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The Determination of Minor Isotope Abundances in Naturally Occurring Uranium Materials. the Tracing Power of Isotopic Signatures for Uranium

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The Determination of Minor Isotope Abundances in Naturally Occurring Uranium Materials. the Tracing Power of Isotopic Signatures for Uranium Mi V Report Series in Radiochemistry 12/1999 FI9900155 THE DETERMINATION OF MINOR ISOTOPE ABUNDANCES IN NATURALLY OCCURRING URANIUM MATERIALS. THE TRACING POWER OF ISOTOPIC SIGNATURES FOR URANIUM. Raisa Qvaskainen 30-42 Helsinki 1999 TT • •, flI,T FI9900155 University of Helsinki Faculty of Science Department of Chemistry Laboratory of Radiochemistry Finland THE DETERMINATION OF MINOR ISOTOPE ABUNDANCES IN NATURALLY OCCURRING URANIUM MATERIALS. THE TRACING POWER OF ISOTOPIC SIGNATURES FOR URANIUM. Raisa Ovaskainen Academic Dissertation To be presented with the permission of the Faculty of Science of the University of Helsinki for public criticism in the Main lecture hall AI10 of the Kumpula Chemistry Department on May 22nd, 1999, at 12 o'clock noon. Helsinki 1999 Aina liikkeella. Aina ikava kotiin. Tuolla kaukana soutaa avomerta pain tulipunainen laiva. Kurohito With deepest gratitude to Aino and Men Omnia vincit amor PREFACE The present study was carried out in the Stable Isotope Measurement (S.I.M.) unit in the Institute for Reference Materials and Measurements (ERMM), European Commission, JRC-Geel, Belgium, during 1994-1997 with a scholarship from the European Commission. Part of the work was carried out in the framework of the European Commission's support programme to the International Atomic Energy Agency (IAEA). It was completed in the Laboratory of Radiochemistry, University of Helsinki. I wish to express my deepest thanks to Professor Timo Jaakkola for his support and encouragement in the course of this work. I am grateful to Professor Paul De Bievre (S.I.M. unit) for discussions and support. I am indebted to my supervisor Dr. Klaus Mayer (Institute for Transuranium Elements, JRC-Karlsruhe) for his guidance and advice. I am greatly obliged to Mr. Willy De Bolle for highly valuable collaboration in the absolute «(235U)/«(238U) ratio measurements by Gas Source Mass Spectrometry. I wish to thank the IRMM staff for assistance in many practical matters. I wish to extend my warm thanks to Dr. Stein Deron and Ph.D. David Donohue in the IAEA for initiating the project and for providing the basic ore concentrate samples for this study. I also wish to thank my friends in Finland and abroad for their support and help. I wish to express my deep thanks to Mr. Hannu Petrelius for his help in data processing. My most deepest gratitude is due to my daughters, Aino and Meri and to my mother Martta for their love, endless patience and understanding during this work. ABSTRACT The mass spectrometric determination of minor abundant isotopes, 234U and 236U in naturally occurring uranium materials requires instruments of high abundance sensitivity and the use of highly sensitive detection systems. In this study the thermal ionisation mass spectrometer Finnigan MAT 262RPQ was used. It was equipped with 6 Faraday cups and a Secondary Electron Multiplier (SEM), which was operated in pulse counting mode for the detection of extremely low ion currents. The dynamic measurement range was increased considerably combining these two different detectors. The instrument calibration was performed carefully. The linearity of each detector, the deadtime of the ion counting detector, the detector normalisation factor, the baseline of each detector and the mass discrimination in the ion source were checked and optimised. A measurement technique based on the combination of a Gas Source Mass Spectrometry (GSMS) and a Thermal Ionisation Mass Spectrometry (TIMS) was developed for the accurate determination of isotopic composition in naturally occurring uranium materials. The absolute n(235U)/rc(238U) amount ratio (precision 0.05 percent) determined by GSMS by Mr. Willy De Bolle was used as an Internal Ratio Standard for TIMS measurements. Because the expected ratio of «(234U)/ra(238U) exceeded the dynamic measurement range of the Faraday detectors of the TIMS instrument, an experimental design using a combination of two detectors was developed. The n(234U)/rc(235U) and «(236U)/«(235U) ratios were determined using ion counting in combination with the decelerating device. The n(235U)/«(238U) ratio was determined by the Faraday detector. This experimental design allowed the detector cross calibration to be circumvented. Precisions of less than 1 percent for the n(234U)/n(235U) ratios and 5-25 percent for the n(236U)/ra(235U) ratios were achieved. The purpose of the study was to establish a register of isotopic signatures for natural uranium materials. The amount ratios and isotopic composition of 18 ore concentrates, collected by the International Atomic Energy Agency (IAEA) from uranium milling and mining facilities (Australia, Canada, Gabon, Namibia, Czech Republic, France), were determined. These signatures form the basic register. The isotopic signatures are feasible in identifying the sample origin and in separating naturally occurring or background contributions from local anthropogenic sources. With the comparison of fingerprints of unknown samples to the isotopic fingerprints of samples of known origin, it is possible to trace back unknown samples to their origin or at least to exclude suspected origins in the case of non-identity of fingerprints. This was successfully demonstrated with a number of samples of unknown origin, which were measured during the study. Generally, no significant variability was observed in the n(235U)/n(238U) ratios except in the well known case of samples originating from Oklo (Gabon). Small variations in the n(234U)/n(238U) amount ratios were understood from the radiochemical mother-daughter relationship of the two isotopes involved. The detection limit for the n(236U)/n(235U) amount ratio (DL = 0.000001) was derived from blank measurements. The limit of quantitation 0.000003 was calculated as LQ = 3DL. When the measured ratio exceeded the quantitation limit, the presence of 236U is explained. TABLE OF CONTENTS PREFACE 1 ABSTRACT 2 1.1. INTRODUCTION 6 1.1. Uranium mining and ore processing 7 1.2. Isotope mass spectrometry 8 1.2. LITERATURE 9 2.1. Natural Uranium 9 2.2. Uranium geochemical cycle and geochemistry 11 2.3. Fractionation of uranium isotopes 13 2.3.1. n(235U)/n(238U) amount ratios 15 2.3.1.1. Oklo phenomenon 16 2.3.2. 234U/238U activity ratios in nature 17 2.3.3. 234U/238U ratios of ore concentrates 20 II. EXPERIMENTAL PART 22 1. Natural uranium samples 22 11.2. Measurement technique using a combination of GSMS and TIMS 27 2.1. Gas Source Mass Spectrometry 27 2.2. Thermal Ionisation Mass Spectrometry 29 11.3. Parameters essential for high-accuracy and high-precision with the TIMS instrument 30 3.1. Mass discrimination in the ion source of the TIMS instrument 30 3.2. Instrumental calibration using IRMM 184 32 3.3. The linearity of the Finnigan MAT 262RPQ 32 3.3.1. Linearity of Faraday cups 33 3.3.2. Linearity of the Ion Counter System 35 3.3.3. Ion Counting parameters 38 11.4. Sample preparation and conditioning for TIMS measurements 41 4.1. Sample loading for TIMS measurements 42 11.5. TIMS measurements of uranium 42 5.1. Data evaluation. Internal Ratio Standard Method 46 5.1.1. Example of data treatment 47 5.1.2. Statistical evaluation of results 48 5.1.3. Uncertainty calculation 50 in. RESULTS AND DISCUSSION 52 1. GSMS measurement results of «(235U)/«(238U) amount ratios 53 2. n(234U)/n(235U) amount ratios measured by TIMS 55 3. n(236U)/n(235U) amount ratios measured by TIMS 56 4. Evaluation of results for establishing the Isotopic Identity Card 63 IV. APPROACHES TO SOME SPECIFIC MEASUREMENT PROBLEMS 66 1. Certified reference materials for quality control of environmental measurements 66 2. Samples of high 238U content 68 3. Enriched vagabonding materials 68 CONCLUSIONS 70 REFERENCES 72 Appendix A Appendix B Appendix C Appendix D 1.1. INTRODUCTION Uranium is relatively abundant in nature, as much as 3.5 u.g/g in most regions of the earth's crust, and even higher in areas where uraniferous ores are present. For this reason, it is difficult to detect anthropogenic U contamination. While a change of the major isotopes, 235U and 238U, whose natural abundance is 1/137.8, provides the best chance of determining anthropogenic activity, high concentration of these isotopes can mask the presence of other isotopic compositions. The occurrence of uranium in nature is presented in Table 1 [1]. Table 1. The occurrence of uranium in nature. Occurrence U concentration (ug/g ) Igneous rocks - basalts 0.6 - granites (normal) 4.8 - ultrabasic rocks 0.003 sandstones, shales, limestones 1.2-1.3 Earth's crust 2.1 - oceanic 0.64 - continental 2.8 Earth's mantle -0.01 seawater 0.002 - 0.003 meteorites 0.05 - chondrites 0.011 uraniferous materials - high-grade veins (3 - 8.5) • 10s - vein ore (2 - 10) • 103 - sandstone ores (0.5- 4) • 103 - gold ores (South Africa) 150 - 600 - uraniferous phosphates 50 - 300 - Chattanooga shales (USA) 60 - uraniferous granites 15 - 100 Uranium has two other long-lived isotopes, 234U and 236U. 236Uranium is generated by neutron capture on 235U, and is present in almost all anthropogenic uranium associated with nuclear fuel or weapons materials, in concentrations ranging from a few parts in 10"7 up to 10~2. In addition, the natural 234U concentration also changes when either depleted or enriched material is added to it. These two isotopes present two additional chances for detecting the presence of anthropogenic insertions into the environment. The abundances of naturally occurring uranium isotopes 238U, 235U and 234U are 99.2745, 0.7200 and 0.0055 atom percentage, respectively. Natural uranium samples have small variations in the isotopic composition due to natural isotopic fractionation, nuclear reactions or contamination by non-naturally occurring uranium. The problem in measuring is found in the accurate determination of the amount ratios, detection of small variations within these ratios and quantification of these differences, all of which pose a challenge to a mass spectromist due to the large dynamic measurement range required.
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