|Year : 2020 | Volume
| Issue : 1 | Page : 31-35
Estimation of radon exhalation rate from a brick wall during various stages of construction by measuring exhalation rates from various building materials
Manish Kumar1, BK Sahoo2, Rajesh Kumar3, Navjeet Sharma4
1 Department of Physics, Kanya Maha Vidayalaya, Jalandhar, Punjab, India
2 Radiological Physics and Advisory Division, Bhabha Atomic Research Center, Mumbai, Maharashtra, India
3 Department of Physics, DAV College, Amritsar, Punjab, India
4 Department of Physics, DAV College, Jalandhar, Punjab, India
|Date of Submission||09-Dec-2019|
|Date of Decision||27-Mar-2020|
|Date of Acceptance||29-Mar-2020|
|Date of Web Publication||12-May-2020|
Department of Physics, DAV College, Jalandhar - 144 008, Punjab
Source of Support: None, Conflict of Interest: None
Radon concentration in a room can be predicted from accurate knowledge of exhalation rates from six surfaces of the room. Walls of a room occupy the maximum surface area of the room. Hence, the contribution of walls to indoor radon concentration is significant. Since the direct measurement of exhalation rate from the walls is difficult, generally, the typical building materials used for wall construction are analyzed for exhalation rates, and from these measurements, exhalation rates from walls are estimated. In the present work, an attempt has been made to estimate the exhalation rate from a wall during various stages of construction by measuring exhalation rates from various building materials and to study the effect of various layers of coatings on exhalation rates. This analysis can help in selecting right materials for construction to mitigate the indoor radon concentrations.
Keywords: Exhalation, radon, radium
|How to cite this article:|
Kumar M, Sahoo B K, Kumar R, Sharma N. Estimation of radon exhalation rate from a brick wall during various stages of construction by measuring exhalation rates from various building materials. Radiat Prot Environ 2020;43:31-5
|How to cite this URL:|
Kumar M, Sahoo B K, Kumar R, Sharma N. Estimation of radon exhalation rate from a brick wall during various stages of construction by measuring exhalation rates from various building materials. Radiat Prot Environ [serial online] 2020 [cited 2020 Aug 14];43:31-5. Available from: http://www.rpe.org.in/text.asp?2020/43/1/31/284229
| Introduction|| |
Exposure to radon and its pry in indoor environment is recognized as one of the health hazards humans., Radon and their decay products in air can enter the human body through inhalation and. Soil and building materials are the major contributors to indoor radon. In the past few years, many studies on the measurement of exhalation rates and specific radioactive contents of building materials have been published and many researchers have attempted to predict the indoor radon concentration using exhalation rates from the walls and floor of a room, based on different models available in the literature.,,,,,,, In a typical house, bare brick walls are applied different types of coatings such as plastering, putty, and finally paints for better finish. Exhalation rate of radon from a medium depends on the strength of source term, i.e. concentration of226 Ra, emanation factor, and diffusion coefficient of medium. Applying different types of coatings on the wall can significantly alter the radon flux from a wall. Concentration of indoor radon can be reduced by controlling the exhalation from the interior surfaces in a room. For this purpose, knowledge of the contribution of various components of building materials to exhalation rate is required. In the present study, we have mainly focused on variation in exhalation from a wall during different stages of construction, with a view to study the contribution of various materials being used during the construction of dwellings.
| Materials and Methods|| |
During the construction of a dwelling, fired bricks made from the local soil are the most preferred materials for the construction of walls. In a typical living room, the maximum surface area is contributed by the walls of the room. After the floor, walls are the next major contributors to the indoor concentration of radon. For this, an attempt has been made in this study to estimate the contribution of wall to radon concentration per unit area per unit time, i.e. surface exhalation rate. For this reason, fired brick was chosen as the sample in the present study. The size of chosen was 0.10 m × 0.08 m. Closed chamber method was used to measure the exhalation rate of radon. Setup for the measurement of exhalation rate of radon is shown in [Figure 1].
|Figure 1: Experimental setup used for the measurement of exhalation rate of radon|
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A lot of fired bricks, used in construction by locals in the area (Jalandhar district, Punjab), were procured from a local brick kiln. One randomly chosen brick was placed in the exhalation chamber and the chamber was sealed from outside. The concentration of radon in the chamber was allowed to build up and recorded at an interval of 1 h using scintillation-based radon/thoron detector SMART RnDuo. It is based on the principle of detection of alpha particles by scintillations with ZnS: Ag. Radon is sampled into the scintillation cell (~153 cm3) through a “progeny filter ” and “Pin hole plate ” eliminating radon progenies and thoron. The “diffusion-time delay ” given by pinhole plate does not allow the short-lived thoron (220 Rn, half-life: 55.6 s) to pass thorough them acting as “thoron discriminator. ” The alpha scintillations from radon and its decay products formed inside the cell are continuously counted for a user programmable counting interval by the photomultiplier tube and the associated counting electronics. The alpha counts obtained are processed by a microprocessor unit as per the developed algorithm to display the concentration of radon. Every sample has been measured for longer period of time to achieve the complete growth of radon and saturation. The Smart RnDuo monitor has minimum detection limit of 8 Bq/m3 and upper detection limit of 50 MBq/m3. The sensitivity factor of the instrument is 1.2 counts/h/Bqm3. Details about SMART RnDuo can be found elsewhere.,,, Experimentally measured values of radon were fitted to exponential equation (1) and exhalation rates (Jm)were calculated.,,,
where Jm is the radon mass exhalation rate for the sample (mBq/kg1/h1), C0 is the initial radon concentration present in the chamber before starting the experiment t = 0 (Bq/m3), M is the mass of the sample (kg), V is the effective volume (volume of the chamber + internal volume of SMART RnDuo monitor – volume of the sample) (m3), λe is the effective decay constant (radon decay, back diffusion, and leakage) (h-1), t is the measurement time (h), and C (t) is the radon concentration inside the chamber measured at time t. The radon surface exhalation rate, Jb(m/Bqm /h1), from the brick was estimated using surface area of the brick (A) instead of mass (M) in equation (1).
To check the reproducibility of the result, the same experiment was repeated with another brick from the lot following the same procedure.
Exhalation of radon in a single brick is a three-dimensional process where all the six surfaces of the brick are available for radon exhalation. However, in case of a wall, exhalation is a one-dimensional process, in direction perpendicular to the wall plane. Hence, the exhalation rates (Jm) measured from the bricks were used to calculate the exhalation rate from a wall made from bricks using the model developed by Sahoo et al. (2011). Mass exhalation rate (Jm) can be related to exhalation rate from the wall (Jw) through the equation.
Where ρ is density of building material, L is radon diffusion length, and d is half thickness of the surface.
In the next step, the two bricks used in previous step were plastered with a mixture of sand and cement mixed in the ratio 3:1, normally used in the plastering. These plastered bricks were allowed to dry normally in air for 10 days and were analyzed for exhalation rate by the same procedure. Using the same mixture of sand and cement, one brick of approximately same dimensions as of regular size brick was formed and was analyzed for exhalation rate after allowing it to dry for 10 days in air. In the next step, all the bricks were coated with thin layer of wall putty and exhalation rates were measured again. Furthermore, all the building materials, i.e. brick powder, sand, cement, wall putty, and mixture of sand and cement in the ratio 3:1 were analyzed for mass exhalation rates in the powdered form. All these materials in powdered form were also analyzed for radium (226 Ra), thorium (232 Th), and potassium (40 K) using gamma spectrometry. Growth curves for radon in a closed chamber, for two samples S3 and S5, are shown in [Figure 2] and [Figure 3], respectively.
|Figure 2: Growth of radon in closed chamber for cement-plastered fired brick|
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|Figure 3: Growth of radon in closed chamber for cement-plastered fired brick coated with putty|
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The effective mass exhalation rate of radon from the brick plastered with sand and cement was calculated by the equation.
Whereis the predicted radon mass exhalation rate (mBq/kg1/h1), J1 is mass exhalation rate of brick, and J2 is exhalation rate of mixture of sand and cement.
Where, m1 mass of the brick (kg) and m2 mass of the material (kg).
Measurement of radioactive contents in building materials
The building materials used in the study, i.e. brick, cement, sand, and wall putty, were also analyzed for radium, thorium, and potassium contents using a gamma spectrometry technique. The samples in the powder form were filtered through 150 μm size sieve dried in oven at 110°C for 24 h to remove the moisture contents. The samples were packed in sealed air tight containers for 1 month to achieve secular equilibrium between the226 Ra,232 Th, and their progenies.
The measurement setup consisted of scintillation smart probe with NaI (Tl) detector of 63 mm × 63 mm size. The instrument is precalibrated with standard known activity source, and SPTR-ATC (AT-1315) software (Scientific and Production Enterprises ATOMTEX, Minsk Belarus) was used to calculate the activity concentration by taking counting time, mass of the sample, and geometry. The energy calibration was performed using standard point disc sources, containing241 Am (59.6 keV),57 Co (122 keV),139 Ce (165.9 keV),113 Sn (391.7 keV),137 Cs (661.7 keV),54 Mn (834.8 keV),22 Na (1274.5 keV), and88 Y(1836 keV),238 Tn (2614.5 keV) covered an energy range of 59–2600 keV. The spectrometer was calibrated for efficiency using the International Atomic Energy Agency reference materials such as RGU-1 (U-ore), RGTh-1 (Th-ore), and RGK-1 (K2 SO4) packed in a same geometry in the Marinelli beakers. The accumulation time for gamma-ray spectra measurements was 10,800 s. The background activity was measured for the same counting and subtracted from the sample activity.
| Results and Discussion|| |
The measured values of radon mass exhalation rates from the bricks with different coatings are summarized in [Table 1]. The measured values for two set of samples agree closely indicating the reproducibility of results. The results of measurements reveal that the bricks plastered with a mixture of sand and cement reported exhalation rate several times higher as compared to unplastered bricks. Furthermore, exhalation rate for bricks made from the mixture of sand and cement was also found to be several times higher than fired bricks. These observations can be justified on the basis of the fact that activity concentration of226 Ra was reported to be highest for cement samples. Second, since the grain size of sand is more, the bricks made from the mixture of sand and cement have more porosity leading to an increased exhalation rate. This observation is very significant in view of the fact that in modern residential apartments, cemented blocks are being preferred over fired mud bricks. Furthermore as mentioned in Section 2, bricks made from the mixture of sand and cement and plastered bricks were allowed to dry in air for 10 days. Hence, it is possible that some moisture might still be present in samples. The presence of moisture can also be the reason for increased value of exhalation rates observed. The coating of thin layer of putty on the bricks leads to a slight decrease, approximately 5%, in exhalation rate. Since the putty is in form of a very fine powder, its coating acts as a barrier to release of radon, leading to a slight decrease in exhalation rate. The estimated exhalation rates also closely matched with the measured exhalation rates, indicating the additive behavior in case of multiple layers.
|Table 1: Measured mass and surface exhalation rates of radon from building materials samples of regular geometry|
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Wall exhalation rates computed from measured mass exhalation rates are also summarized in [Table 1]. The minimum value of wall exhalation rate has been calculated for walls made from fired bricks and maximum value for walls made from the blocks of cement and sand. The measured values of radon mass exhalation rates from different building materials in the powder form are summarized in [Table 2], while the measured activity concentrations of226 Ra,232 Th, and40 K are summarized in [Table 3].
|Table 2: Measured mass exhalation rates of radon from building materials samples in powder form|
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|Table 3: Activity concentration of226Ra,232Th,40K, and radium equivalent for building materials|
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Analyzing the results of measurements of mass exhalation rates from building materials in powdered form reveal that the minimum mass exhalation rate has been recorded for putty, while the maximum value has been recorded for the mixture of sand and cement. Mass exhalation rate for sand is more than the exhalation rate for wall putty, although the226 Ra concentration was more in wall putty than sand because of larger grain size of sand.
| Conclusions|| |
Exhalation rate of radon for a wall plastered with mixture of sand and cement was much higher than a bare wall made of fired mud bricks. Application of thin layer of wall putty on a surface can reduce the exhalation rate by approximately 5%. Radioactive contents, grain size, and moisture are the prime factors affecting the exhalation rates from different materials. While applying the different coating on the walls, materials with low radioactive contents and smaller grain size should be chosen, as exhalation rates from different layers add up to form total exhalation rate.
The authors are thankful to the Board of Research in Nuclear Science, Department of Atomic Energy, BARC Mumbai, India, for providing the financial support in the form of project 2013/36/54- BRNS to carry out this work.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]