Characterization of neutron spectrum parameters at irradiation channels for neutron activation analysis after full conversion of the Dalat nuclear research reactor to low enriched uranium fuel
Nuclear Science and Technology, Vol. 4, No. 1 (2014), pp. 70-75
Characterization of neutron spectrum parameters at irradiation
channels for neutron activation analysis after full conversion of the
Dalat nuclear research reactor to low enriched uranium fuel
C.D. Vu1*, T.Q. Thien1, H.V. Doanh1, P.D. Quyet2, T.T.T. Anh3, and N.N. Dien1
1 Nuclear Research Institute, 01 Nguyen Tu Luc St., Dalat, Lamdong
2 Chu Van Anhigh school, Ductrong, Lamdong
3 The University of Dalat, 01, Phu Dong Thien Vuong St., Dalat, Lamdong
(Received 12 March 2014, accepted 9 May 2014)
Abstract: In the framework of the program on Russian Research Reactor Fuel Return (RRRFR) and
the program on Reduced Enrichment for Research and Test Reactor (RERTR), the full core
conversion of the Dalat Nuclear Research Reactor (DNRR) to low enriched uranium (LEU, 19.75%
235U) fuel was performed from November 24, 2011 to January 13, 2012. The reactor is now operated
with a working core consisting of 92 WWR-M2 LEU. After the full core conversion, the neutron
spectrum parameters which are used in k0-NAA such as thermal neutron flux (th), fast neutron flux
(fast), f factor, alpha factor (), and neutron temperature (Tn) have been re-characterized at four
different irradiated channels in the core. Based on the experimental results, it can be seen that the
thermal neutron flux decreases by 6÷9% whereas fast neutron flux increases by 2÷6%. The neutron
spectrum becomes‘harder’ at most of irradiated positions. The obtained neutron spectrum parameters
from this research are used to re-establish the procedures for Neutron Activation Analysis (NAA)
according to ISO/IEC 17025:2005 standard at NuclearResearch Institute.
Keywords: Neutron Activation Analysis (NAA), k-zero method, neutron flux, HEU, LEU.
I. INTRODUCTION
core configuration consisting of 92 WWR-M2
LEU fuel assemblies [1, 2]. Since March 2012,
the reactor has been continuously operated
about 100÷130 hours per month at nominal
power of 500 kW for radioisotopes production,
activation analysis and other researches.
Dalat nuclear research reactor was
upgraded from the TRIGA Mark-II designed
and constructed by the United States. The
project of reconstruction and upgrade of the
reactor was started in March 1982. The
criticality was reached at 19:50 on November
01, 1983 and its regular operation at nominal
power of 500 kW was started from March 1984
with the core loaded with 88 WWR-M2 fuel
assemblies enriched to 36% (HEU- Highly
Enriched Uranium) [1].
At the DNRR, there are four irradiated
channels used for NAA (Fig. 1): (1) the fast
pneumatic transfer system for short irradiation
at the channel 13-2 and thermal column (Ti<45
sec); (2) another pneumatic transfer system for
short and medium irradiation at the 7-1 channel
(Ti: 45÷1200 sec); (3) the rotary rack with 40
irradiated holes placed inside the graphite
reflector for long irradiation (Ti>20 min).
Through the full core conversion project
performed from November 24, 2011 to January
13, 2012, the DNRR now is operated with a
©2014 Vietnam Atomic Energy Society and Vietnam Atomic Energy Institute
CAO DONG VU et al.
II. EXPERIMENTAL
To standardize the irradiation channel, it
is necessary to identify three basic parameters
such as thermal neutron flux (th), the ratio of
thermal to epi-thermal neutron flux (f), and the
coefficient describing the deviation of neutron
spectrum distribution from the 1/E shape (α).
In addition, two other parameters, fast neutron
flux (fast), and neutron temperature (Tn) are
also considered to be the characteristic
parameters of the neutron spectra in the
irradiation channel [3].
Fig. 1. Dalat research reactor cross-section.
The neutron spectrum parameters used
in k0-NAA including thermal neutron flux, fast
neutron flux, and the factors of f, alpha, and
neutron temperature have been re-characterized
at four irradiated channels after full core
conversion to LEU fuel. The obtained neutron
spectrum parameters from this research are
used to re-establish the procedures for Neutron
Activation Analysis using k0-IAEA software.
In this study, bare multi-monitor method
[3, 4, and 5] using set of four monitors (Al-
0.1%Au, Al-0.1%Lu, 99.98%Ni and 99.8%Zr)
was applied to determine the parameters of the
neutron spectra at four irradiated positions of
the reactor. The experimental conditions are
described in Table I.
Table I. The irradiation, decay and counting times for the monitors with Au, Lu, Ni, and Zr.
Irradiation time
(Ti)/position + Monitors
(weight)
Decay
time
(Td)
Products
[T1/2, E (keV)]
Counting time (Tc)
- 1200s for Ni and Lu monitors
- 1800s for combination
- 15 m/channel 7-1 and 13-
2
65Ni [2.5h, 366.3, 1115.5,
1481.8], 176mLu [3.6h, 88.4]
4÷6 h
~1 d
97Zr [16.7h, 743.4]
97Nb [16.7h, 657.9]
- 3 h/Thermal column
- 1h/Rotary rack
- 7200s for Zr monitor and
combination
+Al-0,1%Au wire (~5mg)
+Al-0,1%Lu wire (~5mg)
+ 99.8%Zr foil (~10mg)
+99.98%Ni foil (~30mg)
198Au [2.7d, 411,8]; 177Lu
[6.7d, 112.9, 208.4]; 95Zr
[64d, 756.7]; 95Nb [64d,
765.8]; 58Co [70.8d, 810.8]
- 900s for Au monitor
- 7200s for Zr monitor
~3 d
- 10800s for Ni and
Lumonitors, and combination
After an appropriate decay time for each
isotope, the samples were measured with the
gamma spectrometry using HPGe detector
(FWHM ~ 2.2 keV at 1332 keV). The samples
were placed at 14 cm from the detector surface.
In order to determine the neutron spectrum
parameters simultaneously, monitors were
combined and measured at 0.5 hours, 1 hour,
and 3 hours with the decay time of 6 hours, 1
day and 3 days, respectively. The k0-IAEA
software was employed for the treatment of
experimental data. For the purpose of quality
71
CHARACTERIZATION OF NEUTRON SPECTRUM PARAMETERS AT …
control of the analytical procedure, 30 mg, 70
assigned value; and (5) 3.28<|Uscore|, the
laboratory result is significantly different from
the assigned value [4].
mg and 100 mg samples of the standard
reference material named NIST-679 (Brick
Clay) were irradiated at 45 sec, 1 hour, and 10
hours, respectively. The U-score is calculated
III. RESULTS AND DISCUSSION
according to the following equation: 푈푠푐표푟푒
=
A. Neutron spectrum parameters at the
irradiated channels of the DNRR after full core
conversion to LEU fuel
(푋퐴푛푎 − 푋퐶푒푟푡 )/ 휎퐴2푛푎 + 휎퐶2푒푟푡
,
where:
XAna, and XCert are the analytical results, and
certificated values, Ana,and Cert are the
uncertainty of XAna, and XCert. The results of
the laboratory are interpreted according to the
5 possible evaluation classes as follows: (1)
|Uscore|1.64, the laboratory result does not
differ significantly from the assigned value; (2)
1.64<|Uscore|<1.96, the laboratory result
probably does not differ significantly from the
assigned value; (3) 1.96<|Uscore|<2.58, it is not
clear whether the laboratory result differs
significantly from the assigned value; (4)
2.58<|Uscore|<3.28, the laboratory result is
probably significantly different from the
Neutron spectrum parameters at the
channel 13-2, thermal column, channel 7-1,
and rotary rack of the DNRR after full core
conversion to LEU fuel are given in Table II,
III, IV and V. In order to study the stability of
the neutron field at irradiated channels, the
experiments at channels 13-2, 7-1, and
thermal column (Table II, III and IV) were
repeated three times in three different
operation cycles of the reactor. However, at
the rotary rack (Table V), the parameters were
obtained only from two experiments (in
March, and April 2012).
Table II. Neutron spectrum parameters at the channel 13-2 after core coversion of the DNRR.
Parameters
Experimental period
Feb. 2013
Average ± SD
Aug. 2012
4.21 0.17
6.220.39
Mar. 2013
4.070.09
th(× 1012 n/cm2/s)
fast(× 1012 n/cm2/s)
4.34 0.17
4.21 0.14
6.610.87
7.610.75
6.010.39
-0.073 0.009
13.1 0.3
-0.068 0.019
10.8 0.2
-0.067 0.004
8.3 0.7
-0.069 0.003
10.7 2.4
f
Tn (K)
317 5
307 9
312 11
312 5
Table III. Neutron spectrum parameters at the thermal column after core coversion of the DNRR.
Parameters
Average ± SD
Experimental period
Mar. 2013
Jul. 2012
1.26 0.54
8.99 0.06
-
Apr. 2013
1.27 0.09
8.03 0.06
-0.094 0.167
198 2
th(× 1011 n/cm2/s)
fast(× 108 n/cm2/s)
1.24 0.03
1.21 0.27
8.29 0.11
-0.140 0.015
197 4
8.440.49
-0.117 0.032
195 4
190 8
306 6
f
Tn (K)
298 7
291 8
297 3
72
CAO DONG VU et al.
Table IV. Neutron spectrum parameters at the channel 7-1 after core coversion of the DNRR.
Parameters
Experimental period
Apr. 2012
Average ± SD
Mar. 2012
4.30 0.14
3.860.35
May2012
4.24 0.12
4.140.23
th(× 1012 n/cm2/s)
fast(× 1012 n/cm2/s)
4.12 0.18
4.22 0.04
3.900.23
3.690.10
-0.022 0.032
9.6 0.9
-0.041 0.025
10.2 0.4
-0.031 0.028
9.3 0.7
-0.031 0.009
9.7 0.5
f
Tn (K)
300 5
300 5
301 5
300 0.6
Table V. Neutron spectrum parameters at the rotary rack after core coversion of the DNRR.
Parameters
Experimental period
Average ± SD
Mar. 2012
Apr. 2012
3.84 0.15
0.32 0.04
0.104 0.010
30.0 1.0
th(× 1012 n/cm2/s)
fast(× 1012 n/cm2/s)
3.68 0.04
0.31 0.05
0.099 0.010
30.1 2.5
3.760.11
0.32 0.01
0.102 0.003
30.1 0.4
f
Tn (K)
294 6
297 6
295 2
B. Comparison of the neutron spectrum
parameters before and after full core conversion to
LEU fuel
results in Table VI the absolute value of α at
channel 7-1 increases by approximately 1.7
times at negative side after full core
conversion. This means that the neutron
spectrum at the channel 7-1 becomes 'harder'
rather than that of before conversion. At the
rotary rack, the α factor significantly increases
by 2.5 times at positive sign. This means that
epi-thermal neutron spectrum at this position
tends to deviate below the 1/E distribution [5].
Table VI shows the thermal (th) and
fast (fast) neutron fluxes, coefficient, and f
at the channel 7-1, and rotary rack measured
before [3] and after the full core conversion.
The obtained results in Table VI show that
after full core conversion, thermal neutron flux
reduces 8% at channel 7-1, and 6% at rotary
rack whereas the fast neutron flux at channel
7-1 and rotary rack increases by 2% and 6%,
respectively. This means that epi-thermal
neutron flux also increases (f decreases)
leading to the occurrence of the interference
reactions in k0-NAA such as (n, p), (n, n')
etc.[4]. On the other hand, also from the
As the old channel 13-2 was removed
from the core in November 2006, and a new
pneumatic transfer system together with the
channel 13-2 was reinstalled in June 2012,
therefore, there are no data for neutron
spectrum
at
channel
13-2
during
2006÷2011 period.
73
CHARACTERIZATION OF NEUTRON SPECTRUM PARAMETERS AT …
Table VI. The thermal and fast neutron flux at channel 13-2 and Rotary rack before
and after full core conversion of the DNRR.
th (n/cm2/s)
fast(n/cm2/s)
f
α
[3], measured in 2010 with HEU-LEU fuel
Channel 7-1
Rotary rack
4.59 × 1012
4.01 × 1012
3.81 × 1012
0.30 × 1012
-0.019
0.040
11.09
42.28
This work, measured in 2012 with LEU fuel
Channel 7-1
Rotary rack
4.22 × 1012
3.76 × 1012
0.92
3.90 × 1012
0.32 × 1012
1.02
-0.031
0.102
1.67
9.70
30.10
0.87
This work/[3], 7-1
This work/[3], Rotary rack
0.94
1.06
2.54
0.71
Table VII presents the thermal neutron
flux values at the channel 13-2 and thermal
column measured in 2003 [5] and after full
core conversion. The results from Table VII
show that after the core conversion, the thermal
neutron fluxes at channel 13-2 reduce 9% and
increase approximately 21 times at thermal
column. This unusual change at the thermal
column does not result from the core
conversion, but mainly relates to the
modification of structure of the thermal column
which was installed together with a new
pneumatic transfer system in 2012. The new
facility for thermal column was put close to the
graphite reflector in which the sample was
placed 10.8 cm deeper in contrast to the old
irradiated position.
Table VII. Themal neutron flux before and after full core conversion.
Thermal column
Channel 13-2
4.62 × 1012
4.21 × 1012
0.91
HEU (2003) [5]
LEU (2012)
LEU/HEU
5.80 × 109
1.24 × 1011
21.38
C. Analysing of SRM NIST-679 (Brick clay)
using obtained neutron parameters
Tables VIIIa and VIIIb show that the
|Uscore| for all analytical values are less than
1.64, which means that all results are
acceptable. This analysis also shows that it is
necessary to re-characterize the neutron
spectrum parameters after the core conversion.
Nevertheless, the data obtained from this study
are reliable and can be used to calibrate the
irradiated channels for k0-NAA at the DNRR.
To assess the quality of the neutron
spectrum data set obtained through this study,
the SRM named NIST-679 was analyzed by k0-
NAA. The analytical results obtained before
and after the core conversion are given in
Table 8a and Table 8b, respectively.
74
CAO DONG VU et al.
Table VIIIa. Analytical results of SRM NIST-679 before the core conversion.
Analyzed value
Certified value
No. Element
Uscore
Position
Conc.
103500
6.95
1764
8.9
Unc.
Conc.
Unc.
3400
0.27
45
7-1
7-1
7-1
RR
RR
RR
RR
RR
1
2
3
4
5
6
7
8
Al
Dy
Mn
As
La
Fe
5208
1.98
436
110100
7.15
1852
9.5
-1.06
-0.10
-0.20
-0.19
0.01
0.25
-0.30
0.01
3.1
0.2
50.0
92133
22.1
12.4
6168
2.3
49.9
0.5
90500
22.8
2100
0.2
Sc
Th
13.47
1.63
13.46
0.12
Table VIIIb. Analytical results of SRM NIST-679 after the core conversion.
Analyzed value
Certified value
No. Element
Uscore
Position
Conc.
106500
6.4
Unc.
Conc.
Unc.
3400
0.27
45
7-1
7-1
7-1
RR
RR
RR
RR
RR
1
2
3
4
5
6
7
8
Al
Dy
Mn
As
La
Fe
8758
1.3
110100
7.15
1852
9.5
-0.38
-0.56
-0.88
-0.84
-1.56
0.65
1742
8.3
45.5
92880
21.9
116
1.42
2.77
3001
2.5
0.2
49.9
0.5
90500
22.8
2100
0.2
Sc
Th
-0.36
-1.11
13.2
0.2
13.46
0.12
IV. CONCLUSION
REFERENCES
Re-establishment of the neutron spectrum
parameters including th, fast, , f, and Tn at
four irradiated channels for NAA at the DNRR
after full core conversion to LEU fuel was
carried out.
[1] N.N. Dien, Project of fuel conversion at Dalat
research reactor, Dalat Nuclear Research
Institute (2011).
[2] N.N. Dien, Report on the physics start-up for
conversion to LEU fuel at Dalat research reactor,
Dalat Nuclear Research Institute, (2012).
After replacement of the core with LEU
fuel assemblies, the thermal neutron flux in
most of irradiated channels decreases by 6÷9%
while the epi-thermal neutron flux and fast
neutron increase by 2÷6%; neutron spectrum
becomes‘harder’ in most of the investigated
positions.
[3] C.D. Vu, Project report (code CS/09/01-01)
Study on application of k0-IAEA at Dalat
research reactor, Vietnam Atomic Energy
Institute (2010).
[4] H.M. Dung*, M.C. Freitas, J.P. Santos, J.G.
Marques, Re-characterization of irradiation
facilities for k0-NAA at RPI after conversion to
LEU fuel and re-arrangement of core
configuration, Nuclear Instruments and Methods
in Physics Research A 622, 438–442 (2010).
New neutron spectrum parameters
obtained through this study will be useful for
characterization of the irradiation channels in
k0-NAA analytical procedure at the DNRR
after full core conversion to LEU fuel.
[5] H.M. Dung, Study for development of k-zero
Neutron Activation Analysis for multi-element
characterization, PhD thesis, the Natural
Science University, Hochiminh city (2003).
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