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Year : 2017  |  Volume : 40  |  Issue : 1  |  Page : 3-8  

Measurement and validation of uranium isotope ratio in uranium ore for isotopic fingerprinting

1 Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
2 Fukushima Project Headquarters, National Institutes for Quantum and Radiological Science and Technology, Anagawa 4-9-1, Inage-ku, Chiba 263-8555, Japan

Date of Submission27-May-2016
Date of Decision26-Dec-2016
Date of Acceptance13-Feb-2017
Date of Web Publication24-Apr-2017

Correspondence Address:
Suchismita Mishra
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/rpe.RPE_36_16

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Accurate determination of uranium isotope ratio can act as an efficient fingerprint in the nuclear forensics to identify source and intended use of illicit trafficking of uranium material. In this context, a rapid chemical separation technique to isolate uranium from uranium ore sample was developed using commercial extraction chromatographic resin. Uranium isotope ratio was measured using thermal ionization mass spectrometry (TIMS). Standard reference material (NBS U010) was used for validating the accuracy and precision of isotope ratio measurement by TIMS. The method is successfully applied to a natural ore (uraninite ore) for the determination of naturally occurring uranium isotope ratio.

Keywords: Fingerprint, thermal ionization mass spectrometry, uranium isotope ratio, uranium ore

How to cite this article:
Mishra S, Sahoo SK, Chaudhury P, Pradeepkumar K S. Measurement and validation of uranium isotope ratio in uranium ore for isotopic fingerprinting. Radiat Prot Environ 2017;40:3-8

How to cite this URL:
Mishra S, Sahoo SK, Chaudhury P, Pradeepkumar K S. Measurement and validation of uranium isotope ratio in uranium ore for isotopic fingerprinting. Radiat Prot Environ [serial online] 2017 [cited 2023 Jan 28];40:3-8. Available from: https://www.rpe.org.in/text.asp?2017/40/1/3/205053

  Introduction Top

Uranium has wide spectrum of applications in various fields. It plays an important role for human life in supplying electrical energy as fuel in the nuclear reactor. Same time, it is being used in making of depleted uranium (DU) munitions and nuclear weapons. Out of 25 known isotopes of uranium (mass ranging from 217 to 241), only three occur naturally at significant concentrations, i.e., 238 U,235 U, and 234 U with natural abundance of about 99.274%, 0.7204%, and 0.00548%, respectively. The content of 235 U is an important feature of uranium application.235 U (natural uranium [NU or Unat] - 0.720 atom% or 0.711 weight%) is expected to be found in undisturbed natural samples (uranium ore or uranium ore concentrate [UOC]). The main use of NU is in pressurized heavy water reactors as fuel for electricity production. DU with 235 U (0.2–0.4%) is used mainly in armor piercing ammunition, in reactive armor of tanks, and in radiation shielding and is also used as ballast weights in aircraft. Uranium that is used to fuel light water nuclear power reactors is generally enriched to a level of 3–5% and is considered as low-enriched uranium and sometimes with as much as 19.75%235 U. High-enriched uranium (HEU) with > 20%235 U is mainly used for nuclear weapons or devices. The weapon-grade uranium typically contains around 90%235 U (HEU). Therefore, the composition of uranium isotopes is an indicator of the intended use of the material.

Isotope ratios of 54.8 × 10−6 and 72.5 × 10−4 are expected for 234 U/238 U and 235 U/238 U atom ratios, respectively, in NU samples (e.g., uranium ores, UOC) and are observed in some uranium containing mineral deposits also. However, the variations in the isotope ratios in natural samples exist due to various physical, chemical, or even biological processes, including mass fractionation, redox transitions, radioactive decay, radioactive disequilibrium, alpha recoil, and neutron capture. Variation in 234 U/238 U in water has been observed long ago due to preferential leaching of 234 U. Yamamoto et al., 2003[1] reported 234 U/238 U value to be several times higher (almost 51 times) than the equilibrium value in hot water springs in Japan. Karpas et al., 2006[2] showed that the extent of the disequilibrium can serve as a natural isotopic tracer for proving exposure pathways. Until very recently, it was assumed that the current 235 U/238 U ratio was a constant value (=72.5 × 10−4) in our solar system because uranium was thought to be too heavy to undergo significant isotope fractionation. Recent work has in fact shown variability in the terrestrial 238 U/235 U ratio over a range of 1.3% with the help of modern analytical techniques [3],[4] and also pointed out their location-specific isotope ratio variations. Bopp et al., 2009[5] examined uranium ore from six uranium mines, sampling two low-temperature deposits, and four high-temperature deposits, and the study concluded that 238 U/235 U fractionation takes place during the low-temperature redox transition, citing higher 238 U/235 U ratios from low-temperature deposits. In another study, Brennecka et al., 2010[6] showed that the data obtained from UOCs of mining facilities around the world show clear evidence that the depositional redox environment, in which uranium is precipitated is the primary factor affecting 238 U/235 U fractionation. Low-temperature uranium deposits are, on average, isotopically ~ 0.4% heavier than uranium deposited at high temperatures or by nonredox processes.238 U/235 U ratios coupled with 235 U/234 U ratios in the same sample provide evidence that the redox transition, U(VI)→U(IV), at low temperatures is the primary mechanism of 238 U/235 U fractionation and that aqueous alteration plays a very limited, if any, role in fractionation of the 238 U/235 U ratio. Chernyshev et al., 2014[7] described the 238 U/235 U isotope ratio variations in minerals from hydrothermal uranium deposits. The natural isotopic variation of uranium is therefore a potential signature that can be used to trace the origin of UOC.

IAEA Illicit Trafficking Database [8] states many incidents of nuclear trafficking involving low-grade material including NU. This shows early fuel cycle stage materials, for example, uranium ores or uranium concentrates as a commonly intercepted material. Thus, there is a strong need to understand and make forensic signatures of different ore types to trace the origin of such materials. In studies by Varga et al., 2011,[9] the important analytical methodologies in the nuclear forensic investigation of UOCs (yellow cakes) for origin assessment have been presented. Uranium, lead, or strontium isotopes were used for the attribution of nuclear materials in previously reported studies.[10],[11],[12] A recent nuclear forensic investigation of an UOC sample by Keegan et al., 2014[13] shows that out of extensive range of measured parameters, the key “nuclear forensic signatures” used to identify the material were the uranium isotopic composition, lead and strontium isotope ratios, and the rare earth element pattern. Therefore, it is important to measure the isotope ratios of uranium samples (or isotopic composition) with high precision and accuracy to identify the origin or the source of the sample and act as isotopic fingerprint in radiation protection, nuclear safeguards, and nuclear forensics.

Since NU samples either ore or ore concentrate contain high concentration of inherent NU, it is more difficult to detect small quantities of uranium from artificial nuclear sources. There is a great challenge to measure accurately the small isotopic variation to identify the origin of uranium deposits. Inorganic mass spectrometry, for example, thermal ionization mass spectrometry (TIMS), is one of the robust analytical techniques over radiometry and electron microscopy to quantify the isotopic abundance of uranium in natural samples. The advantages of TIMS over α counting include its higher precision, smaller amount of sample, and shorter analytical procedure. However, to achieve accurate analyses, good isolation of uranium is indispensable because ionization efficiency of uranium in TIMS is so low that interference by coexisting elements is a serious problem. Most recent separation techniques, which utilize the anion-exchange resin AG1X8, are mainly derived from Chen et al., 1986.[14] However, Adriaens et al.,1992[15] reported that not all iron and lead can be separated from uranium using this method. Since uranium forms stable chloride complexes which have high distribution coefficients (Kd: 103 at 9M HCl condition), purification of uranium is usually carried out in HCl media. However, Fe 3+, Co 2+, Cu 2+, and Zn 2+ also form chloride complexes similar to those of uranium and are adsorbed on the resin simultaneously, making it difficult to efficiently separate uranium in silicate samples. Use of the commercial extraction chromatographic resin (UTEVA) avoids the problem of interference from coexisting anions because this resin is of the chelate-exchange type.[16]

A rapid chemical separation procedure has been developed to isolate uranium from soil samples affected by Fukushima daiichi nuclear power plant (FDNPP) accident using ion exchange resin (Dowex1X8) and UTEVA, and uranium isotope ratios were measured using TIMS in earlier studies.[17] The present study will describe a rapid method for uranium separation using only extraction chromatographic resin (UTEVA), considering the high concentration of uranium in case of uranium ore. This work has been initiated with an objective to apply extensively for characterization of various uranium ores to make a baseline data and potential application for radiation protection and nuclear forensic studies.

  Experimental Top

Sample description and processing

Uranium ore samples were collected from Narwapahar (NUO) and Banduhurang (BUO) uranium mines situated at Jharkhand, India. The samples of Jharkhand uranium ores belong to a typical high temperature magmatic/migmatic type and also occur as inclusions within gangue minerals and found associated with oxides, sulfides, and rare earth and rare metal minerals [18] and the uranium deposit is of vein type.[19] Samples were oven dried and powdered (< 150 μm). The powdered samples (about 250 mg) were then chemically digested using microwave digestion system (Milestone, MLS 1200 Mega) for elemental analysis.

Instrumentation and measurement

The total uranium content was measured using inductively coupled plasma mass spectrometry (ICPMS; Agilent 7500, Agilent Technologies, USA) at National Institute of Radiological Sciences (NIRS) with rhodium as an internal standard. The interference of other elements in uranium separation also tested using ICPMS.

Uranium isotope ratios (234 U /238 U and 235 U/238 U) were measured using TIMS (VG Sector 54-30 TIMS, VG Isotopes Ltd., UK) at NIRS. VG Sector 54-30 TIMS is equipped with nine Faraday cup collectors and Daly-ion detection system positioned behind axial Faraday and wide aperture retardation potential (WARP) energy filter at NIRS was used for the isotopic measurement of uranium. A triple filament assembly was used for the thermal ionization of uranium isotopes. The filament material was 5-pass zone refined rhenium ribbon (H Cross, 99.999%) with 0.003 cm thickness and 0.07 cm width. Triple rhenium filaments were prepared by degassing for 1 h with 4 A current under a vacuum better than 5 × 10–6 mbar. Pure isolates were dissolved in nitric acid and mounted, by evaporation, on the side filament of a triple filament. In case of triple filament assembly, the central filament is very hot and it ionizes and causes the side filaments to evaporate. Since the sample is not directly ionized, it was noticed that mass fractionation is slower than that for a single filament. The central filament was heated to produce 187 Re ion current of 0.1 V and then side filaments were heated to produce ion current of 0.03 V for 235 U. Uranium masses 234, 235, and 238 were measured dynamically using Daly-ion counting and Faraday cup collectors with mass jumps.

234 U/238 U and 235 U/238 U isotope ratios were determined by static data collection on the Daly-ion counting and Faraday cup collectors. All ratios were taken as the grand mean of seven blocks of ten measurements over a period of 80 min. The vacuum during data acquisition was better than 2 × 10–8 mbar in the flight tube as well as that in the ion source. Accuracy and precision of uranium isotope ratios measurement depend on the linearity of the detection system and mass fractionation of the isotopes during the run. The advantage of WARP filter is the enhanced transmission (~100%) of U + ion, abundance sensitivity of 10 ppb at 1 amu with respect to 238 U, and suppression of tailing effect of adjacent strong ion.

Uranium separation for isotope measurement

A rapid methodology using extraction chromatographic resin (UTEVA) only was used for the separation of uranium for uranium isotope ratio measurement. Analytical separation scheme for uranium separation using this method is given in [Figure 1]. Uranium was separated and preconcentrated using extraction chromatography with UTEVA resin from the mixed standard solution (XSTC-1 and XSTC-13). Column was packed with 0.5 ml of precleaned UTEVA resin, 100–150 μm particle size in MUROMAC polypropylene column (7 mm × 60 mm size). The UTEVA resin contains diamylamylphosphonate as a specific extractant. The UTEVA column was conditioned with 1.5 ml of 4M HNO3. The microwave digested sample was evaporated completely and taken in 1 ml of 4M HNO3 to pass through UTEVA, followed by a washing of 5 ml of 4M HNO3. Then, column was washed with 6 ml of 5M HCl to remove thorium and finally uranium was eluted from UTEVA column with 6 ml of 0.1M HNO3. Complete organic destruction was carried out in the eluent collected from UTEVA column by treating with H2O2 and HNO3 before loading onto TIMS filament.
Figure 1: Extraction scheme for uranium separation in uranium ore by extraction chromatography

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  Results and Discussion Top

Measurement of total uranium concentration

ICPMS at NIRS, Japan, was used for the total uranium concentrations in different environmental matrices, which yielded detection limits 0.001 μg/L for uranium. The ICPMS detection limit was calculated as three times the standard deviation of the calibration blank measurements (1:1 v/v HNO3: Milli-Q water, n = 10). The relative error of ICPMS results for the reference sample (lake sediment JLK-1) for 238 U was 0.56%. The parameters for data acquisition and optimization conditions are reported elsewhere.[20] The total uranium concentrations in uranium ore vary from 274 ppm to 291 ppm. These values are compared with previously reported data from the same locality and found to be within the range.[21],[22]

Uranium separation and isotope ratio measurement

Recovery of uranium and interference of the presence of different metal ions during separation was checked with mixed standard solution (XSTC-1 and XSTC-13).

The chemical separation of uranium is done using a single UTEVA resin to remove iron, cobalt, copper, and zinc since they form chloro complexes similar to those of uranium and absorb in resin.[16] The recovery of uranium using the single UTEVA method was found to be 90 ± 5%. This method is applied for the separation of uranium in microwave digested uranium ore sample. Uranium concentration being higher in uranium ore, only 250 mg of sample was taken for complete digestion. Single UTEVA was found to be suitable for removing the interfering elements. The method was found to be fast compared to the method used in our previous study.[17]

Uranium isotope ratios (234 U /238 U and 235 U/238 U) were measured using TIMS. The precision and accuracy of the measured uranium isotope ratios 235 U/238 U and 234 U/238 U in TIMS was checked against the analyses of certified standard reference material (NBS U010) as shown in [Figure 2].
Figure 2: Precision and accuracy of uranium isotope ratio measurement for 235U/238U and 234U/238U by TIMS with NBS U010

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Application for the origin determination

The isotope ratios of 234 U/238 U and 235 U/238 U in standard and samples are shown in [Table 1].234 U/238 U profiles obtained in our investigation are in proximity to secular equilibrium.235 U/238 U ratio observed in the samples shows of natural origin. In this particular study, the uranium ore (uraninite) considered is mainly from vein deposit type. The uranium atom ratios observed in the present study were compared with the previously reported data of same type of uranium deposits from different countries (high temp redox and vein type deposit)[6] and were found to have distinct variations as shown in [Table 2]. Recent studies of uranium ores and UOCs have shown significant elemental and isotopic heterogeneity from a single mine site such that some sites have shown higher variation within the mine site than that seen between multiple sites.[23] Therefore, to avoid these localized elemental variations, it is recommended that representative sampling for an area has to be undertaken before establishing the uranium isotopic ratio or elemental distribution to identify the originating mine for an unknown ore sample.
Table 1: Uranium isotope ratios from standard and samples by TIMS

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Table 2: Comparison of uranium isotope ratios in uranium ore from present study with other reported values of same kind of uranium deposits around the world

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  Conclusions Top

The present methodology was found to be rapid and very effective for uranium separation for isotope ratio analysis by TIMS. The method is successfully applied in uranium ore for uranium isotope ratio measurement. However, extensive measurement in a large number of samples of uranium ore and UOCs is required to establish uranium isotope ratio as a fingerprint for origin assessment. For this, a detailed study has been planned and will be carried out in our future studies.


SM sincerely acknowledges Japan Society for the Promotion of Science (JSPS) for facilitating the analysis of the samples at NIRS, Japan during her JSPS postdoctoral fellowship tenure.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Figure 1], [Figure 2]

  [Table 1], [Table 2]

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