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Neutron spatial flux profile measurement in compact subcritical system using miniature neutron detectors
Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Neutron spatial flux profile measurement in compact subcritical system using miniature neutron detectors Mayank Shukla b,n, Shraddha S Desai a, Tushar Roy b, Yogesh Kashyap b, Nirmal Ray b, Shefali Bajpai b, Tarun Patel b, Amar Sinha b a b
Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India Neutron and X-ray Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India
art ic l e i nf o
a b s t r a c t
Article history: Received 16 July 2014 Received in revised form 24 September 2014 Accepted 25 October 2014
A zero power multiplying assembly in subcritical regime serves as a benchmark for validating subcritical reactor physics. The utilization of a subcritical assembly for the determination of nuclear parameters in a multiplying medium requires a well-defined neutron flux to carry out the experiments. For this it is necessary to know the neutron flux profile inside a subcritical system. A compact subcritical assembly BRAHMMA has been developed in India. The experimental channels in this assembly are typically less than 8 mm diameter. This requires use of miniature detectors that can be mounted in these experimental channels. In this article we present the thermal neutron flux profile measurement in a compact subcritical system using indigenously developed miniature gas filled neutron detectors. These detectors were specially designed and fabricated considering the restrictive dimensional requirements of the subcritical core. Detectors of non-standard size with various sensitivities, from 0.4 to 0.001 cps/nv were used for neutron flux of interest ranging from 103 to 107 n-cm 2 s 1. A comparison of measured neutron flux using these detectors and simulated Monte Carlo calculations are also presented in this article. & 2014 Published by Elsevier B.V.
Keywords: Accelerator driven subcritical system Neutron flux measurement Miniature neutron detector
1. Introduction Accelerator driven sub critical systems (ADS) [1–4] are attracting worldwide attention due to their role in incineration of transuranic elements from spent nuclear fuel, energy production and utilization of thorium besides superior safety characteristics. The current research on ADS is being pursued in several countries and involves mainly an experimental feasibility study with the use of critical assemblies/test facilities [4–14]. As a part of Indian ADS program, a zero power subcritical assembly driven by an indigenously developed deuteron accelerator has been developed. The subcritical core named BRAHMMA (Beryllium oxide Reflected And HDPE Moderated Multiplying Assembly) [15] is a modular compact assembly which consists of natural uranium as fuel, high density polyethylene (HDPE) as moderator and beryllium oxide (BeO) as reflector. The assessment of the feasibility of ADS systems requires experimental verification of the kinetic response features as well the determination of the integral parameters such as reactivity. The monitoring of the sub-criticality level of the reactor is of particular importance and for this purpose robust on-line and
n
Corresponding author. E-mail address: [email protected] (M. Shukla).
continuous monitoring of the subcritical assembly reactivity is a must to yield valuable information concerning the rapid relative change of the reactivity. For example, one of the techniques of online reactivity monitoring is the current-to-flux measurement technique. As the reactivity is a direct function of the accelerator intensity (beam current), the core power (neutron flux) and the source strength, the necessary acquisition system to monitor these three quantities need to be developed. Most of these experiments involve the measurement of neutron flux in the subcritical system using suitable neutron detectors with neutron source operating in either pulsed or continuous mode [9,16–19]. The commonly used neutron detectors are either fission detectors or gas filled. Fission detectors are routinely being used in reactor for in-core and ex-core flux monitoring20. These detectors have the advantage that they can be used in high temperature environment. However they require special facility for precision coating of 235U for inhouse development. For the present application of neutron flux measurement in the subcritical system where the temperature rise is negligible above ambient, 3He gas filled detectors were chosen. These detectors are economical and can be fabricated in house with tailor-made design, considering subcritical system requirements. Gas filled detectors also offer flexibility in size, radiation hardness and gamma tolerance. In this article, we present the neutron flux measurements inside BRAHMMA ADS core, both in axial and radial directions of
http://dx.doi.org/10.1016/j.nima.2014.10.059 0168-9002/& 2014 Published by Elsevier B.V.
Please cite this article as: M. Shukla, et al., Nuclear Instruments & Methods in Physics Research A (2014), http://dx.doi.org/10.1016/j. nima.2014.10.059i
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the core. Specially designed miniature gas filled detectors were developed considering the restrictive dimensional requirements of the subcritical core. The minimum separation between the individual fuel rods in the assembly is 10 mm and the diameter of the experimental channels inside the polyethylene is 7.5 mm in radial direction. Miniature detectors were required to carry out measurements. These detectors were specially designed and developed for online reactivity measurement experiments. Detectors of nonstandard size with various sensitivities 0.4, 0.1, 0.01 and 0.001 cps/nv were used for neutron flux measurements of interest ranging from 103 to 107 n-cm 2 s 1. The experimental data obtained were then compared with those obtained through Monte Carlo simulations in order to validate the calculations.
2. BRAHMMA subcritical core The subcritical core BRAHMMA consists of natural uranium as fuel with high density polyethylene as moderator and beryllium oxide as reflector. The fuel rods are arranged in a 13 13 square lattice with a 48 mm pitch as depicted in Fig. 1. The core is surrounded by reflector made of BeO with a thickness of 200 mm. The core is surrounded on the outside by neutron shielding to isolate the system from scattered neutrons from the surrounding. The central 3 3 part of the lattice (144 mm X 144 mm) of the subcritical assembly serves as the cavity for inserting the neutron source. Seven experimental channels (EC) are located at different axial and radial positions as shown in Fig. 1. The relative positions of the experimental channels are such that their influence on each other is minimized (Fig. 1). Three axial experimental channels (EC1, EC2 and EC3) of diameter 10.0 mm have been provided at radial distances of 122 mm, 238 mm and 265 mm respectively for measurement. EC1 is close to the source whereas EC3 is near the reflector. Four experimental channels (EC4, EC5, EC6 and EC7) of
diameter 7.5 mm have been provided in the moderator/reflector region along the radial direction. Channels EC5, EC6 and EC7 are located in the mid-elevation plane and run up to the cavity only. Channel E4 is located above the mid-elevation plane and covers the full length of the moderator.
3. Miniature neutron detector: design and performance As mentioned in section 2, the experimental channels in the sub-critical assembly have a minimum diameter of 7.5 mm. Therefore miniature detectors for flux measurement inside the core have been designed considering the dimensions of these channels. Neutron flux of interest is ranging from 103 to107 ncm 2 s 1. A single detector cannot cover the entire flux range with accuracy. Thus the detectors, with various sensitivities 0.4, 0.1, 0.01 and 0.001 cps/nv, to cover the flux range of interest have been designed and fabricated. The sensitive volume of the detectors with active length of 70 mm was fabricated using Cu tube cathode of diameter 6.3 mm. Each detector was filled with 3He gas pressure suitable for desired neutron sensitivity [21] and additive Kr gas to maintain proton range [22–24] within detector volume (Table 1). Choice of detector on the basis of sensitivity [25,26] was carried out for a particular neutron flux of external neutron source (as mentioned in section 5). Schematic design and completely assembled miniature detectors are shown in Fig. 2. Detectors were characterized for pulse height distribution (PHD) and high voltage bias curve at various neutron flux ranging from 102 to 106 n-cm 2 s 1. The block diagram of the detector readout electronics is shown in Fig. 3. It consists of a charge sensitive pre-amplifier with fall time of 5 ms, shaping amplifier (1 ms time constant) with single channel analyzer which generates 5 V TTL pulses. These pulses are fed to a counter or MCS as the case may be. Detectors were initially characterized using Pu-Be
EC4
EC5
EC7
EC2
EC1 EC3
EC6
Fig. 1. (a) Front view (b) side view of BRAHMMA. The positions of the axial and radial experimental channels are also shown.
Table 1 Specifications and operating parameters of the detectors. Det. no
Fill gas
Fill pressure (bar)
Sensitivity (CPS/nv)
Plateau range (V)
Plateau slope %/100 V
Detector flux range (ncm 2 s 1)
3 A-1 3 A-2 3 A-3 3 A-11
3
2 þ1.5 0.5 þ2.0 0.06 þ2.5 0.005 þ2.5
0.4 0.1 0.01 0.001
850–1050 900–1150 900–1100 900–1050
1.5 2 4.5 12
4 102–4 105 102–105 103–106 104–107
He þ Kr He þ Kr 3 He þ Kr 3 He þ Kr 3
Please cite this article as: M. Shukla, et al., Nuclear Instruments & Methods in Physics Research A (2014), http://dx.doi.org/10.1016/j. nima.2014.10.059i
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moderated neutron source with thermal neutron flux of 100 n-cm 2 s 1. Source was an extended 20 cm long Pu-Be pin enclosed in an Al-lined paraffin box (10 cm x 10 cm x 70 cm). The performance of low sensitivity detectors (such as 3 A-3 and 3 A-11) was tested using a high flux collimated thermal beam from reactor with thermal neutron flux of 2 106 n-cm 2 s 1 and gamma background of 100 mR / hr. Fig. 4a and b show the PHD spectrum and plateau characteristics of detector 3 A-1, recorded using reactor neutron beam. The spectrum indicates full energy peak with distinct valley for noise and gamma discrimination. PHD shows 30% of Full Width Half Maximum (FWHM) at 764 keV
3
corresponding to Q energy of 3He (n, p) t reaction. Number of full energy events indicates well suppressed wall effects, in spite of a very small cathode diameter. Similar results were obtained with other detectors. Fig. 5 shows the high voltage counting curve of detector 3 A-2, 3 A-3 and 3 A-11 recorded using reactor neutron beam. Plateau length is 200 V and slope 2% / 100 V. The plot shows acceptable slope for stable operation. The detector specifications and operating parameters are listed in Table 1. 3 He filled neutron detectors have temperature tolerance up to 200 1C [19]. For the present application of neutron spatial flux profile measurement the temperature inside the sub-critical
Fig. 2. Schematic and assembled Miniature gas filled detectors.
Fig. 3. Block diagram of readout electronics for miniature detector.
Fig. 4. a) Pulse height distribution of 3 A-1 detector using reactor neutron beam, b) Plateau characteristics of detector 3 A-1, using reactor neutron beam.
Please cite this article as: M. Shukla, et al., Nuclear Instruments & Methods in Physics Research A (2014), http://dx.doi.org/10.1016/j. nima.2014.10.059i
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system was monitored using RTD thermal sensors and was measured to be 29 1C (ambient temperature). The developed detectors were tested for time as well as temperature stability using Pu-Be source. Fig. 6a shows the time stability of detector 3 A-1 measured over the duration of 20 h by integrating counts over 60 sec. Counting statistics or plateau characteristics of detector indicated no change up to 60 1C as shown in Fig. 6b. Similar results have been obtained for other detectors. These characteristics of detectors ensure the suitability for the required application. Gamma sensitivity of the detector was calculated to be 10 3 cps using the photon absorption cross section of Cu and Kr gas [27]. Estimated gamma background within sub-critical assembly is 40 mR/hr. The PHD of detector in 100 mR/hr gamma background of reactor neutron beam (as shown in Fig. 4a) is acceptable for the estimated background of the sub-critical system.
both in axial and radial directions at various positions is shown in Fig. 7a and b respectively. The axial flux profile is centrally for channels EC5 and EC6 show a dip at the center. This is due to the fact that position of peaked due to the present of the external source at the center of the core. The peak flux in EC1 is higher than that in EC2 and EC3 as it is closer to the source. The radial flux profiles for channels EC5 and EC6 show a dip at the center. This is due to the fact that positions of these channels correspond to the non-fuel central region. The flux peaks near the edge on either sides of the central activity. For channel EC4 (which does not see the central cavity), the thermal flux peaks at the center. The effect of the reflector can also be seen as the thermal flux shows an increase near the reflector-moderator interface.
5. Flux profile measurement of subcritical assembly 4. Simulation results: Monte Carlo simulations were carried out for estimating the neutron flux profile in the Neutrons of energy 2.45 MeV (D-D) produced at the source location are transported throughout the volume and all possible interactions and energy ranges are taken into account. Also, the energy spectrum in the subcritical core is predominantly thermal. Simulated thermal neutron flux profile
Fig. 5. Plateau characteristics of detectors 3 A-2, 3 A-3 and 3 A-11 using reactor neutron beam.
The flux profile measurement in the sub-critical assembly was carried out using DD neutron source as external driver. This neutron source is a DC accelerator based neutron source. It consists of a RF ion source, operated at 13.56 MHz with extraction electrode capable of giving ion current of 150 mA, Einzel lens focusing electrode, accelerating column, faraday cup and the water cooled Deuterium Target. The ion source and the Einzel lens electrode are floated on a high voltage dome with Deuterium target at ground potential. Detector 3 A-1 was used for the initial tests at the subcritical assembly. Experimental channel EC1 closest to the central cavity was used for the measurements D-D neutron source coupled to the subcritical assembly. Detector 3 A-1 was also tested at EC1 for DD pulsed source. Fig. 8 shows the response of detector to pulsed source with neutron pulse width of 25 ms and time period of 50 ms. Data was acquired for 10000 sweeps. Similar results were obtained for other detectors. The calibrated detectors were then used to measure the flux profile inside the core for the external neutron source operated in D-D mode with neutron source strength of 3 107 n/s. Detector 3 A-1 was used for the measurement of neutron flux. It was biased at its plateau voltage of 900 V. The detector was coupled to a pre-amplifier followed by amplifier and scalar/counter to measure the counts at different locations inside the experimental channels. The detector was moved inside the channels manually and the counts were recorded every 70 mm (active length of the detector) throughout the length. Fig. 9 shows the measured axial flux profiles for experimental channel EC1, EC2 and EC3. The axial flux profiles peak at the center where the source is located. Thermal flux at EC1 is more
Fig. 6. a) Time stability of 3 A-1 detector measured for 20 hrs using Pu-Be neutron source, b) Counting Statistics of detector 3 A-1 with change in temperature.
Please cite this article as: M. Shukla, et al., Nuclear Instruments & Methods in Physics Research A (2014), http://dx.doi.org/10.1016/j. nima.2014.10.059i
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5
Fig. 7. a) Simulated axial flux profiles with D-D neutrons, b) Simulated radial flux profiles with D-D neutrons.
Fig. 8. 3 A-1 response for pulsed D-D neutron source. Fig. 10. Experimental radial profile with D-D neutrons for EC7.
6. Conclusion The neutron flux profile measurement has been carried out in a subcritical system using indigenously developed miniature gas filled detectors. These miniature detectors were specially designed considering the dimensional requirement of experimental channels in the sub-critical core. Detectors with various sensitivities (0.4, 0.1, 0.01 and 0.001 cps/nv) were calibrated using neutrons from Pu-Be source and reactor. Performance of detectors for PHD, HV bias curve, gamma tolerance, temperature tolerance and time stability indicate the suitability for the present application. The neutron flux profile has been measured in both axial and radial direction using these detectors. The experimental data obtained corroborates well with the Monte Carlo simulations.
Fig.9. Experimental axial profile with D-D neutrons.
compared to other channels as it is nearest to the central cavity. The estimated error bars are around 73%. This was estimated by determining standard deviation of counts from the mean/average value for series of measurements. These profiles corroborate well with the simulated Monte Carlo profiles as shown in Fig. 7a. We have carried out the radial flux profile measurement for EC7. This channel covers approximately half the length of the core. The experimental radial flux profile for channel EC7 is shown in Fig. 10. This matches well with the simulation results obtained as shown in Fig. 7b.
Uncited reference [20].
Acknowledgment Authors acknowledge CDM, BARC for precision machining of the delicate detector components. Authors thank SSPD staff Shri S.M. Patkar for skillful assembly and Mrs Shyalaja Devan and Miss Amrita Das for help in testing of detectors.
Please cite this article as: M. Shukla, et al., Nuclear Instruments & Methods in Physics Research A (2014), http://dx.doi.org/10.1016/j. nima.2014.10.059i
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1 References 2 3 [1] C. Rubbia, S. Buonolil, E Gonzalez, Y. Kadi, A. Rubio, A realistic plutonium elimination scheme with fast energy amplifiers and thorium-plutonium fuel, 4 CERN / AT/ 95-53 (ET). 5 Q5 [2] M. Salvatores, J. Phys. IV France 9 7-17 (1999) 7. 6 [3] J.L. Munoj Cobo, Y Rugama, T.E. Valentine, J.T. Mihalczo, R.B. Perez, Ann. Nuc. Energ. 28 (15) (2001) 1519. 7 [4] Dmitry G. Naberezhnev, Yousry Gohar, James Bailey, Henry Belch, Nucl. Instr. 8 Meth. Phys. Res. A. 562 (2006) 841. 9 [5] R. Soule, W. Assal, P. Chaussonnet, C. Destouches, C. Domergue, C. Jammes, J.M. Laurens, J.F. Lebrat, F. Mellier, G. Perret, G. Rimpault,, H. Servière, Nucl. Sci. 10 Eng. 148 (1) (2004) 124. 11 [6] M. Plaschy, C. Destouches, G. Rimpault, R. Chawla, J. Nucl. Sci. Techno. 42 (9) 12 (2005) 779. [7] J.F. Lebrat, G. Aliberti, A. D’Angelo, A. Billebaud, R. Brissot, H. Brockmann, 13 M. Carta, C. Destouches, F. Gabrielli, E. Gonzalez, A. Hogenbirk, R. Klein14 Meulenkamp, C. Le Brun, E. Liatard, F. Mellier, N. Messaoudi, V. Peluso, 15 M. Plaschy, M. Thomas, D. Villamaran, J. Vollaire, Nucl. Sci. Eng. 158 (1) 16 (2008) 49. [8] Y. Gohar, G. Aliberti, I Bolshinsky, H. Kiyavitskaya, A. Talamo, T. Am. Nucl. Soc. 17 101 (2009) 39. 18 [9] C.M. Persson, A. Fokau, I. Serafimovich, V. Bournos, Y. Fokov, C. Routkovskaia, 19 H. Kiyavitskaya, W. Gudowski, Ann. Nucl. Energ. 35 (12) (2008) 2357. [10] M. Tesinsky, C. Berglof, T. Back, B. Martsynkevich, I. Serafimovich, V. Bournos, 20 A. Khilmanovich, Y. Fokov, S. Korneev, H. Kiyavitskaya, W. Gudowski, Ann. 21 Nucl. Energ. 38 (6) (2011) 1412. 22 [11] H.H. Xia, ICFA Beam Dynam. Newsl. (2009) 49 (72–80). [12] C.H. Pyeon, Y. Hirano, T. Misawa, H. Unesaki, C. Ichihara, T. Iwasaki, S. Shiroya, 23 J. Nucl. Sci. Technol. 44 (11) (2007) 1368. 24 25
[13] C.H. Pyeon, M. Hervault, T Misawa, H. Unesaki, T. Iwasaki, S. Shiroya, J. Nucl. Sci. Technol. 45 (11) (2008) 1171. [14] L. Mercatali, A. Serikov, P. Baeten, W. Uyttenhove, A. Lafuente, P. Teles, Energ. Conversion Manage. 51 (2010) 1818. [15] Amar Sinha, Tushar Roy, Yogesh Kashyap, Nirmal Ray, Mayank Shukla, Tarun Patel, Shefali Bajpai, P.S. Sarkar, S. Bishnoi, P.S. Adhikari, BRAHMMA: a compact experimental accelerator driven subcritical facility using D-T/D-D neutron sources, Ann. Nucl. Energ. Manuscript accepted. [16] P. Seltborg Carl-Magnus Persson, A. Ahlander, W. Gudowski, T. Stummer, H. Kiyavitskaya, V. Bournos, Y. Fokov, I. Serafimovich,, S. Chigrinov, Nucl. Instr. Meth. Phys. Res. A554 (2005) 374. [17] Christian Jammes, Benoît Geslot, George Imel, Nucl. Instr. Meth. Phys. Res. A. 562 (2006) 778.. [18] B.E. Simmons, J.S. King, Nucl. Sci. Eng. (1958) 595. [19] N.G. Sjöstrand, Arkivförfysik 11 (1956) 13. [20] Glenn F. Knoll, Radiation Detection and Measurement, third ed., Wiley India Pvt. Ltd, 2012. [21] William J. Price Nuclear Radiation Detection, second ed., McGraw Hill Company, p. 314. [22] Herman Cember, Introduction to Health Physics, second ed., Pergamon press. [23] L.C. Northcliffe,, R.F. Schilling, Nucl. Data Tables A7 (1970) 233. [24] Neutron data booklet, Editors Albert-Jose Dianoux and Gerry Lander, ILL, France (2002). [25] S.S. Desai, A.M. Shaikh, Nucl. Instru. Methods A557 (2006) 607. [26] J. Als-Nielsen, A. Bahnsen,, W.K. Brown, Precision measurement of thermal neutron beam densities using a 3He proportional counter, Nucl. Instru. Meth. A. 50 (1967) 181. [27] XCOM: Photon cross section database, (version 1.3). 〈http://physics.nist.gov/ xcom〉, 2005 (accessed 31.10.05). National Institute of Standards and Technology, Gaithersburg, MD, 2005.
Please cite this article as: M. Shukla, et al., Nuclear Instruments & Methods in Physics Research A (2014), http://dx.doi.org/10.1016/j. nima.2014.10.059i
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Neutron spatial flux profile measurement in compact subcritical system using miniature neutron detectors Mayank Shukla b,n, Shraddha S Desai a, Tushar Roy b, Yogesh Kashyap b, Nirmal Ray b, Shefali Bajpai b, Tarun Patel b, Amar Sinha b a b
Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India Neutron and X-ray Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India
art ic l e i nf o
a b s t r a c t
Article history: Received 16 July 2014 Received in revised form 24 September 2014 Accepted 25 October 2014
A zero power multiplying assembly in subcritical regime serves as a benchmark for validating subcritical reactor physics. The utilization of a subcritical assembly for the determination of nuclear parameters in a multiplying medium requires a well-defined neutron flux to carry out the experiments. For this it is necessary to know the neutron flux profile inside a subcritical system. A compact subcritical assembly BRAHMMA has been developed in India. The experimental channels in this assembly are typically less than 8 mm diameter. This requires use of miniature detectors that can be mounted in these experimental channels. In this article we present the thermal neutron flux profile measurement in a compact subcritical system using indigenously developed miniature gas filled neutron detectors. These detectors were specially designed and fabricated considering the restrictive dimensional requirements of the subcritical core. Detectors of non-standard size with various sensitivities, from 0.4 to 0.001 cps/nv were used for neutron flux of interest ranging from 103 to 107 n-cm 2 s 1. A comparison of measured neutron flux using these detectors and simulated Monte Carlo calculations are also presented in this article. & 2014 Published by Elsevier B.V.
Keywords: Accelerator driven subcritical system Neutron flux measurement Miniature neutron detector
1. Introduction Accelerator driven sub critical systems (ADS) [1–4] are attracting worldwide attention due to their role in incineration of transuranic elements from spent nuclear fuel, energy production and utilization of thorium besides superior safety characteristics. The current research on ADS is being pursued in several countries and involves mainly an experimental feasibility study with the use of critical assemblies/test facilities [4–14]. As a part of Indian ADS program, a zero power subcritical assembly driven by an indigenously developed deuteron accelerator has been developed. The subcritical core named BRAHMMA (Beryllium oxide Reflected And HDPE Moderated Multiplying Assembly) [15] is a modular compact assembly which consists of natural uranium as fuel, high density polyethylene (HDPE) as moderator and beryllium oxide (BeO) as reflector. The assessment of the feasibility of ADS systems requires experimental verification of the kinetic response features as well the determination of the integral parameters such as reactivity. The monitoring of the sub-criticality level of the reactor is of particular importance and for this purpose robust on-line and
n
Corresponding author. E-mail address: [email protected] (M. Shukla).
continuous monitoring of the subcritical assembly reactivity is a must to yield valuable information concerning the rapid relative change of the reactivity. For example, one of the techniques of online reactivity monitoring is the current-to-flux measurement technique. As the reactivity is a direct function of the accelerator intensity (beam current), the core power (neutron flux) and the source strength, the necessary acquisition system to monitor these three quantities need to be developed. Most of these experiments involve the measurement of neutron flux in the subcritical system using suitable neutron detectors with neutron source operating in either pulsed or continuous mode [9,16–19]. The commonly used neutron detectors are either fission detectors or gas filled. Fission detectors are routinely being used in reactor for in-core and ex-core flux monitoring20. These detectors have the advantage that they can be used in high temperature environment. However they require special facility for precision coating of 235U for inhouse development. For the present application of neutron flux measurement in the subcritical system where the temperature rise is negligible above ambient, 3He gas filled detectors were chosen. These detectors are economical and can be fabricated in house with tailor-made design, considering subcritical system requirements. Gas filled detectors also offer flexibility in size, radiation hardness and gamma tolerance. In this article, we present the neutron flux measurements inside BRAHMMA ADS core, both in axial and radial directions of
http://dx.doi.org/10.1016/j.nima.2014.10.059 0168-9002/& 2014 Published by Elsevier B.V.
Please cite this article as: M. Shukla, et al., Nuclear Instruments & Methods in Physics Research A (2014), http://dx.doi.org/10.1016/j. nima.2014.10.059i
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the core. Specially designed miniature gas filled detectors were developed considering the restrictive dimensional requirements of the subcritical core. The minimum separation between the individual fuel rods in the assembly is 10 mm and the diameter of the experimental channels inside the polyethylene is 7.5 mm in radial direction. Miniature detectors were required to carry out measurements. These detectors were specially designed and developed for online reactivity measurement experiments. Detectors of nonstandard size with various sensitivities 0.4, 0.1, 0.01 and 0.001 cps/nv were used for neutron flux measurements of interest ranging from 103 to 107 n-cm 2 s 1. The experimental data obtained were then compared with those obtained through Monte Carlo simulations in order to validate the calculations.
2. BRAHMMA subcritical core The subcritical core BRAHMMA consists of natural uranium as fuel with high density polyethylene as moderator and beryllium oxide as reflector. The fuel rods are arranged in a 13 13 square lattice with a 48 mm pitch as depicted in Fig. 1. The core is surrounded by reflector made of BeO with a thickness of 200 mm. The core is surrounded on the outside by neutron shielding to isolate the system from scattered neutrons from the surrounding. The central 3 3 part of the lattice (144 mm X 144 mm) of the subcritical assembly serves as the cavity for inserting the neutron source. Seven experimental channels (EC) are located at different axial and radial positions as shown in Fig. 1. The relative positions of the experimental channels are such that their influence on each other is minimized (Fig. 1). Three axial experimental channels (EC1, EC2 and EC3) of diameter 10.0 mm have been provided at radial distances of 122 mm, 238 mm and 265 mm respectively for measurement. EC1 is close to the source whereas EC3 is near the reflector. Four experimental channels (EC4, EC5, EC6 and EC7) of
diameter 7.5 mm have been provided in the moderator/reflector region along the radial direction. Channels EC5, EC6 and EC7 are located in the mid-elevation plane and run up to the cavity only. Channel E4 is located above the mid-elevation plane and covers the full length of the moderator.
3. Miniature neutron detector: design and performance As mentioned in section 2, the experimental channels in the sub-critical assembly have a minimum diameter of 7.5 mm. Therefore miniature detectors for flux measurement inside the core have been designed considering the dimensions of these channels. Neutron flux of interest is ranging from 103 to107 ncm 2 s 1. A single detector cannot cover the entire flux range with accuracy. Thus the detectors, with various sensitivities 0.4, 0.1, 0.01 and 0.001 cps/nv, to cover the flux range of interest have been designed and fabricated. The sensitive volume of the detectors with active length of 70 mm was fabricated using Cu tube cathode of diameter 6.3 mm. Each detector was filled with 3He gas pressure suitable for desired neutron sensitivity [21] and additive Kr gas to maintain proton range [22–24] within detector volume (Table 1). Choice of detector on the basis of sensitivity [25,26] was carried out for a particular neutron flux of external neutron source (as mentioned in section 5). Schematic design and completely assembled miniature detectors are shown in Fig. 2. Detectors were characterized for pulse height distribution (PHD) and high voltage bias curve at various neutron flux ranging from 102 to 106 n-cm 2 s 1. The block diagram of the detector readout electronics is shown in Fig. 3. It consists of a charge sensitive pre-amplifier with fall time of 5 ms, shaping amplifier (1 ms time constant) with single channel analyzer which generates 5 V TTL pulses. These pulses are fed to a counter or MCS as the case may be. Detectors were initially characterized using Pu-Be
EC4
EC5
EC7
EC2
EC1 EC3
EC6
Fig. 1. (a) Front view (b) side view of BRAHMMA. The positions of the axial and radial experimental channels are also shown.
Table 1 Specifications and operating parameters of the detectors. Det. no
Fill gas
Fill pressure (bar)
Sensitivity (CPS/nv)
Plateau range (V)
Plateau slope %/100 V
Detector flux range (ncm 2 s 1)
3 A-1 3 A-2 3 A-3 3 A-11
3
2 þ1.5 0.5 þ2.0 0.06 þ2.5 0.005 þ2.5
0.4 0.1 0.01 0.001
850–1050 900–1150 900–1100 900–1050
1.5 2 4.5 12
4 102–4 105 102–105 103–106 104–107
He þ Kr He þ Kr 3 He þ Kr 3 He þ Kr 3
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moderated neutron source with thermal neutron flux of 100 n-cm 2 s 1. Source was an extended 20 cm long Pu-Be pin enclosed in an Al-lined paraffin box (10 cm x 10 cm x 70 cm). The performance of low sensitivity detectors (such as 3 A-3 and 3 A-11) was tested using a high flux collimated thermal beam from reactor with thermal neutron flux of 2 106 n-cm 2 s 1 and gamma background of 100 mR / hr. Fig. 4a and b show the PHD spectrum and plateau characteristics of detector 3 A-1, recorded using reactor neutron beam. The spectrum indicates full energy peak with distinct valley for noise and gamma discrimination. PHD shows 30% of Full Width Half Maximum (FWHM) at 764 keV
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corresponding to Q energy of 3He (n, p) t reaction. Number of full energy events indicates well suppressed wall effects, in spite of a very small cathode diameter. Similar results were obtained with other detectors. Fig. 5 shows the high voltage counting curve of detector 3 A-2, 3 A-3 and 3 A-11 recorded using reactor neutron beam. Plateau length is 200 V and slope 2% / 100 V. The plot shows acceptable slope for stable operation. The detector specifications and operating parameters are listed in Table 1. 3 He filled neutron detectors have temperature tolerance up to 200 1C [19]. For the present application of neutron spatial flux profile measurement the temperature inside the sub-critical
Fig. 2. Schematic and assembled Miniature gas filled detectors.
Fig. 3. Block diagram of readout electronics for miniature detector.
Fig. 4. a) Pulse height distribution of 3 A-1 detector using reactor neutron beam, b) Plateau characteristics of detector 3 A-1, using reactor neutron beam.
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system was monitored using RTD thermal sensors and was measured to be 29 1C (ambient temperature). The developed detectors were tested for time as well as temperature stability using Pu-Be source. Fig. 6a shows the time stability of detector 3 A-1 measured over the duration of 20 h by integrating counts over 60 sec. Counting statistics or plateau characteristics of detector indicated no change up to 60 1C as shown in Fig. 6b. Similar results have been obtained for other detectors. These characteristics of detectors ensure the suitability for the required application. Gamma sensitivity of the detector was calculated to be 10 3 cps using the photon absorption cross section of Cu and Kr gas [27]. Estimated gamma background within sub-critical assembly is 40 mR/hr. The PHD of detector in 100 mR/hr gamma background of reactor neutron beam (as shown in Fig. 4a) is acceptable for the estimated background of the sub-critical system.
both in axial and radial directions at various positions is shown in Fig. 7a and b respectively. The axial flux profile is centrally for channels EC5 and EC6 show a dip at the center. This is due to the fact that position of peaked due to the present of the external source at the center of the core. The peak flux in EC1 is higher than that in EC2 and EC3 as it is closer to the source. The radial flux profiles for channels EC5 and EC6 show a dip at the center. This is due to the fact that positions of these channels correspond to the non-fuel central region. The flux peaks near the edge on either sides of the central activity. For channel EC4 (which does not see the central cavity), the thermal flux peaks at the center. The effect of the reflector can also be seen as the thermal flux shows an increase near the reflector-moderator interface.
5. Flux profile measurement of subcritical assembly 4. Simulation results: Monte Carlo simulations were carried out for estimating the neutron flux profile in the Neutrons of energy 2.45 MeV (D-D) produced at the source location are transported throughout the volume and all possible interactions and energy ranges are taken into account. Also, the energy spectrum in the subcritical core is predominantly thermal. Simulated thermal neutron flux profile
Fig. 5. Plateau characteristics of detectors 3 A-2, 3 A-3 and 3 A-11 using reactor neutron beam.
The flux profile measurement in the sub-critical assembly was carried out using DD neutron source as external driver. This neutron source is a DC accelerator based neutron source. It consists of a RF ion source, operated at 13.56 MHz with extraction electrode capable of giving ion current of 150 mA, Einzel lens focusing electrode, accelerating column, faraday cup and the water cooled Deuterium Target. The ion source and the Einzel lens electrode are floated on a high voltage dome with Deuterium target at ground potential. Detector 3 A-1 was used for the initial tests at the subcritical assembly. Experimental channel EC1 closest to the central cavity was used for the measurements D-D neutron source coupled to the subcritical assembly. Detector 3 A-1 was also tested at EC1 for DD pulsed source. Fig. 8 shows the response of detector to pulsed source with neutron pulse width of 25 ms and time period of 50 ms. Data was acquired for 10000 sweeps. Similar results were obtained for other detectors. The calibrated detectors were then used to measure the flux profile inside the core for the external neutron source operated in D-D mode with neutron source strength of 3 107 n/s. Detector 3 A-1 was used for the measurement of neutron flux. It was biased at its plateau voltage of 900 V. The detector was coupled to a pre-amplifier followed by amplifier and scalar/counter to measure the counts at different locations inside the experimental channels. The detector was moved inside the channels manually and the counts were recorded every 70 mm (active length of the detector) throughout the length. Fig. 9 shows the measured axial flux profiles for experimental channel EC1, EC2 and EC3. The axial flux profiles peak at the center where the source is located. Thermal flux at EC1 is more
Fig. 6. a) Time stability of 3 A-1 detector measured for 20 hrs using Pu-Be neutron source, b) Counting Statistics of detector 3 A-1 with change in temperature.
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Fig. 7. a) Simulated axial flux profiles with D-D neutrons, b) Simulated radial flux profiles with D-D neutrons.
Fig. 8. 3 A-1 response for pulsed D-D neutron source. Fig. 10. Experimental radial profile with D-D neutrons for EC7.
6. Conclusion The neutron flux profile measurement has been carried out in a subcritical system using indigenously developed miniature gas filled detectors. These miniature detectors were specially designed considering the dimensional requirement of experimental channels in the sub-critical core. Detectors with various sensitivities (0.4, 0.1, 0.01 and 0.001 cps/nv) were calibrated using neutrons from Pu-Be source and reactor. Performance of detectors for PHD, HV bias curve, gamma tolerance, temperature tolerance and time stability indicate the suitability for the present application. The neutron flux profile has been measured in both axial and radial direction using these detectors. The experimental data obtained corroborates well with the Monte Carlo simulations.
Fig.9. Experimental axial profile with D-D neutrons.
compared to other channels as it is nearest to the central cavity. The estimated error bars are around 73%. This was estimated by determining standard deviation of counts from the mean/average value for series of measurements. These profiles corroborate well with the simulated Monte Carlo profiles as shown in Fig. 7a. We have carried out the radial flux profile measurement for EC7. This channel covers approximately half the length of the core. The experimental radial flux profile for channel EC7 is shown in Fig. 10. This matches well with the simulation results obtained as shown in Fig. 7b.
Uncited reference [20].
Acknowledgment Authors acknowledge CDM, BARC for precision machining of the delicate detector components. Authors thank SSPD staff Shri S.M. Patkar for skillful assembly and Mrs Shyalaja Devan and Miss Amrita Das for help in testing of detectors.
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Please cite this article as: M. Shukla, et al., Nuclear Instruments & Methods in Physics Research A (2014), http://dx.doi.org/10.1016/j. nima.2014.10.059i
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