Article Cite This: Environ. Sci. Technol. 2019, 53, 7363−7370
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Hydrothermal Alteration of Nuclear Melt Glass, Colloid Formation, and Plutonium Mobilization at the Nevada National Security Site, U.S.A. Mavrik Zavarin,* Pihong Zhao, Claudia Joseph,† James D. Begg, Mark A. Boggs,‡ Zurong Dai, and Annie B. Kersting
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Glenn T. Seaborg Institute, Physical & Life Sciences, Lawrence Livermore National Laboratory, Livermore, California 94550, United States S Supporting Information *
ABSTRACT: Approximately 2.8 t of plutonium (Pu) has been deposited in the Nevada National Security Site (NNSS) subsurface as a result of underground nuclear testing. Most of this Pu is sequestered in nuclear melt glass. However, Pu migration has been observed and attributed to colloid facilitated transport. To identify the mechanisms controlling Pu mobilization, longterm (∼3 year) laboratory nuclear melt glass alteration experiments were performed at 25 to 200 °C to mimic hydrothermal conditions in the vicinity of underground nuclear tests. The clay and zeolite colloids produced in these experiments are similar to those identified in NNSS groundwater. At 200 °C, maximum Pu and colloid concentrations of 30 Bq/L and 150 mg/L, respectively, were observed. However, much lower Pu and colloid concentrations were observed at 25 and 80 °C. These data suggest that Pu concentrations above the drinking water Maximum Contaminant Levels (0.56 Bq/L) may exist during early hydrothermal conditions in the vicinity of underground nuclear tests. However, formation of colloid-associated Pu will tend to decrease with time as nuclear test cavity temperatures decrease. Furthermore, median colloid concentrations in NNSS groundwater (1.8 mg/ L) suggest that the high colloid and Pu concentrations observed in our 140 and 200 °C experiments are unlikely to persist in downgradient NNSS groundwater. While our experiments did not span all groundwater and nuclear melt glass conditions that may be present at the NNSS, our results are consistent with the documented low Pu concentrations in NNSS groundwater.
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INTRODUCTION
Approximately one-third of the underground nuclear tests at the NNSS were conducted near or below the groundwater table. Nuclear melt glass is subject to dissolution and radionuclide release in the presence of groundwater. The factors affecting rhyolitic glass dissolution rates have been studied extensively and include pH, silica content, temperature, and surface area.4,5 Importantly, the temperature dependence of glass dissolution rates is exponential,4 suggesting that the high temperatures that exist in the first years after a nuclear detonation may substantially exacerbate the rate of nuclear melt glass dissolution and Pu release. Underground nuclear test-related thermal anomalies may exist for 25 years or more.6 The temperature history will depend on the size of the underground nuclear test, its depth of burial, the permeability of the surrounding rock, and the groundwater velocities through the nuclear melt glass. For example, Carle et
The testing of nuclear weapons at the Nevada National Security Site (NNSS), formerly the Nevada Test Site, U.S.A., has led to the deposition of substantial quantities of plutonium (Pu) into the environment. While tritium (3H) is the most abundant anthropogenic radionuclide deposited in the NNSS subsurface from an activity standpoint (4.7 × 106 TBq decay corrected to September 23, 1992), Pu is the most abundant anthropogenic element by mass. Between 1951 and 1992, 828 underground nuclear tests were performed and approximately 2.8 t (3.1 × 104 TBq) of Pu remains in the NNSS subsurface.1 The extreme temperatures associated with underground nuclear tests and the refractory nature of Pu results in most of the Pu (98%) being sequestered in melted rock, referred to as nuclear melt glass.2 The composition of nuclear melt glass is closely related to the composition of the host rock in which the test was conducted. The majority of NNSS tests were conducted in rhyolitic rock. Under those conditions, the melt glass composition will be dominated by SiO2 (∼75 wt %) and Al2O3 (∼15 wt %).3 © 2019 American Chemical Society
Received: Revised: Accepted: Published: 7363
December 20, 2018 June 11, 2019 June 13, 2019 June 13, 2019 DOI: 10.1021/acs.est.8b07199 Environ. Sci. Technol. 2019, 53, 7363−7370
Article
Environmental Science & Technology al.6 simulated the hydrothermal conditions at the CHESHIRE site (200−500 kt test, 1167 m depth of burial, 542 m below water table) and predicted resaturation of the nuclear test cavity with groundwater 20 days after the detonation (test locations identified in Supporting Information (SI), Figure S1). At that time, the nuclear melt glass temperature was predicted to be 160 °C. Seven hundred days later, the nuclear melt glass temperature was predicted to drop to 90 °C. The simulated temperatures were consistent with measured borehole peak temperatures at this site (150, 140, and 70 °C at 154, 201, and 2356 days after the detonation, respectively).7 At the ALMENDRO site (200−1000 kt test, 1064 m depth of burial, 378 m below water table), the unusually low permeability of the surrounding rock led to much slower resaturation of the nuclear test cavity. The nuclear melt glass zone was predicted to fully resaturate only after 7 years. At that point, the measured nuclear melt glass temperature was 215 °C, and it was still 160 °C 23 years after detonation.8 Smaller tests detonated in high permeability sediments (e.g., the CAMBRIC test, 0.75 kt test, 294 m depth of burial, 74 m below water table) result in much shorter test-related thermal anomalies. CAMBRIC nuclear melt glass temperatures were predicted to be as high at 170 °C in the days following the detonation but decreased to near ambient conditions within 10 years.9 Radionuclides represent a very small mass fraction of the NNSS nuclear melt glass. On the basis of the total NNSS Pu inventory (2.8 t), an estimated 700 t nuclear melt glass produced per kiloton explosive yield,10 and an estimated total maximum explosive yield of 67000 kt for all underground tests at the NNSS,11 we calculate an average Pu concentration of 60 ppb in nuclear melt glass (160 Bq/g 239+240Pu). This is in stark contrast to high-level waste (HLW) glass used for long-term storage of nuclear waste. For example, an HLW borosilicate glass developed in France, R7T7 may be loaded with as much as 19 wt % fission products, zirconium, and actinide oxides12 of which as much as 1 wt % may be Pu.13 Thus, actinide releases as a result of nuclear melt glass dissolution are not comparable to the behavior of HLW glasses. Furthermore, the presence of small quantities of Pu in nuclear melt glass are unlikely to affect dissolution rates. Dissolution of high silica rhyolitic glass typical of NNSS nuclear melt glass leads to the formation of clay and zeolite secondary minerals.14 Some of those clays and zeolites may be present in the form of colloids. In 1999, Kersting et al.15 identified Pu concentrations as high as 0.03 Bq/L in groundwater 1.3 km downgradient from the BENHAM test (1150 kt test, 1402 m depth of burial, 800 m below water table). The Pu isotopic ratios attributed the source of the migrating Pu plume to the BENHAM test. Most of the Pu was associated with colloids which consisted of clays (Illite and smectite), zeolites (mordenite and clinoptilolite/heulandite), and cristobalite. As a result of the observed colloid facilitated transport of Pu, significant research has focused on characterizing Pu interaction with aluminosilicate minerals,16−20 quantifying its sorption and desorption behavior,21,22 evaluating the role of natural organic matter and other ligands,23−26 and simulating its colloid facilitated transport behavior.27,28 Recent groundwater samples collected from a number of wells at the NNSS have demonstrated Pu groundwater contamination at concentrations ranging from 0.00022 to 2.0 Bq/L.29 While most Pu concentrations reported in NNSS groundwater fall below the Maximum Contaminant Level (MCL) established by the U.S. Environmental Protection Agency (EPA) for drinking
water (0.56 Bq/L), we do not yet understand what factors limit the Pu concentration or its subsequent transport. To address this, we performed a series of 3 year nuclear melt glass hydrothermal alteration experiments across a range of temperatures (25−200 °C). The experimental conditions were chosen to represent the range of hydrothermal conditions in underground nuclear test cavities when nuclear melt glass is in contact with groundwater. Pu concentrations, colloid concentrations and mineralogy, and Pu partitioning to colloids were monitored. The concentrations of 60Co, 137Cs, and 152Eu were determined as well. The intent of our effort was to demonstrate a mechanism for the generation of the Pu colloids identified by Kersting et al.,15 establish an upper limit for Pu concentrations at the NNSS, and provide context regarding the Pu concentrations observed to date and the Pu concentrations that may be observed in the future.
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MATERIALS AND METHODS Glass Preparation and Characterization. Nuclear melt glass was retrieved from underground nuclear test debris samples available at Lawrence Livermore National Laboratory (LLNL). Four glass samples (from a single test) were selected due to their uniform appearance as solid black glass fragments with little if any secondary minerals visually present. The four glass samples, comprising a total of about 10 g, were crushed and homogenized using mortar and pestle (in MQ water (Milli-Q Gradient System, > 18 MΩ·cm) to minimize dispersion of radioactivity) and wet sieved through a 90 μm sieve. As a result of the wet sieving, a total of about 200 mL of glass-containing water was produced. To establish a lower limit particle size in our stock glass, we used a 10 min settling cutoff (repeated three times in MQ water) in which the supernatant, containing claysized glass particles, was discarded. This produced a glass stock with a calculated lower limit particle diameter of ∼15 μm (based on Stoke’s Law). HF was used to clean the glass and remove any secondary minerals from the surface of the glass fragments. This was accomplished by exposing the glass to a 1% HF solution for 1 min. At the end of the HF reaction, the 10 min settling time cutoff was repeated 3 times in MQ water. This produced a clean stock glass with a calculated particle size range of 15−90 μm. Upon completion of the cleaning and size separation, the glass was dried. The stock glass was characterized by scanning electron microscopy (SEM; JEOL JSM-7401F), transmission electron microscopy (TEM; Philips CM 300 FEG operating at 300 kV and equipped with a Gatan Imaging Filter (GIF) with a 2k × 2k CCD camera and an EDX detector), X-ray diffraction (XRD; Bruker D8 X-ray diffractometer using radiation from a copper target tube (Cu Kα) and operating at 40 kV, 40 mA, and a scanning rate of 0.5°/min), gamma counting (high-purity germanium detectors, Ortec; Ortec’s Maestro data acquisition software combined with gamma-ray spectrum analysis using LLNL-developed software package GAMANAL), and total Pu analysis after dissolution in 3 M HNO3/0.05 M HF by inductively coupled plasma−mass spectrometry (Thermo Fisher Scientific iCAP-Q quadrupole ICP-MS). SEM images indicated a homogeneous glass morphology (SI Figure S2), and XRD revealed no major crystalline phases. TEM images indicated the presence of minor amounts of magnetite inclusions in the nuclear melt glass (SI Figure S3). The magnetite nanoparticles were the only crystalline phase observed in the glass and were present at trace levels. Gamma counting 7364
DOI: 10.1021/acs.est.8b07199 Environ. Sci. Technol. 2019, 53, 7363−7370
Article
Environmental Science & Technology
Figure 1. SEM image of melt glass after 994 days of reaction at room temperature.
solution. Approximately 400 mL of solution was removed from each experiment for analysis. The Parr vessels were replenished with fresh 5 mM NaCl/0.7 mM NaHCO3 solution (pH 8) to a total volume of 500 mL before they were placed back in their respective ovens. The removal of fluids (and cooling of samples) at each sampling time most likely impacted the equilibrium state of the solution in the reactor. As a result, any time dependence of fluid composition should be interpreted with caution. At each sampling time, a small amount of altered nuclear melt glass (the >15 μm solid) was also collected from each experiment for mineralogic characterization. Sample Characterization. At each sampling point, pH, Eh, and major anion/cation composition was measured. The solution Eh remained constant at ∼350 mV (vs SHE) at all temperatures, indicating a mildly oxidizing solution condition in all cases. The solution pH was relatively constant throughout the experiments. However, the pH was elevated at the higher temperatures (average pH of 8.1, 8.3, 9.5, and 9.6 in the 25, 80, 140, and 200 °C experiments, respectively). Significant changes in solution anion/cation composition were not observed. Each ∼400 mL solution was initially gamma counted to determine the activity of gamma emitting radionuclides. A portion of each sample was then taken for total Pu analysis by Multi−Collector ICP−MS (Nu Plasma II MC−ICP−MS). The remaining portion was vacuum filtered through a 20 nm Whatman Anodisc membrane filter. The filter was washed with several milliliters of MQ water and dried to determine the 20 nm−15 μm particle concentration in solution that we hereafter refer to as “colloidal”. The filtrate was gamma counted to determine the “aqueous” activity of gamma emitting radionuclides, and Pu concentration was determined by MC−ICP− MS. It should be noted that the composition of the
