Article pubs.acs.org/est
Plutonium Isotopes in the Terrestrial Environment at the Savannah River Site, USA: A Long-Term Study Christopher R. Armstrong,* Patterson R. Nuessle, Heather A. Brant, Gregory Hall, Justin E. Halverson, and James R. Cadieux Nonproliferation Technology Section, Savannah River National Laboratory, Aiken, South Carolina 29808, United States ABSTRACT: This work presents the findings of a long-term plutonium (Pu) study at Savannah River Site (SRS) conducted between 2003 and 2013. Terrestrial environmental samples were obtained at the Savannah River National Laboratory (SRNL) in the A-Area. Plutonium content and isotopic abundances were measured over this time period by α particle and thermal ionization mass spectrometry (3STIMS). We detail the complete process of the sample collection, radiochemical separation, and measurement procedure specifically targeted to trace plutonium in bulk environmental samples. Total plutonium activities were determined to be not significantly above atmospheric global fallout. However, the 238Pu/239+240Pu activity ratios attributed to SRS are substantially different than fallout due to past 238Pu production on the site. The 240Pu/239Pu atom ratios are reasonably consistent from year to year and are lower than fallout indicating an admixture of weapons-grade material, while the 242Pu/239Pu atom ratios are higher than fallout values, again due to actinide production activities. Overall, the plutonium signatures obtained in this study reflect a distinctive mixture of weapons-grade, heat source, and higher burn-up plutonium with fallout material. This study provides a unique opportunity for developing and demonstrating a blue print for long-term low-level monitoring of trace plutonium in the environment.
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INTRODUCTION
As the cold war dissipated, all of the SRS reactors were gradually decommissioned. The last reactor was shut down in 1988. Today, ongoing operations at SRS include the H-Canyon used nuclear fuel (UNF) reprocessing facility, a tritium facility (Figure 1), and processing of radioactive waste from the legacy of the production era in facilities such as the Defense Waste Processing Facility (DWPF). For over 60 years, the Savannah River National Laboratory (SRNL), located in A-Area at SRS (Figure 1), has performed applied research and development to support the site production facilities and locally tested other processes that never evolved to the “plant” scale. Many of the site releases to the environment occurred during the early operations in the 1950−1960s. That early operational history is recorded in the trace signatures of plutonium and other actinide isotopes in the laboratory environment along with perturbations from ongoing work. During that time, the low level radiochemistry group has tracked plutonium levels and composition in the environment using highly sensitive radioanalytical techniques to concentrate, purify, and assay these minute concentrations. The group’s mandate has been to detect and measure anthropogenic radionuclides (fission products and actinides) at or below the natural background to provide quantitative data to study the effects of site operations on the terrestrial environment. For
Background. The Savannah River Site (SRS), formerly the Savannah River Plant (SRP), began operation in the early 1950s. The site, located close to the Savannah River in South Carolina (Figure 1), encompasses roughly 830 km2. Its primary function was to produce special nuclear materials for national defense, specifically weapons-grade plutonium (Pu) and tritium (3H) required for thermonuclear devices.1 Therefore, these were the major materials produced at SRS in the first several decades of operation. At its peak, five heavy water moderated reactors (Figure 1), fueled with highly enriched uranium, were operated for isotope production. Targets placed inside these high-flux reactors contained lithium (6Li) to produce tritium and depleted uranium to make weapons-grade (240Pu concentration of less than 7%) plutonium. Over its history, the site produced metric tons of plutonium comprising Pu isotopes with masses of 238 through 244. The unique versatility of the SRS reactors for isotope production was also exploited for production of other actinide materials in bulk along with smaller amounts of novel transuranic isotopes. For example 237Np bred in the highly enriched uranium fuel was separated and processed into targets to produce significant quantities (over 300 kg) of 238Pu as heat sources (∼0.5 W per gram) for thermoelectric generators for space probes. Intense, extended irradiations of plutonium targets generated heavier plutonium isotopes including tens of kilograms of 242Pu and 244Cm, tens of grams of 244Pu, and hundreds of milligrams of 252Cf. © 2014 American Chemical Society
Received: Revised: Accepted: Published: 1286
August 29, 2014 December 22, 2014 December 23, 2014 December 23, 2014 DOI: 10.1021/es504147d Environ. Sci. Technol. 2015, 49, 1286−1293
Environmental Science & Technology
Article
conducted Pu analysis of soils and vegetation in the vicinity of FDNPP.5 This study confirmed the presence of commercial reactor-grade plutonium attributed to FDNPP but called for additional investigations, particularly the need for more background data. A critical review by Zheng et al. reported the presence of Pu isotopes released from the damaged reactors but also highlighted the need for more detailed plutonium investigations.6 It is clear that, to better assess plutonium mobility in the environment, detailed long-term monitoring of Pu activity, i.e., involving adequate bulk sample collection and both alpha spectrometry and mass spectrometry measurements, is required. For example, the benefit of detailed monitoring before (background), during, and after an accidental release would enable a more complete assessment of the extent of plutonium activity in the affected environment. In the context of an accidental release such as the FDNPP event, long-term Pu monitoring according to the methodology described herein would enable assessment of possible releases originating from different sources, e.g., spent fuel pools and/or damaged reactors. Moreover, the complexities of plutonium isotopic signatures in the terrestrial environment make data interpretation and accurate determination of the original source(s) a challenge. Thus, further investigation into plutonium fate and transport, particularly focusing on subtle differences in Pu isotopic behaviors in the environment, is critical from radiological, environmental, and nonproliferation perspectives. Aim. Owing to its rich history of plutonium production, the Savannah River Site (SRS) provides a unique test bed to investigate plutonium content and isotopic (238Pu, 239Pu, 240Pu, 241 Pu, and 242Pu) behavior in a highly complex background. The SRS environment is one of the few locations worldwide where weapons-grade, heat source, and reactor-grade (higher burn-up) plutonium signatures abound.7 This study details a long-term investigation of trace level plutonium content and isotopic composition in the SRS environment. Here, we report the methodology for plutonium collection in bulk environmental samples, analysis by alpha spectrometry and mass spectrometry, and data interpretation, i.e., examining Pu isotopic signatures to gain insights into possible sources of plutonium. Few studies investigating plutonium in the terrestrial environment provide both alpha spectrometry and mass spectrometry results. A complementary analytical approach using both of these techniques offers both Pu activity data from alpha spectrometry and abundance data from mass spectrometry.8 The goal of this report is 2-fold: (1) to present a complete procedure for the collection, radiochemical processing, and analysis of plutonium in bulk environmental samples and (2) to determine the sources of Pu particularly in light of the legacy of plutonium production and processing at SRS. The methodology presented herein provides a template for effective low level plutonium collection, processing, and measurement.
Figure 1. Map of Savannah River Site (SRS) located in South Carolina, USA. Triangles show previous reactor locations (C, K, L, P, and R reactors). Circles show other locations of interest including, A: A-Area comprising the Savannah River National Laboratory (SRNL); F: F-Canyon fuel reprocessing facility (decommissioned); H: HCanyon fuel reprocessing and tritium facilities; M: fuel fabrication facility (decommissioned).
plutonium isotopes, this has mainly consisted of sorting out site produced isotopes from the ubiquitous global fallout. Over the years, this has included detection of off-site events such as the releases at Chernobyl and Tomsk. In the past decade, activity levels along with the global fallout from the era of above ground testing have decreased to the point where we have had to adapt our analytical methodology to detect a few femtograms in a nominal 10 to 100 g sample. This is roughly equivalent to finding a couple of pennies in the US national debt. The extraordinary sensitivity of radioanalytical techniques, particularly for the measurement of plutonium, enables one to track concentrations at these levels. The distinctive site composition has also allowed us to routinely validate the technique for all of the plutonium isotopes from 238Pu through 244Pu. Need for More Detailed and Long-Term Plutonium Studies. Remediation efforts associated with the Fukushima Daiichi Nuclear Power Plant (FDNPP) event require a rigorous assessment of the extent of environmental contamination associated with this accidental release. These investigations must be both long-term, e.g., several years, and detailed in terms of the diversity and concentration, i.e., low level monitoring, of possible contaminants.2,3 Although detailed investigations have been undertaken for the elements contributing to the majority of the radionuclide release, e.g., 131 132 I, Te, 134Cs, 136Cs, 137Cs, 133Xe, 85Kr, etc., to date studies reporting data for the actinides are sparse.4 Schneider et al.
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EXPERIMENTAL SECTION Methodology. The environmental samples in this study consist of foot borne solids, e.g., soil, mineral fragments, vegetation debris, etc., removed from the bottom of visitors’ footwear via a mechanical shoe brush9 before entering the clean laboratories at Savannah River National Laboratory (SRNL). The shoe brush (Liberty Shoe Brush 2010SC; Liberty Industries Inc., East Berlin, CT, USA) mechanically removes foot borne debris via four rotating brushes that contact the 1287
DOI: 10.1021/es504147d Environ. Sci. Technol. 2015, 49, 1286−1293
Environmental Science & Technology
Article
columns (5 mL bed volume) are prepared by initial rinsing and loading with 0.1 M hydrochloric acid. After a small addition of sodium nitrite, the sample solutions are then loaded onto the columns and rinsed with 8 M nitric acid and 9 M hydrochloric acid, respectively. The samples are eluted with additions of 9 M hydrochloric acid and 0.1 M ammonium iodide. The eluate is treated with 8 M nitric acid, and the solutions are heated and evaporated to incipient dryness. The residues are redissolved with 0.36 M sodium bisulfate and 8 M nitric acid, and the solutions are heated on a hot plate to incipient dryness. With this separation and purification technique, chemical Pu recoveries of 80% or greater are typically observed. Electrodeposition Preparation. Electrodeposition is carried out according to a long-established method.14 The samples are brought into solution with 0.75 M sulfuric acid with a small amount of thymol blue indicator and transferred to an electrodeposition vial containing a platinum disk (1 cm source diameter). The pH is adjusted by dropwise addition of concentrated ammonium hydroxide to be slightly acidic (salmon color, pH ∼ 2−2.3); the vials are positioned in the electrodeposition unit, and the current is set to 0.75 A. After 1 h, 2 mL of concentrated ammonium hydroxide is added to each vial and deposition proceeds for 1 min after which power is turned off and the platinum anodes are removed from the vials. The electroplated disk is then removed, rinsed with deionized water, and air-dried before alpha spectrometry measurements. Mass Spectrometry Sample Preparation. The electrodeposited material is dissolved from the platinum discs by gently warming the disks in individual beakers containing 8 M hydrochloric acid. Sodium nitrite is added to the sample solutions while stirring. A prepacked column of AG1X4 anion exchange resin (0.5 mL bed volume) is conditioned by rinsing each column with 0.1 M hydrochloric acid, 8 M nitric acid, and 8 M hydrochloric acid, respectively. The columns are loaded with the samples, and the beakers are rinsed with small amounts of 8 M hydrochloric acid. Each column is rinsed successively with 8 M hydrochloric acid, 8 M nitric acid, and 8 M hydrochloric acid. The beakers containing the rinse solutions are removed and replaced with leached 30 mL beakers. The columns are eluted by adding 9 M hydrobromic acid to each column in separate portions.15 The samples are evaporated to incipient dryness on a hot plate and allowed to cool. Then, 8 M nitric acid is added to each sample while stirring, and the samples are transferred to acid-leached Teflon vials with 8 M nitric acid. The vials are capped and submitted for mass spectrometry analysis as described below. Alpha Spectrometry. Alpha-spectrometry was performed using the EG&G Ortec Octête PC Alpha Spectrometer. Passivated implanted planar silicon (PIPS) detectors with an active area of 450 mm2 were used. The samples were measured for several weeks (sample to detector distance is ∼1 mm, and the detector counting efficiency is 33 ± 3%), and the received spectra were evaluated using the software program Maestro (Advanced Measurement Technology, Inc.,Oak Ridge, TN, USA). Three Stage Thermal Ionization Mass Spectrometry (3STIMS). Plutonium measurements were conducted with a 1960s KAPL (Knolls Atomic Power Laboratory) design Three Stage Thermal Ionization Mass Spectrometer (3STIMS) fabricated in-house in the 1970s. The instrument uses three 90° × 30.5 cm sectors arranged in magnetic/magnetic/ electrostatic order (momentum/momentum/energy filter order) with a single ion counting detector. This arrangement
front, sides, and bottom of the footwear in conjunction with a vacuum (suction) system. Material is dislodged from footwear by the brushes and collected by vacuum into a nearby compartment. Samples are taken periodically (typically annually) when the shoe brush compartment is emptied. During this time scale, hundreds of workers and visitors have passed through the shoe brushes. The sheer volume of traffic and distance over which these people travel provides ample material that is representative of A-Area. Because this is a mixture of fine particle materials integrated over a relatively long period of time, this is an ideal method to collect true bulk samples of the nearby environment. Moreover, the amount of sample collected (∼100 g) ensures an adequate amount of plutonium activity and sufficient sensitivity to attain an acceptable uncertainty (generally
