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Analysis of Isotope Ratio of 232U/233U in Irradiated Thoria by Alpha Spectrometry

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Analysis of Isotope Ratio of 232U/233U in Irradiated Thoria by Alpha Spectrometry ANALYSIS OF ISOTOPE RATIO OF 232U/233U IN IRRADIATED THORIA BY ALPHA SPECTROMETRY A.Dakshinamoorthy, P.V.Achuthan, D.S.Divakar, R.Kannan, U.Jambunathan, A.Ramanujam, P.B.Gurba*, R.K.Babber* Fuel Reprocessing Division Bhabha Atomic Research Centre Department of Atomic Energy, Trombay, Mumbai- 400 085. Abstract Indian nuclear program envisages the use of vast deposits of thorium in the country at the second stage for the production of 233U, a fissile material. Among the other isotopes produced, 232U content is of greater significance to fuel reprocessors as its daughter products contribute to high level of gamma dose during reprocessing. Alpha spectrometric method for the isotope ratio of 232U/233U in irradiated thoria has been successfully developed in our laboratory. The report describes the separation procedure followed using Tri-n-Octyl Phosphine Oxide (TOPO) for the purification of uranium from its daughter products and alpha spectrometric method followed for determining 232U content. Alpha spectrum evaluation is also explained in detail. Comparison of spectra from sources prepared prior and after separation forms the method for determining 228 Th activity in uranium. The method is simple and fast. Key words: Alpha spectrometry, Uranium, 232U, 233U, Thorium, 228Th, Reprocessing, Irradiated fuels * Power Reactor Fuel Reprocessing Plant, Bhabha Atomic Research Centre, Tarapur 1. INTRODUCTION India has adopted a three stage nuclear program1 wherein large amount of thorium available in the country will be utilised in the second stage for the production of 233U, which in turn will be used as fuel for the third stage. 232Th is converted to 233U by nuclear reaction in a reactor. In addition to 233U, other isotopes of uranium namely 232, 234, 235 and 236 are also produced. Determination of 232U content is important as it undergoes decay as per the decay chain shown in figure 1. Some of the daughter products are high energy gamma emitters and contributes significantly to the gamma dose during processing. Hence determination of 232U i.e. isotopic composition of uranium produced is one of the important parameters in 233U processing. Thermal ionisation mass spectrometry is normally followed for precise and accurate isotope ratio measurement of uranium2. Difficulty arises in the measurement of 232U content by mass spectrometry due to isobaric interference of 232Th, the major constituent in the sample. Hence alpha spectrometry has been followed in the present work for the determination of 232U present along with 233U in the sample. Alpha energies and branching intensities of uranium isotopes of interest and various daughter products of 232U decay chain are shown in table 1. It is evident that 232U and 228Th are having alpha energies closeby. 224Ra is also contributing to the 232U peak to the extent of 5%. Therefore, presence of these daughter products will interfere in the estimation of 232U by alpha spectrometry. Hence a separation of uranium from thorium is necessary prior to alpha spectrometry. An extractive spectrophotometric and radiometric method has already been reported from our laboratory3 for the determination of 233U. The radiometric method requires specific activity of uranium for finding the uranium concentration. Presence of small amount of 232U will drastically alter the specific activity of uranium. Our present work by alpha spectrometry will be useful in determining the specific activity of uranium in the sample. Separation procedure followed for removing the major constituent i.e. thorium is the same one reported earlier3. The separation steps involve complexing thorium with fluoride followed by solvent extraction using TOPO in xylene. Alpha spectrum of the source prepared from the TOPO extract was used for evaluation. The paper describes the procedure followed for separation of irradiated thorium sample and alpha spectrometric analysis carried out to find out the alpha activity ratio of 232U/233U. The atom ratio of 232/233 calculated from the activity ratio measured. 228Th is also estimated from the spectra obtained prior and after separation. 2. EXPERIMENTAL 2.1 Instruments 2.1.1 Alpha spectrometer Alpha spectrometer used consists of a vacuum chamber designed and fabricated in-house by Technical Physics & Prototype Engineering Division, Bhabha Atomic Research Centre, Trombay, connected to a rotary pump and fitted with pyrani gauge to measure the pressure in the chamber. A vacuum of better than 1x10-2 mbar is obtained in the chamber. A Silicon Surface Barrier detector of 300 mm2, with a resolution of 30 keV at 5.5 MeV is used. DC voltage of +100V is applied. The output of the detector is connected to a preamplifier, spectroscopy amplifier and PC based 8 K multichannel analyser (MCA) card Emacplus with MCA emulation software. 2.1.2 Sample preparation Irradiated thorium oxide pellet is dissolved in conc. HNO3 containing HF and aluminium nitrate. After dissolution it is made upto 25 ml. Suitable dilutions are made from the stock solution for further analyses. 2.1.3 Tri-n-octyl posphine oxide (TOPO) 0.05 M TOPO in xylene is prepared by dissolving 1.9338 g of TOPO, Fluka AG in 100 ml of xylene. 2.1.4. Sodium fluoride solution 4% NaF solution is prepared by dissolving 4 g of NaF (AR) in 100 ml of water and filtered. 2.1.5 2 M HNO3 By suitable dilution of AR grade conc. HNO3 . 2.1.6 Source material: 1” stainless steel disc of 0.3 mm thick having polished surface is used as backing material for preparing source to carry out alpha spectrometry work. 2.2 Procedure 2.2.1 Source preparation Suitable aliquot of the sample containing ~10 microgram of 233U along with milligram amount of thorium is conditioned to 2 M HNO3 and 2 ml of 4% fluoride solution is added and subjected to solvent extraction separation using 0.05 M TOPO as per the procedure reported3. 50 microlitre of the organic extract is placed on an SS disc, dried on a hot plate and fired in bunsen burner. A directly evaporated source is also prepared without separation by placing small volume of the sample solution (50 -100 microlitre) on an SS disc, drying & firing in a flame. 2.2.2 Spectrum analysis The source disc is placed in the vacuum chamber that has adjustable sample tray for counting. The distance between source and detector is maintained at 10 mm. The detector chamber is evacuated to a pressure of less than 4x10-2 mbar. Counting is continued till a minimum of 20,000 counts under region of interest (ROI) in the energy regions of 233U and 232U are accumulated. 2.2.3 Spectrum evaluation i) Correction for tail contribution: Difficulty arises in the determination of activity ratio of 232U/233U from the alpha spectrum because of tail contribution of higher energy peak to lower energy peak due to energy degradation of alpha particles before reaching the detector. Eventhough the problem is minimised under vacuum, it is not totally eliminated. Hence the tail contribution has to be corrected. A number of computer programs 4-9 have been developed for the correction of alpha activity ratio of 238Pu/ (239+240)Pu obtained from the evaluation of plutonium spectrum and a geometric progression (GP) decrease method has been followed successfully in our centre10. The method has been used earlier for the determination of 234U/238U alpha activity ratio measurements.11. The same GP correction method is adopted for evaluating 232U/233U activity ratio from the spectrum. Here the spectrum is divided into four regions A,B,C and D, where A is 232U region, B is 233U region and C & D are regions marked for evaluation of the correction required for tail contribution. Each region has same number of channels and number of channel between any two regions is the same. A The corrected ratio R1 = ------------------------- --------1 {B - AC/(B+AD/C)} 2.2.4 Correction for the presence of 234U : Presence of 234U in 233U sample will contribute to the peak of 233U as the alpha energy of 234U (4.77 MeV) and 233U ( 4.78, 4.82MeV) are close by and their half lives are comparable. Hence 233U peak has to be corrected for contribution from small amount of 234U.The correction is done as follows: Activity at 5.32 MeV N232 λ232 ------------------------ = --------------------------- = R1 --------- 2 Activity at 4.82 MeV N233 λ233 + N234 λ234 where Ni and λ i are the number of atoms and decay constants of isotope Activity ratio of 5.32 MeV/4.82 MeV after tail contribution is R1 . Dividing by N233 both denominator & numerator in equation 2, ( N232/N233) λ232 R1 = ----------------------------------- -------- 3 λ233 + (N234/N233 ) λ234 Rearranging 3 ( N232λ232) N234 λ234 R2 = ------------- = R1(1+ ------ x -----) --------- 4 (N233 λ233) N233 λ233 N234/N233 is available from mass spectrometric analysis, and λ234, λ233 are decay constants of 234 233 232 233 U and U available from literature. R2 is the alpha activity ratio of U to U. Isotope ratio of 232U/233U can be determined by rearranging the equation 4, N232/N233= R2 x λ233/λ232 ---------- 5 2.2.5 228Th determination Activity ratio of 228Th to 233U is determined from the spectrum of direct evaporated source. Two methods of evaluation are followed. Method 1 As it is seen from the spectrum of direct evoporated source in Figure 2, 228Th peak at 5.43 MeV is separated from 232U peak at 5.32 MeV. Hence it is possible to calculate the alpha activity and content of 228Th from the alpha spectrum of direct evaporated source. Five regions A’, A, B, C, D are marked where A’ is peak area of 228Th, A is peak area of 232U , B is peak area of 233U peak with minor contribution due to 234U and tail of U232 and C & D are regions on 233U tail contribution, marked for correction purpose as described already.
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