of NNSA/NV, was to determine as accurately as possible a measure of the total radionuclide inventory for calculation of the RST deposited in the subsurface at the Nevada Test Site (NTS). The motivation for the development of a total radionuclide inventory is driven by a need to calculate the amount of radioactivity that will move away from the nuclear test cavities over time, referred to as the hydrologic source term (HST). The HST is a subset of the RST and must be calculated using knowledge of the geochemistry and hydrology of the subsurface environment. This will serve the regulatory process designed to protect human health from exposures to contaminated groundwater. Following the detonation of an underground nuclear test, and depending on the presence of water at the location of the detonation, the residual radionuclides may be found in aqueous or gaseous states, precipitated or chemically sorbed states, or incorporated in melt glass produced by the nuclear test. The decay and in-growth of radionuclides may have geochemical implications for the migration of radionuclides away from underground nuclear test cavities. For example, in the case of a long-lived mobile parent decaying to a shorter-lived and less mobile daughter, the geochemical properties of the parent element may control the migration potential of the daughter nuclide. It becomes important to understand the evolution of the RST in terms of effects on the mobility, solubility, or abundance of radionuclides in the HST that are created by decay and in-growth processes. The total radionuclide inventory and thus the RST changes with time due to radioactive decay. The abundance of a specific radionuclide at any given time is a function of the initial amount of radioactivity, the decay rate and in-growth from parent radionuclides. The in-growth of radioactivity is the additional amount of radioactivity for a given radionuclide that comes from the decay of the parent isotopes. In this report, decay and in-growth of radionuclides from the RST are evaluated over the 1000-year time frame in order to determine whether coupled in-growth and decay affect the relative abundance of any RST radionuclide. In addition, it is also necessary to identify whether any new derivative radionuclides not initially produced by the nuclear test but exist now as a result of in-growth from a parent radionuclide One of the major goals of this report is to simplify the transport modeler's task by pointing out where in-growth is unimportant and where it needs to be considered. The specific goals of this document are to evaluate radionuclide decay chains and provide specific recommendations for incorporating radionuclide daughters of concern in the calculation of the radionuclide inventory.