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Neutron flux parameters for k0-NAA method at the Malaysian nuclear agency research reactor after core reconfiguration
Radiation Measurements 46 (2011) 219e223
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Neutron flux parameters for k0-NAA method at the Malaysian nuclear agency research reactor after core reconfiguration A.R. Yavar a, S. Sarmani b, A.K. Wood c, S.M. Fadzil a, Z. Masood c, K.S. Khoo a, * a
School of Applied Physics, Faculty of Science and Technology, University Kebangsaan Malaysia (UKM), Bangi, Selangor 43600, Malaysia School of Chemical Sciences and Food Technology, Faculty of Science and Technology, University Kebangsaan Malaysia (UKM), Bangi, Selangor 43600, Malaysia c Analytical Chemistry Application Group, Industrial Technology Division, Malaysian Nuclear Agency (MNA), Bangi, Kajang, Selangor 43000, Malaysia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2010 Received in revised form 15 October 2010 Accepted 17 November 2010
The Malaysian Nuclear Agency (MNA) research reactor, commissioned in 1982, is a TRIGA Mark II swimming pool type reactor. When the core configuration changed in June 2009, it became essential to re-determine such neutron flux parameters as thermal to epithermal neutron flux ratio (f ), epithermal neutron flux shape factor (a), thermal neutron flux (4th) and epithermal neutron flux (4epi) in the irradiation positions of MNA research reactor in order to guarantee accuracy in the application of k0-neutron activation analysis (k0-NAA).The f and a were determined using the bare bi-isotopic monitor and bare triple monitor methods, respectively; Au and Zr monitors were utilized in present study. The results for four irradiation positions are presented and discussed in the present work. The calculated values of f and a ranged from 33.49 to 47.33 and 0.07 to 0.14, respectively. The 4th and the 4epi were measured as 2.03 1012 (cm2 s1) and 6.05 1010 (cm2 s1) respectively. These results were compared to those of previous studies at this reactor as well as to those of reactors in other countries. The results indicate a good conformity with other findings. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: k0-NAA Neutron flux parameters TRIGA Mark II reactor
1. Introduction The TRIGA Mark II reactor of the Malaysian Nuclear Agency (MNA) was commissioned in 1982. This reactor uses light-water as moderator, coolant and reflector. The fuel assembly consists of an alloy of uranium enriched to 20% U-235 and zirconium hydride (UeZrH). Several experimental facilities are available in the MNA research reactor. For activation analysis and isotope production, a rotary specimen rack is located around the top portion of the core and inside the reflector. The rotary specimen rack assembly consists of ring-shaped, seal-welded aluminium housing containing an aluminium rack mounted on special bearings. As shown in Fig. 1, the rotary rack (RR) supports 40 evenly spaced tubular aluminium containers that serve as receptacles for the specimen containers. Each receptacle has an inside diameter of 3.17 cm and height of 27.4 cm and can hold two specimen containers (Masood et al., 2008). The k0-NAA method, developed in the 1970s, is increasingly used for multielement accurate analysis of biological, geological, environmental and high purity materials using reactor neutrons (De Corte et al., 1969, 1987, 1993; Simonits et al., 1975, 1982; Moens et al., 1984; De Corte, 1992, 1994, 2000, 2001; Bellemans et al., 1995;
* Corresponding author. Tel.: þ60 389214506; fax: þ60 389213777. E-mail address: [email protected] (K.S. Khoo). 1350-4487/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.11.014
Lin et al., 1997).The k0-NAA method requires the accurate characterization of irradiation and counting facilities as well as the use of composite nuclear constants known as k0-factors. The k0-factors, which are independent of irradiation and measurement conditions, are tabulated and published in literature as generally useful nuclear parameters, as shown in Table 1 (Moens et al., 1984; Simonits et al., 1987; De Corte and Simonits, 2003). The k0-NAA method was successfully developed using the HØGDAHL convention (HØGDAHL, 1962). Its application is restricted to (n, g) cross sections that follow the 1/n law in the thermal neutron energy region (i.e., up to w1.5 eV). According to the HØGDAHL convention, neutron flux spectrum parameters such as thermal to epithermal neutron flux ratio (f ) and epithermal neutron flux shape factor (a) are vital for calculating the concentration of elements in a sample when using the k0-NAA method (Lin et al., 1997; De Corte, 2001). In our work f and a were determined by the bare bi-isotopic monitor and bare triple monitor methods, respectively (De Corte et al., 1969; De Corte, 2001; Simonits et al., 1982; Lin et al., 1997). Calculation of f proceeded according to the following equation:
Asp;1 ko ; Auð1Þ 3p;1 $Q ðaÞ $Q ðaÞ $ Asp;2 0;2 ko ; Auð2Þ 3p;2 0;1 f ¼ Asp;1 ko ; Auð1Þ 3p;1 $ Asp;2 ko ; Auð2Þ 3p;2
(1)
220
A.R. Yavar et al. / Radiation Measurements 46 (2011) 219e223
Fig. 1. Arrangement of the 40 rotary rack (RR) irradiation channels around the core of MNA research reactor.
where 1 ¼ 97Zr/97mNb (743.3 keV) and 2 ¼ 95Zr (724.2 þ 756.7 keV); 3p is the full energy peak efficiency; and k0,Au(a) is the nuclear constant known as k0-factor of analyst a versus the gold (Au) monitor. The Asp is the specific count rate, calculated as Asp ¼ (Np/ tc)/SDCW; where Np is measured gamma net peak area (in counts); S is the saturation factor, calculated as S ¼ 1 elti , where ti is irradiation time; D is the decay factor, calculated as D ¼ eltd , where td is decay time (from end of irradiation to start of counting); C is the counting factor, calculated as C ¼ ½1 eltc =ltc , correcting for decay during counting, where tc is counting time; W is mass of irradiated element (g). The Q0(a) is calculated as:
Q0 ðaÞ ¼
Q0 0:429 a
Er
þ
0:429 ð2a þ 1Þ$ð0:55Þa
(2)
Where Er is effective resonance energy in eV; Q0 ¼ I0/s0, where I0 is the resonance integral for the (n, g) reaction and s0 is the thermal neutron cross section (2200 ms1). The a is calculated according to the following De Corte et al., 1969, 1980, 1981, 1986; Simonits et al., 1982; Lin et al., 1997):
Table 1 Nuclear data of neutron flux monitors (Simonits et al., 1987; De Corte and Simonits, 2003). Target isotope
Eg, keV
T1/2
k0,Au (Literature)
Q0
Er ; eV
Formed isotope
197
411.8 724.2 756.7 (724.2 þ 756.7) 743.3
2.695d 64.02d
1
15.7 5.306
5.65 6260
198
251.6
338
94
96
Au Zr
Zr
16.74h
9.32 1.149 2.0 1.237
10-5 10-4 10-4 10-5
95
Au Zr
97
Zr/97mNb
ða bÞQ0;1 ðaÞ aQ0;2 ðaÞ þ bQ0;3 ðaÞ ¼ 0
(3)
where
a ¼
Asp;2 k0 Auð1Þ 3p;1 1 $ $ Asp;1 k0 Auð2Þ 3p;2
b ¼
1
Asp;3 k0 Auð1Þ 3p;1 $ $ Asp;1 k0 Auð3Þ 3p;3
!1 and !1
Where 1 ¼ 97Zr/97mNb (743.3 keV), 2 ¼ 95Zr (724.2 þ 756.7 keV) and 3 ¼ 198Au (411.8 keV). The thermal neutron flux 4th (cm2 s1) was calculated as follows (Lin et al., 1997; Khoo et al., 2007):
fth ¼
f $Asp;Au $3:47 f þ Q0;Au ðaÞ $3p;Au
(5)
and the epithermal neutron flux 4epi (cm2 s1) was calculated as:
fepi ¼
fth f
(6)
The relevant nuclear data used for determination of neutron flux parameters are presented in Table 1(Simonits et al., 1987; De Corte and Simonits, 2003). In principle, the f , a, 4th and 4epi parameters are subject to change according to the reactor configuration and irradiation positions. After a major core reconfiguration of a nuclear reactor, the determination of these parameters is required before performing k0-NAA analysis (De Wispelaere and De Corte, 2003; Mustra et al., 2003). This paper aims to show the results of the f , a, 4th, 4epi recalculation in the present study at four RR irradiation channels of
A.R. Yavar et al. / Radiation Measurements 46 (2011) 219e223
221
Fig. 2. Efficiency curve for the HPGe detector, “ref” ¼ point geometry at 15.8 cm source detector distance. Table 2 f, a, 4th and 4epi at MNA research reactor in present study. f
a
4th (cm
34.97 33.49 47.33 42.90
0.08 0.07 0.14 0.12
2.07 2.08 2.03 2.05
2
1
s
)
1012 1012 1012 1012
2
4epi (cm 5.92 6.21 4.29 4.78
1
s
)
1010 1010 1010 1010
MNA’s TRIGA Mark II reactor after the last configuration change and to compare the new calculations with measurements used in previous studies done at this reactor to observe possible changes in neutron flux parameters. Furthermore, our results are compared with results of the other TRIGA Mark II reactors in an overview of TRIGA Mark II reactors from countries that have calculated these parameters. Because differences exist between TRIGA MARK I, II and III reactors in terms of thermal power and number of fuel elements, their neutron flux parameters are expected not to be similar. Therefore, the scope of this present work is limited to TRIGA MARK II reactors worldwide.
2. Experimental 2.1. Efficiency calibration of g-spectrometer The g-spectrometry measurements were performed with an HPGe detector coupled with Canberra Accuspec multichannel analyzer (MCA); the computer code Gamma Acquisition Analysis was used for peak area evaluation. Full energy peak efficiency calibration of the detector was carried out using 241Am, 109Cd, 57Co, 137Cs and 60Co point calibration sources placed at the reference position 15.8 cm from the detector where true coincidence effects are negligible. Fig. 2 shows the peak detection efficiency of the HPGe detector plotted in logarithmic
scale. The energy range was from 58.91 keV to 1332.58 keV (De Corte et al., 1993; Alghem et al., 2006; Wasim et al., 2008). 2.2. Characterization of MNA research reactor spectrum Au and Zr monitors were used to determine the f , a, 4th and 4epi parameters. The monitors were made ofAl-0.1%Au alloy wire (IRMM-527a, diameter 1 mm, length 10 mm) and Zr foils (IRMM, 99.9%, 125 mm thick), respectively. The monitors were cut and carefully weighed so that the size range for Au monitors was10.7e12.9 mg while for the Zr monitors the size range was18.5e19.8 mg. The vials were chosen with 1 cm diameter and 3 cm length and then cut back to 1 cm length. The monitors were heat sealed inside the polyethylene vials and were packed in heat resistant plastic so that each vial included one Au monitor and one Zr one. The monitors were irradiated for 6 h in four RR irradiation channels (#1, #11, #21 and # 31 channels, shown in Fig. 1) of the MNA research reactor. Due to the short half-lives of radionuclides of 198 Au and 97Zr/97mNb, both monitors were counted for about 5 min after 24 h cooling time using an HPGe detector placed 15.8 cm away from the detector to minimize true coincidence effects. The irradiated zirconium foils were counted again for measurement of 95Zr about 15 min after 3 days of decay time. Three gamma-lines were used in the estimation of f and a: 411.8 keV of 198Au, 743.4 keV of 97 Zr/97mNb and the sum of the two peaks (724.2 and 756.7 keV) of 95 Zr (De Corte et al., 1993; Alghem et al., 2006; Wasim et al., 2008). 3. Results and discussion Table 2 shows the neutron flux parameters at the four RR irradiation channels of the MNA research reactor. The f and a were determined according to equations (1) and (3), identified as the bare bi-isotopic monitor method and the bare triple monitor method,
Table 3 cimovi c et al., 2003; Dung and Hien, 2003; Wee et al., 2006; Khoo et al., 2007) Average and standard deviation of f, a, 4th and 4epi in TRIGA Mark II reactors worldwide (Ja Location of reactor
Date of experiment
f
Malaysia (Wee et al., 2006) Malaysia (Khoo et al., 2007) Malaysia (Present work) Vietnam Slovenia
2003 2005 2009 2003 2003
17.2 33.55 39.67 35.70 31.51
a 0.9 11.2 6.57 2.41
0.016 0.087 0.102 0.073 0.010
0.005 0.046 0.033 0.0068
4th 1012 (cm2 s1)
4epi 1011 (cm2 s1)
2.29 0.09 2.03 0.27 2.06 0.02 3.61 2.15
1.33 0.61 0.45 0.52 0.09 1.01 0.68
222
A.R. Yavar et al. / Radiation Measurements 46 (2011) 219e223
Fig. 3. Core configurations of the MNA research reactor as of (a) August 2001 and (b) April 2009.
respectively. The f ranged from 33.49 to 47.33. The a was found to be in the range of 0.07 to 0.14. Equations (5) and (6) were used to calculate the 4th and 4epi, which were found to range from 2.03 1012 to 2.08 1012 (cm2s1) and from 4.29 1010 to 6.21 1010 (cm2s1), respectively. The observed average value of f was 39.67 with a relative standard deviation (RSD) of 16.56%. A reactor core may be reconfigured for several reasons: installation or uninstallation of an experimental facility inside the reactor for performing a special experiment, homogenization of thermal power distribution, and/or refilling of fuel elements because of burn up effect. The 11th core reconfiguration of the MNA research reactor was performed with 111 fuel elements in its core on 23 August 2001; the 12th core reconfiguration was carried out on 5 July 2006. Since the previous studies’ experiments (Wee et al., 2006; Khoo et al., 2007) were carried out in 2003 and 2005, respectively, (Table 3), they used the same core configuration (i.e. the 11th core reconfiguration), but present work used the 14th core reconfiguration, with 112 fuel elements in the core, which had been carried out on 20 April 2009. Given that previous studies using the MNA reactor were performed when the reactor was configured differently from the configuration of the time when this study was done, it is not unexpected that there would exist some deviation between our results and theirs (Wee et al., 2006; Khoo et al., 2007). In particular, the differences may have been due to differences in the reactors’ methods of neutron flux distribution since the reactor used for the present work had a rotary system of RR channels for irradiation of monitors; therefore, neutron flux distribution was homogeneous for our study. Previous studies, on the other hand, used non-rotary systems of RR channels during irradiation. Moreover, the variation of f values may have arisen from the shape of the RR container as the length axis of the container was not precisely parallel to the length axis of the fuel elements, leaving some space between the container and the inner side of the irradiation tube (Khoo et al., 2007); in addition, as noted in Fig. 3, the number of fuel elements in the reactor core in our work was 112 (Fig. 3b) but 111 in previous studies (Fig. 3a).The greater number of fuel elements in the core inevitably increases thermal neutron flux, which is in turn correlated with the f parameter. The average value of a was found to be 0.1025 with RSD of 32.36%. The a value depends on the reactor configuration and rises with increasing distance from the core of the reactor. The negative values of a could be attributable to poor thermalization in the reactor during our study (De Corte et al., 1980, 1993).
The average value of 4th was found to be 2.03 1012 (cm2 s1) with RSD of 1.09% while the average value of 4epi was 6.05 1010 (cm2 s1) with RSD of 17.24%. The low RSD values of the 4th and 4epi indicate a homogeneous neutron flux due to the rotation of RR irradiation channels during the irradiation of our monitors; as a result, the use of the four RR irradiation channels #1, #11, #21 and #31 (as shown in Fig. 1) positioned in a cruciform with the rotary system of RR channels during the irradiation is taken to be suitable and sufficient for determination of neutron flux parameters in TRIGA MARK II reactors. Worldwide, 18 TRIGA Mark II research reactors are in operation, but only four of them are used for k0-NAA (TRIGA II VIENNA, Vietnam; TRIGA PUSPATI, Malaysia; MA-R1, Morocco; and TRIGA MARK II LJUBLJANA, Slovenia). The neutron flux parameters of three of them can be found in published papers (Dung and Hien, 2003; Ja cimovi c et al., 2003; Wee et al., 2006; Khoo et al., 2007). Table 3 shows the average values of neutron flux parameters and available standard deviation from two previous studies at the MNA research reactor, from present study and from evaluations of other TRIGA Mark II reactors worldwide. In the first study at MNA (Wee et al., 2006), values of f , a and 4th were calculated as 17.2, 0.0157 and 2.29 1012 (cm2 s1), respectively. The second study at MNA (Khoo et al., 2007) obtained values of f , a and 4th as 33.55; 0.0873 and 2.03 1012 (cm2 s1), respectively. Although both previous studies were performed when the MNA research reactor had the same core configuration, yet, as shown in Table 3, there was variation between their findings. Fuel burn up is clearly the parameter that most affects this fluctuation, but it must also be taken into consideration that the control rods positions can be strikingly impressive (Khoo et al., 2007). Furthermore, another factor in this variability could be the number of irradiation channels used. The first study (Wee et al., 2006) irradiated with just one RR channel with a non-rotary system (resulting in inhomogeneous flux distribution), while the second study (Khoo et al., 2007), also with a non-rotary system, used twenty RR channels to obtain more accurate results. As shown in Table 3, with the exception of the first study (Wee et al., 2006), where the MNA research reactor may have reached very high thermalization, results show an acceptable level of consistency of our results with those of other studies. 4. Conclusion The reconfiguration of the core of the MNA research reactor in April 2009 necessitated the re-determination of the neutron flux
A.R. Yavar et al. / Radiation Measurements 46 (2011) 219e223
parameters for k0-NAA applications. The neutron flux values of f , a, 4th and 4epi were determined using Au and Zr monitors in four RR irradiation channels of the MNA research reactor. Although present results showed a good level of consistency with those of most other studies, nevertheless some deviation was found. This deviation may have been caused by differences in neutron flux distribution, since the present work used a rotary system of RR channels during the irradiation of monitors, unlike the others which used nonrotary systems. In addition, the number of fuel elements in the core may have contributed to the deviation.
Acknowledgments The authors would like to thank the MNA operation group for their kind assistance during the implementation of this project.
References Alghem, L., Ramdhane, M., Khaled, S., Akhal, T., 2006. The development and application of k0-standardization method of neutron activation analysis at Es-Salam research reactor. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 556, 386. Bellemans, F., De Corte, F., Van Den Haute, P., 1995. Composition of SRM and CN U-doped glasses: significance for their use as thermal neutron fluence monitors in fission track dating. Radiat. Meas. 24, 153. De Corte, F., 2001. The standardization of standardless NAA. J. Radioanal. Nucl. Chem. 248 (1), 13. De Corte, F., 1992. Problems and solutions in the standardization of reactor neutron activation analysis. J. Radioanal. Nucl. Chem. 160, 63. De Corte, F., 1994. Neutron activation analysis: an old faithful to cherish. Acta Physica Hungarica 75, 189. De Corte, F., 2000. k0 and comparator NAA: influences and interactions. J. Radioanal. Nucl. Chem. 245, 157. De Corte, F., Simonits, A., 2003. Recommended nuclear data for use in the k0 standardization of neutron activation analysis. Atomic Data and Nuclear Data Tables 85, 47. De Corte, F., Speecke, A., Hoste, J., 1969. Reactor neutron activation analysis by a triple comparator method. J. Radioanal. Nucl. Chem. 3, 205. De Corte, F., Moens, L., Simonits, A., De Wispelaere, A., Hoste, J., 1980. Instantaneous a-determination without Cd-cover in the 1/E1þa epithermal neutron spectrum. J. Radioanal. Nucl. Chem. 58, 401.
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De Corte, F., Hammami, K., Moens, L., Simonits, A., De Wispelaere, A., Hoste, J., 1981. The accuracy and precision of the experimental a-determination in the 1/E1þa epithermal reactor-neutron spectrum. J. Radioanal. Nucl. Chem. 62, 209. De Corte, F., Moens, L., Jovanovic, S., Simonits, A., De Wispelaere, A., 1986. Applicability of the 1/E1þa epithermal spectrum representation and the effective resonance energy Er in NAA. J. Radioanal. Nucl. Chem. 102, 37. De Corte, F., Simonits, A., DeWispelaere, A., Hoste, J., 1987. Accuracy and applicability of the k0-standardization method. J. Radioanal. Nucl. Chem. 113, 145. De Corte, F., Simonits, A., Bellemans, F., Freitas, M.C., Jovanovic, S., Smodis, B., Erdtmann, G., Petri, H., De Wispelaere, A., 1993. Recent advances in the k0standardization of neutron activation analysis: extensions, applications, prospects. J. Radioanal. Nucl. Chem. 169, 125. De Wispelaere, A., De Corte, F., 2003. Recalibration of the irradiation facilities in the Thetis reactor, with an examination of the a versus E behavior in the keV neutron energy range. J. Radioanal. Nucl. Chem. 257, 519. Dung, H.M., Hien, P.D., 2003. The application and development of k0-standardization method of neutron activation analysis at Dalat research reactor. J. Radioanal. Nucl. Chem. 257, 643. HØGDAHL, O.T., 1962. Neutron Absorption in Pile Neutron Activation Analysis. Technical Report. Reactor Division, Brookhaven National Laboratory (BNL), Upton, New York, USA. Ja cimovi c, R., Mau cec, M., Trkov, A., 2003. Verification of Monte Carlo calculations of the neutron flux in typical irradiation channels of the TRIGA reactor, Ljubljana. J. Radioanal. Nucl. Chem. 257, 513. Khoo, K., Sarmani, S., Abugassa, I., 2007. Determination of thermal to epithermal neutron flux ratio (f), epithermal neutron flux shape factor (a) and comparator factor (Fc) in the Triga Mark II reactor, Malaysia. J. Radioanal. Nucl. Chem. 271, 419. Lin, X., Baumgärtner, F., Li, X., 1997. The program MULTINAA for various standardization methods in neutron activation analysis. J. Radioanal. Nucl. Chem. 215, 179. Masood, D.Z., Abu, M.P., Ammad, A.M., Yunus, M.N., 2008. Safety Analysis Report for PUSPATI TRIGA Mark II Reactor Facility. Malaysian Nuclear Agency Research Reactor, Bangi, Malaysia. Moens, L., De Corte, F., De Wispelaere, A., Hoste, J., Simonits, A., Elek, A., Szabo, E., 1984. k0-measurements and related nuclear data compilation for (n, g) reactor neutron activation analysis. J. Radioanal. Nucl. Chem. 82, 385. Mustra, C., Freitas, M., Almeida, S., 2003. Neutron flux and associated k0 parameters in the RPI after the last configuration change. J. Radioanal. Nucl. Chem. 257, 539. Simonits, A., De Corte, F., Hoste, J., 1975. Single-comparator methods in reactor neutron activation analysis. J. Radioanal. Chem. 24, 31. Simonits, A., De Corte, F., Moens, L., Hoste, J., 1982. Status and recent developments in the k0-standardization method. J. Radioanal. Nucl. Chem. 72, 209. Simonits, A., De Corte, F., De Wispelaere, A., Hoste, J., 1987. Nuclear data measurements for zirconium isotopes used for activation analysis and neutron metrology. J. Radioanal. Nucl. Chem. 113, 187. Wasim, M., Zaidi, J., Arif, M., Fatima, I., 2008. Development and implementation of k0-INAA standardization at PINSTECH. J. Radioanal. Nucl. Chem. 277, 525. Wee, B.S., Dung, H.M., Wood, A.K., Salim, N.A., Elias, M.S., 2006. Testing the applicability of the k0-NAA method at the MINT’s TRIGA MARK II reactor. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 564, 716.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Neutron flux parameters for k0-NAA method at the Malaysian nuclear agency research reactor after core reconfiguration A.R. Yavar a, S. Sarmani b, A.K. Wood c, S.M. Fadzil a, Z. Masood c, K.S. Khoo a, * a
School of Applied Physics, Faculty of Science and Technology, University Kebangsaan Malaysia (UKM), Bangi, Selangor 43600, Malaysia School of Chemical Sciences and Food Technology, Faculty of Science and Technology, University Kebangsaan Malaysia (UKM), Bangi, Selangor 43600, Malaysia c Analytical Chemistry Application Group, Industrial Technology Division, Malaysian Nuclear Agency (MNA), Bangi, Kajang, Selangor 43000, Malaysia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2010 Received in revised form 15 October 2010 Accepted 17 November 2010
The Malaysian Nuclear Agency (MNA) research reactor, commissioned in 1982, is a TRIGA Mark II swimming pool type reactor. When the core configuration changed in June 2009, it became essential to re-determine such neutron flux parameters as thermal to epithermal neutron flux ratio (f ), epithermal neutron flux shape factor (a), thermal neutron flux (4th) and epithermal neutron flux (4epi) in the irradiation positions of MNA research reactor in order to guarantee accuracy in the application of k0-neutron activation analysis (k0-NAA).The f and a were determined using the bare bi-isotopic monitor and bare triple monitor methods, respectively; Au and Zr monitors were utilized in present study. The results for four irradiation positions are presented and discussed in the present work. The calculated values of f and a ranged from 33.49 to 47.33 and 0.07 to 0.14, respectively. The 4th and the 4epi were measured as 2.03 1012 (cm2 s1) and 6.05 1010 (cm2 s1) respectively. These results were compared to those of previous studies at this reactor as well as to those of reactors in other countries. The results indicate a good conformity with other findings. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: k0-NAA Neutron flux parameters TRIGA Mark II reactor
1. Introduction The TRIGA Mark II reactor of the Malaysian Nuclear Agency (MNA) was commissioned in 1982. This reactor uses light-water as moderator, coolant and reflector. The fuel assembly consists of an alloy of uranium enriched to 20% U-235 and zirconium hydride (UeZrH). Several experimental facilities are available in the MNA research reactor. For activation analysis and isotope production, a rotary specimen rack is located around the top portion of the core and inside the reflector. The rotary specimen rack assembly consists of ring-shaped, seal-welded aluminium housing containing an aluminium rack mounted on special bearings. As shown in Fig. 1, the rotary rack (RR) supports 40 evenly spaced tubular aluminium containers that serve as receptacles for the specimen containers. Each receptacle has an inside diameter of 3.17 cm and height of 27.4 cm and can hold two specimen containers (Masood et al., 2008). The k0-NAA method, developed in the 1970s, is increasingly used for multielement accurate analysis of biological, geological, environmental and high purity materials using reactor neutrons (De Corte et al., 1969, 1987, 1993; Simonits et al., 1975, 1982; Moens et al., 1984; De Corte, 1992, 1994, 2000, 2001; Bellemans et al., 1995;
* Corresponding author. Tel.: þ60 389214506; fax: þ60 389213777. E-mail address: [email protected] (K.S. Khoo). 1350-4487/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.11.014
Lin et al., 1997).The k0-NAA method requires the accurate characterization of irradiation and counting facilities as well as the use of composite nuclear constants known as k0-factors. The k0-factors, which are independent of irradiation and measurement conditions, are tabulated and published in literature as generally useful nuclear parameters, as shown in Table 1 (Moens et al., 1984; Simonits et al., 1987; De Corte and Simonits, 2003). The k0-NAA method was successfully developed using the HØGDAHL convention (HØGDAHL, 1962). Its application is restricted to (n, g) cross sections that follow the 1/n law in the thermal neutron energy region (i.e., up to w1.5 eV). According to the HØGDAHL convention, neutron flux spectrum parameters such as thermal to epithermal neutron flux ratio (f ) and epithermal neutron flux shape factor (a) are vital for calculating the concentration of elements in a sample when using the k0-NAA method (Lin et al., 1997; De Corte, 2001). In our work f and a were determined by the bare bi-isotopic monitor and bare triple monitor methods, respectively (De Corte et al., 1969; De Corte, 2001; Simonits et al., 1982; Lin et al., 1997). Calculation of f proceeded according to the following equation:
Asp;1 ko ; Auð1Þ 3p;1 $Q ðaÞ $Q ðaÞ $ Asp;2 0;2 ko ; Auð2Þ 3p;2 0;1 f ¼ Asp;1 ko ; Auð1Þ 3p;1 $ Asp;2 ko ; Auð2Þ 3p;2
(1)
220
A.R. Yavar et al. / Radiation Measurements 46 (2011) 219e223
Fig. 1. Arrangement of the 40 rotary rack (RR) irradiation channels around the core of MNA research reactor.
where 1 ¼ 97Zr/97mNb (743.3 keV) and 2 ¼ 95Zr (724.2 þ 756.7 keV); 3p is the full energy peak efficiency; and k0,Au(a) is the nuclear constant known as k0-factor of analyst a versus the gold (Au) monitor. The Asp is the specific count rate, calculated as Asp ¼ (Np/ tc)/SDCW; where Np is measured gamma net peak area (in counts); S is the saturation factor, calculated as S ¼ 1 elti , where ti is irradiation time; D is the decay factor, calculated as D ¼ eltd , where td is decay time (from end of irradiation to start of counting); C is the counting factor, calculated as C ¼ ½1 eltc =ltc , correcting for decay during counting, where tc is counting time; W is mass of irradiated element (g). The Q0(a) is calculated as:
Q0 ðaÞ ¼
Q0 0:429 a
Er
þ
0:429 ð2a þ 1Þ$ð0:55Þa
(2)
Where Er is effective resonance energy in eV; Q0 ¼ I0/s0, where I0 is the resonance integral for the (n, g) reaction and s0 is the thermal neutron cross section (2200 ms1). The a is calculated according to the following De Corte et al., 1969, 1980, 1981, 1986; Simonits et al., 1982; Lin et al., 1997):
Table 1 Nuclear data of neutron flux monitors (Simonits et al., 1987; De Corte and Simonits, 2003). Target isotope
Eg, keV
T1/2
k0,Au (Literature)
Q0
Er ; eV
Formed isotope
197
411.8 724.2 756.7 (724.2 þ 756.7) 743.3
2.695d 64.02d
1
15.7 5.306
5.65 6260
198
251.6
338
94
96
Au Zr
Zr
16.74h
9.32 1.149 2.0 1.237
10-5 10-4 10-4 10-5
95
Au Zr
97
Zr/97mNb
ða bÞQ0;1 ðaÞ aQ0;2 ðaÞ þ bQ0;3 ðaÞ ¼ 0
(3)
where
a ¼
Asp;2 k0 Auð1Þ 3p;1 1 $ $ Asp;1 k0 Auð2Þ 3p;2
b ¼
1
Asp;3 k0 Auð1Þ 3p;1 $ $ Asp;1 k0 Auð3Þ 3p;3
!1 and !1
Where 1 ¼ 97Zr/97mNb (743.3 keV), 2 ¼ 95Zr (724.2 þ 756.7 keV) and 3 ¼ 198Au (411.8 keV). The thermal neutron flux 4th (cm2 s1) was calculated as follows (Lin et al., 1997; Khoo et al., 2007):
fth ¼
f $Asp;Au $3:47 f þ Q0;Au ðaÞ $3p;Au
(5)
and the epithermal neutron flux 4epi (cm2 s1) was calculated as:
fepi ¼
fth f
(6)
The relevant nuclear data used for determination of neutron flux parameters are presented in Table 1(Simonits et al., 1987; De Corte and Simonits, 2003). In principle, the f , a, 4th and 4epi parameters are subject to change according to the reactor configuration and irradiation positions. After a major core reconfiguration of a nuclear reactor, the determination of these parameters is required before performing k0-NAA analysis (De Wispelaere and De Corte, 2003; Mustra et al., 2003). This paper aims to show the results of the f , a, 4th, 4epi recalculation in the present study at four RR irradiation channels of
A.R. Yavar et al. / Radiation Measurements 46 (2011) 219e223
221
Fig. 2. Efficiency curve for the HPGe detector, “ref” ¼ point geometry at 15.8 cm source detector distance. Table 2 f, a, 4th and 4epi at MNA research reactor in present study. f
a
4th (cm
34.97 33.49 47.33 42.90
0.08 0.07 0.14 0.12
2.07 2.08 2.03 2.05
2
1
s
)
1012 1012 1012 1012
2
4epi (cm 5.92 6.21 4.29 4.78
1
s
)
1010 1010 1010 1010
MNA’s TRIGA Mark II reactor after the last configuration change and to compare the new calculations with measurements used in previous studies done at this reactor to observe possible changes in neutron flux parameters. Furthermore, our results are compared with results of the other TRIGA Mark II reactors in an overview of TRIGA Mark II reactors from countries that have calculated these parameters. Because differences exist between TRIGA MARK I, II and III reactors in terms of thermal power and number of fuel elements, their neutron flux parameters are expected not to be similar. Therefore, the scope of this present work is limited to TRIGA MARK II reactors worldwide.
2. Experimental 2.1. Efficiency calibration of g-spectrometer The g-spectrometry measurements were performed with an HPGe detector coupled with Canberra Accuspec multichannel analyzer (MCA); the computer code Gamma Acquisition Analysis was used for peak area evaluation. Full energy peak efficiency calibration of the detector was carried out using 241Am, 109Cd, 57Co, 137Cs and 60Co point calibration sources placed at the reference position 15.8 cm from the detector where true coincidence effects are negligible. Fig. 2 shows the peak detection efficiency of the HPGe detector plotted in logarithmic
scale. The energy range was from 58.91 keV to 1332.58 keV (De Corte et al., 1993; Alghem et al., 2006; Wasim et al., 2008). 2.2. Characterization of MNA research reactor spectrum Au and Zr monitors were used to determine the f , a, 4th and 4epi parameters. The monitors were made ofAl-0.1%Au alloy wire (IRMM-527a, diameter 1 mm, length 10 mm) and Zr foils (IRMM, 99.9%, 125 mm thick), respectively. The monitors were cut and carefully weighed so that the size range for Au monitors was10.7e12.9 mg while for the Zr monitors the size range was18.5e19.8 mg. The vials were chosen with 1 cm diameter and 3 cm length and then cut back to 1 cm length. The monitors were heat sealed inside the polyethylene vials and were packed in heat resistant plastic so that each vial included one Au monitor and one Zr one. The monitors were irradiated for 6 h in four RR irradiation channels (#1, #11, #21 and # 31 channels, shown in Fig. 1) of the MNA research reactor. Due to the short half-lives of radionuclides of 198 Au and 97Zr/97mNb, both monitors were counted for about 5 min after 24 h cooling time using an HPGe detector placed 15.8 cm away from the detector to minimize true coincidence effects. The irradiated zirconium foils were counted again for measurement of 95Zr about 15 min after 3 days of decay time. Three gamma-lines were used in the estimation of f and a: 411.8 keV of 198Au, 743.4 keV of 97 Zr/97mNb and the sum of the two peaks (724.2 and 756.7 keV) of 95 Zr (De Corte et al., 1993; Alghem et al., 2006; Wasim et al., 2008). 3. Results and discussion Table 2 shows the neutron flux parameters at the four RR irradiation channels of the MNA research reactor. The f and a were determined according to equations (1) and (3), identified as the bare bi-isotopic monitor method and the bare triple monitor method,
Table 3 cimovi c et al., 2003; Dung and Hien, 2003; Wee et al., 2006; Khoo et al., 2007) Average and standard deviation of f, a, 4th and 4epi in TRIGA Mark II reactors worldwide (Ja Location of reactor
Date of experiment
f
Malaysia (Wee et al., 2006) Malaysia (Khoo et al., 2007) Malaysia (Present work) Vietnam Slovenia
2003 2005 2009 2003 2003
17.2 33.55 39.67 35.70 31.51
a 0.9 11.2 6.57 2.41
0.016 0.087 0.102 0.073 0.010
0.005 0.046 0.033 0.0068
4th 1012 (cm2 s1)
4epi 1011 (cm2 s1)
2.29 0.09 2.03 0.27 2.06 0.02 3.61 2.15
1.33 0.61 0.45 0.52 0.09 1.01 0.68
222
A.R. Yavar et al. / Radiation Measurements 46 (2011) 219e223
Fig. 3. Core configurations of the MNA research reactor as of (a) August 2001 and (b) April 2009.
respectively. The f ranged from 33.49 to 47.33. The a was found to be in the range of 0.07 to 0.14. Equations (5) and (6) were used to calculate the 4th and 4epi, which were found to range from 2.03 1012 to 2.08 1012 (cm2s1) and from 4.29 1010 to 6.21 1010 (cm2s1), respectively. The observed average value of f was 39.67 with a relative standard deviation (RSD) of 16.56%. A reactor core may be reconfigured for several reasons: installation or uninstallation of an experimental facility inside the reactor for performing a special experiment, homogenization of thermal power distribution, and/or refilling of fuel elements because of burn up effect. The 11th core reconfiguration of the MNA research reactor was performed with 111 fuel elements in its core on 23 August 2001; the 12th core reconfiguration was carried out on 5 July 2006. Since the previous studies’ experiments (Wee et al., 2006; Khoo et al., 2007) were carried out in 2003 and 2005, respectively, (Table 3), they used the same core configuration (i.e. the 11th core reconfiguration), but present work used the 14th core reconfiguration, with 112 fuel elements in the core, which had been carried out on 20 April 2009. Given that previous studies using the MNA reactor were performed when the reactor was configured differently from the configuration of the time when this study was done, it is not unexpected that there would exist some deviation between our results and theirs (Wee et al., 2006; Khoo et al., 2007). In particular, the differences may have been due to differences in the reactors’ methods of neutron flux distribution since the reactor used for the present work had a rotary system of RR channels for irradiation of monitors; therefore, neutron flux distribution was homogeneous for our study. Previous studies, on the other hand, used non-rotary systems of RR channels during irradiation. Moreover, the variation of f values may have arisen from the shape of the RR container as the length axis of the container was not precisely parallel to the length axis of the fuel elements, leaving some space between the container and the inner side of the irradiation tube (Khoo et al., 2007); in addition, as noted in Fig. 3, the number of fuel elements in the reactor core in our work was 112 (Fig. 3b) but 111 in previous studies (Fig. 3a).The greater number of fuel elements in the core inevitably increases thermal neutron flux, which is in turn correlated with the f parameter. The average value of a was found to be 0.1025 with RSD of 32.36%. The a value depends on the reactor configuration and rises with increasing distance from the core of the reactor. The negative values of a could be attributable to poor thermalization in the reactor during our study (De Corte et al., 1980, 1993).
The average value of 4th was found to be 2.03 1012 (cm2 s1) with RSD of 1.09% while the average value of 4epi was 6.05 1010 (cm2 s1) with RSD of 17.24%. The low RSD values of the 4th and 4epi indicate a homogeneous neutron flux due to the rotation of RR irradiation channels during the irradiation of our monitors; as a result, the use of the four RR irradiation channels #1, #11, #21 and #31 (as shown in Fig. 1) positioned in a cruciform with the rotary system of RR channels during the irradiation is taken to be suitable and sufficient for determination of neutron flux parameters in TRIGA MARK II reactors. Worldwide, 18 TRIGA Mark II research reactors are in operation, but only four of them are used for k0-NAA (TRIGA II VIENNA, Vietnam; TRIGA PUSPATI, Malaysia; MA-R1, Morocco; and TRIGA MARK II LJUBLJANA, Slovenia). The neutron flux parameters of three of them can be found in published papers (Dung and Hien, 2003; Ja cimovi c et al., 2003; Wee et al., 2006; Khoo et al., 2007). Table 3 shows the average values of neutron flux parameters and available standard deviation from two previous studies at the MNA research reactor, from present study and from evaluations of other TRIGA Mark II reactors worldwide. In the first study at MNA (Wee et al., 2006), values of f , a and 4th were calculated as 17.2, 0.0157 and 2.29 1012 (cm2 s1), respectively. The second study at MNA (Khoo et al., 2007) obtained values of f , a and 4th as 33.55; 0.0873 and 2.03 1012 (cm2 s1), respectively. Although both previous studies were performed when the MNA research reactor had the same core configuration, yet, as shown in Table 3, there was variation between their findings. Fuel burn up is clearly the parameter that most affects this fluctuation, but it must also be taken into consideration that the control rods positions can be strikingly impressive (Khoo et al., 2007). Furthermore, another factor in this variability could be the number of irradiation channels used. The first study (Wee et al., 2006) irradiated with just one RR channel with a non-rotary system (resulting in inhomogeneous flux distribution), while the second study (Khoo et al., 2007), also with a non-rotary system, used twenty RR channels to obtain more accurate results. As shown in Table 3, with the exception of the first study (Wee et al., 2006), where the MNA research reactor may have reached very high thermalization, results show an acceptable level of consistency of our results with those of other studies. 4. Conclusion The reconfiguration of the core of the MNA research reactor in April 2009 necessitated the re-determination of the neutron flux
A.R. Yavar et al. / Radiation Measurements 46 (2011) 219e223
parameters for k0-NAA applications. The neutron flux values of f , a, 4th and 4epi were determined using Au and Zr monitors in four RR irradiation channels of the MNA research reactor. Although present results showed a good level of consistency with those of most other studies, nevertheless some deviation was found. This deviation may have been caused by differences in neutron flux distribution, since the present work used a rotary system of RR channels during the irradiation of monitors, unlike the others which used nonrotary systems. In addition, the number of fuel elements in the core may have contributed to the deviation.
Acknowledgments The authors would like to thank the MNA operation group for their kind assistance during the implementation of this project.
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