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Measurement of uranium isotope ratios in keratinous materials; a non-invasive bioassay for special nuclear material. John Douglas Brockman, John W. N. Brown, Jonathan S Morrell, and J. David Robertson Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b02144 • Publication Date (Web): 03 Aug 2016 Downloaded from http://pubs.acs.org on August 3, 2016

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Analytical Chemistry

Measurement of uranium isotope ratios in keratinous materials; a non-invasive bioassay for special nuclear material. John D. Brockman*1‡, John W. N. Brown2‡, Jonathan S. Morrell3‡, J. David Robertson1, 2‡ 1. University of Missouri - Columbia Research Reactor, 1513 Research Park Drive, Columbia, Missouri 65211, United States, [email protected] 2. University of Missouri - Columbia Chemistry Department, 601 South College Avenue, Columbia, Missouri 65211, United States 3. Y12 National Security Complex, Oak Ridge, Tennessee 37830, United States Author Contributions

KEYWORDS Uranium biomonitor, multi collector, ICP-MS, nuclear proliferation safeguards, enriched uranium, hair, fingernail, toenail

ABSTRACT

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Hair, toenail, and fingernail are non-invasive, integrative biological monitors routinely used to assess mineral intake.1-4 In this study, we demonstrate the feasibility of distinguishing between exposure to natural, depleted, and enriched U by measuring the 236

235

U/238U,

234

U/238U, and

U/238U ratios in the hair, fingernails, and toenails of occupationally exposed workers and

control volunteers. The exposure history of cases and controls to non-natural U was assessed through voluntary self-reporting using a simple questionnaire. The measured U isotope ratios and U concentration in the hair, toenail, and fingernail of cases were compared to a non-exposed control group. No difference was observed in the uranium concentration between the two groups. Significant differences between the cases and the control group were observed in the and

236

235

U/238U

U/238U isotope ratios but not the 234U/238U. This is the first time that hair, fingernail, and

toenail have been demonstrated to be sensitive to occupational exposure to enriched and depleted U, a result with significant implications for proliferation compliance monitoring.

Introduction The scientific tools used to support nuclear security investigations were recently reviewed by Keegan, Kristo, Toole, et al.5 The review focuses on characterization of signatures associated with interdicted nuclear material for source attribution. Two separate cases described in the review involved high enriched U (HEU) in powder form that was seized by police forces in Paris and Moldova. While the review describes numerous analytical approaches to aid in determining the source attribution of the interdicted material, no mention is made of means to establish human interaction with the HEU. A bioassay sensitive to HEU could be used to establish a suspect’s involvement in handling HEU by law enforcement agencies in nuclear proliferation or smuggling investigations. The goal of this work was to determine if hair, fingernail clippings,


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and toenail clippings could reflect exposure to non-natural sources of U in a small group of occupationally exposed workers using U isotope ratio analysis. U is a primordial, naturally occurring radioactive element comprised of 99.2752 – 99.2739 atom% 238U, 0.7202 – 0.7198 atom% 235U and 0.0059 – 0.0050 atom% 234U.6 The 235U/238U ratio measured in basalts, granites, seawater, corals, shale, ferromanganese crusts, and other terrestrial materials varies by 1.3‰, ranging from 0.007248 in black shale to 0.007257 in manganese nodules.7,8 The largest variation in the natural 235U/238U ratio occurs in U deposits at natural fission reactors, such as the Oklo site in western Africa.9 The isotope 235U is enriched primarily for use in nuclear applications. Most light water reactors in the world use a nuclear fuel that has been enriched to 3-5% 235U. An enrichment of higher than 20% 235U constitutes HEU. An 235U enrichment greater than 90% is used as fuel in specialized nuclear reactors, isotope production targets, and nuclear weapons. Depleted U (DU) has a 235U/238U ratio less than the natural ratio and is a byproduct of U enrichment that is used in military munitions, armor, and radiation shielding. Measurement of the 235U/238U, 234U/238U, and 236

U/238U isotope ratios allows discrimination between natural and anthropogenic U.1,10,11

Human exposure to natural U occurs through the normal consumption of contaminated food and water. In a 2015 review, Karpas reported that within the United States people are exposed to 0.03 – 4 µg per day of natural U through ingestion.12 Occupational exposure to natural U may occur during mining and refining. Occupational exposure to anthropogenic U may occur during U isotope enrichment, fuel production, or other areas of the nuclear fuel cycle. Soldiers and civilians may also be exposed to depleted U in zones where DU munitions have been used.13,14 DU is also present in large quantities at U enrichment facilities.15 A biomonitor for



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anthropogenic U would also be useful for monitoring U exposure in support of law enforcement, following a nuclear accident, nuclear security investigations, and public health studies. A U bioassay that is useful for nuclear security investigations must be able to distinguish between anthropogenic and natural U exposure, be sensitive to U exposure over a useful time period, and be simple to collect. Anthropogenic U used in nuclear programs will have some level of enrichment in 235U, 234U, and 236U that is distinguishable from natural U. Health physics surveillance programs monitor urine and feces in workers at risk for occupational exposure to anthropogenic U in the nuclear industry. The internal radiation dose from U exposure is determined by measuring the concentration of U in urine or feces and coupling the data with a U biokinetic compartment model provided by the International Commission on Radiological Protection.16 A major limitation to this approach for nuclear proliferation and nuclear smuggling investigations is that the compartment model is limited by the rapid excretion of U in urine and feces. U exposure that occurred weeks or months prior to urine collection is obscured by U intake occurring over the previous days or hours.17 In 2009 Li, Karpas, et al. published a compartmental model of U in human hair for estimating internal dose based on U hair concentration.12,17 This work demonstrates that hair, fingernail, and toenail tissues could be useful as long term, non-invasive biomonitors of U exposure. Hair and nail are composed primarily of the protein keratin which is a stable endpoint removed from further metabolic process. U may be incorporated into hair metabolically and reflect internal exposure. U in the hair and nail sample can also result from exogenous contamination of the hair and nail surface. The efficacy of hair analysis for determining exposure to U was reviewed in 2014 by Joksic and Katz.4 In 2005, Mohagheghi et al. described results on a study that examined the use of urine and hair to monitor exposure to U for medical surveillance.18 The study focused on


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measuring background levels of 235U, 236U, 238U, and 232Th in hair, urine, and drinking water using alpha spectrometry. The authors conclude that the higher concentrations of U in hair make it a viable candidate for bioassay but that a bio-kinetic model is needed to determine dose from U exposure for medical physics applications. In 2005 Karpas et al. measured the 234U/238U ratio in well water and in the hair, nail, and urine samples collected from people who consumed the well water.12,19 The 234U/238U ratio measured in the well water and the biological monitors varied from 5.1 x 10-5 to 2.5 x 10-4. In the Karpas study, the 234U/238U ratio measured in hair and nail was highly correlated (r=0.91) with the 234U/238U measured in well water. The 234U/238U ratio in urine was also correlated (r=0.72) with the 234U/238U ratio in well water. This work suggests that hair and nail could be useful for measuring exposure to anthropogenic U by monitoring 235

U/238U, 234U/238U, and 236U/238U isotope ratios.

In this paper the isotope ratios

235

U/238U, 234U/238U, and 236U/238U measured in the hair,

toenail, and fingernail samples of controls from a self- reported population not exposed to special nuclear material were compared to the isotope ratios of cases who self-report occupational, nonnatural U exposure. Occupationally exposed volunteers were recruited, under an Internal Review Board (IRB) approved protocol, from the US Department of Energy Central IRB office, the University of Missouri, and the Y12 National Security Complex (NSC). The Y12 NSC originally housed an electromagnetic separation plant and enriched U from feedstock material that was reclaimed from reactors operating at the Savannah River Site and Hanford. It also used feedstock from the Idaho Chemical Processing plant, the Oak Ridge Gaseous Diffusion Plant (K25), and the Paducah Gaseous Diffusion Plant. Today, workers at the Y12 NSC routinely handle depleted, natural, enriched, and highly enriched U.



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The hair and nail samples collected from this occupationally exposed population, as well as other exposed populations, may be contaminated with non-natural U through surface contamination. Nuclear weapons testing, depleted U munitions, and nuclear accidents release anthropogenic U with an isotope ratio that reflects the source. 20 The released U and other radionuclides are often bound to particles which persist in the environment.21 For example, in 2014, Meyers et al. demonstrated that U isotope ratios measured in soil collected from two former U facilities reflected the work being conducted at the site.22 Individuals who work at or live near a nuclear site could come into contact with soil, dust, or other environmental materials that contain anthropogenic U.

It is common practice to clean the surface of hair and nail

samples prior to analysis to remove external contamination.4 Toenails and fingernails are relatively protected from external contamination compared to hair, particularly at work sites where personal protective equipment such as gloves and close toed shoes are required. In occupationally exposed individuals the greatest levels of exogenous contamination were expected to be present in hair followed by fingernails and then toenails. Regardless of internal or exogenous origin, measurement of 235U/238U, 234U/238U, and 236U/238U ratios that deviate significantly from the natural U ratios can definitively demonstrate that an individual came into contract with anthropogenic sources of U. In the case of monitoring a nuclear site or a nuclear accident, both endogenously deposited U and external contamination could provide useful information about contact with anthropogenic U. Experimental Study Recruitment


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Volunteers were recruited under protocols approved by the University of Missouri Health Science IRB 1202836 and US Department of Energy (DOE) central IRB DOE000010. Those willing to participate in the study were sent a packet that included a simple survey, fingernail/toenail/hair collection kit and a self-addressed return envelope. Volunteers were recruited into the study from the Y12 national security laboratory under the IRB approved protocol. To confirm occupational exposure, all volunteers were asked if they had ever participated in a U urine analysis or fecal analysis program at work. The participants selfreported their exposure history to U by responding to a survey question which asked to choose all that apply: exposure to depleted U, natural U, low enriched U, high enriched U, >90% 235U, 233

U, don’t know, I have not worked with U, and other (please specify). Participants were asked

if they currently worked with U. If the participant responded “no” they were asked how many years it had been since they had last worked with U. Participants were asked to self-report, with a yes or no response, if their U urinalysis or fecal analysis indicated elevated U levels. Participants were not asked in the survey if they had participated in “in-vivo” whole body counting surveillance. Volunteers for the control group, aged 18 years or older, were recruited from University of Missouri faculty and staff. Volunteers in the control group self-reported no occupational U exposure. Participants self-collected samples and returned materials via mail. The study consisted of 6 possible cases and 7 controls. Reagents Trace metal grade nitric acid and hydrochloric acid, ACS grade sulfamic acid, ascorbic acid, sodium nitrite, ferrous nitrate, aluminum nitrate nonahydrate, hydrofluoric acid, 20% titanium trichloride, and all glassware were purchased from Fisher Scientific (Fairlawn, NJ U.S.A.). A Millipore Milli-Q water system (Billerica, Ma U.S.A.) provided the 18MΩ ultra-pure water. A 7 ACS Paragon Plus Environment


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vacuum box and pre-packed UTEVA resin columns were purchased from Eichrom (Lisle, Il U.S.A.) . The certified reference material (CRM) U010 was purchased from the U.S.A, DOE New Brunswick Laboratory (Argonne, IL U.S.A.), and used to make solution standards. A NIST traceable (SRM 3164) U ICP-MS standard was purchased from High Purity Standards in Charleston, SC U.S.A. and used as an ‘in-house’ quality control. The 235U/238U isotope ratio in the U standard was not certified by NIST. Pappas et al. have reported the 235U/238U isotope ratio in NIST 3164 to be 0.72%.23 In this study the measured 235U/238U ratio was 7.1870 x 10-3 (6.8 x 10-6). To our knowledge, a matrix matched (hair or nail) reference material with certified U isotope ratios from a metrology laboratory is currently not available. In addition, a sample of CRM DC73347 hair from the Chinese National Analysis Center for Iron and Steel (NCS) in Beijing, China was prepared and digested with each sample set. The NCS DC73347 hair is not certified for U isotope ratios. Digestion and U Separation Hair, toenail, and fingernail samples were not cleaned prior to analysis. Samples were digested in 3.1 mL of nitric acid and 1 mL of 30% hydrogen peroxide using in a Milestone Ethos Plus microwave digestion system (Shelton, CT U.S.A.). The microwave was set to heat samples to 140 °C at 400 W for 10 minutes and then increase the temperature to 190 °C at 600 W for 25 minutes. The samples were digested in sets of 10. Each sample set included a sample of CRM NCS DC73347 hair reference material and a complete process blank. Following digestion the U was separated from the samples using UTEVA columns. Details of the separation are reported in Brown et al.24 U Isotope Ratio Analysis


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Isotope ratio measurements were made using a Nu-Plasma II MC-ICP MS equipped with a DSN-100 desolvating nebulizer manufactured by Nu Instruments (Wrexham, UK). Instrument signal optimization, ion counter gains, and inter-faraday gain calibrations were performed daily. At the beginning, middle, and end of each analysis, a 10 ng·g-1 U010 standard was analyzed to calibrate mass bias and ion counter gains. The major isotope 238U was measured on a faraday cup while simultaneously measuring 236U, 235U, or 234U using ion counter detectors. The normal 238

U signal measured on a faraday cup for the 10 ng g-1 U010 standard was 4.8 V. The

exponential model was used to calculate the mass bias correction factor.25 The samples were bracketed with acid blanks and analyzed with an analytical blank and the reference material NCS DC73347 hair. The NIST traceable natural U standard (1 ng g-1) was analyzed at the beginning, middle, and end of each set of samples. The concentration of 238U was measured using an external calibration curve constructed from 3 U010 standards ranging in concentration from 100300 pg·g-1. Details of the MC ICP-MS measurement are reported in Brown et al.24 Results and Discussion A matrix spike was used to measure U recovery by splitting two nail samples prior to the U separation and spiking the two samples with 3 ng of a U standard prepared from CRM U010 . The measured average U recovery was 105 (4%). The instrumental limits of detection for each U isotope (in ng·g-1) were 8.1 x 10-3 (238U), 3.2 x 10-7 (236U), 7.2 x 10-5 (235U), and 4.8 x 10-7 (234U). The concentration of U measured in NCS DC 73347 was 70 (18) ng·g-1. The CRM NCS DC 73347 hair provided an informational U concentration value but was not certified for U concentration or U isotope ratios. The NCS DC 73347 material was measured 6 times and the natural U standard was measured 18 times over 6 analysis sets. The 235U/238U measured in the DC 73347 reference material was 7.178 x 10-3 (2.8 x 10-5). The 235U/238U measured in natural U 9 ACS Paragon Plus Environment


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standard was 7.1870 x 10-3 (6.8 x 10-6). The uncertainties reported for the U isotope ratios in NCS DC 73347 and the in-house natural U reference materials is the standard error calculated from the mean value of the measurements made over 6 different analysis sets. These results compare well with the natural 235U/238U ratio of 7.202 – 7.198 x10-3.7,8 The concentration and 235

U/238U, 234U/238U, and 236U/238U isotope ratio measurements for the controls are reported in

Table 1 and the cases, with self-reported exposure information, are reported in Table 2. The uncertainty reported for the isotope ratios of the cases and controls is the expanded uncertainty calculated from the uncertainties of the instrument isotope ratio measurement, the measurement blank, the mass bias correction standard, and the ion counter gain correction, with k=2. The mean 235U/238U ratio with standard error measured in all matrices of the control population was 7.193 x 10-3 (3.6 x 10-5).

The 235U/238U values measured in each case was compared to the

aggregate control group using a two-sided, one sample student-t test. The cases with a 235U/238U ratio that are significantly different than the aggregate control group, p90% 235U, HEU, DU, NU, LEU current worker U urine analysis not elevated >90% 235U, HEU, DU, NU, LEU Current Worker U urine analysis not elevated

error = expanded uncertainty at k=2, N/A = not available * indicates a significant difference from the control, p

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