Radiological Effects of Fly Ash as Concrete Additive: A Study in Vietnam

34 Nowadays, fly ash is recycled to make more eco-friendly building materials and 35 reduce landfill area of coal-fired power plant. However, the high amount of natural 36 radionuclides contained in fly ash could potentially pose radiological risks to people 37 living in buildings made from these materials. The results revealed that the 226 Ra, 232 Th 38 and 40 K activities for the commonly used building material were in the range from 10.1 to 39 254.9 Bq kg -1 , from 16.6 to 176.9 Bq kg -1 and from 21.2 to 1240.3 Bq kg -1 and 569.1 Bq 40 kg -1 , respectively: High gamma activity concentration for fly ash is due to the origin of 41 fly ash and coal enrichment process of coal-fired power plant, in contrast, sand and stone 42 samples which contain high radon concentration. Additional fly ash in concrete can 43 increase or decrease the radioactivity of building materials, in which its variation depends on the percentage of the fly ash and matrix composition of the mixture. Even though the 45 average indoor annual effective doses were lower than the upper limit, the total annual 46 effective doses were slightly higher than the recommended dose of 2.4 mSv -1 due to the 47 exposure of natural sources by UNSCEAR. From this study, radiological effects of fly 48 ash samples as concrete additive in Viet Nam could be evaluated for any practical 49 circumstances before they are used. 50

increase or decrease the radioactivity of building materials, in which its variation depends 44 on the percentage of the fly ash and matrix composition of the mixture. Even though the 45 average indoor annual effective doses were lower than the upper limit, the total annual 46 effective doses were slightly higher than the recommended dose of 2.4 mSv -1 due to the exposure of natural sources by UNSCEAR. From this study, radiological effects of fly Introduction 53 All materials in the environment contain natural radionuclides with their various 54 isotopes. Human is exposed by ionizing radiation about 2.4 mSv y -1 from these natural 55 sources, in which, approximately about 1.0 mSv y -1 is due to the exposure of radon 56 (UNSCEAR, 2000). Materials contain mainly natural radionuclides from the primary 57 sources of 238 U series, 232 Th series and 40 K. The activity concentration of radionuclides in 58 raw building materials greatly affect the annual effective dose due to the ionizing 59 radiation. In 238 U series, 222 Rn is considered as the main factor of exposure. Radon atoms 60 located within solid grains are unlikely to release into the atmosphere, owing to their very 61 low diffusion coefficients in solids. However, if they are located in the interstitial space 62 between grains, they may diffuse into the surface. Therefore, the release of radon from a 63 residue repository to the atmosphere takes place by the following series of processes: 64 emanation, transport and exhalation (Ishimori et al., 2013). Due to its short half-life, 65 indoor radon inhalation could potentially damage lung cells and consequently, cause lung 66 cancer. Because of this radiological hazard, radon concentration needs to be monitored 67 from building materials.    The samples were then transported to the laboratory, dried at room temperature, 128 crushed with a particle size less than 0.2 mm. Following that, samples were dried at 129 105 0 C for 8 hours and packed into a cylinder beaker. Samples were sealed within 30 days 130 to reach radioactive equilibrium between 226 Ra radionuclide and its daughters in uranium 131 series. After this period, the samples were measured by a gamma spectrometry system 132 using HPGe detector for 24 hours.
The amount of radon available for transport to the surface was calculated using Eq.

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The annual effective dose due to radon exposure was calculated using Eq.

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Living or working in a room, people will be exposed to gamma, alpha and radon 266 radiation exposure. Therefore, the total annual effective dose ( γ D , E radon ) needs to be 267 calculated. The results indicated the total annual effective dose for CEN room made of 268 the studied raw building material samples was in the range from 0.69 mSv y -1 (for 269 cement) to 2.21 mSv y -1 (for stone) with an average value of 1.44 mSv y -1 .

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The indoor annual effective doses for concrete containing fly ash 275 Fly ash is coal combustion products from coal-fired power plants. It is used often for 276 building purposes. It may be used to be an addition to concrete or raw component in  stone. Because the radioactivities in the fly ash material were higher than those in the 292 cement and lower than those in the sand and stone samples (see Table 1). Similarly, the estimated of all type concrete samples in this study were found to be higher than the 296 recommended limit of 0.3 mSv y -1 for building material but still lower than the dose 297 limit of 1 mSv y -1 (according to EC, 1999).

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The lowest radon concentration of 2.3 Bq m −3 was found for the concrete with 0%  In general, the radioactivity in concrete depends on the composition of the raw 321 building material in concrete products. Some raw building materials used in construction 322 contain high concentrations of 226 Ra, 40 K and 232 Th depending on the nature of the 323 material origin and composition. People might be exposed from gamma, radon and alpha 324 rays by living in a room containing high concentration of radionuclides. Not only the annual effective dose for each radiation exposure from gamma, alpha, radon needs to be 326 estimated, but also the total annual effective dose due to summing impacts from them 327 should also be calculated to evaluate accurately radiological impacts to human.

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In this study, the average activity concentration of radionuclides were found to be 330 highest in fly ash sample. The radioactivity in fly ash were also higher than the fly ash and other raw materials. However, the total effective doses were slightly higher 343 than the recommended dose of 2.4 mSv -1 for natural sources by UNSCEAR, from 2.56 to 344 2.88 mSv -1 . Therefore, the utilization of the studied building materials needs to be 345 cautiously monitored in practical circumstances to ensure radiation safety for residents.