Applied Radiation and Isotopes 95 (2015) 85–89
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Elemental composition in sealed plutonium–beryllium neutron sources N. Xu n, K. Kuhn, D. Gallimore, A. Martinez, M. Schappert, D. Montoya, E. Lujan, K. Garduno, L. Tandon Los Alamos National Laboratory, Los Alamos, NM 87545, USA
H I G H L I G H T S
A destructive chemical analysis of the PuBe neutron sources includes the solubilization and digestion of the PuBe alloy material. Plutonium was assayed by an electrochemical method. Beryllium assay and trace elemental contents were determined by ICP instruments. A large variation in trace elemental composition was observed among the five PuBe source materials.
art ic l e i nf o
a b s t r a c t
Article history: Received 25 February 2014 Received in revised form 22 June 2014 Accepted 12 October 2014 Available online 22 October 2014
Five sealed plutonium–beryllium (PuBe) neutron sources from various manufacturers were disassembled. Destructive chemical analyses for recovered PuBe materials were conducted for disposition purposes. A dissolution method for PuBe alloys was developed for quantitative plutonium (Pu) and beryllium (Be) assay. Quantitation of Be and trace elements was performed using plasma based spectroscopic instruments, namely inductively coupled plasma mass spectrometry (ICP-MS) and atomic emission spectrometry (ICP-AES). Pu assay was accomplished by an electrochemical method. Variations in trace elemental contents among the five PuBe sources are discussed. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Sealed PuBe neutron source ICP-MS ICP-AES Plutonium assay Beryllium assay Trace elements
1. Introduction Plutonium–beryllium material has been made into sealed neutron sources since excess plutonium became available from nuclear reactors in the late 1940's (Tate and Coffinberry, 1958). PuBe sealed sources are inexpensive, portable neutron generators suitable for applications that require a low neutron flux. Research institutions such as national laboratories and universities utilize PuBe neutron sources frequently for various experiments (Griffin et al., 2008), while the petroleum industry has been employing them for decades in oil well logging around the world (Buryakovsky et al., 2012). PuBe is an alloy formed by heating Pu in either metal or oxide form with Be metal at 1150 1C (Tate and Coffinberry, 1958; Wauchope and Baird, 1959). The intermetallic compound has a single face
n Correspondence to: Los Alamos National Laboratory, P.O. Box 1663, G740, Los Alamos, NM 87545, USA. Tel.: þ 1 505 667 2016; fax: þ1 505 665 4737. E-mail address: [email protected] (N. Xu).
http://dx.doi.org/10.1016/j.apradiso.2014.10.013 0969-8043/& 2014 Elsevier Ltd. All rights reserved.
centered cubic structure with the chemical formula of PuBe13 (Konobeesky, 1955; Coffinberry and Ellinger, 1956). The texture of PuBe depends on the exothermic reaction temperature attained during the formation of the compound and can be either a friable solid or a coalesced mass. Some manufacturers encapsulated the reaction container directly inside a sealed container. Inside a sealed PuBe neutron source, neutrons are produced through an alpha–neutron (α,n) nuclear reaction, i.e. alpha particles generated from the decay of Pu are absorbed by low atomic weight Be which then ejects neutrons. As a small-scale neutron generator, PuBe has operational advantages over radium–beryllium (RaBe), polonium–beryllium (PoBe), americium–beryllium (AmBe), and americium–lithium (AmLi) materials. This is due to the production of a stable and predictable neutron yield owing to the formation of an intermetallic PuBe13 compound with a prolonged operational life time accredited to the 24,360 year half-life of the Pu-239 isotope. In addition, Pu produces lower gamma intensity than other sources which is beneficial for safety reasons.
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Early commercial distribution of PuBe neutron sources was not regulated, causing a lack of registration tracking and nuclear material safeguards. Concerns have been raised regarding nuclear material illicit trafficking and nuclear proliferation by the International Atomic Energy Agency. The Offsite Source Recovery Project championed by the Los Alamos National Laboratory (LANL), USA, has recovered a large number of the unwanted sealed Pu neutron sources. Disposition of these disused neutron sources necessitates the disassembly of several PuBe sources for destructive analysis to address the U.S. Resource Conservation and Recovery Act (RCRA) and other environmental concerns. Research on sealed PuBe neutron sources has been primarily carried out on characterizing the neutron intensity and modeling the neutron spectrum (Kumar and Nagarajan, 1977; Anderson and Bond, 1963), as the source is frequently used to calibrate other measuring instruments. While non-destructive analysis methods have been applied to determine Pu content and Pu isotopic distribution (Nguyen et al., 2007; Lakosi et al., 2006); methods applied to the direct destructive assay of PuBe sources are scarce (Breakall, 1972). Destructive analysis of the chemical composition of PuBe materials in neutron sources has never been reported. The only publication on destructive chemical analysis of a neutron source (Sommers et al., 2009) described an AmBe source material evaluated by Idaho National Laboratory, USA. The lack of a literature on destructive chemical analysis may be due to the extreme challenge in radioanalytical chemistry capability as well as engineering infrastructure support. While handling a sealed PuBe neutron source requires only the minimum radiological precautions, disassembly of a source demands more rigorous engineering controls such as glove boxes to prevent radiological contamination and to minimize the radiation exposure dose to the workers. Despite the commonness of glove box enclosures in many nuclear and radiological facilities, few are equipped to disassemble sealed sources and recover neutron source materials for destructive analyses. LANL possesses the necessary capabilities: the nuclear facility infrastructure to handle bulk quantities of special nuclear materials; skilled machinists to dismantle sealed sources using glove box installed equipment; and a strong analytical chemistry team to complete trace elemental determinations for bulk nuclear materials. The integrated technologies and skills at LANL were essential to accomplish this study. In this study, five sealed PuBe neutron sources from different manufactures were disassembled using a manual Hardinge lathe located in a glove box. Loose Pu contamination was controlled by introducing clean consumables and wiping-down glove box, glove, equipment, and tool surfaces. As a general strategy, sealed source container opening employed lathe cuts along defined or presumed seams defining the placement of a press-fit or welded cap to seal the container. This resulted in cracking along the seam at a lathe cut depth prior to penetrating the inner container wall. Cracked sources were removed from the lathe and caps were separated by manually pulling on the cap using pliers. This approach effectively prevented container turnings from depositing in the source container along with PuBe material.
2. Material and methods
(Solution # S5/31/73) into the multi-element calibration/verification standard solutions. For ICP-AES measurements, multi-element calibration and verification standards were prepared from standard solutions by High Purity Standards (Charleston, SC, USA) and Inorganic Ventures, respectively. The nitric acid, hydrofluoric acid and hydrochloric acid used in the study for sample and standard preparation were Fisher Optima (Pittsburgh, PA, USA) ultrapure grade. Deionized (DI) water at Z 18 MΩ cm 1 resistivity was produced by a Barnstead (Dubuque, IA, USA) E-pure system. An AG MP-1 anion exchange resin (200–400 mesh) from Bio-Rad (Hercules, CA, USA) was converted to the nitrate form with 4 M HNO3 prior to the preparation of chromatographic columns for Pu removal. 2.2. Instrumentation and methods The trace elements were measured by both ICP-MS and ICP-AES. A PlasmaQuad PQ2 þ (Thermo Fisher, USA) ICP-MS instrument manufactured by VG Elemental was used for trace elemental analysis. This instrument was modified to interface with a radiological glove box. Detailed operating conditions are listed in a previous publication (Xu et al., 2013a). Beryllium and other trace elements were analyzed by an IRIS (Thermo Fisher, USA) ICP-AES instrument manufactured by Thermo Scientific. The ICP-AES was interfaced to a glove box so that only the torch box and the sample introduction system were located inside the glove box. The instrument was equipped with a charge injection device (CID) detector and operated in a radial-view mode. The detector covered a wavelength range of 165–1050 nm with a single pixel resolution of 0.005 to 0.012 nm. A fixed cross flow nebulizer with a Teflons spray chamber was used for sample introduction. Instrument operating conditions are listed in Table 1. All working standards and blanks were prepared in 4 M HNO3 and 0.03 M HF solution to match the sample matrix. Pu was assayed by controlled potential coulometry (CPC) (Waterbury et al., 1970). 2.3. Sample preparation PuBe alloy samples were dissolved in 6 M HCl followed by a pressurized closed vessel digestion with 12 M HNO3 and 0.1 M HF at 130 1C for approximately 12 h. The pressurized closed vessel digestion system used in this study has been described elsewhere (Xu et al., 2013b). The sample aliquot for trace elemental measurements by ICP methods contains 0.20 to 0.25 g of PuBe material, depending on the availability from the total amount of PuBe material recovered from the sources. Matrix spike samples were carried out as a QC measure whenever the total amount of material allowed. Concentrated nitric acid was added to the sample solution to adjust the final acid concentration between 8 and 10 M. This acid concentration range maintains Pu in the tetravalent state required for the chromatographic separation of Pu from trace elements. A sub-sample of 0.020 to 0.025 g of PuBe material was aliquoted from the dissolved PuBe solution for trace elemental analysis by ICP-MS. A large dilution was required to minimize the heavy ion effect. Internal standards were added to the diluted solution prior to the instrumental analysis. The remaining solution was loaded onto a packed chromatographic column consisting of 4 mL of AG MP-1 anion exchange resin to
2.1. Reagents and materials For ICP-MS analysis, multi-element calibration standards were prepared daily from standard solutions by SPEX CertiPrep (Metuchen, NJ, USA). Calibration verification standards and single element internal standards were prepared from Inorganic Ventures (Christiansburg, VA, USA) standard solutions. Neptunium standards were prepared by diluting an Amersham 9410 stock solution
Table 1 Thermo Fisher IRIS ICP-AES instrument operating conditions. RF power Nebulizer Ar pressure Auxiliary Ar flow rate Solution uptake
1150 W 30 psi 1.0 L/min 1 mL/min
N. Xu et al. / Applied Radiation and Isotopes 95 (2015) 85–89
remove the Pu. The separation was completed by eluting the column with 2 mL of 10 M HNO3 and 9 mL of 8 M HNO3. The entire volume of eluent was collected and diluted 1:1 with DI water before the determination of trace elements by the ICP-AES method. Additional dilutions were also required for Be determination. Pu retained on the column was recovered by eluting with 0.1 M HCl.
3. Results and discussion 3.1. Sample description and dissolution For this study, five sealed PuBe neutron sources from different manufactures were disassembled and PuBe materials were extracted for analysis of elemental composition. Source 1 was manufactured at LANL. Sources 2 and 3 were made by the Mound Laboratories, USA, and Monsanto Research Corporation, USA, respectively. Sources 4 and 5 were archived sources of possible non-domestic origin. The PuBe materials varied significantly in appearance. Sources 1, 4 and 5 PuBe materials appeared as fine powder to granular lumps, whereas sources 2 and 3 materials were a single coalesced lump. The colors of the PuBe materials ranged from metallic to dull gray to charcoal black. The total amount of PuBe material recovered from each source and their respective appearances are described in Table 2. The sample dissolution process was performed in a glove box. An array of 2 L plastic bottles filled with water was lined up along the front portion of the glove box floor to reduce staff neutron dose. An early attempt to dissolve the PuBe alloy adopted a routine Pu metal dissolution protocol by adding 6 M HCl drop-wise to the solid sample in a plastic test tube. It was quickly discovered that this method was not suitable for PuBe alloy dissolution because the exothermic reaction was so vigorous that it resulted in sample loss due to excessive splattering. Thus, the procedure was modified to first add DI water to the solid sample, followed by slow drop-wise addition of concentrated HCl, and finally a supplement of 6 M of HCl to complete the reaction. Even with the slow, drop-wise addition of concentrated HCl, the initial reaction started vigorously. It was observed that the dark colored PuBe pieces were surrounded by hydrogen bubbles and were floating and swirling in the noble blue colored Pu solution pool, spattering occasionally. To avoid splattering related sample loss, a tall dissolution vessel was used. Undissolved solids were observed at the end of the initial reaction for all of the five PuBe materials; hence, a subsequent pressurized closed vessel digestion procedure was used to complete the digestion process. Since sources 1, 4 and 5 were in granular form, approximately 0.2 to 0.6 g of each sample was processed. For a typical sample weight of 0.6 g, the addition of 5 mL of DI water followed by 5 mL of concentrated HCl, and 3 mL of 6 M HCl solubilized a majority of the PuBe solid. The volume of the reagents was reduced proportionally for smaller sample weights. A subsequent pressurized closed vessel digestion with the addition of 13 mL 12 M HNO3 and 0.1 M HF usually took the Table 2 Recovered PuBe material weight and appearance for the five sealed PuBe neutron sources. Source
PuBe recovered (g)
Appearance
1 2
1.260 2.252
3 4
1.395 2.774
5
7.819
Uniform, fine powder, gray–black Coalesced lump, metallic gray to black Coalesced lump, gray–black Coarse powder to granular lumps, metallic gray–black Coarse powder to granular lumps, metallic gray–black
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dissolution to completion or near completion. Any amount of undissolved solid was separated by centrifugation. The coalesced PuBe mass in sources 2 and 3 was bonded to the inner container. Efforts were made to dislodge the material using small hand tools but were not successful. The entire source was then immersed in HCl for dissolution. At the end of this process the inner container was intact. It was removed from the solution prior to the subsequent nitric acid digestion. The reaction time was longer for the coalesced PuBe materials compared to the granular samples, as the surface area available for reaction was much smaller. The reaction for source 2 took 7 days to complete. Although there was substantial amount of undissolved solid in the solution, no subsequent acid digestion was conducted for this sample. 3.2. Chromatographic separation of Pu and Be In order to assay the Be and determine trace elements in the PuBe material by the ICP-AES method, the Pu in the matrix must be removed to minimize spectral interferences. A chromatographic column packed with an anion exchange resin AG MP-1 is used to retain Pu. Prior to the chromatographic separation, Pu ions must be adjusted to the Pu (IV) oxidation state with HNO3. Beryllium and other cations in the solution have negligible to minimal retention on the AG MP-1 resin; hence, they pass through the column with the use of sufficient wash volume and are collected for ICP-AES determination. Utilizing AG MP-1 resin to separate Pu from trace impurities in dissolved bulk Pu materials, such as Pu metal and oxides, is a routine protocol at LANL's Actinide Analytical Chemistry group (Mahan et al., 2000). At trace concentrations, the Be fraction generally elutes within 2 to 3 bed volumes of acid rinse solution. There was a concern that extra volume of rinsing solution might be required due to percent levels of Be in the samples. However, it was observed that Be formed a distinct yellow band on the chromatographic column right below the emerald colored Pu band during the sample elution. The yellow band eluted down the column quickly with the additional rinse solutions, and was completely off the column at the end of the elution. Nonetheless, an extra volume of 2 mL of 8 M HNO3 rinse solution was administered to the column. The extra volume was collected separately and analyzed for Be by the ICP-AES method. The ICP-AES method detection limit for Be was defined as three times the standard deviations of concentrations from seven reagent blanks that went through the chromatographic separation procedure. No detectable Be was found in the extra volume fraction which confirmed that a complete elution of Be had been achieved using the routine chromatographic separation protocol. 3.3. Pu and Be content in PuBe sources Beryllium concentrations in the dissolved, diluted samples were determined by the ICP-AES method. Dilutions were made to get the Be concentration into the calibration range of the instrument. Replicates of two to four solutions from each aliquot were analyzed. The measurement uncertainty was calculated using a Guide to the Expression of Uncertainty in Measurement (GUM) workbench software (GUM Workbench User Manual, 2009). Pu content was determined by the CPC method in replicates of three to six from each aliquot. CPC is a standard high accuracy and precision method for establishing Pu material purity in safeguarded Pu. The measured Pu and Be weight percentage for five of the PuBe sources and respective measurement uncertainties are shown in Table 3. Be content ranged from 30.1 to 40.8 wt%, whereas Pu assay ranged from 59.3 to 76.3 wt%. The mass balance is the summation of the total mass of the Pu and the Be, as well as all the other trace elements measured. It is noteworthy that an elemental mass balance of near 100% was achieved from two
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different analytical methods: ICP and CPC; each method carries its own measurement uncertainty. The near 100% mass balance suggests that the measurements are accurate in view of measurement uncertainties and that all the significant elemental constituents have been captured. It is also shown in Table 3 that none of the PuBe alloys were made with an exact, stoichiometric PuBe13 amount of reactants. Of the five sources, two of them are rich in Pu, whereas the other three have excess Be. This may be a production choice for the purpose of adjusting the neutron flux. PuBe alloy richer in Pu than PuBe13 would result in a higher neutron yield (Tate and Coffinberry, 1958), although only the exact PuBe13 stoichiometry would provide a predictable neutron yield.
Table 3 Pu and Be assay, the stoichiometric formula and mass balance for the five PuBe materials. Source
Be (wt%)
Pu (wt%)
Apparent compound
Measured element sum (wt%)
1 2 3 4 5
30.7 70.2 30.1 70.4 38.7 70.3 38.4 70.6 40.8 70.3
68.67 0.1 76.3 7 0.1 60.7 7 0.1 61.5 7 0.1 59.3 7 0.1
PuBe11.9 PuBe10.5 PuBe16.9 PuBe16.6 PuBe18.3
100.3 106.9 101.5 100.4 100.8
3.4. Trace element constituents in PuBe sources Trace elemental concentrations for five PuBe samples were determined by both ICP-AES and ICP-MS methods. Matrix interferences due to the high Be content in the sample were shown to affect several elements, such as aluminum, chromium, copper, nickel, and magnesium by the ICP-AES method. Therefore, these elements had to be analyzed by the ICP-MS method. The wavelength used for the ICP-AES analysis and the mass used for the ICP-MS measurement are provided in Tables 4 and 5. The concentrations of RCRA regulated elements in the five PuBe samples are listed in Table 4, and other trace elements are shown in Table 5. RCRA element limits are established in the Toxicity Characteristic Leaching Procedure (TCLP) by the U.S. Environmental Protection Agency (USEPA, 1999). TCLP is a protocol of extraction with a solution that simulates the ground water pH, instead of a total dissolution with strong acids. It was impossible to perform the TCLP test because of the limited amount of PuBe sample available in this study. Nonetheless, the results from the total acid digestion are presented in Table 4 along with the RCRA element limits that are converted to μg g 1 units for approximate comparison. It is noticed that all of the RCRA elements in source 1 (Table 4) are below the RCRA limits, except for lead, which is slightly more than two times the limit. Most elements in sources 2, 4 and 5 are below the RCRA limits, except for chromium, which concentrations are under twice the RCRA limit. For source 3, only cadmium exceeds the RCRA limit by less than three times. Although elevated elemental
Table 4 RCRA limits and elemental constituents measured by the ICP-MS method for the five PuBe sources, mg g 1. Element
Measurement mass
RCRA
Source 1
Source 2
Source 3
Source 4
Source 5
Arsenic Barium Cadmium Chromium Lead Selenium Silver
As_75 Ba_138 Cd_114 Cr_52 Pb_208 Se_78 Ag_107
100 2000 20 100 100 20 100
6.4 7 0.7 377 2 9.7 7 1.0 607 3 2107 11 4.4 7 0.5 217 1
4.3 7 5.7 1.0 7 0.1 o 0.2 1707 14 8.9 7 0.8 3.5 7 1.4 o 0.4
9.1 71.8 10 71 49 73 7772 4.9 70.1 8.0 77.3 0.9 70.3
7.2 72.4 o 0.2 0.4 7 0.1 1307 5 o 0.5 o3 o 0.4
8.4 7 2.4 o 0.2 0.3 7 0.1 1907 5 7.9 7 1.0 o4 o 0.5
Table 5 Trace elemental constituents measured by the ICP-MS and ICP-AES methods for the five PuBe sources, mg g 1. Element
Wavelength(nm)/mass
Source 1
Source 2
Source 3
Source 4
Source 5
Aluminum Antimony Borona Calciuma Cerium Copper Gallium Gold Irona Magnesium Manganese Molybdenum Neptunium Nickel Niobium Silicona Tantalum Tin Titanium Tungsten Zinca Zirconium
Al_27 Sb_121 B249.6 Ca396.8 Ce_140 Cu_65 Ga_69 Au_197 Fe259.9 Mg_24 Mn_55 Mo_95 Np_237 Ni_60 Nb_93 Si251.6 Ta_181 Sn_116 Ti_48 W_184 Zn213.8 Zr_90
11007 51 11 71 o4 220 7 11 o 0.2 18007 180 1.7 7 0.2 227 1 750 7 38 827 5 177 2 1.4 7 0.1 1507 8 3707 30 o 0.2 860 7 43 o 0.3 7.3 7 0.4 267 2 o 0.5 13007 55 6.6 70.3
250 7 14 o 0.2 o4 o 10 o 0.2 437 1 3.4 70.4 347 7 750 7 11 477 3 51 73 3.2 70.4 35 71 340 7 28 o 0.2 807 6 840 7 99 0.5 70.1 1.7 7 0.3 o 0.5 o 10 6.6 70.3
490 712 o 0.2 417 1 3007 13 o 0.2 37 74 1.2 7 0.1 360 7 72 780 7 22 877 3 37 72 127 1 2007 15 837 3 5.0 7 1.0 460 7 12 13,0007 5000 1.2 7 0.3 967 2 9.4 7 2.0 347 1 3.7 7 0.1
150 711 o 0.2 o3 950 7 30 207 4 417 3 6.3 70.4 o 0.2 530 7 25 327 4 737 3 140 75 1017 10 427 7 o 0.2 260 7 10 127 2 3.0 70.1 14 71 8.4 71.0 237 2 227 1
1607 10 o 0.2 o3 217 7 o 0.2 367 3 177 1 o 0.2 680 725 o3 947 6 867 7 1107 9 1407 7 o 0.2 2007 10 187 2 3.3 7 0.1 117 1 117 1 o 10 197 1
a
Elements analyzed by the ICP-AES method.
N. Xu et al. / Applied Radiation and Isotopes 95 (2015) 85–89
concentrations are expected by the complete acid dissolution process as compared to TCLP, none of the elements are over three times the RCRA limits in all of the five materials examined in this study. It is seen in Table 5 that trace elemental compositions vary significantly among the five PuBe sources. The only element that has consistent concentration among the five PuBe materials, i.e., within twice the measured concentrations, is iron. Sources 4 and 5 materials exhibited overall consistency in all elements. The low levels of boron and cadmium in these two sources suggested the ability to and interest in removing neutron absorbers. Elevated levels of molybdenum (over ten times difference in concentration) and zirconium (over six times difference in concentration) were noticed in these two sources but not in the others. Source 4 showed cerium. Elemental contents in PuBe sources 1, 2 and 3 were quite divergent except for gallium, magnesium and zirconium, which are within two times the measured concentrations. Gold was measured at various concentration levels in these three sources with a high of 360 mg g 1 in source 3, but low in the remaining sources. Boron and niobium were detected in source 3, and antimony in source 1. Significantly high levels of copper, zinc and tin in source 1 were noticed. One possible explanation could be that the soldering material used to seal the PuBe capsule was present in the sample. The solder, American Platinum Works Silvaloy No 355, which contains 22% copper, 17% zinc and 5% tin (Tate and Coffinberry, 1958) was used by the manufacturer of source 1 for a period of time in the fabrication of PuBe sources. It was decided that the high tantalum values in both samples sources 2 and 3 were not related to the PuBe source materials but to the tantalum inner container that was included in the sample solubilization. It was likely that a small amount of solid tantalum was removed from the inner container material during the initial dissolution process as the entire source was in direct contact with the HCl. Subsequently, digestion with HNO3 and HF dissolved the previously dislodged solid tantalum into the solution. As a result, 1.3 wt% of tantalum was measured in the solution of source 3. To avoid the same problem, no further nitric acid digestion was applied to source 2. Instead, the undissolved particulates were separated by centrifugation. The reason that the mass balance of source 2 deviates from 100% is unclear at this time. 4. Conclusion An analytical method has been developed to dissolve PuBe alloys using an initial addition of 6 M HCl followed by a pressurized closed vessel digestion with HNO3 and HF at 130 1C for approximately 12 h. The destructive analytical chemistry analysis for the five PuBe materials indicates that the Pu and the Be concentrations vary among different manufacturers, ranging from 59.3 to 76.3 for Pu and from 30.1 to 40.8 wt% for Be, respectively. A mass balance of near 100% confirms complete coverage of the major constituents in the analytical plan and supports the least analytical measurement uncertainty. Trace elemental constituents showed a large variation among the elements analyzed. Although
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some elemental concentrations were consistent among the five PuBe sources, most of the elements remained unique signatures for a specific sample. Most of the RCRA related elements are present below the RCRA limits in all of the five sources, except for cadmium, chromium, and lead. Nevertheless, none of these elements are over three times the RCRA limits.
Acknowledgment The authors would like to thank the Department of Homeland Security, Domestic Nuclear Detection Office (Grant no. IAA HSHQDC-14-X-00028) for funding the project, and Andres Borrego for assisting in the sample preparation work. This publication is LA-UR-14-21135.
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Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Elemental composition in sealed plutonium–beryllium neutron sources N. Xu n, K. Kuhn, D. Gallimore, A. Martinez, M. Schappert, D. Montoya, E. Lujan, K. Garduno, L. Tandon Los Alamos National Laboratory, Los Alamos, NM 87545, USA
H I G H L I G H T S
A destructive chemical analysis of the PuBe neutron sources includes the solubilization and digestion of the PuBe alloy material. Plutonium was assayed by an electrochemical method. Beryllium assay and trace elemental contents were determined by ICP instruments. A large variation in trace elemental composition was observed among the five PuBe source materials.
art ic l e i nf o
a b s t r a c t
Article history: Received 25 February 2014 Received in revised form 22 June 2014 Accepted 12 October 2014 Available online 22 October 2014
Five sealed plutonium–beryllium (PuBe) neutron sources from various manufacturers were disassembled. Destructive chemical analyses for recovered PuBe materials were conducted for disposition purposes. A dissolution method for PuBe alloys was developed for quantitative plutonium (Pu) and beryllium (Be) assay. Quantitation of Be and trace elements was performed using plasma based spectroscopic instruments, namely inductively coupled plasma mass spectrometry (ICP-MS) and atomic emission spectrometry (ICP-AES). Pu assay was accomplished by an electrochemical method. Variations in trace elemental contents among the five PuBe sources are discussed. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Sealed PuBe neutron source ICP-MS ICP-AES Plutonium assay Beryllium assay Trace elements
1. Introduction Plutonium–beryllium material has been made into sealed neutron sources since excess plutonium became available from nuclear reactors in the late 1940's (Tate and Coffinberry, 1958). PuBe sealed sources are inexpensive, portable neutron generators suitable for applications that require a low neutron flux. Research institutions such as national laboratories and universities utilize PuBe neutron sources frequently for various experiments (Griffin et al., 2008), while the petroleum industry has been employing them for decades in oil well logging around the world (Buryakovsky et al., 2012). PuBe is an alloy formed by heating Pu in either metal or oxide form with Be metal at 1150 1C (Tate and Coffinberry, 1958; Wauchope and Baird, 1959). The intermetallic compound has a single face
n Correspondence to: Los Alamos National Laboratory, P.O. Box 1663, G740, Los Alamos, NM 87545, USA. Tel.: þ 1 505 667 2016; fax: þ1 505 665 4737. E-mail address: [email protected] (N. Xu).
http://dx.doi.org/10.1016/j.apradiso.2014.10.013 0969-8043/& 2014 Elsevier Ltd. All rights reserved.
centered cubic structure with the chemical formula of PuBe13 (Konobeesky, 1955; Coffinberry and Ellinger, 1956). The texture of PuBe depends on the exothermic reaction temperature attained during the formation of the compound and can be either a friable solid or a coalesced mass. Some manufacturers encapsulated the reaction container directly inside a sealed container. Inside a sealed PuBe neutron source, neutrons are produced through an alpha–neutron (α,n) nuclear reaction, i.e. alpha particles generated from the decay of Pu are absorbed by low atomic weight Be which then ejects neutrons. As a small-scale neutron generator, PuBe has operational advantages over radium–beryllium (RaBe), polonium–beryllium (PoBe), americium–beryllium (AmBe), and americium–lithium (AmLi) materials. This is due to the production of a stable and predictable neutron yield owing to the formation of an intermetallic PuBe13 compound with a prolonged operational life time accredited to the 24,360 year half-life of the Pu-239 isotope. In addition, Pu produces lower gamma intensity than other sources which is beneficial for safety reasons.
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Early commercial distribution of PuBe neutron sources was not regulated, causing a lack of registration tracking and nuclear material safeguards. Concerns have been raised regarding nuclear material illicit trafficking and nuclear proliferation by the International Atomic Energy Agency. The Offsite Source Recovery Project championed by the Los Alamos National Laboratory (LANL), USA, has recovered a large number of the unwanted sealed Pu neutron sources. Disposition of these disused neutron sources necessitates the disassembly of several PuBe sources for destructive analysis to address the U.S. Resource Conservation and Recovery Act (RCRA) and other environmental concerns. Research on sealed PuBe neutron sources has been primarily carried out on characterizing the neutron intensity and modeling the neutron spectrum (Kumar and Nagarajan, 1977; Anderson and Bond, 1963), as the source is frequently used to calibrate other measuring instruments. While non-destructive analysis methods have been applied to determine Pu content and Pu isotopic distribution (Nguyen et al., 2007; Lakosi et al., 2006); methods applied to the direct destructive assay of PuBe sources are scarce (Breakall, 1972). Destructive analysis of the chemical composition of PuBe materials in neutron sources has never been reported. The only publication on destructive chemical analysis of a neutron source (Sommers et al., 2009) described an AmBe source material evaluated by Idaho National Laboratory, USA. The lack of a literature on destructive chemical analysis may be due to the extreme challenge in radioanalytical chemistry capability as well as engineering infrastructure support. While handling a sealed PuBe neutron source requires only the minimum radiological precautions, disassembly of a source demands more rigorous engineering controls such as glove boxes to prevent radiological contamination and to minimize the radiation exposure dose to the workers. Despite the commonness of glove box enclosures in many nuclear and radiological facilities, few are equipped to disassemble sealed sources and recover neutron source materials for destructive analyses. LANL possesses the necessary capabilities: the nuclear facility infrastructure to handle bulk quantities of special nuclear materials; skilled machinists to dismantle sealed sources using glove box installed equipment; and a strong analytical chemistry team to complete trace elemental determinations for bulk nuclear materials. The integrated technologies and skills at LANL were essential to accomplish this study. In this study, five sealed PuBe neutron sources from different manufactures were disassembled using a manual Hardinge lathe located in a glove box. Loose Pu contamination was controlled by introducing clean consumables and wiping-down glove box, glove, equipment, and tool surfaces. As a general strategy, sealed source container opening employed lathe cuts along defined or presumed seams defining the placement of a press-fit or welded cap to seal the container. This resulted in cracking along the seam at a lathe cut depth prior to penetrating the inner container wall. Cracked sources were removed from the lathe and caps were separated by manually pulling on the cap using pliers. This approach effectively prevented container turnings from depositing in the source container along with PuBe material.
2. Material and methods
(Solution # S5/31/73) into the multi-element calibration/verification standard solutions. For ICP-AES measurements, multi-element calibration and verification standards were prepared from standard solutions by High Purity Standards (Charleston, SC, USA) and Inorganic Ventures, respectively. The nitric acid, hydrofluoric acid and hydrochloric acid used in the study for sample and standard preparation were Fisher Optima (Pittsburgh, PA, USA) ultrapure grade. Deionized (DI) water at Z 18 MΩ cm 1 resistivity was produced by a Barnstead (Dubuque, IA, USA) E-pure system. An AG MP-1 anion exchange resin (200–400 mesh) from Bio-Rad (Hercules, CA, USA) was converted to the nitrate form with 4 M HNO3 prior to the preparation of chromatographic columns for Pu removal. 2.2. Instrumentation and methods The trace elements were measured by both ICP-MS and ICP-AES. A PlasmaQuad PQ2 þ (Thermo Fisher, USA) ICP-MS instrument manufactured by VG Elemental was used for trace elemental analysis. This instrument was modified to interface with a radiological glove box. Detailed operating conditions are listed in a previous publication (Xu et al., 2013a). Beryllium and other trace elements were analyzed by an IRIS (Thermo Fisher, USA) ICP-AES instrument manufactured by Thermo Scientific. The ICP-AES was interfaced to a glove box so that only the torch box and the sample introduction system were located inside the glove box. The instrument was equipped with a charge injection device (CID) detector and operated in a radial-view mode. The detector covered a wavelength range of 165–1050 nm with a single pixel resolution of 0.005 to 0.012 nm. A fixed cross flow nebulizer with a Teflons spray chamber was used for sample introduction. Instrument operating conditions are listed in Table 1. All working standards and blanks were prepared in 4 M HNO3 and 0.03 M HF solution to match the sample matrix. Pu was assayed by controlled potential coulometry (CPC) (Waterbury et al., 1970). 2.3. Sample preparation PuBe alloy samples were dissolved in 6 M HCl followed by a pressurized closed vessel digestion with 12 M HNO3 and 0.1 M HF at 130 1C for approximately 12 h. The pressurized closed vessel digestion system used in this study has been described elsewhere (Xu et al., 2013b). The sample aliquot for trace elemental measurements by ICP methods contains 0.20 to 0.25 g of PuBe material, depending on the availability from the total amount of PuBe material recovered from the sources. Matrix spike samples were carried out as a QC measure whenever the total amount of material allowed. Concentrated nitric acid was added to the sample solution to adjust the final acid concentration between 8 and 10 M. This acid concentration range maintains Pu in the tetravalent state required for the chromatographic separation of Pu from trace elements. A sub-sample of 0.020 to 0.025 g of PuBe material was aliquoted from the dissolved PuBe solution for trace elemental analysis by ICP-MS. A large dilution was required to minimize the heavy ion effect. Internal standards were added to the diluted solution prior to the instrumental analysis. The remaining solution was loaded onto a packed chromatographic column consisting of 4 mL of AG MP-1 anion exchange resin to
2.1. Reagents and materials For ICP-MS analysis, multi-element calibration standards were prepared daily from standard solutions by SPEX CertiPrep (Metuchen, NJ, USA). Calibration verification standards and single element internal standards were prepared from Inorganic Ventures (Christiansburg, VA, USA) standard solutions. Neptunium standards were prepared by diluting an Amersham 9410 stock solution
Table 1 Thermo Fisher IRIS ICP-AES instrument operating conditions. RF power Nebulizer Ar pressure Auxiliary Ar flow rate Solution uptake
1150 W 30 psi 1.0 L/min 1 mL/min
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remove the Pu. The separation was completed by eluting the column with 2 mL of 10 M HNO3 and 9 mL of 8 M HNO3. The entire volume of eluent was collected and diluted 1:1 with DI water before the determination of trace elements by the ICP-AES method. Additional dilutions were also required for Be determination. Pu retained on the column was recovered by eluting with 0.1 M HCl.
3. Results and discussion 3.1. Sample description and dissolution For this study, five sealed PuBe neutron sources from different manufactures were disassembled and PuBe materials were extracted for analysis of elemental composition. Source 1 was manufactured at LANL. Sources 2 and 3 were made by the Mound Laboratories, USA, and Monsanto Research Corporation, USA, respectively. Sources 4 and 5 were archived sources of possible non-domestic origin. The PuBe materials varied significantly in appearance. Sources 1, 4 and 5 PuBe materials appeared as fine powder to granular lumps, whereas sources 2 and 3 materials were a single coalesced lump. The colors of the PuBe materials ranged from metallic to dull gray to charcoal black. The total amount of PuBe material recovered from each source and their respective appearances are described in Table 2. The sample dissolution process was performed in a glove box. An array of 2 L plastic bottles filled with water was lined up along the front portion of the glove box floor to reduce staff neutron dose. An early attempt to dissolve the PuBe alloy adopted a routine Pu metal dissolution protocol by adding 6 M HCl drop-wise to the solid sample in a plastic test tube. It was quickly discovered that this method was not suitable for PuBe alloy dissolution because the exothermic reaction was so vigorous that it resulted in sample loss due to excessive splattering. Thus, the procedure was modified to first add DI water to the solid sample, followed by slow drop-wise addition of concentrated HCl, and finally a supplement of 6 M of HCl to complete the reaction. Even with the slow, drop-wise addition of concentrated HCl, the initial reaction started vigorously. It was observed that the dark colored PuBe pieces were surrounded by hydrogen bubbles and were floating and swirling in the noble blue colored Pu solution pool, spattering occasionally. To avoid splattering related sample loss, a tall dissolution vessel was used. Undissolved solids were observed at the end of the initial reaction for all of the five PuBe materials; hence, a subsequent pressurized closed vessel digestion procedure was used to complete the digestion process. Since sources 1, 4 and 5 were in granular form, approximately 0.2 to 0.6 g of each sample was processed. For a typical sample weight of 0.6 g, the addition of 5 mL of DI water followed by 5 mL of concentrated HCl, and 3 mL of 6 M HCl solubilized a majority of the PuBe solid. The volume of the reagents was reduced proportionally for smaller sample weights. A subsequent pressurized closed vessel digestion with the addition of 13 mL 12 M HNO3 and 0.1 M HF usually took the Table 2 Recovered PuBe material weight and appearance for the five sealed PuBe neutron sources. Source
PuBe recovered (g)
Appearance
1 2
1.260 2.252
3 4
1.395 2.774
5
7.819
Uniform, fine powder, gray–black Coalesced lump, metallic gray to black Coalesced lump, gray–black Coarse powder to granular lumps, metallic gray–black Coarse powder to granular lumps, metallic gray–black
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dissolution to completion or near completion. Any amount of undissolved solid was separated by centrifugation. The coalesced PuBe mass in sources 2 and 3 was bonded to the inner container. Efforts were made to dislodge the material using small hand tools but were not successful. The entire source was then immersed in HCl for dissolution. At the end of this process the inner container was intact. It was removed from the solution prior to the subsequent nitric acid digestion. The reaction time was longer for the coalesced PuBe materials compared to the granular samples, as the surface area available for reaction was much smaller. The reaction for source 2 took 7 days to complete. Although there was substantial amount of undissolved solid in the solution, no subsequent acid digestion was conducted for this sample. 3.2. Chromatographic separation of Pu and Be In order to assay the Be and determine trace elements in the PuBe material by the ICP-AES method, the Pu in the matrix must be removed to minimize spectral interferences. A chromatographic column packed with an anion exchange resin AG MP-1 is used to retain Pu. Prior to the chromatographic separation, Pu ions must be adjusted to the Pu (IV) oxidation state with HNO3. Beryllium and other cations in the solution have negligible to minimal retention on the AG MP-1 resin; hence, they pass through the column with the use of sufficient wash volume and are collected for ICP-AES determination. Utilizing AG MP-1 resin to separate Pu from trace impurities in dissolved bulk Pu materials, such as Pu metal and oxides, is a routine protocol at LANL's Actinide Analytical Chemistry group (Mahan et al., 2000). At trace concentrations, the Be fraction generally elutes within 2 to 3 bed volumes of acid rinse solution. There was a concern that extra volume of rinsing solution might be required due to percent levels of Be in the samples. However, it was observed that Be formed a distinct yellow band on the chromatographic column right below the emerald colored Pu band during the sample elution. The yellow band eluted down the column quickly with the additional rinse solutions, and was completely off the column at the end of the elution. Nonetheless, an extra volume of 2 mL of 8 M HNO3 rinse solution was administered to the column. The extra volume was collected separately and analyzed for Be by the ICP-AES method. The ICP-AES method detection limit for Be was defined as three times the standard deviations of concentrations from seven reagent blanks that went through the chromatographic separation procedure. No detectable Be was found in the extra volume fraction which confirmed that a complete elution of Be had been achieved using the routine chromatographic separation protocol. 3.3. Pu and Be content in PuBe sources Beryllium concentrations in the dissolved, diluted samples were determined by the ICP-AES method. Dilutions were made to get the Be concentration into the calibration range of the instrument. Replicates of two to four solutions from each aliquot were analyzed. The measurement uncertainty was calculated using a Guide to the Expression of Uncertainty in Measurement (GUM) workbench software (GUM Workbench User Manual, 2009). Pu content was determined by the CPC method in replicates of three to six from each aliquot. CPC is a standard high accuracy and precision method for establishing Pu material purity in safeguarded Pu. The measured Pu and Be weight percentage for five of the PuBe sources and respective measurement uncertainties are shown in Table 3. Be content ranged from 30.1 to 40.8 wt%, whereas Pu assay ranged from 59.3 to 76.3 wt%. The mass balance is the summation of the total mass of the Pu and the Be, as well as all the other trace elements measured. It is noteworthy that an elemental mass balance of near 100% was achieved from two
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different analytical methods: ICP and CPC; each method carries its own measurement uncertainty. The near 100% mass balance suggests that the measurements are accurate in view of measurement uncertainties and that all the significant elemental constituents have been captured. It is also shown in Table 3 that none of the PuBe alloys were made with an exact, stoichiometric PuBe13 amount of reactants. Of the five sources, two of them are rich in Pu, whereas the other three have excess Be. This may be a production choice for the purpose of adjusting the neutron flux. PuBe alloy richer in Pu than PuBe13 would result in a higher neutron yield (Tate and Coffinberry, 1958), although only the exact PuBe13 stoichiometry would provide a predictable neutron yield.
Table 3 Pu and Be assay, the stoichiometric formula and mass balance for the five PuBe materials. Source
Be (wt%)
Pu (wt%)
Apparent compound
Measured element sum (wt%)
1 2 3 4 5
30.7 70.2 30.1 70.4 38.7 70.3 38.4 70.6 40.8 70.3
68.67 0.1 76.3 7 0.1 60.7 7 0.1 61.5 7 0.1 59.3 7 0.1
PuBe11.9 PuBe10.5 PuBe16.9 PuBe16.6 PuBe18.3
100.3 106.9 101.5 100.4 100.8
3.4. Trace element constituents in PuBe sources Trace elemental concentrations for five PuBe samples were determined by both ICP-AES and ICP-MS methods. Matrix interferences due to the high Be content in the sample were shown to affect several elements, such as aluminum, chromium, copper, nickel, and magnesium by the ICP-AES method. Therefore, these elements had to be analyzed by the ICP-MS method. The wavelength used for the ICP-AES analysis and the mass used for the ICP-MS measurement are provided in Tables 4 and 5. The concentrations of RCRA regulated elements in the five PuBe samples are listed in Table 4, and other trace elements are shown in Table 5. RCRA element limits are established in the Toxicity Characteristic Leaching Procedure (TCLP) by the U.S. Environmental Protection Agency (USEPA, 1999). TCLP is a protocol of extraction with a solution that simulates the ground water pH, instead of a total dissolution with strong acids. It was impossible to perform the TCLP test because of the limited amount of PuBe sample available in this study. Nonetheless, the results from the total acid digestion are presented in Table 4 along with the RCRA element limits that are converted to μg g 1 units for approximate comparison. It is noticed that all of the RCRA elements in source 1 (Table 4) are below the RCRA limits, except for lead, which is slightly more than two times the limit. Most elements in sources 2, 4 and 5 are below the RCRA limits, except for chromium, which concentrations are under twice the RCRA limit. For source 3, only cadmium exceeds the RCRA limit by less than three times. Although elevated elemental
Table 4 RCRA limits and elemental constituents measured by the ICP-MS method for the five PuBe sources, mg g 1. Element
Measurement mass
RCRA
Source 1
Source 2
Source 3
Source 4
Source 5
Arsenic Barium Cadmium Chromium Lead Selenium Silver
As_75 Ba_138 Cd_114 Cr_52 Pb_208 Se_78 Ag_107
100 2000 20 100 100 20 100
6.4 7 0.7 377 2 9.7 7 1.0 607 3 2107 11 4.4 7 0.5 217 1
4.3 7 5.7 1.0 7 0.1 o 0.2 1707 14 8.9 7 0.8 3.5 7 1.4 o 0.4
9.1 71.8 10 71 49 73 7772 4.9 70.1 8.0 77.3 0.9 70.3
7.2 72.4 o 0.2 0.4 7 0.1 1307 5 o 0.5 o3 o 0.4
8.4 7 2.4 o 0.2 0.3 7 0.1 1907 5 7.9 7 1.0 o4 o 0.5
Table 5 Trace elemental constituents measured by the ICP-MS and ICP-AES methods for the five PuBe sources, mg g 1. Element
Wavelength(nm)/mass
Source 1
Source 2
Source 3
Source 4
Source 5
Aluminum Antimony Borona Calciuma Cerium Copper Gallium Gold Irona Magnesium Manganese Molybdenum Neptunium Nickel Niobium Silicona Tantalum Tin Titanium Tungsten Zinca Zirconium
Al_27 Sb_121 B249.6 Ca396.8 Ce_140 Cu_65 Ga_69 Au_197 Fe259.9 Mg_24 Mn_55 Mo_95 Np_237 Ni_60 Nb_93 Si251.6 Ta_181 Sn_116 Ti_48 W_184 Zn213.8 Zr_90
11007 51 11 71 o4 220 7 11 o 0.2 18007 180 1.7 7 0.2 227 1 750 7 38 827 5 177 2 1.4 7 0.1 1507 8 3707 30 o 0.2 860 7 43 o 0.3 7.3 7 0.4 267 2 o 0.5 13007 55 6.6 70.3
250 7 14 o 0.2 o4 o 10 o 0.2 437 1 3.4 70.4 347 7 750 7 11 477 3 51 73 3.2 70.4 35 71 340 7 28 o 0.2 807 6 840 7 99 0.5 70.1 1.7 7 0.3 o 0.5 o 10 6.6 70.3
490 712 o 0.2 417 1 3007 13 o 0.2 37 74 1.2 7 0.1 360 7 72 780 7 22 877 3 37 72 127 1 2007 15 837 3 5.0 7 1.0 460 7 12 13,0007 5000 1.2 7 0.3 967 2 9.4 7 2.0 347 1 3.7 7 0.1
150 711 o 0.2 o3 950 7 30 207 4 417 3 6.3 70.4 o 0.2 530 7 25 327 4 737 3 140 75 1017 10 427 7 o 0.2 260 7 10 127 2 3.0 70.1 14 71 8.4 71.0 237 2 227 1
1607 10 o 0.2 o3 217 7 o 0.2 367 3 177 1 o 0.2 680 725 o3 947 6 867 7 1107 9 1407 7 o 0.2 2007 10 187 2 3.3 7 0.1 117 1 117 1 o 10 197 1
a
Elements analyzed by the ICP-AES method.
N. Xu et al. / Applied Radiation and Isotopes 95 (2015) 85–89
concentrations are expected by the complete acid dissolution process as compared to TCLP, none of the elements are over three times the RCRA limits in all of the five materials examined in this study. It is seen in Table 5 that trace elemental compositions vary significantly among the five PuBe sources. The only element that has consistent concentration among the five PuBe materials, i.e., within twice the measured concentrations, is iron. Sources 4 and 5 materials exhibited overall consistency in all elements. The low levels of boron and cadmium in these two sources suggested the ability to and interest in removing neutron absorbers. Elevated levels of molybdenum (over ten times difference in concentration) and zirconium (over six times difference in concentration) were noticed in these two sources but not in the others. Source 4 showed cerium. Elemental contents in PuBe sources 1, 2 and 3 were quite divergent except for gallium, magnesium and zirconium, which are within two times the measured concentrations. Gold was measured at various concentration levels in these three sources with a high of 360 mg g 1 in source 3, but low in the remaining sources. Boron and niobium were detected in source 3, and antimony in source 1. Significantly high levels of copper, zinc and tin in source 1 were noticed. One possible explanation could be that the soldering material used to seal the PuBe capsule was present in the sample. The solder, American Platinum Works Silvaloy No 355, which contains 22% copper, 17% zinc and 5% tin (Tate and Coffinberry, 1958) was used by the manufacturer of source 1 for a period of time in the fabrication of PuBe sources. It was decided that the high tantalum values in both samples sources 2 and 3 were not related to the PuBe source materials but to the tantalum inner container that was included in the sample solubilization. It was likely that a small amount of solid tantalum was removed from the inner container material during the initial dissolution process as the entire source was in direct contact with the HCl. Subsequently, digestion with HNO3 and HF dissolved the previously dislodged solid tantalum into the solution. As a result, 1.3 wt% of tantalum was measured in the solution of source 3. To avoid the same problem, no further nitric acid digestion was applied to source 2. Instead, the undissolved particulates were separated by centrifugation. The reason that the mass balance of source 2 deviates from 100% is unclear at this time. 4. Conclusion An analytical method has been developed to dissolve PuBe alloys using an initial addition of 6 M HCl followed by a pressurized closed vessel digestion with HNO3 and HF at 130 1C for approximately 12 h. The destructive analytical chemistry analysis for the five PuBe materials indicates that the Pu and the Be concentrations vary among different manufacturers, ranging from 59.3 to 76.3 for Pu and from 30.1 to 40.8 wt% for Be, respectively. A mass balance of near 100% confirms complete coverage of the major constituents in the analytical plan and supports the least analytical measurement uncertainty. Trace elemental constituents showed a large variation among the elements analyzed. Although
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some elemental concentrations were consistent among the five PuBe sources, most of the elements remained unique signatures for a specific sample. Most of the RCRA related elements are present below the RCRA limits in all of the five sources, except for cadmium, chromium, and lead. Nevertheless, none of these elements are over three times the RCRA limits.
Acknowledgment The authors would like to thank the Department of Homeland Security, Domestic Nuclear Detection Office (Grant no. IAA HSHQDC-14-X-00028) for funding the project, and Andres Borrego for assisting in the sample preparation work. This publication is LA-UR-14-21135.
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