University of Helsinki Faculty of Science Department of Chemistry Laboratory of Radiochemistry Finland DEVELOPMENT AND COMPARISON OF ANALYTICAL METHODS FOR THE DETERMINATION OF URANIUM AND PLUTONIUM IN SPENT FUEL AND ENVIRONMENTAL SAMPLES Dina Solatie Academic Dissertation To be presented with the permission of the Faculty of Science of the University of Helsinki for public examination in the small lecture hall A129 of the Department of Chemistry on 7th June, 2002, at 14 o'clock. Helsinki 2002 Author Dina Solatie Laboratory of Radiochemistry Department of Chemistry P.O. Box 55 FIN-00014 University of Helsinki Supervised by: Dr. Maria Betti European Commission, Joint Research Centre Institute for Transuranium Elements P.O. Box 2340 D-76125 Karlsruhe Reviewed by: Dr. Christos Apostolidis European Commission, Joint Research Centre Institute for Transuranium Elements P.O. Box 2340 D-76125 Karlsruhe and Prof. Timo Jaakkola Laboratory of Radiochemistry Department of Chemistry P.O. Box 55 FIN-00014 University of Helsinki Opponent: Prof. Michel Genet Radiochemistry Group Institute of Nuclear Physics University of Paris Sud F-91405 Orsay ISSN 0358-7746 ISBN 952-10-0547-5 (nid.) ISBN 952-10-0546-7 (PDF) http://ethesis.helsinki.fi Helsinki 2002 Yliopistopaino Nothing in life is to be feared, it is only to be understood. Marie Curie ABSTRACT Radionuclides represent a very important class of environmental contaminants. Therefore, it is necessary to have available accurate, reliable and precise analytical methods to determine their bulk concentrations as well as their isotopic composition. These procedures should also be as fast as possible in order to give results in emergency cases to take decision for radioprotection of the environment and human beings. In this work, sample preparation methods for nuclear spent fuel and environmental samples have been developed and compared in terms of accuracy, precision and time response. Dissolution procedures as well as radiochemical separation based on anion- exchange and extraction chromatography have been investigated along with their applicability in different matrices. Attention has been given to uranium and plutonium. This because uranium, which is chemically toxic and may be enriched through food chain. Plutonium is an anthropogenic element with a high radiotoxicity and accumulates in bone and liver. Moreover, 241Pu decays by β-particle emission to 241Am, which is also a highly radiotoxic α-emitter with a half-life of 433 years. Uranium and plutonium have been determined in environmental and spent fuel samples, and in aqueous leachate solutions from α-doped uranium oxide, by radiometric techniques (alpha- and gamma spectrometry and liquid scintillation counting). The results, whenever possible, were compared to those obtained by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) coupled on-line with ion chromatography (IC). It was found out that the recoveries from the environmental samples are very much affected by matrix effects. This was not the case for the samples obtained from the spent fuel and aqueous leachate solutions. Separations based on the anion exchange, ion and extraction chromatography were demonstrated to be effective for attaining purified fractions of U and Pu before measurements. The minor isotopes 232U and 236Pu were also successfully analysed in spent fuels despite the large separation factors, which needed to be achieved. The comparison of radiometric and IC-ICP-MS confirmed that IC-ICP-MS is a powerful method for the detection of long-lived radionuclides. The i radiometric methods have a detection limit several orders of magnitude lower than IC- ICP-MS in the case of the shorter-lived nuclides. Nuclear track methods have been exploited to locate and separate fuel particles in soil samples. The method has been applied to a soil sample stemming from a radioactively contaminated area after the Chernobyl accident. A comparison of α-track analysis with the total α-activity showed that about 90% of the α-activity was present as clearly detectable α-spots. ii LIST OF PUBLICATIONS This thesis is based on the following publications, which will be referred to in the text by their Roman numerals. I. D. Solatie, P. Carbol, M. Betti, F. Bocci, T. Hiernaut, V. V. Rondinella, J. Cobos, Ion Chromatography Inductively Coupled Plasma Mass Spectrometry (IC-ICP-MS) and radiometric techniques for the determination of actinides in aqueous leachate solutions from uranium oxide, Fresenius Journal of Analytical Chemistry (2000) 368:88-94. II. D. Solatie, P. Carbol, P. Peerani, M. Betti, Investigation on separation/purification methodologies for the determination of 232U and 236Pu in solutions of spent nuclear fuels by α-spectrometry, Radiochimica Acta (2001) 89:551-556. III. D. Solatie, E. Hrnecek, P. Carbol, T. Jaakkola, M. Betti, Sample preparation methods for the determination of plutonium and strontium in environmental samples by low level liquid scintillation counting and α-spectrometry, Radiochimica Acta, accepted. IV. P. Carbol, D. Solatie, N. Erdmann, T. Nylén, M. Betti, Deposition and distribution of Chernobyl fallout fission products and actinides in a Russian soil profile, Journal of Environmental Radioactivity, submitted. V. V. Rondinella, J. Cobos, Hj. Matzke, T. Wiss, P. Carbol, D. Solatie, Leaching behaviour and α-decay damage accumulation of UO containing short-lived actinides, 2 MATERIALS RESEARCH SOCIETY SYMPOSIUM PROCEEDINGS, Scientific Basis for Nuclear Waste Management XXIV, in press. iii TABLE OF CONTENTS ABSTRACT i LIST OF PUBLICATIONS iii TABLE OF CONTENTS iv 1. INTRODUCTION 1 2. ACTINIDES 4 2.1 Chemical properties of plutonium 5 2.2 Chemical properties of uranium 6 2.3 The sources of actinides in the environment 8 2.4 Behaviour of actinides in the environment 12 2.5 Actinides in spent fuel 13 3. SAMPLE PREPARATION 14 3.1 Pre-treatment of the samples 16 3.1.1 Spent fuel samples 16 3.1.2 Environmental samples 16 3.2 Chemical separation 16 3.2.1 Anion exchange 16 3.2.2 Extraction chromatography 17 3.3 Preparation of sources for α-spectrometry 19 3.3.1 Electrodeposition 19 3.3.2 Co-precipitation 20 3.4 α-tracks analysis 21 3.5 Speciation of Pu in aqueous leachate solutions from 238Pu doped uranium dioxide 22 4. MEASUREMENT TECHNIQUES 24 4.1 Radiometric techniques 24 4.1.1 Alpha spectrometry 25 iv 4.1.2 Gamma spectrometry 25 4.1.3 Liquid scintillation counting 25 4.1.3.1 Calibration of the instrument 26 4.2. Inductively Coupled Plasma Mass Spectrometry 28 5. RESULTS AND DISCUSSION 30 5.1 Plutonium 30 5.1.1 Sample dissolution 30 5.1.2 Determination of plutonium by α-spectrometry 31 5.1.3 Determination of 241Pu by liquid scintillation counting 33 5.1.4 Chemical recoveries 33 5.1.5 Comparison of the methods 34 5.1.6 Use of isotopic ratios 36 5.1.7 Speciation of Pu in aqueous leachate solutions from 238Pu doped uranium dioxide 38 5.2 Uranium 38 5.2.1 Comparison of the methods 40 5.3 α-track analysis 42 6. CONCLUSIONS 44 7. ACKNOWLEDGEMENTS 46 8. REFERENCES 47 v 1. INTRODUCTION Both natural and anthropogenic radionuclides occur in the environment. The major sources of natural radioactivity are the nuclides of very long half-lives, which have persisted since the formation of the earth, along with their shorter-lived daughter nuclides and nuclides produced by cosmic rays in the atmosphere at high altitude. Anthropogenic radionuclides can enter into the environment through a variety of different processes. The most relevant sources of artificial radioactivity are global fallout from atmospheric weapons tests, local fallout accidentally released from nuclear power plants and spent fuel reprocessing plants as well as short-lived nuclides from the production of radioisotopes for medical applications. The release of radionuclides from nuclear accidents and storage of nuclear wastes has also to be considered. Nowadays, man-made radionuclides constitute an important class of environmental contaminants. Therefore, it is very important to have accurate, reliable and precise analytical methods available to measure their concentration and isotopic composition in different matrices. For example, from their isotopic composition it is possible to understand their source term and determine eventual clandestine nuclear activities [Donohue, 2002; Donohue and Ziesler, 1993]. From the knowledge of their concentration levels in spent fuels, the conditions for spent fuel waste repository, as well as leaching behaviour under specific environmental conditions can be investigated. Plutonium is one of the major concerns of the artificial transuranium elements released into the environment. From an environmental point of view isotopes of importance are 238Pu, 239Pu, 240Pu and 241Pu. All of these decay by alpha particle emission, with the exception of 241Pu, which decays by beta particle emission to 241Am. The alpha emitting isotopes of plutonium have long half-lives, they accumulate in animals and man in bone and liver, and they are highly radiotoxic. However, the largest contribution to the total plutonium radioactivity in environmental samples is due to the beta emitting 241Pu. Though its radiotoxicity is much lower than that of the α-emitting Pu isotopes, the decay product 241Am is a highly radiotoxic alpha-emitting nuclide with a half-life of 433 years. It also has a long residence time in animals and humans, and it is much more mobile in the biosphere than Pu [Shelley et al., 2001; Iwasaki et al., 1998; Hakanen et al., 1984]. 1