Journal of Nuclear Energy Science & Power Generation TechnologyISSN: 2325-9809

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Research Article, J Nucl Ene Sci Power Generat Technol Vol: 2 Issue: 1

Investigation of Heat Effect on the Shielding Properties of Some Local Concretes

W. A. Kansouh*
Reactor Physics Department, Reactors Division, Nuclear Research Centre, Atomic Energy Authority, Cairo, Egypt
Corresponding author : W. A. Kansouh
Reactor Physics Department, Reactors Division, Nuclear Research Centre, Atomic Energy Authority, Cairo, Egypt
Tel: 012-2993-3383
E-mail: [email protected]
Received: September 19, 2012 Accepted: December 10, 2012 Published: December 14, 2012
Citation: Kansouh WA (2013) Investigation of Heat Effect on the Shielding Properties of Some Local Concretes J Nucl Ene Sci Power Generat Technol 2:1. doi:10.4172/2325-9809.1000102

Abstract

Investigation of Heat Effect on the Shielding Properties of Some Local Concretes

The effect of heat on gamma ray and fast neutron shielding properties of two types of the local concretes (basalt concrete; ρ=2.2 g/cm3 and magnetite concrete; ρ=3.9 g/cm3) with two different water to cement ratios for both were investigated. The samples were exposed to temperatures between 20 to 300°C and 800°C.

Small losses in attenuation coefficients for gamma rays and fast neutrons were observed for the studied types of concretes. Investigations have been performed using a narrow beam of Cs-137 source and a collimated beam from ET-RR-1 reactor. A gamma spectrometer with NaI (Tl) detector and a neutron-gamma spectrometer with stilbene scintillator were used during measurements.

 

Keywords: Magnetite concrete; Basalt concrete; Heated samples

Keywords

Magnetite concrete; Basalt concrete; Heated samples

Introduction

Concrete is specified in building and civil engineering projects for several reasons, sometimes cost; sometimes speed of construction or architectural appearance, but one of concrete’s major inherent benefits is its performance in fire, which may be overlooked in the race to consider all the factors affecting design decisions [1]. Concrete is said to have a high degree of fire resistance and, in the majority of applications, concrete can be described as virtually ‘fireproof’. This excellent performance is due in the main to concrete’s constituent materials (i.e. cement and aggregates) which, when chemically combined within concrete, form a material that is essentially inert and, importantly for fire safety design, has a relatively poor thermal conductivity. It is this slow rate of heat transfer (conductivity) that enables concrete to act as an effective fire shield not only between adjacent spaces, but also to protect itself from fire damage. The use of concrete in nuclear facilities for containment and shielding of radiation and radioactive materials has made its performance crucial for the safe operation of the facility [2].
Long-term degradation of concrete structures under permanently humid environmental conditions is mainly influenced by interacting chemical and mechanical processes leading to the destruction of the microstructure by the dissolution of cement constituents and the propagation of micro-cracks [3]. Mechanical damage accelerates the chemical degradation as a consequence of improving transport properties [4]. Early reported research results, for example, were unable to distinguish between damage caused by heat and damage caused by radiation [2].
Since the year 2000, there have been many reports published about the deterioration of concrete [5-10]. Some of these have reported the results of experimental work, and others have proposed models that might be used to predict the deterioration of concrete over periods of time. Many of the predictive models have focused on the micro-scale processes. Few of these have addressed the effects of radiation on the concrete.
Tests on the effects of long-term exposure to elevated temperatures showed that the compressive strength in general tended to decrease with increasing temperature and with the length of exposure. Temperature affects the modulus of elasticity [11]. The moisture content in the concrete near a hot surface decreases because of pressure-induced flow [12]. Heating of refractory concrete causes physical and chemical changes mainly because of removal of water. The compressive strength is reduced after exposure to about 540°C [13]. In conventional concrete, long-term exposure to high temperatures can cause changes in compressive strength, modulus of elasticity, creep resistance, conductivity, diffusivity, and shrinkage/ expansion characteristics. General speaking, the threshold of degradation in the concrete is approximately 95°C [13].
In some investigations, radiation led to an increase in temperature of the concrete up to 250°C. Avecedo and Serrato [14] reported that several studies conducted explain that major deterioration of concrete occurs after being exposed to high levels of radiation. Granata and Motagnint [15] reported that at nuclear fluxes of the order 109 neutrons/cm2 (at temperature of 130°C) had minimal effects on Portland 730 and limestone aggregate (standard mortar) and Portland 730 cement and barite aggregate (BHT mortar); the gamma dose equivalent used during the experiment was measured to be 1011 roentgens (1 roentgen ~ 1 rad). However, upon increasing the temperature to 280°C, (commonly found in heavy water reactors, and neutron integrated flux up to 1020 neutrons/cm2), the concrete samples were heavily damaged [15]. A similar experiment was conducted by Elleuch et al. [16] using serpentine granulate and aluminate cement paste while maintaining a temperature of 200°C. The density of the concrete was 2.51, which was lower than that used in other high density concretes, especially that of barite. The samples were exposed to doses of 1.2×1019 to 1.2×1020 n/cm2 fast flux, resulting in dehydration of the cement paste and decreasing bending strength by 50% [16]. Such a temperature increase may cause considerable damage to the concrete even if there is no radiation effect [17,18]. The effect of temperature is even more pronounced on the tensile strength of concrete. A temperature increase from 20 to 100°C may cause a reduction of tensile strength of concrete by as much as 50%.
In the present work a comparative study of the heating effect on the attenuation coefficients for fast neutrons and gamma rays distributions through basalt concrete and that of magnetite concrete having different values of water to cement ratios.. These concretes were made from local materials in Egypt. Investigations were performed using different sources of radiation and two types of spectrometers. The obtained results show that this magnetite concrete is better than the basalt concrete under investigation in their attenuation coefficients at different temperatures for gamma rays and fast neutrons. The results were put in tables and figures to get a conclusion from the discussion.

Experimental Methods

Materials and sample preparation
The basalt and magnetite concrete shields were made using local materials with densities 2.2 and 3.9 g/cm3 and with different water to cement ratios. Basalt concrete has the values of water to cement ratios equal to 0.35 and 0.4 and magnetite concrete has the values of water to cement ratios equal to 0.28 and 0.35. The elemental composition of concrete depends mainly on the mix proportions and the chemical composition of the materials used. Mix proportions by weight of basalt and magnetite concretes are given in Table 1. The choice of relative percentage ingredients, compressive strength, tensile strength and flexural strength to obtain the mentioned suitable density for basalt and magnetite concretes was done by the Structural Engineering Department, Faculty of Engineering, Cairo University, Cairo, Egypt. Chemical composition as a percentage by weight of the different constituents in the basalt and magnetite ores used in the two mentioned concretes is presented in Table 2.
Table 1: Mix proportions by weight of two types of concretes.
Table 2: Chemical composition as a percentage by weight for basalt and magnetite ores.
For radiation test, special specimens of 10cm×10cm×10cm were prepared to measure the attenuation coefficient of different types of concretes to gamma ray penetration from Cesium-137 point source and also the attenuation coefficient to reactor beam penetration (fast neutrons and total gamma rays (primary and secondary) to investigate the heat effect on the shielding properties of the investigated concretes.
Test procedures
Heating exposure technique: A big furnace chamber was used. The samples were placed unloaded in the cooled furnace chamber and the temperature was increased to reach certain degrees (100, 200, 300, 800°C). After an exact period (2h), the furnace switched off until cooling, and then the samples were tested.
Radiation test:
Cesium-137 gamma ray source test: Shielding tests of gamma rays were performed using cesium-137 point source of 3.7×104 Bq (1 μCi) activity. Tests were carried out on four different samples of concrete (two samples of basalt concrete and two samples of magnetite concrete) using 4 in. in diameter of NaI (sodium iodide crystal) detector connected to a computerized Multichannel Analyzer (MCA) [19]. Figure 1 shows the schematic diagram of the experimental arrangement using NaI(Tl) Detector. The different samples of each type of concrete heated to different temperatures (100, 200, 300°C) were subjected to Cs137 point source for 1000 seconds to investigate the shielding properties against gamma radiation. The relation between temperature and attenuation coefficients was concluded. The attenuation coefficient was calculated from the following equation:
μ=1/10 [ln Io/I)]
where Io is the intensity of gamma rays before the shield material and I is the intensity of gamma rays after the shield material and μ is the attenuation coefficient factor.
Figure 1: Schematic diagram of the experimental arrangement using NaI(Tl) Detector.
The ET-RR-1 Reactor: This reactor is of the type WWR-S, 2 MW research reactor. A complete information and description of the basic characteristics for this type of reactor are given in details in several references [20]. It is a tank type reactor with fuel of 10% enriched uranium oxide. The moderator, coolant and reflector are ordinary water. The maximum power is 2 MW which corresponds to a maximum thermal flux of 2×1013 neutrons/cm2s and average thermal neutron flux of 1×1013 neutrons/cm2s.
Reactor beam test: Attenuation properties of fast neutrons and total gamma rays for basalt concrete (w/c=0.4) and magnetite concrete (w/c=0.28) and heated to 300 and 800°C) have been examined. Tests were carried out by measuring the transmitted fast neutron and gamma ray spectra through different samples of concrete barriers. Concrete barriers of cubic shape of 10 cm x 10 cm x 10 cm were used to perform the experimental measurements.
Measurements have been carried out for the four types of concrete using a reactor collimated beam (18 mm dia. and 58.5 cm length) of neutrons and gamma rays. The collimated beam is emitted from the horizontal channel number 2 of the (2MW) research reactor ET-RR-1. Special beam filters and collimators were used to provide a beam of specific intensity and geometry suits the measuring purposes. In addition, a special detector collimator and filter were used to eliminate the side scattered radiation and to reduce the thermal neutrons to enhance the discrimination capability of the discriminating techniques. Experimental arrangement layout was shown elsewhere [21,22] and is shown here in Figure 2a. The neutron-gamma spectrometer with stilbene scintillator (4×4 cm) and fast photomultiplier tube were used to measure the proton and electron pulses amplitude distributions. The undesired pulses of recoil proton and electron due to neutrons and gamma rays were achieved using the pulse shape discrimination technique (PSD) based on the zero-cross over method. Spectrometer setup and the description of the discrimination technique were given elsewhere [23,24]. Spectrometer linearity, discrimination capabilities and energy scaling were tested and checked before the course of experimental measurements by measuring the spectra of gamma rays emitted from 22Na, 137Cs, as well as, from Pu-α-Be neutron source. The diagram of the measuring system was shown elsewhere [21,25] and is shown here in Figure 2b. The measured pulse amplitude distributions of recoil protons and electrons were converted to energy spectra of fast neutrons and gamma rays using two unfolding codes (N Spec and G Spec) based on double differentiation and matrix correction methods respectively [26,27]. A ZnS (Ag) scintillator was used to check the reactor power fluctuations during the course of the experimental measurements.
Figure 2a: Experimental arrangement layout using stilbene detector.
Figure 2b: A block diagram of the electronic equipments of the neutron-gamma spectrometer with a dynode chain of the photomultiplier tube.

Results and Discussion

Variation of Cesium-137 gamma ray attenuation coefficient against temperature
Table 3 and Figure 3 show the variation in gamma ray attenuation coefficient (μ, cm-1) for Cesium-137 source against temperature changes for the four types of concrete. At 300°C the attenuation coefficient for all studied mixtures lost between 9 and 10% of their initial values at 25°C. Magnetite concrete with water to cement ratio equals 0.28 has shown more attenuation coefficient than other types of concrete mixtures at room and at different temperatures under investigation (Figure 3). It is clear from Table 3 that the concretes with their lower water to cement ratios for the two types of concretes generally show the more attenuation coefficients than the others two concrete mixtures.
Table 3: Variation of gamma radiation (Cesium-137) attenuation coefficients ( μ, cm-1) for some local concretes against temperature with uncertainties (Unc.).
Figure 3: Variation of gamma radiation (Cesium-137) attenuation coefficients for some local concretes against temperature.
Linear attenuation coefficients (μ(Eg), cm-1), mass attenuation coefficients (μ/ρ, cm2/g), relaxation length (λ, cm), half value thicknesses (HVT, cm) and tenth value thicknesses (TVT, cm) of gamma rays for these concretes have been achieved from these attenuation relations and are given in Table 4.
(μ(Eg))=[Ln (Io/I)]/t
Where: Io is the total flux intensity of gamma rays of the required energy range before leaking through the sample and I is the total flux intensity of gamma rays of the same energy range after leaking through the concrete sample and t is certain thickness of the concrete sample.
(λ)=1/(μ(Eg))
HVT=0.693/(μ(Eg))
TVT=2.303/(μ(Eg))
Table 4: Radiation shielding parameters of gamma rays for concretes under investigation with uncertainties (Unc.).
Reactor total gamma ray attenuation index
For the region of reactor total gamma rays energy from 1.262 to 7.197 MeV Table 5 and Figure 4 show variation of reactor total gamma attenuation coefficient (μ, cm-1) against temperature changes (300°C and 800°C) for basalt concrete (w/c=0.40) and magnetite concrete (w/c=0.28). At 300°C the linear attenuation coefficient is nearly the same as that at room temperature for basalt concrete mixture. But for magnetite concrete mixture at 300°C it lost nearly 2% of its initial value at room temperature. At 800°C for basalt and magnetite concrete mixtures linear attenuation coefficients lost 7% of its initial values at room temperature. It is clear from Table 5 that the magnetite concrete mixture has shown more attenuation coefficients for reactor total gamma ray than basalt concrete mixture at room and at different temperatures under investigation.
Table 5: Variation of reactor total gamma attenuation coefficient (μ, cm-1) for two different types of concrete against temperature with uncertainties (Unc.).
Figure 4: Variation of reactor total gamma ray attenuation coefficients against temperature degrees in different concrete shields.
It is clear from Table 3 and Table 5 that the attenuation coefficients of gamma rays of (Cs-137) point source for two types of concrete (magnetite concrete with w/c=0.28 and basalt concrete with w/c=0.40) are more than that for reactor total gamma rays for the same types of concrete. This can be attributed to the fact that reactor total gamma rays have energy spectra with energy range from 1.262 to 7.197 MeV and consisted of primary gamma rays from the reactor and secondary gamma rays resulted from the interactions of neutrons with the samples which decrease the values of the attenuation coefficients. But gamma rays from Cs-137 point source are primary gamma rays only of energy 0.662 MeV. Linear attenuation coefficients (μ, cm-1), mass attenuation coefficients (μ/ρ, cm2/g), relaxation lengths (λ, cm), half value thicknesses (HVT, cm) and tenth value thicknesses (TVT, cm) of reactor total gamma rays for these concretes have been achieved from the previous attenuation relations of the integral fluxes and are given in Table 6.
Table 6: Radiation shielding parameters of reactor total gamma rays for concretes under investigation with uncertainties (Unc.).
For the region of neutron energies from 1.2-13.6 MeV Table 7 and Figure 5 show the variation of reactor fast neutron removal cross section against temperature changes (300°C and 800°C) for basalt concrete (w/c=0.40) and magnetite concrete (w/c=0.28). At 300°C the reactor fast neutron removal cross section lost nearly 3.5% of its initial value at room temperature (25°C) for the two mixtures of concretes. At 800°C for both concrete mixtures the reactor fast neutron removal cross sections lost nearly 17.5% of its initial values at room temperature (25°C). This can be attributed to the fact that both concretes had lost more of their water content. It is clear from Table 7 that the magnetite concrete mixture shows more attenuation coefficient for fast neutrons than basalt concrete mixture at room and at different temperatures under investigation.
Figure 5: Variation of reactor fast neutron removal cross section against temperature degrees in different concrete shields.
Table 7: Variation of reactor fast neutron removal cross section (ΣR, cm-1) for two different types of concrete against temperature with uncertainties (Unc.).
Removal ΣR (En) cross sections, relaxation lengths (λ), half value thicknesses (HVT) and tenth value thicknesses (TVT) of fast neutrons for these concretes have been achieved from these attenuation relations and are given in Table 8.
<Sigma>R=[Ln (Io/I)]/t
Where: Io is the total flux intensity of fast neutrons of the required energy range before leaking through the sample and I is the total flux intensity of fast neutrons of the same energy range after leaking through the concrete sample and t is certain thickness of the concrete sample.
<Lamda>=1/<Sigma>R
HVT=0.693/<Sigma>R
TVT=2.303/<Sigma>R
Magnetite concrete has clearly better attenuation properties for fast neutrons than basalt concrete.
Table 8: Radiation shielding parameters of fast neutrons for concretes under investigation with uncertainties (Unc.).

Conclusion

This work provides a preliminary macroscopic comparison between two types of concrete for radiation shielding concretes as a function of temperature and initial moisture content. It is only investigatory as noted by the title and in no way constitutes a qualification for use. It can be concluded that:
a) Magnetite concretes with their different water to cement ratios under investigation have better attenuation properties for gamma rays, total gamma rays (primary and secondary) and fast neutrons than basalt concretes with their two water to cement ratios at room and at different temperatures under investigation.
b) Concretes under investigation with lower water to cement ratios have more attenuation properties for gamma rays radiation shielding than concretes with higher water to cement ratios at room and at different temperatures under investigation.
c) Produced results for heat effect on different types of concrete may be useful for nuclear research community and shielding designers.

Recommendations

More samples need to be tested to show statistical variation in the results, especially in construction variation in the mixtures. Also work needs to be performed to measure differences in thermal conductivity in spite of its poor values generally for concretes and to show concrete spalling effects caused by high temperature gradients due to fire, unlike the uniform heating followed by cooling provided by the furnace. Further testing of concrete under reactor-beam conditions should be performed while concrete samples are simultaneously undergoing heating. This should provide a continuous time curve of shielding decrease.

Acknowledgement

The author would like to thank Prof. R.M. Megahid, Reactor Physics Department, Reactors Division, Nuclear Research Center, Atomic Energy Authority, Cairo, Egypt for his valuable discussion and H. M. Hassanien, Structural Engineering Department, Faculty of Engineering, Cairo University, Cairo, Egypt, for the preparation of the different types of concrete under investigation.

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