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Direct multielemental trace determinations in plutonium samples by Total reflection X-Ray Fluorescence Spectrometry using very small sample amount Kaushik Sanyal, Sangita Dhara, and Nand Lal Misra Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02917 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018
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Analytical Chemistry
Direct multielemental trace determinations in plutonium samples by Total reflection X-Ray Fluorescence Spectrometry using very small sample amount Kaushik Sanyal1, 2, Sangita Dhara1, 2 and N.L. Misra1,2,* 1
Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India 2
Homi Bhabha National Institute, Mumbai 400 094
* Corresponding Author, Email:
[email protected],
[email protected], Tel: 0091-22-25594565
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Abstract A simple, safe and sensitive method for direct multielemental trace determinations in plutonium samples using Total reflection X-ray Fluorescence (TXRF) spectrometry has been developed. A very small volume (2 µL) of the sample solutions were deposited on TXRF supports after separation of plutonium matrix from these solutions. Since the amount of the plutonium deposited on the supports was in ng level only fixed on the supports and the specimen spots were not disturbed during the sample preparation, the samples could be analyzed directly without putting the instrument in a glove box. This approach avoided cumbersome operation of the instrument in glove box which is normally utilized for Pu based samples using other techniques. Similarly, the requirement of small amounts of the samples minimized the radiation dose to the operator as well as cumbersome problem of management of radioactive analytical waste of plutonium samples. The samples were analyzed using the TXRF spectra of the specimens, concentration of the internal standard Se or Ga and predetermined sensitivity values. The elemental detection limits for the elements K-Sr varied from 1.06 to 0.09 ng. The elements K, Ca, Cr, Mn, Fe, Ni, Cu, Zn, Sr, Ba, Tl, and Pb were analyzed at µg/mL level. The analytical results of TXRF determinations showed average relative standard deviation (RSD) value of 4.5 % (1σ, n=3) and the TXRF determined results deviated from the expected values by 5.9 % on average for samples prepared by adding multielements in plutonium solutions. Two real plutonium samples were also analyzed in similar manner. For the real plutonium sample solution the average RSD values of TXRF determinations were 10.6% (1σ, n=3) for the elemental concentrations in the range of 0.2 to 61 µg/mL. These values are comparable with conventional trace element analytical techniques with added advantages mentioned above.
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Analytical Chemistry
Introduction Plutonium is one of the key elements in nuclear reactor fuels for the production of energy. It is highly hazardous, radio-active and toxic element which does not exist in nature [1, 2]. Due to its radio toxicity, it cannot be handled without proper enclosure. Plutonium based fuels are very important for fast breeder reactors, because of its nuclear characteristics. At present, mixed oxides (MOX) fuel of uranium and plutonium oxides in form of pellets of different composition are used in many LWRs (Light Water Reactors) and PFRs (Prototype Fast Reactors) [3]. In space exploration, plutonium based fuels are very important energy source and superior to the electrical batteries for such purpose, as the batteries are very heavy and have very small lifetime. The solar cells cannot be used for this purpose as they do not operate at very long distance from sun. The compact Pu-238 is well suited as thermoelectric power source for the power supply in satellites [4, 5]. Thus plutonium has several peaceful applications for the advancement of human civilization in addition to its use as nuclear weapon. Due to above reasons, plutonium is a very important technological material despite its radiotoxic nature. In nuclear industry, the production and fabrication of the fuel involves several processes e.g. conversion of plutonium to oxide form, grinding, mixing, pelletization, sintering etc. During these fuel processing operations, it can get contaminated with various trace impurities. The presence of impurities beyond specified limits in fuel affects the neutron economy as well as the physical and chemical characteristic of the fuel. A detailed description of the effects of different trace impurities on the fuel characteristic during the operation of the reactor is available in the literature [6, 7]. Due to above reasons, stringent specifications of maximum tolerable limits of different trace impurities in different type of reactor fuels have been laid down for safe and
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efficient operation of the reactors. In order to satisfy these specifications, it is very important to determine the trace impurities before using the fuels in the reactor core. Different techniques are available for trace determinations in uranium and thorium based fuels. Spectrometric techniques e.g. atomic emission spectrometry, mass spectrometry etc. are more popular for the trace determination as many elements can be determined simultaneously in a short span of time [8]. These techniques require separation of metallic impurities by column elution or solvent extraction before their determination. However, only metallic impurities can be determined by these techniques [9, 10]. Another important trace element determination technique is DC-arc carrier distillation technique which is generally used for volatile impurities. The sensitivity and reproducibility of this technique is also very poor [11]. ICP-AES (Inductively Coupled Plasma – Atomic Emission Spectroscopy) and ICP-MS (ICP-Mass Spectrometry) have been used for the determination of trace impurities in different uranium, thorium and plutonium matrices after separation of the major matrix. These techniques require comparatively large amount of sample (approximately 1-10 mL) which is not a problem for U and Th based samples [12]. However, for Pu based samples even this amount of sample cannot be handled in open atmosphere because plutonium is highly radiotoxic element which primarily decays by alpha emission. It has high specific activity as well as high biological half life. Hence it is always desirable to handle minimum amount of plutonium based materials during the analysis for the safety of the analyst and avoiding cumbersome radioanalytical waste management. In order to analyze such samples in a safe manner by these techniques the instrument is put inside a glove box. Though these techniques are being used for Pu analysis by keeping the instrument inside a glove box, a comparatively large amount of radioanalytical waste is generated during such analysis. Operation and maintenance of instrument in glove box is difficult and slow and the
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Analytical Chemistry
disposal of the instrument after its life completion is very difficult. The running costs of both the instruments to maintain the plasma and Ar gas flow during the sample analysis are very high. On the other hand, TXRF (Total reflection X-Ray Fluorescence) has many good features like multielemental analytical capability, analysis of metals non metals alike, high detection power, simple, cost-effective for material characterization especially nuclear materials etc. [13, 14]. The spectral interferences in XRF/TXRF are very less compared to those in ICP-AES and ICP-MS. In our laboratory, studies have shown that this technique has a huge potential for the analysis of radioactive nuclear materials [15-18]. The main features which make TXRF an excellent candidate for the analysis of radiotoxic elements is the requirement of very small amount of sample during TXRF analysis, its multielemental analytical capability, ease of operation, fast analysis without requirement of matrix matched standards [19-21]. A few nano-gram of sample on the sample support is enough to give very good TXRF spectrum [15]. Despite above advantages of TXRF for radioactive materials, no trace elemental analysis study of plutonium samples are reported in literature till now. In the year 1989,an article regarding analysis of highly diluted simulated waste solutions from reprocessing plant by a TXRF spectrometer partially kept inside a glove box was published [22]. Recently a method of measuring TXRF spectra of Pu based samples was developed in our laboratory for the Pu solutions with Pu concentration in the range of 50-100 mg/mL. For such spectra measurements the dried sample on the TXRF support was covered with collodion layer. However, such method may not work for the elements which are present in trace level and have lower energy X-ray lines. Since major matrix is separated for trace element determinations in any matrix, when this approach shall be applied for Pu samples the amount of Pu in such solutions after its separation shall be very small (about
