Novel Shielding Mortars for Radiation Source Transportation and Storage

Abstract

New types of mortar, M1 (60% sand, 25% cement, 10% ball clay, and 15% WO₃), M2 (50% sand, 25% cement, 10% ball clay, and 25% WO₃), M3 (60% sand, 25% cement, 10% Barite, and 15% WO₃), and M4 (50% sand, 25% cement, 10% Barite, and 25% WO₃), were prepared and the impact of WO₃ and barite on their radiation shielding performance and mechanical properties was evaluated. The radiation attenuation factors were evaluated using five radioactive point sources, and a sodium iodide (NaI) scintillation detector (3″ × 3″) was used to detect the attenuation of gamma ray photons emitted from radioactive sources. The density values of the mortar samples lie within the range of 2.358 and 2.602 g/cm³. The compressive strength and the tensile strength of the prepared mortars increased with the increasing percentage of WO₃. The M4 mortar had the highest linear attenuation coefficient (LAC) value. The LAC results demonstrated that adding barite and a high percentage of WO₃ into the mortars notably enhanced the radiation shielding performance of the prepared mortar. The relationship between the half value layer (HVL) and the energy is direct, and so was used to calculate the thickness of mortar needed to absorb or scatter half the number of low-energy photons falling on the samples. At 0.06 MeV, the HVL values of the samples were 0.412, 0.280, 0.242, and 0.184 cm for samples M1–M4, respectively. The highest HVL values, obtained at 1.408 MeV, were 5.516, 5.202, 5.358, and 5.041 cm. Thus, a thinner layer of the M4 sample provided comparable attenuation of photons and radiation protection to the thicker M1–M3 samples. The new material is promising as an effective shield of radiation-emitting sources during transportation and long-term storage.

1. Introduction

Effective ionizing radiation protection is regarded as one of the most critical difficulties confronting nuclear scientists and engineers in the twenty-first century, particularly with the increased use of radiation in medicine, industry, agriculture, and other everyday uses [1,2]. As a result of the increased usage of radioactive sources, researchers have attempted in recent years to create materials that can be used as containers for burying radioactive sources, as well as materials that can be utilized as radiation shields [3,4]. These shields must have a number of qualities, the most essential of which is their capacity to absorb radiation. Their constituent materials should be easily accessible, inexpensive, and easy to prepare, as well as being non-toxic (environmentally friendly).

Concrete is a typical building material that is utilized in the design of medical facilities, nuclear power plants, research reactors, research centers, universities, and other institutions [5,6]. Radiation protection materials are employed in practical applications in two ways: directly covering the radiation source or installing it on the surface of the wall. Particular types of concrete must provide appropriate radiation protection from both gamma rays and neutrons. As new technologies are adopted, novel materials will be critical in future years and will outperform standard building materials (such as regular concrete or lead barriers) as ionizing radiation shields [7,8,9].

Traditional radiation protection construction materials are heavy and make the transportation of contained radioactive sources challenging. Lightweight construction materials that have similar shielding capabilities to the materials now in use are highly desired. Modifying cement with bitumen can enhance its physical, chemical, mechanical, and shielding qualities [10,11].

The safe isolation of radioactive materials during transportation and storage procedures, as well as the shielding of nuclear and medical buildings and facilities must achieve a number of goals including the immobilization of radioactive materials inside the container, the reduction of radiation exposure to medical personnel and unintended parts of patients during the radiotherapy, and the reduction of radiation leakage, particularly to areas adjacent to nuclear facilities and nuclear power plants [12].

In contrast to concrete, cement mortar has been widely employed as a fundamental construction material in many recently constructed facilities and is frequently used by civil engineers due to its adaptability to different designs, low material cost, high strength, and remarkable durability [13]. Mortar, generally, is a construction paste used to bind bricks or stone and to fill gaps between them. It hardens when it dries and is composed of many elements such as clay, argillite, cement, and is sometimes mixed with sand [14,15].

The effect of heavy metals, namely barium sulfate, lead slag, and heavy iron metal wastes, on the density and radioactive attenuation coefficients of geopolymer mortars has been determined [16]. Two radioactive isotopes, Cs¹³⁷ and Co⁶⁰, were used to determine the radiation protection properties at three energies, namely 662, 1173, and 1332 keV. The density of the geopolymer mortar increased when the sand was replaced with fine lead slag aggregates and the mortar was highly efficient at attenuating photons. Garnet residue was also an effective sand replacement [17]. The incorporation of solid F-Ba and Pb-Zn tailings into the mortar increased the attenuation of gamma rays to an effective shielding level, especially at energies between 0.122 and 0.622 MeV [18], with HVL increasing with loading, and most pronounced at the higher energy of 0.15 MeV. Gd₂O₃ and ulexite increased the neutron protection properties of Portland cement and good long-term strength was maintained [19]. An optimal ratio of water to cement of 0.3 was determined for shielding the mortar, but this was increased to 0.4 when fly ash was included to increase the shielding properties [20]. Mortars were successfully combined with the fine aggregate mineral and ores, which did not affect attenuation up to 30% loading [21].

Although the toxicity of many of the attenuating additives can be an issue, as described above, the efficacy of heavy metals in mortar is clear. Of the available metals, tungsten (W), a heavy element with a large atomic number and high density, has led to outstanding shielding characteristics when added as WO₃ to glassy materials [2]. Therefore, the addition of WO₃ to the mortar will increase its effective atomic number which will enhance the radiation attenuation performance for the prepared mortars. In this report, the WO₃-containing mortars were evaluated mechanically, and their shielding properties (via radiation attenuation coefficients) were determined using a NaI detector and a set of radioisotopes.

2. Materials

2.1. Cement and Sand

Ordinary Portland cement OPC-CEM IIA/L was supplied by the Elmasria Cement Company, Egypt. Natural sand used as a fine aggregate of less than 1 mm in size and potable water were obtained from local sources.

2.2. Barite and Ball Clay

Barite is barium sulfate (BaSO₄), with a white color, a specific weight range from 4.2 to 5, and is already sometimes used as aggregate for concrete. Finely powdered barite (particle size distribution of micro scale) was added to cement mortar. Ball clay is the main plastic material used in clay bodies of all types. It has a specific gravity of 2.6 and is composed predominately of SiO₂ (50–60%), Al₂O₃ (30–40%), Fe₂O₃ (5–10%), and TiO₂ (2–5%). When this clay is used to replace a small proportion of sand it gives the mortar more softness and density.

2.3. Tungsten Oxide (WO₃)

Tungsten (VI) oxide, also known as tungsten trioxide (WO₃) or tungsten anhydride, has a density of 7.15 g/cm³ and is a known radiation shielding oxide that mixes well with mortar.

2.4. Composition

The chemical compositions of the base materials were determined using energy-dispersive X-ray or EDX analysis with a scanning electron microscope unit (

Table 1. The chemical composition of base materials used in the present work.

Oxides (%)	Cement	Sand	Barite	Ball Clay
CaO	61.77	0.08	0.07	--
SiO2	22.21	99.29	2.26	58.26
Al2O3	5.01	0.35	0.36	35.08
CaSO4	4.11	--	--	--
Fe2O3	2.97	0.09	0.17	4.16
MgO	2.05	--	0.21	--
SO3	1.98	--	32.04	--
Na2O	--	0.09	--	--
K2O	--	0.08	--	--
TiO2	--	0.01	--	2.5
SrO	--	--	2.68	--
BaO	--	--	62.20	--

).

3. Experimental

Four samples were prepared according to the cement test method EN 196-1, by replacing the sand in certain proportions with other additives (

Table 2. Mixture percentage of prepared mortars.

Sample	Composition
	Cement (g)	Water (g)	Nature Sand (g)	Barite (g)	Ball Clay (g)	WO3 (g)
M1	250	125	750	-	100	150
M2	250	125	650	-	100	250
M3	250	125	750	100	-	150
M4	250	125	650	100	-	250

). The samples were poured into the plastic tubes (50 × 50 × 150 mm) and kept in a humid place for 24 h. The samples were removed from the molds and then stored under water at room temperature until strength testing.

The density of mortar samples was determined by the general law for homogeneous cylindrical samples ($\rho =\mathrm{M}/\mathrm{V}$) where M is the mass (g) and V is the volume (cm³) of the sample $(\raisebox{1ex}{$4$}\!\left/ \!\raisebox{-1ex}{$3$}\right.{\mathsf{\pi }\mathrm{R}}^3$), and where R (cm) is the radius of the sample. The tensile strength was determined according to the NF P 18- 408 [18] and calculated by the equation:

$$\tau =2P/\pi DL,$$

(1)

where $\tau$ is the tensile strength, P is the applied load, D is the diameter of the mortar sample, and L is the height of the sample. The attenuation factor was determined using five radioactive point sources (

Table 3. The specification of point sources used in this work.

Nuclide	Energy (keV)	Emission Probability (%)	Activity (Bq)	Uncertainty (kBq)
Am-241	59.52	35.90	259,000	±2.6
Cs-137	661.66	84.99	385,000	±4.0
Eu-152	121.78	28.37	290,000	±4.0
	244.69	7.53
	344.28	26.57
	778.91	12.97
	964.13	14.63
	1112.0	13.54
	1408.1	20.85
Ba-133	80.99	32.90	275,300	±1.5
	356.01	62.05

) as described in [22,23,24,25]. A sodium iodide (NaI) scintillation detector (3″ × 3″) was used to detect the gamma ray photons emitted from the radioactive sources with and without mortar shielding [26,27,28]. The detector crystal was connected to the photomultiplier tube, preamplifier, amplifier, and multichannel analyzer, and the applied voltage was 800 volts. The source-detector distance was 50 cm and the mortar sample was placed between them 15 cm away from the detector (Figure 1).

The mass attenuation coefficient (MAC) was determined experimentally by calculating the net area under the full energy peak which represents the peak intensity. The MACs were determined for each sample by the exponential attenuation law shown in the following equation [29,30,31,32]:

$$\mathrm{MAC}=\frac{1}{x.\rho }\ln \left(\frac{I_{Trans}}{I_{Incid}}\right),$$

(2)

where I_Trans and I_Incid are the transmitted and incident peak intensities corresponding to a specific energy, ρ is the density of the mortar sample, and x is its thickness. The linear attenuation coefficient (LAC), defined as the probability of photon interaction within a matter through a certain path-length, can be determined by multiplying the MAC by the material density as follows:

$$\mathrm{LAC}=\mathrm{MAC}\times \rho .$$

(3)

The half value layer (HVL) and the mean free path (MFP) were calculated by Equations (3) and (4) [33,34]:

$$\mathrm{HVL}=Ln\left(2\right)/ \mathrm{LAC},$$

(4)

$$\mathrm{MFP}=1/ \mathrm{LAC}$$

(5)

The shielding efficiency of the samples can be as the expressed as the Radiation Protection Efficiency (RPE) calculated using the following equation [35]:

$$\mathrm{RPE} \left(\%\right)=\left[1-\frac{I_{Trans}}{I_{Incid}}\right]\times 100$$

(6)

The MACs were also theoretically calculated based on the chemical composition of each material using XCOM software, and compared to the experimental results.

4. Results and Discussion

4.1. Density

The density values of the mortar samples were 2.358 ± 0.0015, 2.507 ± 0.0022, 2.442 ± 0.0025, and 2.602 ± 0.0018 g/cm³ for M1, M2, M3, and M4, respectively. In general, increasing WO₃ increased mortar density. Barite slightly increased mortar density because it has a higher specific weight than the ball clay.

4.2. Compressive and Tensile Strength

The compressive strength of the prepared mortars increased as the combined loading of WO₃ and barite (Table 2) increased (Figure 2a). These values are on the high end of the accepted range of 28 days compressive strength (33–53 MPa). WO₃ in the mixtures affects mortar particle bonding by reducing the incidence of pores in the solidified mortar, which consequently increased the strength of the mortar (Figure 2b). The overall tensile strength of the mortar is approximately 10% of the compressive strength, as expected.

4.3. Morphological Images

A scanning electron microscope (SEM) of the JSM-5300, JEOL model was used to produce morphological images of the prepared mortar samples, as shown in Figure 3. The figure shows that the distribution of WO₃ is more homogeneous when its percentage is increased. WO₃ is more well distributed in Figure 3d (M4) than in Figure 3b (M2), which indicates that the barite-WO₃ composite has better properties than ball clay-WO₃ composite.

4.4. Radiation Shielding Features

In this study, the mass attenuation coefficient (MAC) values of the prepared mortars were calculated by using five radioactive sources in the energy range of 0.06 to 1.408 MeV. In addition, the MAC values for the same samples were determined theoretically using the XCOM software [36] in order to confirm the accuracy of the experimental results. The correlation coefficients (R²) between the practical and theoretical values (Figure 4) demonstrated excellent correspondence of the two sets of MAC values at most energy levels. Minor differences observed were attributed to variation in experimental data. Thus, the experimental set-up was confirmed as a valid method to determine the MAC values of the mortars prepared in this work, which is important because the experimental method can be used to determine the effects of additives and manufacturing conditions not modelled in XCOM. The error, ∆%, between the experimental and theoretical results was calculated (

Table 4. The experimental and theoretical mass attenuation coefficients for the prepared mortars.

Energy (MeV)	MAC (cm2/g) M1			MAC (cm2/g) M2
	XCOM	EXP	∆%	XCOM	EXP	∆%
0.060	0.7129	0.7011	1.68	0.9861	0.9647	2.22
0.081	1.0890	1.1174	−2.54	1.6730	1.6556	1.05
0.122	0.4583	0.4543	0.88	0.6579	0.6446	2.07
0.245	0.1619	0.1586	2.05	0.1918	0.1887	1.62
0.344	0.1197	0.1180	1.47	0.1314	0.1300	1.05
0.356	0.1166	0.1177	−0.94	0.1273	0.1288	−1.18
0.662	0.0797	0.0775	2.85	0.0813	0.0806	0.88
0.779	0.0730	0.0724	0.74	0.0739	0.0726	1.66
0.964	0.0652	0.0662	−1.55	0.0654	0.0641	2.07
1.111	0.0604	0.0621	−2.75	0.0604	0.0595	1.62
1.173	0.0587	0.0582	0.85	0.0586	0.0581	0.95
1.333	0.0548	0.0541	1.44	0.0547	0.0538	1.70
1.408	0.0533	0.0532	0.21	0.0532	0.0518	2.52
Energy (MeV)	MAC (cm2/g) M3			MAC (cm2/g) M4
	XCOM	EXP	∆%	XCOM	EXP	∆%
0.060	1.1750	1.1524	1.96	1.4480	1.4134	2.45
0.081	1.2930	1.2745	1.45	1.8770	1.9008	−1.25
0.122	0.5232	0.5119	2.21	0.7228	0.7238	−0.14
0.245	0.1705	0.1755	−2.87	0.2004	0.1987	0.85
0.344	0.1226	0.1245	−1.51	0.1344	0.1321	1.74
0.356	0.1193	0.1174	1.62	0.1299	0.1271	2.22
0.662	0.0797	0.0786	1.47	0.0814	0.0805	1.05
0.779	0.0729	0.0734	−0.75	0.0737	0.0746	−1.18
0.964	0.0649	0.0636	2.05	0.0652	0.0646	0.88
1.111	0.0601	0.0619	−2.87	0.0601	0.0595	0.95
1.173	0.0584	0.0593	−1.51	0.0583	0.0573	1.7
1.333	0.0545	0.0541	0.85	0.0544	0.0531	2.52
1.408	0.0530	0.0537	−1.25	0.0528	0.0535	−1.18

). ∆% for the M1 sample fell between 0.21 and 2.85%, while ∆% for the M2 sample was between 0.88 and 2.52% and ranged between 0.75 and 2.21% for the M3 sample. The M4 sample had the lowest and highest values of ∆%, of 0.14 and 2.5%, respectively. Overall, the ∆% of the four samples is small (less than 3%), further confirming the accuracy of the practical results as well as the compatibility of the experimental and theoretical results.

The linear attenuation coefficients (LAC) of the four mortars were generally negatively correlated with energy (Figure 5) except when the energy level increased from 0.060 to 0.081 MeV. This can be explained by the W k-absorption edge which appears at an energy of about 70 keV. The LAC values at 0.06 MeV for the M1–M4 mortars are 1.68, 2.47, 2.87, and 3.77 cm⁻¹, respectively. The decrease in the LAC values as energy increases above 0.081 MeV is a clear indication that the mortars have excellent efficiency in attenuating low energy photons, but that their ability to attenuate photons decreases as photon energy increases. This encourages the use of these mortars in applications where low-energy radiation is used. In addition, the values of LAC change with the change in the chemical composition of the mortars (Figure 5), as M4 has the highest LAC and M1 the lowest. Since the sand and cement levels are similar in all four mortars (Table 2) these differences are attributed to the WO₃, barite, and ball clay content. The M4 mortar, which has the highest LAC value, contains 20% WO₃ and 8% barite. The higher LAC of M4 compared to M3, and of M2 compared to M1, demonstrates the strong contribution of WO₃ to the LAC values, as well as its contribution to improving the mortar’s ability to attenuate photons. The second highest LAC was obtained by the M2 mortar demonstrating that the barite contributed to the LAC value more than the ball clay. The barium in barite has a relatively large atomic number (137, compared to 184 for tungsten and 207 for lead) and so improves the radiation shielding properties of mortar. The combination of WO₃ and barite in M4 combines radiation shielding with the best physical performance of the mortar.

The half value layer (HVL) was calculated from the LAC values and is an important physical parameter when studying the radiation attenuation properties of materials since it indicates the thickness of the mortar sample required to halve the number of photons emitted from the source. The lower the HVL, the more suitable the material is for practical shielding applications. The relationship between the HVL and photon energy is direct, so only a thin layer of mortar is required to absorb or scatter half the low-energy photons reaching the samples. As expected, as the energy of the photons increased, their ability to penetrate mortars increased, and in this case, the value of HVL also increased (Figure 6). Thus, thicker mortar layers must be used to attenuate high-energy than low energy radiation. For example, at a photon energy of 0.06 MeV, the HVL values of the M1–M4 mortars were 0.412, 0.280, 0.242, and 0.184 cm, respectively. However, at the higher energy level of 0.245 MeV the HNVL M1–M4 values rose to 1.816, 1.442, 1.665, and 1.329 cm, while at the highest energy of 1.408 MeV 5.516, 5.202, 5.358, and 5.041 cm HNVL distances were required, respectively (Figure 6). The HVL at a given energy clearly depended on the amount of WO₃, ball clay, and barite in the mortar and the combination of 20% WO₃ and 8% barite (M4) provided the best shield at a standard thickness and, to achieve identical attenuation, the thinnest layer.

The mean free path (MFP) determines the distance over which a photon travels between the two successive interactions within the irradiated material. Small MFP values are preferred in shielding applications. The MFP of all the four mortars increased with the increasing energy level (Figure 7), and the MFP values for the four samples ranged from highest to lowest at all energy levels from M1–M4, respectively. The probability of photons interacting with mortar atoms decreases with increasing energy levels, which increases their ability to penetrate the mortar. Thus, the mortar thickness required ranges from below 1 cm, at the lowest energy, to 7–8 cm at the highest energy. The better performance of M4 was attributed to its density of 2.6 g/cm³, the highest of all four mortars (Section 4.1). It is known that a dense material has a greater chance of photons interacting with its atoms than a less dense substance, and thus can absorb more photons.

Radiation protection efficiency (RPE) is a direct measurement of the relationship between the number of photons that are emitted from a source and the number of photons that reach a detector on the other side of the mortar. The more photons arriving at the detector, the smaller the RPE value and the weaker the attenuating ability of the mortar. On the other hand, the fewer photons arriving at the detector, the higher the RPE value and the stronger the shielding properties. RPE values of the prepared mortar samples decreased with increasing energy levels (Figure 8), consistent with the LAC results.

In addition, consistent with the HLV and LAC data, the RPE values were the highest for the M4 mortar and lowest for the M1 mortar (Figure 6). At the higher energy values, the M2 sample had the next highest RPE value, followed by M3 and then M1, corroborating the importance of WO₃ and barite as a combined shielding material for applications of photons with energy ranges from 0.06 to 1.408 MeV.

Finally, we compared the tenth value layer (TVL) (the amount of material needed to absorb 90% of the radiation) of the four WO₃-containing mortars in with other mortars reported in the literature (Figure 9). These mortars are WL13 (slag mortar having a fine lead slag aggregate), WA13 (slag mortar having fine aggregates of air-cooled slag), WS13 (slag mortar having fine aggregates of sand), and PC13 (normal cement mortar) [16]. At 0.662 MeV, the TVL values for the present mortars are 12.258, 11.299, 11.825, and 10.877 cm, all smaller than those reported for WL13, WA13, WS13, and PC13.

5. Conclusions

New types of mortar were developed, lighter than conventional lead and concrete shields, and were tested for their ability to attenuate radiation. The two methods used in this investigation demonstrate that the best shielding was produced by a mortar containing 50% sand, 25% cement, 8% barite, and 25% WO₃. This mortar provided the best shield at a standard thickness and, to achieve identical attenuation, the thinnest layer. Thus, this shield would be lighter than shields that are currently used because it can be made thinner. The strength of this mortar (compressive strength 53 MPa and tensile strength 4.2 MPa) indicates that it could be used as an effective shield of radiation emitting sources during transportation and long-term storage.