Journal of Nuclear and Radiation Physics, Vol. 4, No. 2, 2009, pp. 65-73
NON DESTRUCTIVE ASSAY FOR 235U DETERMINATION IN
REFERENCE MATERIALS OF URANIUM OXIDE
M.H. Nassef, W. El Mowafi, and M.S. El Tahawy
National Center for Nuclear Safety and Radiation Control
Atomic Energy Authority of Egypt
E-mail: mnassef@yahoo.com
Rec. 10/2/2009
Accept. 20/10/2009
A non-destructive technique using hyper-pure germanium (HPGe) gamma ray spectrometry was used
for the analysis of uranium oxide standards with different enrichment values. The activity ratio of
235
U/238U in the studied samples were calculated on the basis of the measurement of the count rate of
185.7 keV gamma line of 235U and 1001 keV gamma line of 234mPa, a first daughter of 238U. It was found
that, the specific count rates of 235U, and 238U were constant regardless of 235U enrichment. A good
agreement was found between the calculated and the measured values of the activity ratios of 235U/238U.
A strong correlation (0.9982) was found between the certified and the measured enrichments. The
results obtained from these analyses shows that this suggested analytical method can be used to
determine the uranium concentration, and the isotopic abundance, especially in the case of depleted,
natural and low enriched uranium samples.
Keywords: Non-destructive Assay, Uranium Enrichment, Activity Ratio of 235U/238U.
INTRODUCTION
The non-destructive analysis (NDA) of materials has become a popular analytical technique
in nuclear and non-nuclear industries, after the advent of high-resolution gamma-ray spectrometric
techniques.
Gamma-ray spectrometry has found increased application in recent years for determining
uranium isotopic abundances. With the use of high-resolution germanium detectors, it was possible to
achieve high accuracy in gamma-ray counting. However, the measurement accuracy and precision,
obtainable by this technique, are dependent upon the reference materials being used as standards. If
these reference materials were poorly or improperly evaluated, then the final measured results will
suffer. In order to ensure that this will not occur, the measurements leading to the development and
production of reference materials should be of sufficient quality to improve the precision and
accuracy of analyses performed in the field [1, 5].
Gamma-ray spectrometric methods can be used for samples in the analytical laboratory for
performing both direct and following laboratory processing measurements. Samples can often be
prepared for gamma-ray spectrometric measurements with considerably less effort than is required for
measurement by other methods [4].
High-purity Germanium (HPGe) detectors, cooled by liquid nitrogen, is widely used in
nuclear safeguards to verify the isotopic composition of uranium in non-irradiated materials [5].
66
M.H. Nassef, W.A. El Mowafi, and M.S. El Tahawy
The traditional uranium enrichment measurement method is based on the measurement of the
185.7 keV peak and is commonly referred to as the “enrichment meter” method [6]. The 235U
enrichment can be determined by destructive or non-destructive means. For high-quality
accountability, it may be necessary to invest the time and money to determine the enrichment by
destructive means (mass spectrometry) in order to achieve the smallest possible limit of error on the
inventory result. Non-destructive assay (NDA) method, involving gamma-ray spectroscopy, can be
used to determine 235U enrichment on a more timely basis. NDA methods are called for in cases
where:
1 - high measurement throughput is needed,
2- Measurement of a larger sample is required to obtain an average enrichment, because of sample
inhomogeneities, or
3 - Measured items are sealed or are products that cannot be opened or sampled [7].
If the uranium sample is large enough, then the 185.7-keV gamma rays originating from deep
levels within the sample are completely absorbed and do not contribute to the gamma-ray intensity
observed at the surface of the sample. Thus, with increasing sample thickness the 185.7-kev gammaray intensity at the surface reaches an equilibrium value, which is almost independent on the physical
form of the sample. For pure uranium compounds this value is proportional to the 235U enrichment of
the sample, and generally only small corrections for chemical composition have to be applied. This is
the “enrichment meter” principle, and its application requires that the sample under assay is thick
enough to be opaque for 186-keV gamma radiation [7].
The objective of uranium enrichment measurement methods is to determine the 235U/U isotopic
ratio which, for most samples, is the determination of the ratio 235U/(234U+235U+238U). In general, this
determination is based on the analysis of one or more X-or γ-ray peaks, mainly, in the 60 to 200 keV
range. Among the different methods developed in the last years, the oldest and the most widely
employed method uses the 185.7 keV full energy peak. These methods require calibration of the
instruments with standards, by fixing the measurement conditions which provide that the
measurement samples are similar to the standards and the number of counts in the 185.7 keV peak is
proportional to the enrichment [8].
In passive gamma counting, the 235U enrichment is correlated with the count rate of the 185.7
keV γ-ray emitted by 235U. The well-known 185.7 keV gamma ray from 235U occurs in about 55% of
the alpha decays of this isotope. Weaker gamma rays at 143.8, 163.4, and 205.3 keV are also
characteristic of 235U. The 185.7 keV gamma line is commonly used to determine 235U enrichment.
Uranium-238 doesn't have a direct gamma-ray signature. Gamma rays from 234mPa and 234Pa daughters
are used to identify 238U. The characteristic gamma-ray energies used to identify 238U are 766.4 and
1001.0 keV from 234mPa [2, 3].
In the present work, the enrichment in the standard reference material samples (SRM-969)
was obtained by measuring the activity ratio of 235U/238U based on the gamma-rays associated with the
decay of 235U at 185.715 keV and the 1001.03 keV 234mPa gamma-ray from the decay of 238U using
gamma spectrometer based on HPGe detector.
EXPERIMENTAL PROCEDURE
Description of the Reference Materials EC NBL (SRM-969)
The reference materials provide two functions in non-destructive assay (NDA). They are used
to calibrate NDA instruments and verify the constancy of the calibrations [9].
NON DESTRUCTIVE ASSAY FOR 235U…
67
This standard reference material intended for use in the calibration and evaluation of gamma
–ray spectrometric procedures for nondestructive determination of the 235U/U isotopic abundance in
uranium bulk material, consists of a set of five different U3O8 powders, encased in aluminum cans
that were manufactured to rigid specifications. Each sample is made up of 200g of U3O8 powder
contained in aluminum cans of 2mm bottom and wall thickness. The cans are 80 mm in diameter by
90 mm height with nominal 235U enrichments of 0.31%, 0.71%, 1.94%, 2.95%, and 4.46% mass
percent [14&15]. For unique identification and checking the integrity of the subunit, the plugs used
for sealing the cans are equipped with ultrasonic seals, each having a unique ultrasonic spectrum.
Additional information about these samples [15] are given in Table 1.
Table 1. Material characteristics and specifications of the certified nuclear material SRM-969
Material
ID
Enrichment
(wt%)
Total weight
of U3O8 ( g)
U3O8 Density
(g/cm2)
Uranium Isotopic
Abundances in SRM 969
234
NBS 031-111
NBS 071-111
NBS 194-111
NBS 295-111
NBS 446-111
0.31
0.71
1.94
2.95
4.46
200.1±0.2
200.1±0.2
200.1±0.2
200.1±0.2
200.1±0.2
5.2±0.3
5.2±0.3
5.2±0.3
5.2±0.3
5.2±0.3
U/U
0.0020
0.0052
0.0171
0.0279
0.0359
235
U/U
0.0146
<0.00002
0.0003
0.0033
0.0068
238
U/U
99.6688
99.2828
98.0406
97.0196
95.4950
Gamma-Ray Detection System
In this study a gamma-ray spectrometer (Canberra type), equipped with hyper- pure
germanium (HPGe) detector was used. The system is used for the measurement of the energy
spectrum of the emitted gamma rays in the energy range between 29 keV and 2615 keV, with
an efficiency of approximately 40%. A side view of the nuclear material cans (the
dimensions are given in cm) located along the coaxial axis of the collimated HPGe detector is
shown in Figure (1).
7.996
0.517
3.00
3.499
1.581
5.596
7.003
U3O8
0.1
1.0
3.0
0.199
3.0
8.0
Coaxial HPGe Detector
14.5
Figure 1. A side view of the nuclear material located along the coaxial axis of the collimated HPGe
detector [15].
68
M.H. Nassef, W.A. El Mowafi, and M.S. El Tahawy
The used gamma ray spectrometer system was energy calibrated by different gamma emitters.
The energy and efficiency calibration of the system were carried out before the analysis using some
standard point sources.
This work was carried out in the KMP-E of MBA (ET-Z) at National Center for Nuclear
Safety and Radiation Control (NCNSRC) of the Egyptian Atomic Energy Authority.
RESULTS AND DISCUSSION
In this work the energy lines of gamma rays of 235U and 238U that were used in this analysis
were 185.715 keV and 1001 keV, respectively. A lead collimator with slit area of 0.785cm2×3cm was
used in this analysis as shown in Figure (1). The mass of the 235U and 238U were calculated using the
equations (1&2) and are given in Table (2).
The uranium molar mass, used in this analysis is 238.029 g/mol, suggesting an isotopic
abundance of 0.72% for 235U. The total weight of uranium oxide (200.1g) as mentioned in the user`s
manual was used in this analysis. The weight of the total uranium in the U3O8 sample was calculated
using equation 1.
Wu = [Ws] × [Mu / M U3O8]
(1)
where : Wu:
is the weight of total uranium in the sample,
Ws : is the total weight of the sample,
Mu : is the molar mass of 238U and
MU3O8 : is the molar mass of U3O8 .
Then, the weight of 235U is calculated using equation (2) by knowing the weight of total uranium in
the sample and its enrichment as indicated in the certificate.
WU-235 = [WUT] × [E]
(2)
where
WU-235: is the weight of 235U,
WUT : is the weight of total uranium and
E:
is the enrichment.
Count Rate (cps) at 185.7 keV gamma ray energy
10
R=0.99738
8
6
4
2
0
0
1
2
3
4
5
Uranium Enrichm ent
Figure 2. Count rate as a function of the 235U enrichment by the 185.7 keV gamma-ray photo-peak.
NON DESTRUCTIVE ASSAY FOR 235U…
69
Table (2) shows the total count rate for all the measured samples at the photo peaks 185.7
keV for 235U and 1001keV for 238U. The specific count rates (in cps/g) were measured and are listed
also in Table (2). The specific count rates of 235U and 238U for the given photo peaks were found to be
almost constant regardless of 235U enrichment as shown in Table (2). The count rates (CR) for all the
measured samples were plotted as a function of the 235U enrichment at 185.7 keV and were found to
be linear with a strong correlation of R= 0.997 as shown in Figure(2).
Table 2. Gamma spectrometry results of U3O8 isotopic standards
Material
ID
Total Uranium
wt. (169.598)g
235
NBS 031-111
NBS 071-111
NBS 194-111
NBS 295-111
NBS 446-111
Ug
0.526
1.204
3.290
5.003
7.564
238
η
(185.7
kev)
Ug
169.072
168.394
166.308
164.595
162.034
2.33×10-5
2.22×10-5
2.16×10-5
2.58×10-5
2.56×10-5
cps of U- cps of U- Specific
235 at
238 at
count-rate
the 185.7 the 1001 (cps/g) of
235
keV
keV
U
at 185.7
keV
0.55±1.6
1.2±0.4
3.2±2.0
5.8±2.3
8.7±1.8
7.1±1.6
7.4±2.5
7.8±2.9
8.1±1.2
8.4±0.2
1.046
0.997
0.973
1.159
1.150
Specific
countrate
(cps/g)
of 238 U
at 1001
keV
0.042
0.044
0.047
0.049
0.052
Table 3. Activity ratio for each sample as a function of the enrichment
Activity (Bq)
Material
ID
NBS 031-111
NBS 071-111
NBS 194-111
NBS 295-111
NBS 446-111
ε
Calculated
Activity of
U-235
Calculated
Activity of U238
0.31
0.71
1.94
2.95
4.46
41500.57
94993.71
259575.84
394728.85
596787.73
2100772.59
2092335.86
2066416.81
2045132.37
2013311.33
Calculated
Activity
Ratio
(235U/238U)
0.0198
0.0454
0.1256
0.1930
0.2964
Measured
Activity
Ratio
(235U/238U)
0.0194
0.0425
0.1146
0.2099
0.3198
Measured
Enrichmen
t wt %
0.27
0.66
1.83
3.23
4.69
ε: The Certified Enrichment (wt%)
Besides, the observed count rate as a function of the uranium enrichment at 1001 keV was
found to be linear with a strong correlation (almost reaches unity) of R= 0.983 as shown in Figure (3).
M.H. Nassef, W.A. El Mowafi, and M.S. El Tahawy
Count Rate (cps) at 1000.1 keV gamma ray energy
70
8.6
8.4
R =0.98267
8.2
8.0
7.8
7.6
7.4
7.2
7.0
0
1
2
3
4
5
U ran ium E nrich m en t
Figure 3. Count rate as a function of the 238U by the 1001 keV gamma-ray photo-peack.
The count rate of 235U and 238U were used to calculate the activity in (Bq) by using the
general equation of the activity concentration using equation 3.
A = [W/A] × NA × [ln2 /T1/2]
where
W:
A:
NA :
T1/2:
(3)
is the total weight of 238U or U-235,
is the atomic weight of U-238 or U-235,
is the Avogadro's number = 6.02×1023,and
is the half-life time of U-238 or U-235.
Knowing the half-life times of 235U (7.038× 108 y) and 238U (4.468× 109 yr), and the
Avogadro’s number (6.02×1023 mole) as well as the activity (in Bq) of 235U and 238U, the activity
ratios 235U/238U were calculated and listed in Table (3). It was found that, for natural uranium,
the activity ratio calculated value (0.0454) is close to the certified value (0.045). The efficiency
at the photo peak 185.7 keV was calculated by knowing the branching ratio and the count rate
of each sample and its activity concentration using equation (4).
The activity values (in Bq) are calculated for both 235U and 238U using the following
equation.
A = CR/(B.R ×η)
where
(4)
B.R.: is the branching ratio or the gamma-ray emission rate,
CR : is the count rate, and
η : is the full energy peak efficiency for the specific experimental system condition [11].
A strong correlation was found between the calculated and the measured activity ratios of
U/238U as depicted in Table (3). The measured activity ratio of 235U/238U was plotted as a function of
the 235U enrichment and was found to be linear with a strong correlation of R= 0.9999 as shown in
Figure(4).
235
Calculated Activity Ratio of (U-235/U-238)
NON DESTRUCTIVE ASSAY FOR 235U…
0.30
71
R = 0.9 99 92
0.25
0.20
0.15
0.10
0.05
0.00
0
1
2
3
4
5
U ranium E nrichm ent
Figure 4. Activity ratio calculation of (235U/238U) as a function of uranium enrichment for the
reference material samples.
The measured activity ratios of 235U/238U on the 185.7 keV gamma line of 235U and 1001
keV gamma line of 234mPa were used to calculate the enrichment of 235U in the sample [13 ] using
equation (5).
Log (wt%235U) = 1.065 - 0.7317 (log 238U/235U Activity) – 0.129 (log 238U/235U Activity)2
(5)
The measured enrichment values were compared with the certified values and are given in Table (3).
Measured Enrichment Wt%
5
R = 0 .9 9 82
4
3
2
1
0
0
1
2
3
4
5
C ertified E nrichm ent W t%
Figure 5. Correlation between the certified and the measured enrichment.
A strong correlation (0.9982) was found between the certified and the measured enrichment
values as shown in Figure (5).
CONCLUSINS
It was possible to define the enrichment through the measurement of the activity ratio of
U/ U using the described gamma-ray analysis method. Measuring the activity ratio, one can
deduce the enrichment especially for the low enriched uranium (LEU). A strong correlation (0.9982)
235
238
72
M.H. Nassef, W.A. El Mowafi, and M.S. El Tahawy
was found between the certified and the measured enrichment values .The precision of the U-235
measurements is primarily dependent upon the
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[3] P.E. Fehlau and W.H. Chambers "Perimeter Safeguards Techniques for Uranium Enrichment
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[4] E. Arnold Hakkila, Nuclear Safeguards Analysis, Nondestructive and Analytical Chemical
Technique, ACS Symposium Series 79, American Chemical Society, Anaheim, CA, March 13-17,
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Sharma; Journal of Radioanalytical and Nuclear Chemistry, 1-6( 2006).
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Using Gamma Rays Above 100 keV, Symposium on International Safeguards; Verification and
Nuclear Material Security, Vienna, Austria, October 29-November 1, 2001
[7] H.A. Smith Jr., P. Schillebeeckx, Nondestructive Assay of Special Nuclear Material for Uranium
Fuel-Fabrication Facilities, http:// lib-www.lanl.gov/la-pubs/00412677.pdf, LA-UR-97-1195.
[8] J. Morel, M. Etcheverry, and G. Riazuelo; Appl. Radiat.Isot. Vol. 49, No 9-11, pp. 1251-1257,
(1998).
[9] J.K. Sprinkle, R.N. Likes, J.L. Parker, H.A. Smith, Reference Materials for Nondestructive Assay
of Special Nuclear Material, volume 1: Uranium Oxide Plus Graphite Powder, LA-9910-MS.
NUREG/CR-3522.
[10] B.B. Shriwastwa, Anil Kumar, B. Raghunath, M.R. Nair, M.C. Abani, R. Ramachandran,
S.Majumdar, J.K.Ghosh; Applied Radiation and Isotopes,V. 54, 941-945(2001).
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[14] S.T. Hsue et al., Guide to Nondestructive Assay Standards: Preparation Criteria, Availability,
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[15] A Users Manual for the Certified Reference Material SRM-969. National Bureau of Standards
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‫‪73‬‬
‫…‪NON DESTRUCTIVE ASSAY FOR 235U‬‬
‫ﻃﺮﯾﻘﺔ ﻻ إﺗﻼﻓﯿﺔ ﻟﺘﻌﯿﻦ ﺗﺮﻛﯿﺰ اﻟﯿﻮراﻧﯿﻮم‪ 235-‬ﻓﻲ ﻋﯿﻨﺎت اﻛﺴﯿﺪات اﻟﯿﻮراﻧﯿﻮم اﻟﻌﯿﺎرﯾﺔ ﻣﺨﺘﻠﻔﺔ اﻷﺛﺮاء‬
‫م‪.‬ح‪ .‬ﻧﺎﺻﻒ‪ ،‬و‪.‬ع‪ .‬اﻟﻤﻮاﻓﻲ‪ ،‬م‪.‬س‪ .‬اﻟﻄﺤﺎوي‬
‫ھﯿﺌﺔ اﻟﻄﺎﻗﺔ اﻟﺬرﯾﺔ ‪ ,‬اﻟﻤﺮﻛﺰ اﻟﻘﻮﻣﻲ ﻟﻸﻣﺎن اﻟﻨﻮوي واﻟﺮﻗﺎﺑﺔ اﻹﺷﻌﺎﻋﯿﺔ‬
‫ﺒﺎﺴﺘﺨﺩﺍﻡ ﻁﺭﻴﻘﺔ ﻻ ﺇﺘﻼﻓﻴﺔ ﻟﺘﺤﻠﻴل ﻋﻴﻨﺎﺕ ﺃﻜﺴﻴﺩﺍﺕ ﺍﻟﻴﻭﺭﺍﻨﻴﻭﻡ ﺍﻟﻌﻴﺎﺭﻴﺔ ﻤﺨﺘﻠﻔﺔ ﺍﻹﺜﺭﺍﺀ ﺘﻡ ﺤﺴﺎﺏ ﺍﻟﻨﺸﺎﻁ ﺍﻹﺸﻌﺎﻋﻲ ﺍﻟﻨﺴﺒﻲ‬
‫ﻟﻠﻴﻭﺭﺍﻨﻴﻭﻡ‪/235-‬ﺍﻟﻴﻭﺭﺍﻨﻴﻭﻡ‪ 238-‬ﻟﻠﻌﻴﻨﺎﺕ ﺍﻟﻌﻴﺎﺭﻴﺔ ﻭﺫﻟﻙ ﺍﻋﺘﻤﺎﺩﺍ ﻋﻠﻲ ﻗﻴﺎﺱ ﻤﻌﺩل ﺍﻟﻌﺩ ﻟﻠﺨﻁ ﺍﻟﺠﺎﻤﻲ ‪ 185.7‬ﻙ‪.‬ﺃ‪.‬ﻑ‪.‬‬
‫ﺍﻟﻤﻨﺒﻌﺙ ﻤﻥ ﺍﻟﻴﻭﺭﺍﻨﻴﻭﻡ‪ 235-‬ﻭﺍﻟﺨﻁ ﺍﻟﺠﺎﻤﻲ ‪ 1001.0‬ﻙ‪.‬ﺃ‪.‬ﻑ‪ .‬ﺍﻟﻤﻨﺒﻌﺙ ﻤﻥ ﺍﻟﺒﺭﻭﺘﻜﺘﻨﻴﻭﻡ‪234-‬ﻡ ‪ ،‬ﻭﺍﻟﻤﺘﺯﻥ ﻤﻊ‬
‫ﺍﻟﻴﻭﺭﺍﻨﻴﻭﻡ‪ .238-‬ﻭﻗﺩ ﻭﺠﺩ ﺘﻁﺎﺒﻕ ﺠﻴﺩ ﺒﻴﻥ ﻗﻴﻡ ﻨﺴﺒﺔ ﺍﻟﻨﺸﺎﻁ ﺍﻹﺸﻌﺎﻋﻰ ﻟﻠﻴﻭﺭﺍﻨﻴﻭﻡ‪ 235-‬ﺇﻟﻰ ﺍﻟﻴﻭﺭﺍﻨﻴﻭﻡ‪ 238-‬ﺍﻟﻤﺤﺴﻭﺒﺔ‬
‫ﻭﺍﻟﻤﻘﺎﺴﺔ‪ .‬ﻭﻜﺫﻟﻙ ﻭﺠﺩ ﺘﻭﺍﻓﻕ ﻗﻭﻯ )‪ (0.9982‬ﺒﻴﻥ ﻨﺴﺒﺔ ﺍﻹﺜﺭﺍﺀ ﺍﻟﻤﻘﺎﺴﺔ ﻭﺍﻟﻤﺩﻭﻨﺔ ﺒﺸﻬﺎﺩﺓ ﺍﻟﻌﻴﻨﺎﺕ ﺍﻟﻌﻴﺎﺭﻴﺔ‪.‬‬
‫ﺘﺒﻴﻥ ﺍﻟﻨﺘﺎﺌﺞ ﺍﻟﺘﻲ ﺘﻡ ﺍﻟﺤﺼﻭل ﻋﻠﻴﻬﺎ ﺃﻥ ﺍﻟﻁﺭﻴﻘﺔ ﺍﻟﺘﺤﻠﻴﻠﻴﺔ ﺍﻟﻤﻘﺘﺭﺤﺔ ﻴﻤﻜﻥ ﺍﺴﺘﺨﺩﺍﻤﻬﺎ ﻟﺘﺤﺩﻴﺩ ﺘﺭﻜﻴﺯ ﺍﻟﻴﻭﺭﺍﻨﻴﻭﻡ ﻭﻨﺴﺒﺔ‬
‫ﺍﻟﺘﻭﺍﺠﺩ ﺍﻟﻨﻅﺎﺌﺭﻯ ﻭﺫﻟﻙ ﻟﻌﻴﻨﺎﺕ ﺍﻟﻴﻭﺭﺍﻨﻴﻭﻡ ﺍﻟﻤﻨﻀﺒﺔ ﻭﺍﻟﻁﺒﻴﻌﻴﺔ ﻭﻤﻨﺨﻔﻀﺔ ﺍﻻﺜﺭﺍﺀ ‪.‬‬