Gamma Shielding Properties for Rock Fiber Reinforcement Concrete for Nuclear Applications

Gamma Shielding Properties for Rock Fiber Reinforcement Concrete for Nuclear Applications

El-Sayed M. El-Refaie a Sherin El Essawy b, Elham M. Hegazi b , Mohammed Fayizc,

Amal Abdel Razek El-Gamel d, Karim El-Adham e

a Faculty of Engineering, Helwan Univ., Cairo, Egypt

b QC and QA Depart., Egyptian Atomic Energy Authority, Cairo, Egypt

c Exp. Nucl. Phys. Dep., N. R. C., Egyptian Atomic Energy Authority, Cairo, Egypt d Radiation Safety Department, Egyptian Atomic Energy Authority, Nasr City, Egypt e Egyptian Atomic Energy Authority, Cairo, Egypt

Abstract:- In this study, the linear attenuation coefficient (µ cm-1), half value layers (HVL) has been modified and transmission curves have been plotted with different loading of Rock Fiber (RF) by weight (0, 2, 7.5, 12.5, 17.5 and 30) % were studied in gamma radiation shielding application. The Monte Carlo N-Particle (MCNP) transport code and XCOM were used to verify the experimental results of concrete with and without different percentages of RF. Measurements were carried out using 70% High Purity Germanium (HPGe) detector at different gamma radiation energies (185, 240.7, 294, 305.78, 608.7, 1121 and 1767.6) keV for radiation shielding design. The results concluded that the linear attenuation coefficient (µ) of the all prepared samples increase with increasing the rock fiber content.

Keywords: Concrete, Rock Fiber, Linear Attenuation Coefficient, Half Value Layers, XCOM, MCNP5.

1. INTRODUCTION

   Shielding from ionizing radiation plays a vital role in protecting the man and environmental from these radiation. Nuclear safety depends on the safe operation of all parts and components of a Nuclear Power Plants (NPP) during its designed operational life. One of the important aspects of safe operation is ensuring the safe operation of fixed parts during its life, in addition to minimizing the radiation levels in the surrounding environment. Different types of concretes, such as ordinary concrete, heavy weight concrete (e.g., barite, serpentine, steel magnetite, limonite, magnetite) etc. are used in the manufacturing of these fixed parts depending on the energy, radiation types [1]. In recent years, several studies had been made for different construction materials used for gamma radiation shielding [2-6]. Cracks are a major cause of deterioration in concrete structures in a nuclear power plant [7]. Cracks lead to moisture penetration into the concrete resulting in its gradual destruction. Previous studies indicate that reinforced concrete with fiber reduces the rate of crack formation in reinforced concrete. Recently, other types of fibers such as Polymer Fibers and Basalt Fibers have been developed and produced for use in the concrete reinforcement of a nuclear power plant resulting in increased safety for the surrounding environment of nuclear power. Basalt Fibers reinforced concrete are a new construction materials established through broad research and progress during the past years. The addition of fibers to concrete had proved the enhancement the properties of concrete structures as tensile and shears strength, fracture toughness and crack resistance [8 – 12]. Basalt Fibers have different properties from other high-tech fibers (Polypropylene Fiber, Carbon Fibers, Organic Fibers, etc.) due to its high chemical and thermal stability and do not produce any harmful gases. Basalt Fiber has a strength that far exceeds other natural and synthetic fibers and it also greatly meets the requirements of environmental protection. Basalt Fibers have a high modulus of elasticity that is very similar to the modulus of elasticity of Carbon Fibers [13]. The aim of this study is to determine the shielding properties of local Rock Fiber reinforced concrete which will help in the shielding design. Different gamma radiation energies (185 1767.6) keV were used to determine attenuation parameters experimentally and theoretically.

2. EXPERIMENTAL

   2.1 Used Materials

   Ordinary Portland cement was used. The chemical composition of cement is shown in Table 1. The coarse and fine aggregates are chosen according to the Egyptian requirements and ASTM C33 (2013). The water used for all the concrete mixes are normal drinking water. Rock fiber used in this work is provided from a local factory Egyptian Rock Wool Factory, Cairo, Egypt. All materials used are locally produced.

   The concrete samples were casted in the laboratories with rock fiber of 6.5 m diameter and 0.5 mm length with different weight ratios (2, 7.5, 12.5, 17.5 and 30) % Table 2. Samples formed as disks with 2 cm thickness were used to study radiation shielding properties Fig. 1.

   Table 1: Rock Fiber Chemical Composition

   Sio2	AlO3	Fe2O3	MgO	Na2O	K2O	TiO2
   49.79%	14.02%	10.23%	5.25%	1.83%	1.01%	2.66%

   Table 2: The mix proportions of all attenuation specimens.

   Mix No	Cement	Fine Aggregate	Coarse Aggregate	RF (wt %) of Cement
   1	350	656	583	0
   4	348	656	583	2
   5	342.5	656	583	7.5
   6	337.5	656	583	12.5
   7	332.5	656	583	17.5
   8	320	656	583	30

   Fig. 1: Prepared Samples with Rock Fibers

   2.2. Measurement of Gamma Ray Intensity

   The linear attenuation measurements of the prepared Concrete/ RF samples with thickness 2 cm were carried out using narrow beam gamma-ray transmission geometry. Radium-226 source was used to apply strong and well resolved gamma rays of energies (185, 240.7, 294, 305.78, 608.7, 1121, 1767.6) keV.

   A high-resolution spectroscopic system (in Nuclear Reaction Units Laboratory at Atomic Energy Authority, NRC, Inshas) is used for the measurement of the energy spectrum of the emitted gamma rays Fig.2. The system consists of a 70% Hyper-Pure Germanium (HPGe) detector (closed end coaxial cylinder of 54.5 mm in diameter and 49 mm in length). The detector is mounted on a cryostat which is dipped into a 30-L Dewar filled with liquid Nitrogen. The detector is surrounded by a special heavy lead shield cylinder of 5 cm thickness with 1 cm Copper layer inside, which provides an efficient suppression of background gamma radiation present at the laboratory site.

   Figure 2: Experimental measurements of -ray attenuation coefficient

   The primary data came from the counter registering the gamma-ray intensity. The incident intensity (I0) of the gamma ray was reduced to intensity (I) after the passing through the specimens. The Radiation shielding parameters like the linear attenuation coefficient is obtained values using Beer-Lambert's equation (1) as follows [14];

   I = Io e x (1)

   where I0 and I are the unattenuated and attenuated photon intensities respectively, and µ(cm-1) is the linear attenuation coefficient of the material.

   Once the linear attenuation coefficients values were determined, additional radiation shielding parameers, such as the Half- Value Layer (HVL), were used to evaluate the gamma ray shielding ability of the prepared samples. The HVL values represent the thickness of an absorber that will reduce the gamma radiation intensity to one-half of its original value, as the following equation (2):

   HVL = ln2/µ (2)

3. THEORETICAL CALCULATION

   XCOM and MCNP5 are used to calculate the linear attenuation coefficient and Half-Value Layer.

   1. (XCOM)

      XCOM program is a calculation program and its input parameter specifications are quite flexible and easy to access. First, the types of shielding material were determined by elementary mass fractions. XCOM is used to evaluate cross-sections of photon for scattering, photoelectric absorption and attenuation coefficients for energies ranging from (185 1.767.6) keV. Then, the shielding parameters were determined by elementary mass fractions. The total attenuation factors for all components are calculated as the sum of the quantities of the corresponding atomic components. Then the defined weights are calculated as fractions of the weight of the components by XCOM from the chemical formula [13].

   2. Radiation Shielding Modeling MCNP

   MCNP is a general code used for modeling interactions of radiations with materials and tracking all particles at all energies. Being fully 3D, it uses a random nuclear cross-section and uses physical models of particle types [15]. In order to confirm a validation of modeled MCNP5 code for the used geometry. The experimental geometry was simulated using MCNP5 Monte Carlo code as in Fig. 3, which was used to calculate the attenuation parameters for the concrete composites. The gamma rays are collimated by the lead shielding, attenuated by the sample of the Concrete with different weight ratio (0, 2, 7.5, 12.5, 17.5 and

   30) % RF and then collimated again and detected by the detector.

   Fig. 3: experimental geometry simulated using MCNP5 Monte Carlo code

4. RESULTS AND DISCUSSION

The linear attenuation coefficients were calculated in photon energy range from (1851767.6) keV. The values of the linear attenuation coefficients for concrete samples with different ratio of Rock Fiber are listed in Table 3. It shows that linear attenuation coefficient increases with fiber loading for all photon energies, and decrease with the increase of gamma ray energy. This is due to different interaction mechanism of photons at different energies. The strength of these interactions depends on the energy and the elemental composition of the material, but not much on chemical properties, since the photon energy is much higher than chemical binding energies. [16]. Fig. 4 shows a linear attenuation coefficients for concrete samples with (0, 2, 17.5 and 30) % RF, which recorded the highest linear attenuation values. The Half-Value Layer (HVL) thicknesses were calculated using Eq. (2) for gamma shielding. HVL thicknesses for concrete specimens as a function in Rock Fiber ratios are shown in Table 4. As it can be seen, the required thickness decreases with increases in Rock Fiber ratio.

Table 3: Linear Attenuation Coefficients (cm-1) for the prepared concrete/RF samples.

0 2 17.5 30

0 2 17.5 30

0.4

0.35

0.3

0.25

(cm-1)

(cm-1)

0.2

0.15

0.1

0.05

0

0 500 1000 1500 2000

Photon Energy, keV

Fig. 4: Linear Attenuation Coefficient for samples with (0, 2, 17.5 and 30) % RF Table 4: HVL (mm) parameters for the prepared concrete/RF samples

Fig. 5 shows the linear attenuation coefficients of Concrete /RF by using MCNP5. It is observed that the linear attenuation of the cement with 17.5 % RF has higher values. The measured results are compared with calculated values of linear attenuation coefficient in table 3 for samples with different rock fiber ratios at different photon energies is presented in Table 5. It can be seen that the results of () show good agreement between experimental and calculated results for high energies. Moreover, there is a rearkable deviation between the results values of () for low energies.

Linear Attenuation Coeff. (Cm-1)

Linear Attenuation Coeff. (Cm-1)

0.30

0.25

0.20

0.15

0.10

0.05

0 % RF

2 % RF

17.5% RF

30 % RF

0 % RF

2 % RF

17.5% RF

30 % RF

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

photon energy (MeV)

Fig. 5: linear attenuation coefficent of cement /RF versus incident photon energy using MCNP5.

Table 5: Comparison between experimental and theoretical linear attenuation coefficient results.

For a better view of the comparison between XCOM theoretical results and experiment results values for the concrete reinforcement with Rock Fibers (0, 2, 17.5 and 30) % is plotted in Fig. 6 (a, b, c), respectively. It is clear that there is a high deviation between experimental and theoretically results for all energies. The important reason for this deviation is that the composite elements do not distribute uniformly. XCOM homogeneous mixture defined as any mixture part has the same proportions of its components and the mixture has only one phase. The attenuation coefficient for mixture is calculated by XCOM as the sum of the corresponding quantities for the atomic constituents (17). Specifically, fibers contains of many elements such as (Sio2, Fe2O3, MgO), therefore, this elements not distribute in the composite as it defined in the XCOM theory, and the photon rays path traveled in the material was changed due to the RF agglomerates. The RF tends to agglomerate as more is added, resulting deviation between measured and theoretical attenuation. Fig. 7 shows the transition factor of cement with different ratios of RF and different thickness of the samples. It is observed that the cement with 17.5 % RF is the best compound as shielding for gamma radiation. Also, a thickness of 4 cm of the proposed compound may reduce up to 2.75% from gamma intensity. Fig. 8 shows the Half value layer versus Incident Photon Energy for concrete / RF calculated by MCNP5. Its observed that the optimum value of HVL recorded by 17.5 % RF ratio for all energies.

0.24

Linear attenuation Cofficient Cm-1

Linear attenuation Cofficient Cm-1

0.22

0.20

0.18

0.16

0.14

EXP. XCOM

a

a

17.5 %

0.60

Linear Attenuation Coefficient (Cm-1)

Linear Attenuation Coefficient (Cm-1)

0.55

0.50

0.45

0.40

0.35

0.30

0.25

b exp.

XCOM

RF 2%

0.12

0.10

0.08

0.20

0.15

0.10

0.05

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

photon energy (MeV)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Photon energy (MeV)

Linear Attenuation Coefficient( Cm-1)

Linear Attenuation Coefficient( Cm-1)

0.35

0.30

c exp.

XCOM

RF 30%

0.25

0.20

0.15

0.10

0.05

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Photon energy (MeV)

0% RF

2% RF

17.5% RF

30% RF

0% RF

2% RF

17.5% RF

30% RF

Fig. 6: Comparison between experimental and theoretical linear attenuation coefficient values for (a) 2% RF, (b) 17.5% RF, (c) 30% RF

0.75

transmission of gamma rays

transmission of gamma rays

0.70

0.65

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

2 3 4 5 6

Thickness(cm)

Fig. 7: the variation of transimation of cement /RF samples versus thickness at 0.185 MeV. using MCNP5

0% RF

2% RF

14 17.5% RF

30% RF

12

Half-Value Layer (HVL)

Half-Value Layer (HVL)

10

8

6

4

2

0

0.2 0.4 0.6 0.8 1.0 1.2 1.4

Photon energy (MeV)

Fig. 8: Half value layer versus Incident Photon Energy for concrete / RF using MCNP5

4. CONCLUSION

The effect of rock fiber on shielding properties of concrete matrices were studied. Concrete samples were prepared using different ratio of rock fiber with length 0.5 mm. Shielding properties were determined experimentally and theoretically used XCom program and Monte Carlo Simulations. The main conclusions of this study can be summarized:

• The gamma shielding properties of concrete is significantly improved by reinforcement by Rock Fiber for different photon energies

• the linear attenuation coefficient decreases with photon energy increasing,

• A linear attenuation coefficients for concrete samples with (0, 2, 17.5 and 30) % RF recorded the highest linear attenuation values.

• High value Layer decreases with increases of Rock Fiber content.

• There is observed that deviation between experimental and theoretically results for low energies, this is due to Irregular distribution of fiber in concrete matrix.

• Theoretically results are in good agreement with experimental results for the optimum ratios of RF (0, 2, 17.5 and 30)

% RF.

DECLARATION OF COMPETING INTEREST

The authors declare that they have no known competing financial intercessor personal relationships that could have appeared to influence the work reported in this paper.

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