Nondestructive method for determination of isotopic composition of uranium Rahul Shankara, Ashwani Kumar, D.B.Paranjape and B.S.Tomar* Radioanalytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085 a Amity University, Noida, U.P. *Corresponding author: Email,
[email protected] A methodology has been developed for the determination of isotopic composition of uranium nondestructively using low energy photon spectrometer. The gamma lines at 53, 58 and 63 KeV were used 235 238 for the isotopes 234 92𝑈 , 92𝑈 and 92𝑈 respectively. The uncertainty in the isotopic abundances obtained by this method were found to be in the range of 5-10%. The method is suitable for uranium bearing fuels as well as waste packets containing natural Uranium as well as low enriched uranium.
Introduction Nondestructive determination of isotopic composition of uranium is a challenging problem, particularly with regard to finished nuclear fuel pellets, rods and bundles, as well as in waste packets, as a part of quality control measures and nuclear material control and accounting. 185 KeV gamma line of 235 92𝑈 is used for determination of enrichment of 235 92𝑈 as a part of nondestructive assay (NDA) of finished nuclear fuels wherein infinite source method is used. For isotopic 238 abundances of 235 92𝑈 and 92𝑈 185 KeV and 1004 KeV gamma lines can be used. However, the large difference between the gamma ray energies precludes relative efficiency calibration. Further, the 120 KeV gamma line of 234 92𝑈 is quire far from the above two gamma energies. Thus determination of isotopic 235 238 abundances of 234 92𝑈 , 92𝑈 and 92𝑈 by NDA has been quite a challenge for the nuclear chemists. In the present paper a method has been developed for the determination of isotopic composition of uranium using the close lying low energy gamma rays of 234 92𝑈 238 (53 KeV), 235 92𝑈 (58 KeV) and 92𝑈 (63 KeV). The nuclear data of different isotopes of uranium are given in Table 1.
Gaussian with lower exponential tail and polynomial background. The peak area (P) for the gamma line of any one isotope (i) can be expressed as, 𝑃𝑖 = 𝑁𝑖 𝜆𝑖 𝑇𝐼𝑖 𝜀𝑖
Where 𝑁𝑖 and 𝜆𝑖 are the number of atoms and decay constant of the radioisotope (𝑖), 𝐼 and are the gamma ray intensity and detection efficiency. 𝑇 is the time of counting. The only unknown quantities are 𝑁 and . As the sample geometry and uranium content is not known, the absolute efficiency cannot be determined accurately. Therefore, relative efficiency in the gamma energy range of interest was determined using the different gamma lines of the isotope 235 92𝑈 given in Table 1. Table 1: Nuclear data of Uranium isotopes used in the present work Isotope 𝐸𝛾 (𝑘𝑒𝑉) 𝐼𝛾 (%) 𝑡1⁄2 (𝑦) 234 92𝑈 235 92𝑈
2.455 × 105 7.04 × 10
8
Experimental Uranium samples of known isotopic composition (mass spectrometry) of varying composition were counted on a low energy photon spectrometer (10cm3) based on planar HPGe detector coupled to a 4096 channel analyzer. The energy resolution of the detector at 121.8 KeV gamma line was 600 eV. Table 2 gives the different samples analyzed for their isotopic composition. The 235 92𝑈 percentage in the samples varied from 0.7% (natural) to 17% (LEU). The counting time for each sample was varied so as to acquire statistically good data. The gamma spectra were analyzed for the peak areas of the gamma lines by the peak fitting program (PHAST) employing
(1)
53.2
0.123
25.64( 231 90𝑇ℎ )
14.1
58.57( 231 90𝑇ℎ )
0.462
84.214( 231 90𝑇ℎ )
6.6
0.436 102.27( 231 90𝑇ℎ ) 238 92𝑈
4.468 × 109
63.29( 234 90𝑇ℎ )
3.7
234 The ratio of number of atoms of 234 92𝑈 and 92𝑈 with 238 respect to that of 92𝑈 were determined using the formula:
𝑁𝑖 𝑃𝑖 𝜆𝑗 𝐼𝑗 𝜀𝑗 = 𝑁𝑗 𝑃𝑗 𝜆𝑖 𝐼𝑖 𝜀𝑖
(2)
The isotopic fraction of a particular isotope (say 238 92𝑈 ) was determined using the formula:
1 𝑓238
=
𝑓234 + 𝑓235 + 𝑓238 𝑓234 𝑓235 = + +1 𝑓238 𝑓238 𝑓238
(3)
𝑓234 𝑓 were determined using the ⁄𝑓 and 235⁄𝑓 238 238 equation (2). Once 𝑓238 is obtained from equation (3), 𝑓234 and 𝑓235 could be calculated.
Table 2: 235 92𝑈 enrichment measured by gamma ray spectrometry Sample Accuracy % 235 % 235 92𝑈 92𝑈 (%) MS GS
Results and Discussion
Counts
Figure 1 shows the gamma spectrum of one of the uranium samples obtained using the LEPS. 6x10
5
5x10
5
4x10
5
3x10
5
2x10
5
1x10
5
0.72
0.85 0.013
17.63
𝑆2
1.6
1.690.043
5.32
𝑆3
2.2
2.030.06
3.25
𝑆4
2.66
2.690.095
1.23
𝑆5
10.1
10.350.248
3.53
𝑆6
14.8
14.980.196
1.24
𝑆7
17.1
16.580.310
2.45
Conclusion
0 10
20
30
40
50
60
70
80
E(keV) Figure 1. Gamma spectrum of LEU sample
The relative efficiency curve for the region of interest is given in figure 2.
1.0 Relative efficiency
𝑆1
Methodology has been developed for determination of isotopic composition of uranium nondestructively using low energy photon spectrometer. The method can be applied to any type of sample, including finished fuels and waste packets of wide range of enrichment.
Acknowledgements The support from Nuclear Fuel Complex in supplying the samples no. 2, 3 and 4 is gratefully acknowledged.
References
0.8
1.
Passive Nondestructive Assay of Nuclear Materials, eds. Doug Reilly, Norbert Ensslin and Hastings smith Jr., LA-UR-90-732 (1991).
2.
C.A. Papachristodoulou, P.A. Assimakopoulos, N.E. Patronis, K.G. Ioannides, Environmental Radioactivity, 64, 195 (2003).
3.
Cong Tam Nguyen, Jozsef Zsigrai, Nucl. Inst. Meth. B 246, 417 (2006).
4.
N.C. Tam, J. Zsigrai, L. Lakosi, E. Jozsef, J. Safar, Nucl. Inst. Meth. A 515, 644 (2003).
5.
Y. Nir-El, Appl. Radiat. Isotopes 52, 753 (2000).
6.
B.S. Tomar, T.C. Kaushik, Sanjay Andola, Ramniranjan, R.K. Rout, Ashwani Kumar, D.B.Paranjape, Pradeep Kumar, K.L. Ramakumar, Satish C. Gupta, and R. K. Sinha, Nucl. Instr. Meth. A 703, 11-15 (2013).
0.6 0.4 0.2 0.0 20
30
40
50
60
70
80
90 100 110
E(keV) Figure 2. Relative efficiency for LEU sample
The isotopic compositions obtained, following the prescription given above, are given in table 2. The uncertainty shown on the percentage of different isotopes, is propagated error due to counting statistics and efficiency fits. The % 235 92𝑈 obtained by GS (present work) is close to the MS values within 5-7%. The methodology in combination with active neutron activation followed by delayed neutron counting (AIDNEC) can be used for determination of total uranium content in the sample [5].
