a new analytical Large Geometry â Secondary Ion Mass Spectrometry (LG-SIMS) laboratory that has been established at JRC-ITU, jointly funded by the JRC and ...
Enhancing the EC's analytical capabilities for environmental sample analysis for nuclear safeguards purposes by the establishment of a new Large Geometry Secondary Ion Mass Spectrometry (LG-SIMS) laboratory. P.M.L. Hedberg1, P. Peres2, T. Fanghaenel1, K. Luetzenkirchen1, K. Mayer1, P. Meylemans3, P. Schwalbach3, M. Wallenius1. 1
European Commission, Joint Research Centre (JRC), Institute for Transuranium Elements, P.O. Box 2340, D-76125 Karlsruhe, Germany 2 Cameca, 29 Quai des Grésillons, 92622 Gennevilliers-Cedex, France 3 European Commission, Directorate General for Energy, Directorate Nuclear Safeguards, L-2920 Luxembourg, Luxembourg
Abstract Nuclear activities unavoidably leave fingerprints in the environment. Such fingerprints may consist of characteristic building structures, of typical supply lines or of minute releases of process material. In particular, the release of micron-sized aerosol particles to the immediate environment is very difficult to avoid completely, especially when nuclear materials are processed in larger quantities. These micro-particles contain the isotopic signature of the handled nuclear materials. This allows nuclear safeguards authorities with specialised sampling techniques and analytical laboratories to verify the completeness of a state's declaration and to check the consistency of measured material properties (i.e. isotopic composition) with declared operations at nuclear facilities. This methodology has been in particular applied to uranium enrichment facilities. At the European Commission (EC) Joint Research Centre's (JRC), Institute for Transuranium Elements (ITU), uranium particle analysis for nuclear safeguards purposes has been performed on environmental samples since the late 90's with the EC Directorate General Energy (DG-ENER) as its main user. In recent years, significant efforts have been made to enhance the analytical techniques and processes. Recently, significant enhancements have been achieved in collaboration with a leading manufacturer of analytical equipment by implementing new purpose built systems for particle analysis in the field of Secondary Ion mass Spectrometry (SIMS). This paper describes the purpose and outlines the performance of a new analytical Large Geometry – Secondary Ion Mass Spectrometry (LG-SIMS) laboratory that has been established at JRC-ITU, jointly funded by the JRC and DG-ENER. The laboratory will mainly be used for analysing uranium bearing aerosol particles collected on cotton swipes during nuclear safeguards inspections, but it will also be involved in other safeguards related applications and nuclear forensics. This paper gives an overview of the capabilities and enhancements that can be expected from this new laboratory. It also describes the use of environmental sampling, followed by high performance trace analysis of particles, in the context of European nuclear safeguards.
1. Introduction nuclear safeguards The need to safeguard, track and detect nuclear material is of increasing interest with the current security concerns over potential terrorist acts, undeclared nuclear activities and political issues with new nations seeking advanced nuclear capabilities. The main objective for nuclear safeguards is the timely detection of diversions of significant quantities of nuclear material like Uranium (U) and Plutonium (Pu). Traditional safeguards work mainly focuses on material accountancy measures, physical verification of declared quantities and containment and surveillance. However, safeguards authorities also apply methods for detection of non-declared nuclear activities by the use of environmental sampling and other tools like satellite imaging etc. Under the Euratom Treaty, the European Commission has the duty to assure that nuclear material is only used for declared purposes. The European Commission Directorate General for Energy (DG-ENER) and in particular its Directorate for Nuclear Safeguards with support from the Commission's Joint Research Centre (JRC), have the task to ensure that nuclear material within the EU is not diverted from its intended use, and that the safeguarding obligations that have been agreed with third parties are complied with. At an international level, DG-ENER cooperates with the International Atomic Energy Agency (IAEA) and does the control of nuclear materials and facilities within the 27 member states of the European Union. DG-ENER has approximately 160 inspectors that perform around 1500 inspections at nuclear facilities each year. Of particular interest for the inspectors are those nuclear materials that are proliferation sensitive, a focus is thus on plutonium and highly enriched uranium. 239
238
Pu is produced in nuclear reactors from U by neutron capture. The plutonium can be extracted from the irradiated fuel elements in a reprocessing facility, which makes reprocessing facilities of particular concern for safeguards authorities. The dissolved nuclear fuel at the start of the reprocessing process offers the first opportunity to the safeguards authorities to gain precise knowledge of the contained nuclear materials since the construction of the fuel element. For this reason, DG-ENER established two on-site analytical laboratories at the reprocessing plants at La-Hague and Sellafield. These laboratories are staffed by personnel from the JRC Institute for Transuranium Elements (ITU). A main advantage of the on-site laboratories is the timely analysis of the U and Pu content and isotopic composition in samples taken for safeguards purposes from the accountancy tanks containing the dissolved nuclear fuel1. 235
U is present in small quantities (0.7%) in natural uranium. Enrichment facilities increase the proportion of 235U to an agreed level of 4 to 6 %, which is required to make it usable as fuel for the most common power reactors today. However, these enrichment facilities can enrich uranium to much higher levels of 235U, which would make it a weapons usable material. This is why enrichment facilities are also of high concern for safeguards authorities. On the other hand, details of the enrichment processes and of the enrichment technology are highly confidential. Access to the facilities is typically managed in agreed ways to avoid the disclosure of confidential engineering to the inspectors. In addition to the inspections based on such managed access, DG-ENER together with the IAEA are routinely using environmental sampling at enrichment facilities to verify that the plants adhere to the declared enrichment levels. This verification thus assures also the absence of high enriched uranium production.
2. High Performance Trace Analysis (HPTA) in environmental samples When uranium is processed in industrial quantities, it is very difficult to avoid the release to the immediate environment of micron to submicron-sized aerosol particles containing the isotopic signature of the handled materials. This allows nuclear safeguards authorities with specialised sampling techniques and analytical laboratories to monitor the nuclear materials handled at nuclear facilities. The particle samples are most commonly dust samples collected on cotton swipes. These samples typically contain several hundreds of millions of particles. Secondary Ion Mass Spectrometry (SIMS) has been a mainstay technique in particle analysis for nuclear safeguards for more than a decade. The SIMS analytical work consists of two main sub tasks; the first one is to find the particles of interest in a matrix of other dust materials, the second is to perform accurate and precise isotopic measurements of the individual particles. In summary, the SIMS technique features numerous advantages for small uranium particle analysis: 1. Sample preparation is fast and requires little effort using the vacuum impactor technique2. 2. Very high sensitivity for small amounts of material. The high ion yield production for Uranium (around 1%) allows for analysis of particles in sub micron size range. 3. Fast and effective particle search capabilities: SIMS instruments with ion imaging provide the exact location of uranium particles within a sample matrix. In instruments equipped with a screening software, uranium particles are localised in fully automated mode, and an estimate of the enrichment is computed for every particle. 4. All-in-one-instrument: the screening measurements can be followed immediately by precise and accurate isotopic micro beam measurements of individual particles using the same instrument. 5. Timely analysis compared to other techniques like the fission track TIMS method4-7. 3. The new Large Geometry – SIMS laboratory at ITU Until recently, particle analyses have predominantly been performed using the small geometry CAMECA IMS 3F-7F instrument series4. Analysis has previously been performed at ITU using an IMS 6F and an IMS 4FE6. These instruments provide both particle screening and isotope ratio capabilities. The performance of these instruments is however often limited by the occurrence of isobaric interferences, in particular for the minor isotopes (234U, 236U), that often could not be resolved without compromising the transmission of the instrument, resulting in lower sensitivity. A recent breakthrough to solve this problem has been the implementation of Large Geometry SIMS, the CAMECA IMS 1270 / 1280 / 1280-HR models, for this type of analysis5-8. Basically, LG-SIMS instruments were developed in response to a demand for high mass resolution – high transmission instruments in geosciences and cosmochemistry. These instruments are, like the small geometry SIMS, based on a double focusing mass spectrometer, but with the implementation of a large-radius magnetic sector and improved secondary ion optics. This allows measurements to be performed at high mass resolution while maintaining high transmission. The implementation of the LG-SIMS for safeguards purposes was made by ITU in close collaboration with CAMECA (Paris, France), NORDSIM laboratory (Stockholm, Sweden) and the University of Western Australia (Perth, Australia). In short, the LG-SIMS today provides isotopic data for particle analysis that are at a state of the art level, combining highest quality with high throughput. The latter is important for the safeguards application, where timely analysis can be critical, especially in facilities with a high throughput.
The new LG-SIMS laboratory at ITU, equipped with a CAMECA IMS 1280-HR has jointly been established by DG-ENER and JRC to enhance the EU's capabilities for environmental sample analysis. The new laboratory will primarily be used for safeguards samples (DGENER, IAEA) and nuclear forensic samples. In addition, the instrument will be used in research aiming at further enhancing the possibilities to determine a variety of parameters of the samples, to analyse particles more in depth and in general to improve the information gained from the analytical work. The quality assurance in particle analysis will be improved by making reference particles with certified isotopic and by producing monodispersed particles certified for isotopic and total uranium content. The latter will be produced in collaboration between ITU and the JRC Institute for Reference Materials and Measurements (IRMM). The analytical safeguards work at ITU is made under accreditation of ISO 17025. For a highly sensitive instrument like the IMS 1280-HR, it is important to fulfil all the requirements regarding the environment for the instrument and its operators. Even small floor vibrations can affect the imaging capabilities. External electromagnetic fields, poor supply power grid and/or poor grounding can affect the quality of the measurements. In addition, a stable room temperature is essential for guaranteeing the stability of the ion sources and of the magnetic fields used for mass separation of both primary and secondary ion beams. It is also recommended to separate the instrument control from the instrument itself to reduce the disturbance for the operators from the noise produced by vacuum pumps, uninterrupted power supplies, fans, cooling units etc. In the case of particle analysis it is also recommended (or essential) to maintain a high level of cleanliness in the instrument room. The new laboratory at ITU consists of a control room, the CAMECA IMS 1280-HR instrument room and a sample preparation room, see figures 1 and 2. The instrument room is maintained at minimum as a class 1000 clean room. The sample preparation room is maintained to a level of better than class 100. All sample handling is in addition performed in HEPA filtered laminar flow boxes.
Figure 1. The 20m2 control room is separated from the instrument room to minimise the noise level for the operator. This also minimizes the contact with the instrument and reduces the particle levels in the instrument room. A window allows the operator to have a clear view of the instrument. There is a small changing area between the control room and the instrument room for the operators to suit up in clean room clothes before entering the instrument room.
Figure 2. The 54m2 instrument room with the IMS 1280-HR is maintained as a minimum at a class 1000 clean room level. Eight H14 HEPA filter modules continuously clean the air. The room has been verified to fulfil all CAMECA requirements.
4. Application in nuclear forensics As previously mentioned, a main application for the new LG-SIMS laboratory is in uranium particle analysis to verify declared activities in enrichment plants and to search for undeclared nuclear material handling. Particle analysis can, however, also be used in nuclear forensic investigations for trying to establish the origin of seized or found nuclear material. A classic case of applying nuclear forensics methodology with particle analysis in safeguards was IAEA's finding of highly enriched uranium particles at different locations in Iran in 20039. The uranium isotopic composition of particles found in Iran was compared with particles found from equipment that had been sold by a large clandestine nuclear trading network10,11 and contaminated centrifuge equipment previously used in an Asian country for the making of weapon grade uranium. The investigation showed that the material found in Iran could be a contamination originating from an enrichment facility used for high enriched uranium production outside Iran12,13. Other, more recent cases where particle analysis has been part of nuclear forensic investigations are the finding of uranium contaminated scrap metal in a few European scrap yards. The contaminated scrap was very likely imported from outside the EU. As the isotopic composition is one of the key signatures telling about the intended use of the material as well as in narrowing down the possible production facilities, isotopic composition measurements are always performed on nuclear forensic samples. In the case of the contaminated scrap metal it was observed that the 235U enrichment varied between 10 and 33 wt% in different bulk samples. The particle analysis revealed however, a more complex isotopic range of signatures and that the 235U enrichment ranged from natural to weapon grade level. 5. Conclusions The establishment of the new state of the art laboratory at ITU will strongly enhance JRC/ITU's capabilities in environmental sample analysis in support of main safeguards authorities like DG-ENER and its partner IAEA as well as improving sample analyses in nuclear forensic investigations.
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