Preliminary studies on the 232U and 228Th radioactivity levels in reprocessed
uranium samples
Anilkumar. S, Rekha. A.K, Narayani. K, Patre. D.K, Belhe. M.S, D.A.R. Babu
and D.N. Sharma.
Radiation Safety Systems Division
Bhabha Atomic Research Centre
Mumbai – 400085., INDIA
Abstract. Uranium oxide (U3O8) from a reprocessing plant was taken up for benefaction and purification for further
fuel cycle applications. The reprocessing of spent fuel from power reactors are normally taken up after a cooling
period of about 10 years. So many by product actinides are produced in the fuel because of various (n,γ), (n,2n)
reactions and subsequent decays. It was reported that 232U is formed in the fuel in significant quantities and will go
along with the depleted uranium during reprocessing. 232U is a short lived (68.9y) radionuclide of very high
radiological concern in advanced thorium based fuel cycle programs. The daughter products of this nuclide form a
radioactive series similar to that of thorium series containing high energy gamma emitting nuclides. Uranium oxide
samples were taken to assess the radio nuclides contributing to the higher surface gamma dose rate observed on the
container. High resolution gamma spectrometry is a proven analytical tool for the non destructive qualitative and
quantitative analysis of radio nuclides. The gamma spectrometric analysis of U3O8 powder is carried out to see the
radionuclide content in the sample contributing to the high external gamma dose. The gamma spectrometry analysis
is based on the activity of daughter products built up with time from the decay of 232U. The activity of 228Th, the
daughter product of 232U, estimated in the uranium oxide sample is 1028.02±24.53 Bq/gm. For the direct estimation
of 232U activity in the sample alpha spectrometry technique was employed. The activity of 232U is estimated by
analyzing the alpha energy peak of 5.26 MeV and 5.32 MeV. The 232U activity estimated in uranium oxide samples
is 1006±32.34 Bq/gm. The results of the analysis clearly show that 232U is present in the reprocessed uranium oxide
in radioactive equilibrium with its daughter products. This paper discusses in detail the methodology employed for
the estimation of 232U and 228Th in uranium oxide samples and the results of the analysis.
Key words: Reprocessed uranium, Gamma ray spectrometry, 232U
1. Introduction
In a closed fuel cycle program, the recycling of fissile and fertile material is an important process for
improving the efficiency of natural resource management and reducing the long lived radioactive waste
accumulation. Environmental and radiological considerations will be important in the management of
reprocessed uranium (RepU) from the recycling. Significant developments were reported in management
of reprocessed uranium such as purification and conditioning for storage, reenrichment and direct
utilization [1]. Huge quantity of reprocessed uranium is stocked from the civil reprocessing of irradiated
fuel on an industrial scale in several countries with different types of reactors. It will be of interest to
assess the radiological concerns of this reprocessed uranium arising from the presence of some uranium
isotopes formed in the reactor. This paper deals with the preliminary studies on the concentration of 232U
and 228Th present in the reprocessed uranium oxide samples obtained from the reprocessing of PHWR
reactor fuel.
2. Production of 232U in reactor
The production of 232U in the high burn up power reactor is investigated by many authors. It is formed in
the reactor by various neutron capture reactions and decay involving isotopes of uranium, thorium,
neptunium and plutonium. Many production routes are proposed for the formation of this nuclide in the
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reactor. One route proposed is due to the irradiation of natural thorium (232Th) present in the UO2 fuel as
impurity up to the level of 60 ppm [2,3]. In this case 232U is formed in the reactor by the following
reaction
β
231
β
232
Th (n, 2n ) 231Th →
Pa (n, γ ) 232Pa →
U
232
The first reaction is a fast neutron reaction and the production of 231Th will be more in harder neutron
spectrum because of the threshold nature of the (n,2n) reaction. The second reaction in the chain is
thermal neutron reaction with high cross section. Significant quantity of 232Th must be present in the fuel
for the production of 232U in this route. The impurity elements data obtained from different sources
suggest a maximum concentration of thorium in nuclear grade uranium as 10 ppm [4,5,6].
The other route for the production of 232U is through the (n,2n) reaction of 238U and 237Np in
reactor core by fast neutrons suggested by many authors [7,8,9]. The experimental observations by them
suggested significant reaction cross section for 238U and 237Np in fast neutron spectrum. Through this
route 232U can be formed by the following reaction.
β
237
β
236
α
232
U (n,2n ) 237U →
Np (n,2n ) 236Np →
Pu 
→
U
238
In the above reaction chain the half lives of beta active 237U (6.75d) and 236Np (22.5 hr) are very short
compared to the alpha active 236Pu (2.86 y). By taking into consideration the abundance of the isotope
238
U in the core and availability of fast neutron reactor spectrum, it is possible to have significant
production of 232U in the fuel during the high burn up reactor operation. Other production route involving
the (n,3n) reactions on 238Pu and 234U is also suggested [9]. The prediction of the levels of 232U affords a
severe test of computer programs (such as ORIGEN) because of the variety of formation routes and their
sensitivity to the complexity of the reactor spectrum. Detailed calculations in this aspect are required to be
carried out.
3. Experimental Methods
High resolution gamma spectrometry is a proven analytical tool for the non destructive analysis
of samples for its contents. To begin with it was decided to carry out the gamma spectrometric analysis of
reprocessed U3O8 powder to see the radionuclide content in the sample contributing to the high external
gamma dose. The samples collected from the drum were packed in standard counting geometry and
doubly sealed before taken up for measurement. The high resolution gamma ray spectrometry system
consists of p-type coaxial germanium detector with 50% relative efficiency and about 2 keV resolution
for 1332 keV gamma energy of 60Co. The output of the detector was analysed using a PC based 16k
multichannel analyser. The detector is shielded with 3” lead to reduce the background. The samples were
counted by keeping at a distance of 10cm from the detector for a counting time of 10000 sec. The
efficiency calibration of the system was carried out using 152Eu standard source at the same geometry.
From the analysis of spectral data it was observed that major radioactivity contributing to gamma
emissions are from the daughter products of 228Th in addition to 234Th, 234Pa, and 235U. Trace amount of
fission product activities such as 137Cs and 106Ru was also observed in the sample. The typical gamma
spectrum of the reprocessed U3O8 is shown in fig.1.
The activity of 228Th is estimated using the gamma lines of its daughter products 224Ra, 212Pb, 212Bi and
208
Tl. From the activity analysis, it is clear that all the daughter nuclides in the chain are in equilibrium up
to 208Tl. It is observed from the analysis that the major gamma activity was contributed by short lived
gamma emitting radio nuclides in the thorium decay chain. But the absence of gamma lines of 228Ac in
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the spectrum suggests that the source of activity is due to the presence of 232U, an isotope of uranium
(T½=68.9y) formed in the reactor. Some of the earlier studies reported the presence air borne activity due
to thoron and its daughter products in the storage area of reprocessed uranium [2,3]. The decay scheme of
232
U is similar to that of natural thorium series with a difference that it starts with the daughter 228Th. The
decay scheme of this nuclide is shown in fig.2. As 232U does not have any significant gamma emission it
is very difficult to estimate its activity by direct gamma spectrometry methods. Alpha spectrometry
method is the best choice to analyse this radionuclide in samples.
8000
238 keV Pb-212
7000
3000
2000
1000
2614 keV Tl-208
4000
583 keV Tl-208
185 keV U-235
Counts
5000
1001 keV Pa-234
6000
0
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
Energy in keV
Fig.1 Gamma spectrum of U3O8 powder
74.81 (9.6), 77.1 (17.5)
57.2 (6.3), 89.8 (1.75)
115.2 (0.58), 176.6 (0.05)
238.6 (43.1), 300.1 (3.27)
415.2 (0.02)
57.78 (0.2)
232 α 5.26 (31.7)
92 U 5.32 (68)
68.9 y
NO GAMMA
549.7 (0.1)
α 5.42 (71.1)
228
90 Th 5.34 (28.2)
1.913y
84.4 (1.19)
131.6 (0.11)
166.4 (0.08)
205.6 (0.03)
215.9 (0.27)
224 Ra α 5.68 (95)
88
5.45 (5)
3.66 d
220
86 Rn
55.6 s
81.1(12), 83.8 (0.21), 94.7(0.08)
97.6 (0.02), 241 (3..9)
α 6.29 (99.9)
216 Poα 6.78 (100)
84
0.145s
212
82
Pb
10.64h
β 0.24(83), 0.57(12), 0.41(5)
α 6.05 (27),6.3(26), 6.33(35)
212
Bi
36%
83
60.55m
64% β 155(10), 0.45(85)
3.1m
β 1.8 (48.8)
1.29 (23.9)
1.52 (22.7)
2.3(63), 0.93 (7.5)
288.1(0.34),
452.8(0.36),
785.48(1.1),
1078.8(0.54),
1620.6(1.51),
327.9(0.14)
727.2(6.15)
952.1(0.18)
1512.7(0.31)
1806.7(0.11)
212
84 Po
3.04 x 10-7 s
α 8.78(100)
Fig. 2
232U
208
Pb
82
STABLE
NO GAMMA
208
Tl
81
72.8 (2), 74.97 (3.5)
87.3 (0.35), 233.5 (0.3)
252.6 (0.7), 277.36 (6.5)
510.7 (22.5), 583.1 (86)
722.3 (0.27), 763.3 (1.7)
860.5 (12), 982.8 (0.2)
1093.9 (0.38), 2614.5 (100)
Decay series
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By using alpha spectrometry method the concentration of 232U in reprocessed uranium oxide and pure
uranium metal was estimated. For alpha spectrometry of uranium, the samples have to be prepared by
electro deposition on stainless steel planchets. The activity of 232U in the sample is estimated by analyzing
the alpha energy peak of 5.26 MeV and 5.32 MeV. The corrections for the possible contribution due to
228
Th and 224Ra on above energies also carried out. The samples were counted in the Quad Alpha
Spectrometry system employing ion implanted silicon detectors. Samples were counted for a period of
20000 sec. The alpha spectrum of pure RepU sample showing the peak of 232U is shown in fig.3. The
measurements were also repeated and the average activity concentration of 232U is estimated. The results
of the measurements of activities due to 228Th and 232U are shown in Table.1.
Table-1: Activities of 228Th and 232U
228
232
Sample Name
Th (Bq/gm)
( By Gamma spectrometry)
U (Bq/gm)
(By Alpha spectrometry)
U3O8
1028.02±24.53
1006.02±32.34
RepU
164.25±14.98
1223.03±28.5
The benefaction process involves the conversion of natural uranium oxide and diuranate cake to pure
nuclear grade uranium metal. For the reprocessed uranium oxide, the same procedure has been followed
and pure uranium ingot is produced as final product. Final product consists of mainly the pure uranium
metal ingot and the magnesia slag. All the other impurities and daughter nuclides present in the
reprocessed uranium oxide powder get separated and go in the slag route. Because of that surface dose
rate on the slag side can be up to 120 mR/Hr. Gamma spectrometry analysis was also carried out on the
slag and ingot samples.
80
70
60
40
30
20
U-232
Counts
50
10
0
3500
4000
4500
5000
5500
6000
6500
7000
Enrgy in keV
Fig. 3 Alpha spectrum of RepU metal
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4. Results and conclusion
It is seen that a very high clean up is observed in the RepU metal after purification, the final product, from
228
Th and its daughter products. But because of the unavoidable presence of the isotope of 232U in the
uranium metal there will be a build up of 228Th and its daughter products with time. The daughter
products of 228Th will reach equilibrium with in a month. Hence the dose rate due to high energy gamma
radiations of thorium daughters will depend only on the build up of 228Th from 232U as shown in fig.4.
Very high specific activity was observed in slag sample due to the presence of 228Th and its daughter
products. It is also noticed that activities of 234Th and 234Pa, the daughter products of 238U in equilibrium,
also shows an increased concentration. But this activity will decay out very fast because of the short half
life of 24 days for 234Th. At the same time during the purification of RepU the slag carries significantly
long lived (1.9y) and unsupported 228Th, with its daughters. The 228Th and its daughter product will
contribute external gamma field in slag for long period of time. In natural thorium the specific activity of
228
Th in equilibrium with the parent is only 4046 Bq/gm because of the long half life of the parent 232Th.
But in RepU the source of 228Th is highly specific active 232U (8.28 x10 11Bq/gm). The studies on the
impact of this radionuclide in thorium fuel cycle programs are in progress. In addition to the high external
gamma dose from the slag samples, it also forms a potential source for thoron and its daughters in the
working environment causing occupational radiological hazard [10]. During the large scale operation, the
management of large quantities of slag is an important radiological issue to be addressed.
1.00E+07
U-232
1.00E+06
Activity (Bq )
1.00E+05
1.00E+04
1.00E+03
Th-228
1.00E+02
1.00E+01
Ra-224
Rn-220
Po-216
Pb-212
Bi-212
Tl-208
1.00E+00
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
1.00E+05
1.00E+06
1.00E+07
1.00E+08
Time in hours
Fig.4 Activity buildup of daughter nuclides from
232
U
The concentration of 232U estimated from the present work in the RepU samples is 1.5 ppb. The result is
with in the range of values reported from the analysis of RepU from different type of reactors [1]. The
amount of this uranium isotope present in RepU depend on a number of factors like the type of the fuel
used for different reactors, the degree of initial enrichment, level of burn up at the time of fuel discharge
from the reactor and aging periods of spent fuel in cooling water ponds. In short the presences of 232U
along with its daughter products create a significant radiological impact for the storage of RepU and its
subsequent processing for the fuel manufacturing operations. Purification and benefaction of natural
uranium from MDU/ADU is the standard process and the radiological safety practices also well in place.
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But in the case of operations involving reprocessed uranium some more radiological safety steps have to
be taken based on the concentration of 232U, 228Th and its daughter products. Attention also has to be
given for the presence of thoron and its daughters in the working environment causing occupational
radiological hazard. During the dry operations, the 228Th can be air born and create radiological hazard
while considering the annual limit of intake (600 Bq) of 228Th. During the large scale operation, the
management of large quantities of slag is an important radiological issue to be addressed.
Acknowledgements
The authors are thankful to Shri.H.S.Kushwaha, Director , HS&E Group for his keen interest and
encouragement in this work.
REFERENCES
[1] INTERNATIONAL ATOMIC ENERGY AGENCY, Management of Reprocessed Uranium, Current
Status and Future prospects, IAEA-TECDOC-1529, 2007.
[2] M.D. DESHPANDE, et al, Airborne activity due to thoron daughters and associated problems in a
fuel reprocessing plant, Bulletin of Radiation Protection, Vol.17, No.1, (1994) pp 23.
[3] R.K. SHIVADE, Assessment of airborne thoron progeny in a nuclear fuel reprocessing facility and
its associated hazards, M.Sc. Thesis,,(2002), Mumbai University.
[4] S.B. ROY, Benefaction and refining of U/Th, INCAS Bulletin, July 2005.
[5] ASTM, Standard specification for sintered uranium dioxide pellets, ASTM Designation C776-2000.
[6] ASTM, Standard specification for Nuclear Grade, Sinterable uranium dioxide powder, ASTM
Designation C 753 – 2004.
[7] K. LINDEKE, et al , Determination of the 237Np(n,2n)236Np cross section at 15 MeV neutron energy,
Physical Review C, Vol 12, No.5, (1973) pp1507.
[8] JERRY H. LANDRUM, et al, (n,2n) Cross sections for 238U and 237Np in the region of 14 MeV,
Physical Review C, Vol-8, (1973) pp 1938.
[9] R. WELLUM et al, The determination of 232U and 236Pu in solutions of irradiated reactor fuel by
alpha spectrometry, Nuclear Instruments and Methods in Physics Research 223 (1984) 523-527.
[10] M.J. DUGGAN, Some aspects of the hazard from airborne thoron and its daughter products, Health
Physics, Vol-24, ,(1973) pp301.
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