|Year : 2015 | Volume
| Issue : 3 | Page : 109-114
Comparison of neutron attenuation properties of ferro boron slabs containing 5% natural boron with other high density materials
D Venakata Subramanian, Adish Haridas, Subhrojit Bagchi, D Sunil Kumar, A John Arul, RS Keshavamurthy, P Puthiyavinayagam, P Chellapandi
Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India
|Date of Web Publication||10-Nov-2015|
D Venakata Subramanian
RSS/RSDD, CDG/RDG, IGCAR, DAE, Kalpakkam - 603 102, Tamil Nadu
Source of Support: None, Conflict of Interest: None
Modeling and designing cost-effective neutron attenuation along with shield volume reduction is a challenging task in fast reactors. It involves reducing the neutron energy and absorbing them with suitable materials. A series of experiments were conducted in the South beam end of Kalpakkam Mini reactor with powders of ferro boron (FeB), ferrotungsten (FeW), boron carbide, slabs of FeB, and mild steel plates to study their neutron attenuation characteristics. In one of the experiments, FeB slab cast with 5% natural boron was used, and neutron attenuation measurements were carried out. The attenuation factors were found over a thickness of 28 cm for the measured reaction rates of 195 Pt (n, n') 195m Pt, 111 Cd (n, n') 111m Cd, 103 Rh (n, n') 103m Rh, 115 In (n, n') 115m In, 180 Hf (n, n') 180m Hf, 63 Cu (n,g) 64 Cu, 23 Na (n,g) 24 Na, 55 Mn (n,g) 56 Mn, and 197 Au (n,g) 198 Au reactions representative of fast, epithermal, and thermal neutron fluxes. A comparative analysis of the neutron attenuation behavior measured with various materials is presented. In case of attenuation of both thermal and fast fluxes, FeB is better than other high density materials such as mild steel and FeW. The outcome of the experimental study is that FeB slab cast with 5% natural boron can be utilized as cost-effective neutron shield in streaming paths in nuclear reactors.
Keywords: Ferro boron, Kalpakkam Mini, neutron attenuation
|How to cite this article:|
Subramanian D V, Haridas A, Bagchi S, Kumar D S, Arul A J, Keshavamurthy R S, Puthiyavinayagam P, Chellapandi P. Comparison of neutron attenuation properties of ferro boron slabs containing 5% natural boron with other high density materials. Radiat Prot Environ 2015;38:109-14
|How to cite this URL:|
Subramanian D V, Haridas A, Bagchi S, Kumar D S, Arul A J, Keshavamurthy R S, Puthiyavinayagam P, Chellapandi P. Comparison of neutron attenuation properties of ferro boron slabs containing 5% natural boron with other high density materials. Radiat Prot Environ [serial online] 2015 [cited 2020 Jul 13];38:109-14. Available from: http://www.rpe.org.in/text.asp?2015/38/3/109/169389
| Introduction|| |
Shields around core and blankets form the major part of reactor assembly in fast breeder reactors. Modeling and designing cost-effective neutron attenuation in fast reactors is a challenging task. It involves reducing the neutron energy and absorbing them with suitable materials. The reduction of neutron strength, need to be achieved through least volume of shields. A series of experiments were conducted with various materials in the South beam end of Kalpakkam Mini  (KAMINI) reactor to study their neutron attenuation characteristics. Materials such as powders of ferro boron (FeB),  ferrotungsten (FeW),  boron carbide,  10% slab cast of FeB,  and mild steel plates  were used in the experiments. It was observed that due to the high boron content in the 10% FeB slabs, the stability of the slabs had reduced and cracks were formed. New slab cast with less boron content was made and with 5% natural boron the stability improved appreciably. This paper describes the details of the neutron attenuation measurements carried out using FeB slabs with 5% boron and its comparison with other high density materials.
FeB is an alloy of iron and boron, and it is indigenously available. It is used as a master alloy in the steel industry as an additive for boron. It contains 15-18 wt% boron. Apart from iron and boron it also contains <1 wt% of silicon, aluminum, carbon, sulfur, and phosphorus. Commercial grade material has a density around 6-7 g/cc. It is available in lumps, granules, and coarse powder form. When subjected to X-ray diffraction studies,  it is observed that three phases, namely FeB, Fe 2 B, and Fe 3 (B, C) are there in FeB. Of these, the orthorhombic FeB constitutes the major phase, the other two being present in smaller volume fractions. Its melting point  is ~1450°C. Since the material will be used as a shielding material, it will be filled in stainless steel (SS) subassemblies and placed inside reactor core surrounding fuel subassemblies. When accelerated tests are performed with the material filled in SS capsules for 10,000 h at higher temperatures of 550°C, 600°C, 700°C, and 800°C, it is found that the boron present in the material penetrated the SS clad only up to 20-45 μ depending on the temperature. The material is showing good compatibility to use in liquid sodium environment  up to 1000 K without any constraint.
KAMINI is a 233 U-fueled, light water moderated; natural convection cooled, and beryllium oxide reflected research reactor. It is located at the Indira Gandhi Centre for Atomic Research at Kalpakkam, India. Because of the highly efficient reflector material BeO, it has a very low fuel inventory (~612 g). The reactor is designed to operate at a nominal power of 30 kW. As shown in [Figure 1], the reactor has three horizontal hollow beam tubes (North, South, and West) that extract neutrons from the core-reflector interface. The neutron flux available at the South beam end is ~10 6 -10 7 n/(cm 2 s).
| Experimental details|| |
In this experiment, five FeB slabs of dimension 300 mm × 200 mm × 50 mm were used. The overall thickness of the shield model was 280 mm. The density of the slab was 6.95 g/cm 3 . The experiment was conducted by stacking the FeB slabs in front of KAMINI South end neutron beam as shown in [Figure 2], with the help of a stand. Activation foils  such as indium, gold, rhodium, copper, platinum, hafnium, cadmium, and pellets of MnSO 4 and NaCl were placed in between the FeB slabs. The weight of the foils ranges from 0.1 g to 1.8 g. The foils become activated when subjected to neutron flux and the measurement of their activities from different locations provided information on the neutron attenuation behavior of thermal, epithermal, and fast neutrons with respect to FeB slabs. [Table 1] lists the foils used, nuclear reaction, the half-life of product nucleus, threshold energies, and gamma energies measured. The foils were so selected that their combined sensitivities covered the energy spectrum from epithermal to fast neutron region. The first location was at the exit of the South end beam slit and at a distance of 50 mm from the slit. The second, third, fourth, fifth, and sixth locations were at a distance of 56 mm, 112 mm, 168 mm, 224, and 280 mm, respectively from the first location. The positioning of the foils was made in such a way that, the center of the beam slit should coincide with the center of the slabs so that the neutron flux subjecting on the foils would be maximum. The irradiation of the foils was carried out for 2 h at the reactor power of 25 kW.
|Figure 2: Arrangement of ferro boron slabs with foil holders in the south beam end of Kalpakkam Mini reactor|
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The irradiated foil activities were measured in coaxial type high-purity germanium (HPGe) detector as well as in planar HPGe detector. The absolute energy calibration of the detectors was done using the certified standard sources such as 152 Eu, 241 Am, 22 Na, and 137 Cs. After shutdown of the reactor, irradiated foils were taken out after sufficient cooling for the gamma dose to die out completely in the surrounding area. The gamma dose due to the irradiated foils of different positions were measured using a teledetector which was found to be <25 mR/h. The foils were taken to the counting set-up. The detector system used for counting gamma energies above 100 keV is HPGe P-type coaxial detector. Using it, irradiated gold, indium, cadmium, sodium, manganese, copper, and hafnium were measured. The HPGe detector with preamplifier is connected to a spectroscopic amplifier with a gain of about 1000. In addition to amplification, the spectroscopic amplifier converts the signal into a form suitable for measurement. The amplified output is fed to 8K multi channel analyzer (MCA) card with built in analog to digital converter (ADC). The gamma spectrum software is used to analyze the acquired signal. It searches and identifies the product nucleus gamma energies, from which the net counts due to the energy peak of interest are observed. Rhodium and platinum foils were counted in planar HPGe detector. Planar HPGe detector is used for counting gamma energies below 100 keV. All foils were counted after sufficient delay to have dead time errors <3%. The delay time varied for different sets of foils starting from 2 to 24 h. While counting, precautions were taken to keep the foils of different locations on the same position over the detector surface to minimize the errors.
The following expression  is used to calculate the measured reaction rate due to the activation of foils kept in between the FeB slabs.
Here, C = total counts due to the peak of interest, Φ = total neutron flux in n/cm 2 /s, σa = microscopic activation cross section in barns, N o = Avogadro number, w = weight of foil in g, I a = isotopic abundance, y = yield of gamma rays emitted from the activated foils, ε = detector efficiency at a particular energy, λ = decay constant, A = atomic weight, G = detector geometry factor (1.0 for foil put on the top of the detector), t d = delay between the end of irradiation and the time of counting in s, t r = length of irradiation in s, and t c = length of counting in s.
Results of measurements (i.e., the net counts due to the energy peak of interest) are given in [Table 2]. [Table 3] gives the measured reaction rates (normalized with respect to weight and time of shutdown) and [Table 4] provides the ratio between the measured reaction rates at different locations.
|Table 2: Measured net count rate of foils irradiated at different locations |
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|Table 3: Measured reaction rates at 25 kW (with normalized time of shutdown and weight) at different locations |
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|Table 4: Ratios between measured reaction rates (neutron flux reduction) at 25 kW (with normalized time at shutdown and weight) of different locations |
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| Discussions|| |
The attenuation factors were obtained in the activation of gold, manganese, copper, sodium, indium, cadmium, platinum, rhodium, and hafnium. Capture reactions on gold, manganese, copper, and sodium represent attenuation of thermal and epithermal fluxes whereas inelastic reactions on indium, cadmium, platinum, rhodium, and hafnium represent the reduction of fast neutron fluxes. Since these reactions are threshold reactions, they occur in the fast neutron energy range.
Measured thermal attenuation for 63 Cu (n,g) 64 Cu, 23 Na (n,g) 23 Na, 197 Au (n,g) 197 Au, and 55 Mn (n,g) 55 Mn reactions for a thickness of 22.4 cm are 397, 1204, 1357, and 1396, respectively. The measured reaction rates are higher than expected at the last location (28 cm) due to the reflection effect (backscattering) of neutrons from the walls of the experimental pit. The neutrons emerging from the South beam end slit, after passing through the shield models hit the concrete wall at a distance of 23 cm from the last slab. After hitting the wall, the neutrons are reflected (backscattering) and lose their energy. These neutrons induce further activity in the foils which are kept after last shield model (last location). The increase in activity due to reflection is measured only in foils which are responding to thermal neutron fluxes. Due to this reason the measured reaction rates of last location are more than previous location for foils such as gold, copper, sodium, and manganese [Figure 3] gives comparison of measured thermal flux attenuation pattern of FeB (5%) slabs with other high density materials with backscattering effect.
|Figure 3: Comparison of measured thermal flux attenuation pattern of ferro boron (5%) slabs with other high density materials (with back reflection)|
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The measured fast neutron attenuation over a thickness of 22.4 cm for the reaction rates of 180 Hf (n, n') 180m Hf, 195 Pt (n, n') 195m Pt, 111 Cd (n, n') 111m Cd, 103 Rh (n, n') 103m Rh, and 115 In (n, n') 115 mIn are 41, 35, 21, and 28, respectively. The effect of backscattering is not observed in fast flux attenuation.
[Figure 4] shows the comparison between attenuations in FeB (5%) slabs and other high density materials as a function of areal density (defined as thickness multiplied by density) for Au (n,g) reactions representative of thermal/epithermal fluxes for measured and fitted values (based on curve fitting). The fits were done after removing the data corresponding to the outermost foils, which have reflection component. From [Figure 4] it is observed that the experimental data points fall on a straight line in a log-log plot. The extracted parameters are reported in [Table 5]. The availability of simple exponent for characterizing neutron attenuation will be useful in routine shield thickness estimates. [Figure 5] shows comparison for FeB slabs, boron carbide, and other high density materials as a function of areal density for In (n, n') reactions representative of fast fluxes for measured values.
|Figure 4: Comparison of measured and fitted thermal flux attenuation of ferro boron (5%) slabs with other high density materials|
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|Figure 5: Comparison of measured fast flux attenuation of ferro boron (5%) slabs with other high density materials and boron carbide|
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| Conclusions|| |
The neutron attenuating properties of FeB slabs with 5% natural boron are studied in this work. For the attenuation of both thermal and fast fluxes, FeB is better than other high density materials such as mild steel and FeW. The percentage of boron in the slabs plays an important factor in the attenuation of neutrons. Earlier studies with 10% FeB slabs show, about 15% better neutron attenuation than that of 5 % slabs. FeB slabs containing 10% boron content are less stable (crack formation at high temperature) compared to 5% boron content. The 5% FeB slab is recommended as cost-effective shield material, as the difference in attenuation characteristics between 5% FeB and 10% FeB is not very different. Along with attenuation, the backscattering (reflection) of thermal neutrons is observed in the end of the shield model. This effect is not seen with respect to the reactions representing fast neutron flux attenuation. From the ratio of measured reaction rates for various activation reactions, it is clearly observed that the neutron attenuation increases as the energy decreases.
Financial support and sponsorship
The work has been performed under XII plan project of Reactor Design Group (RDG), IGCAR, KALPAKKAM, INDIA.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]
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