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

2 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 
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]. 
Through the full core conversion project 
performed from November 24, 2011 to January 
13, 2012, the DNRR now is operated with a 
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. 
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). 
©2014 Vietnam Atomic Energy Society and Vietnam Atomic Energy Institute 
CAO DONG VU et al. 
71 
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. 
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]. 
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) 
Counting time (Tc) 
Products 
[T1/2, E (keV)] 
- 15 m/channel 7-1 and 13-
2 
- 3 h/Thermal column 
- 1h/Rotary rack 
+Al-0,1%Au wire (~5mg) 
+Al-0,1%Lu wire (~5mg) 
+ 99.8%Zr foil (~10mg) 
+99.98%Ni foil (~30mg) 
4÷6 h 
- 1200s for Ni and Lu monitors 
- 1800s for combination 
65
Ni [2.5h, 366.3, 1115.5, 
1481.8], 
176m
Lu [3.6h, 88.4] 
~1 d 
- 7200s for Zr monitor and 
combination 
97
Zr [16.7h, 743.4] 
97
Nb [16.7h, 657.9] 
~3 d 
- 900s for Au monitor 
- 7200s for Zr monitor 
- 10800s for Ni and 
Lumonitors, and combination 
198
Au [2.7d, 411,8]; 
177
Lu 
[6.7d, 112.9, 208.4]; 
95
Zr 
[64d, 756.7]; 
95
Nb [64d, 
765.8]; 
58
Co [70.8d, 810.8] 
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 
CHARACTERIZATION OF NEUTRON SPECTRUM PARAMETERS AT  
72 
control of the analytical procedure, 30 mg, 70 
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 
according to the following equation: 𝑈𝑠𝑐𝑜𝑟𝑒 =
(𝑋𝐴𝑛𝑎 − 𝑋𝐶𝑒𝑟𝑡 )/ 𝜎𝐴𝑛𝑎
2 + 𝜎𝐶𝑒𝑟𝑡
2 , 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 
assigned value; and (5) 3.28<|Uscore|, the 
laboratory result is significantly different from 
the assigned value [4]. 
III. RESULTS AND DISCUSSION 
A. Neutron spectrum parameters at the 
irradiated channels of the DNRR after full core 
conversion to LEU fuel 
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 
Average ± SD 
Aug. 2012 Feb. 2013 Mar. 2013 
th(× 10
12
 n/cm
2
/s) 4.21 0.17 4.34 0.17 4.07 0.09 4.21 0.14 
fast(× 10
12
 n/cm
2
/s) 6.22 0.39 7.61 0.75 6.01 0.39 6.61 0.87 
 -0.073 0.009 -0.068 0.019 -0.067 0.004 -0.069 0.003 
f 13.1 0.3 10.8 0.2 8.3 0.7 10.7 2.4 
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 Experimental period Average ± SD 
Jul. 2012 Mar. 2013 Apr. 2013 
th(× 10
11
 n/cm
2
/s) 1.26 0.54 1.24 0.03 1.27 0.09 1.21 0.27 
fast(× 10
8
 n/cm
2
/s) 8.99 0.06 8.44 0.49 8.03 0.06 8.29 0.11 
 - -0.117 0.032 -0.094 0.167 -0.140 0.015 
f 190 8 195 4 198 2 197 4 
Tn (K) 306 6 298 7 291 8 297 3 
CAO DONG VU et al. 
73 
Table IV. Neutron spectrum parameters at the channel 7-1 after core coversion of the DNRR. 
Parameters Experimental period 
Average ± SD 
Mar. 2012 Apr. 2012 May2012 
th(× 10
12
 n/cm
2
/s) 4.30 0.14 4.12 0.18 4.24 0.12 4.22 0.04 
fast(× 10
12
 n/cm
2
/s) 3.86 0.35 3.69 0.10 4.14 0.23 3.90 0.23 
 -0.022 0.032 -0.041 0.025 -0.031 0.028 -0.031 0.009 
f 9.6 0.9 10.2 0.4 9.3 0.7 9.7 0.5 
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 
th(× 10
12
 n/cm
2
/s) 3.68 0.04 3.84 0.15 3.76 0.11 
fast(× 10
12
 n/cm
2
/s) 0.31 0.05 0.32 0.04 0.32 0.01 
 0.099 0.010 0.104 0.010 0.102 0.003 
f 30.1 2.5 30.0 1.0 30.1 0.4 
Tn (K) 294 6 297 6 295 2 
B. Comparison of the neutron spectrum 
parameters before and after full core conversion to 
LEU fuel 
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 
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]. 
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. 
CHARACTERIZATION OF NEUTRON SPECTRUM PARAMETERS AT  
74 
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/cm
2/s) fast(n/cm
2/s) α f 
[3], measured in 2010 with HEU-LEU fuel 
Channel 7-1 4.59 × 10
12
 3.81 × 10
12
 -0.019 11.09 
Rotary rack 4.01 × 10
12
 0.30 × 10
12
 0.040 42.28 
This work, measured in 2012 with LEU fuel 
Channel 7-1 4.22 × 10
12
 3.90 × 10
12
 -0.031 9.70 
Rotary rack 3.76 × 10
12
 0.32 × 10
12
 0.102 30.10 
This work/[3], 7-1 0.92 1.02 1.67 0.87 
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 
HEU (2003) [5] 5.80 × 10
9
 4.62 × 10
12
LEU (2012) 1.24 × 10
11
 4.21 × 10
12
LEU/HEU 21.38 0.91 
C. Analysing of SRM NIST-679 (Brick clay) 
using obtained neutron parameters 
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. 
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. 
CAO DONG VU et al. 
75 
Table VIIIa. Analytical results of SRM NIST-679 before the core conversion. 
No. Element 
Analyzed value Certified value 
Uscore Position 
Conc. Unc. Conc. Unc. 
1 Al 103500 5208 110100 3400 -1.06 7-1 
2 Dy 6.95 1.98 7.15 0.27 -0.10 7-1 
3 Mn 1764 436 1852 45 -0.20 7-1 
4 As 8.9 3.1 9.5 0.2 -0.19 RR 
5 La 50.0 12.4 49.9 0.5 0.01 RR 
6 Fe 92133 6168 90500 2100 0.25 RR 
7 Sc 22.1 2.3 22.8 0.2 -0.30 RR 
8 Th 13.47 1.63 13.46 0.12 0.01 RR 
Table VIIIb. Analytical results of SRM NIST-679 after the core conversion. 
No. Element 
Analyzed value Certified value 
Uscore Position 
Conc. Unc. Conc. Unc. 
1 Al 106500 8758 110100 3400 -0.38 7-1 
2 Dy 6.4 1.3 7.15 0.27 -0.56 7-1 
3 Mn 1742 116 1852 45 -0.88 7-1 
4 As 8.3 1.42 9.5 0.2 -0.84 RR 
5 La 45.5 2.77 49.9 0.5 -1.56 RR 
6 Fe 92880 3001 90500 2100 0.65 RR 
7 Sc 21.9 2.5 22.8 0.2 -0.36 RR 
8 Th 13.2 0.2 13.46 0.12 -1.11 RR 
IV. CONCLUSION 
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. 
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. 
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. 
REFERENCES 
[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). 
[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). 
[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).