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. B region contains counts due to 233U and tail contribution of 232U. Tail contribution is corrected by Geometric Progression method already described. Counts at 4.82 MeV after tail correction, B’ = {B - AC/(B+AD/C)}

Activity at 5.43 MeV/Activity at 4.82 MeV , R3 = A’/B’ ------6 234U contribution to 233U peak is corrected as explained earlier,

R4 = R3 (1+(N234/N233) x (λ234/λ233) ------7 228 Th activity at 5.43 MeV has to be corrected for its branching intensity (BI228) as well as 0.0504 fraction of activity due to 224Ra as shown, 228 233 Activity of Th/Activity of U, Rm1 = R4 /(BI 228 × 1.0504) ------8

BI 228 = 0.7616

Method 2

This method of evaluation is simpler than the first one and uses peak area at 5.68 MeV due to 224Ra. This peak does not have any interference and hence additional region of interest A’’ is marked at 5.68 MeV. 228Th and 224Ra are in secular equilibrium as seen from figure1. In secular equilibrium, activity of parent is equal to the activity of daughter. Hence activity ratio of 224Ra/233U will directly give the activity ratio of 228Th to 233U. Peak area of 233U is corrected for tail contribution and 234U presence as explained earlier. After tail correction, R5 = A’’/B’ 234 After correction for U, R6 = R5 {1+(N234/N233) x (λ234/λ233) } ------9 224 After correcting for branching intensity of Ra at 5.68 (BI224), 224 233 Ra to U activity, R7 = R6 / BI224 ------10 228 233 Activity of Th / Activity of U, Rm1 = R7 ------11

Rm1 and Rm2, the activity ratio calculated by two methods are compared. Concentration of 233U is measured by isotope dilution mass spectrometry and activity of 233 U(A233) is calculated.

228 Activity of Th in the sample(A228) = Rm1 or Rm2 x A233 in the sample ------12

2.2.6 Determination of specific activity of Uranium

Radiometric method is normally employed for concentration measurement of uranium in samples drawn from various stages of the process to separate uranium(233) formed in irradiated Thorium fuels. The method requires specific activity of uranium and presence of very small amount of 232U will drastically change the specific activity of uranium formed due to its lesser half life. In fact, in the present work it is observed that alpha activity due to 232U is more than 233U. Since major alpha activity is due to 232 & 233 isotopes of uranium, Alpha spectrum obtained from purified source i.e. TOPO source is evaluated for determining the specific activity of uranium as described here,

Activity at 5.32MeV N232 λ232

------= ------= R1 ------2

Activity at 4.82MeV N233 λ233 + N234 λ234

A232 /(A233 +A234) = R1 ------13

1+( A232 /(A233 +A234)) = 1+R1

(A232 +A233 +A234)/(A233 +A234) = 1+R1

(A232+A233 +A234) = (1+R1) × (A233 +A234) = total activity of sample , ------14 as 235 to 238 are minor isotopes not contributing to total alpha activity. Dividing by mass of uranium(m) in sample to both sides of the equation, (A232 +A233 +A234)/m = (1+R1) × (A233 +A234)/m ------15 Activity per unit mass is specific activity and since 233U + 234U are major constituents (>96%), their mass in sample also can be taken as m .Both have comparable specific activity, their specific activity can be approximated to that of 233U. Then, 233 specific activity of U in sample = (1+R1) specific activity of U. ------16

Thus from the ratio R1 obtained from the spectra, specific activity of sample can be calculated.

3. RESULTS & DISCUSSION Fig 2, 3 and 4 show the alpha spectra obtained for direct evaporated source and TOPO separation source. TOPO source shows only two peaks of 233U & 232U whereas direct evaporated source shows in addition to these peaks higher energy peaks of its daughter products at 5.43 MeV of 228Th, 5.683 MeV of 224Ra, 6.28 MeV of 220Rn, 6.78 MeV of 216Po, 6.34 MeV of 212Bi and 8.785 MeV of 212Po. This conclusively proves the effectiveness of the purification followed using TOPO in removing the daughter products of 232U. Hence TOPO source can be easily evaluated for the isotope ratio measurement of 232U/233U. Precision of the method was checked by repeating the spectrum measurement 10 times & a precision within + 0.7% is obtained. Table 2 gives the comparison of isotope ratio and specific activity results obtained by alpha spectrometry with that of with mass spectrometric method developed subsequently12 and the inter method difference in both the measurements is within 4.5 %. Specific activity of uranium in the sample is found to 785 Bq/µg by alpha spectrometry. The result by alpha spectrometry is higher due to assumption that mass of 233U+234U equals to total uranium mass in arriving equation 16. Specific activity of uranium can be determined by the method without the requirement of costly mass spectrometer. Table 3 gives the result of activity ratio of 228Th to 233U obtained in the irradiated fuel solution by the two evaluation methods and the difference between the two evaluation procedures is within 3.2 %. Alpha spectrometry is a simple and rapid method for the determination of 232U/233U isotope ratio in the irradiated thoria dissolved solution. The method also gives the 228Th content. It is possible to determine other daughter products present

4. ACKNOWLEDGEMENTS

The authors are grateful to Shri.V.P.Kansra, Director, Nuclear Recycle Group for his keen interest, encouragement and unstinting support in carrying out this work. Thanks are due to Shri.A.K.venugopal, Supdt., PC&R Lab and Shri.P.K.Dey, Plant Supdt., PREFRE, Tarapur for their cooperation in carrying this work. It is a great pleasure to thank our colleaques, Shri.K.Vijayan, Dr.P.S.Dhami, Dr.V.Gopalakrishnan and members of mass spectrometric group of PREFRE for their help at various stages of this work.

5. REFERENCES 1) Anil Kakodkar, Indian Nuclear Society twelfth annual conference INSAC-2001(2001) Beyrich,W., Drosselmeyer,E., Kernforschungszentrum Karlsruhe, report 2) KFK 1905 I and II (1975) 3) DS.Divakar, K.J.Parikh, S.S.Pandit, S.K.Singh, U.Jambunathan, Indian Nuclear society 12th annual conference, INSAC-2001 (295), 2001 4) Kirby,HW., J.Inorg. Nucl. Chem. 32, 2823 (1970) 5) De Regge.P , Nucl. Inst. And methods 102, 269 (1972) 6) Bortels,G., A set of FORTRAN programmes for calculating the ratio 238Pu/(239Pu+240Pu) in Alpha particle spectra, CBNM/RN/16/75(1975) 7) Baba,H., Nucl. Inst. And methods 148, 173 (1978) 8) Watzig,W., Westmeier,W., Nucl. Inst. And methods, 153, 517 (1978) 9) Aggarwal,S.K.,Jai,H.C., Mathews,C.K., and Ramaniah,M.V. BARC-824 (1975) 10) Aggarwal,S.K., Chitamber,S.A., Kavimandan,V.D., Alamaula,A.I., Shah,P.M., Parab,A.R., Sant,V.L., Jain,H.C., and Ramaniah,M.V., Radiochim. Acta 27, 1 (1980) 11) Umeoto,S., Radiochim. Acta 8, 107 (1967) 12) Ramanujam,A., “ Mass spectrometric method for isotope ratio measurement of 232U/233U in irradiated Thorium fuels” under publication.

Table 1 Table of Nuclides

Nuclide Half life Alpha energy Branching Ratio (Kev) % 232U 68.9 y 5263.4 31.70 5320.2 68.00 233U 1.592E05 y 4729.2 1.61 4783.5 13.20 4824.2 84.4 234U 2.45E05 y 4722.6 27.5 4774.9 72.5 235U 7.038E08 y 4218.0 5.70 4324.9 4.40 4345.5 1.50 4365.3 17.0 4400.0 55.0 4414.0 2.10 4502.0 1.70

236U 2.3415E07 y 4445.0 26.0 4494.0 74.0 238U 4.468E09 y 4147.0 21.0 4197.0 79.0 228Th 1.9131 y 5340.3 28.2 71.1 5423.2 224Ra 3.62 d 5448.7 5.06 94.94 5685.4 220Rn 55.6 s 6281.9 99.89 216Po 0.145 s 6778.3 100 212Bi 60.55 m 6050.8 25.13 6089.9 9.75 212Po 0.298 µs 8784.9 100

Table 2 Comparison of Results

AS- Alpha spectrometry MS- Mass spectrometry

Sr. Measurement carried out Results obtained Diff in % No AS MS (AS-MS) × 100/MS 1 Isotope ratio of 232/233 0.000572 0.000599 -4.5 2 Specifc activity of U (Bq/µg) 785 752.3 4.3

Table 3

Comparison of actiity ratio of 228Th/233U by two evaluation methods

Method 1: 224Ra peak at 5.68MeV Method 2: 228Th peak at 5.42

228Th/233U By method 1 228Th/233U by method 2 Diff in % 1.050 1.016 3.2

Fig.1 232U Decay Chain

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Fig.2. Direct evaporated source spectrum

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Fig.3. Spectrum of TOPO separated source 1400

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Fig.4. Expanded spectrum of TOPO source