TY - JOUR
AU1 - Madruga, M, J
AU2 - Miró,, C
AU3 - Reis,, M
AU4 - Silva,, L
AB - Abstract Building materials from Iberian Peninsula (Portugal and Spain) were collected and analysed for 226Ra, 232Th and 40K using HPGe gamma-ray spectrometers. The results show that the highest mean value of 226Ra and 232Th activities are 2168 and 390 Bq kg−1, respectively, measured in zircon. For 40K, this value is 1290 Bq kg−1, measured in granite. The mean concentrations of the three radionuclides in the different building materials, excluding the zircon and the industrial by-products (ashes, gypsum and phosphogypsum), are 62, 31 and 519 Bq kg−1 for 226Ra, 232Th and 40K, respectively. The radiological health hazard parameters: radium equivalent activity (Raeq), activity concentration index (I) absorbed and effective dose rates, associated with these radionuclides, were evaluated. These values are within the EU recommended limits in building materials, except for same samples of aggregates, granites, ceramics, phosphogypsum and zircon. This study will contribute for the worldwide data pooling on the radioactivity of the building materials. INTRODUCTION The radionuclides of the natural radioactive series Uranium-238 (238U) and Thorium-232 (232Th) and the primordial radioactive isotope Potassium-40 (40K) are present in the Earth’s crust in the earth’s environment(1–4). Gamma-rays are emitted by the members of the decay series of 238U and 232Th and directly by 40K. Therefore, the materials taken from the Earth’s crust and used in building materials are sources of external and internal exposure in dwellings, workplaces and industrial buildings. The external radiation exposure is caused by the gamma radiation since the internal radiation exposure, mainly affecting the respiratory tract, is due to the short-lived decay products of radon, which are exhaled from building materials into indoor air. The radiation exposure of the population may be increased appreciably by the use of building materials containing above-normal levels of natural radionuclides(5). From a radiological point of view, building material means any construction product for incorporation in a permanent manner in a building or parts thereof and the performance of which has an effect on the performance of the building with regard to exposure of its occupants to ionising radiation as defined at Directive 2013/59/Euratom(6). Since building materials are one of the sources which cause direct radiation exposure, there are many studies which report the natural radionuclide content of building materials over the world mainly since 1970(7–15). Raw material and processed building products can vary greatly in radionuclide content, reflecting their origin and geological conditions at the site of their production(16). The content of 226Ra (238U daughter), 232Th and 40K in these materials is of major interest with regard to the radioactivity indoors. Some building materials such as, brick, cement, gypsum and granite stone may produce significant external dose rates in the range of nGy h−1(17). In addition to the materials used in the building construction, materials used in rooms’ "decoration have also to be considered as possible sources for radioactivity indoors, for example ceramics tiles used for floors and walls due to the inclusion of zircon sand in the tile glaze. Zircon sand can act as an opacifier in glasses and enamels and as an additive in the glazing on ceramic tiles(18). The reported values of indoor absorbed dose rate around the world presents a wide variation, with national averages ranging from 20 to 200 nGy h−1. The highest values (95–115 nGy h−1) in Europe are reported for Hungary, Portugal, Italy, Spain and Sweden, reflecting the wide use of stone and masonry materials in the construction of buildings(19). In the last years, the building industry requires large quantities of low-cost raw materials, and there is increasing interest in industrial and extractive wastes as substitutes for natural products. This practice may also cause radiological concern. One of those materials is the fly ash, a by-product from the coal-fired thermal power plants that is used since 1980 as a substitute of cement in concrete and it may contain elevated radioactivity levels(20). The phosphogypsum, another by-product resulting from phosphate production from phosphate ore or fluoroapatite, can be used as a plasterboard and for the manufacture of bricks and blocks(21). The Eucalyptus forests can be considered as the main source of raw material associated with wood for the building material industry of Iberian Peninsula. Radionuclides can be transferred into the pulp mills, where they are concentrated by industrial processes, becoming the cause of the radiation doses(22–24). The European Directive(6) presents in Annex XIII an indicative list of types of building materials (natural materials and materials incorporating residues from industries processing naturally occurring radioactive material) to be considered with regard to their emitted gamma radiation. The list of industrial sectors involving naturally occurring radioactive material that should be taken into account from a radiation protection point of view is presented in Annex VI. The aim of this work is to determine the activity concentrations of 226Ra, 232Th and 40K as well as the radiological health hazards associated to building materials from Iberian Peninsula (Portugal and Spain) in order to establish a baseline data for radiation exposure due to the natural radioactivity. This study will help to identify the concern, from the radiation protection point of view, for people living in dwellings constructed with these materials. EXPERIMENTAL Sample collection and preparation Samples of building materials, weighing from 1.0 to 5.0 kg, were collected from quarries and from suppliers and/or factories suppliers. The collected materials can be included in two categories: natural materials (NM) and materials incorporating residues from industries processing naturally occurring radioactive material (PM) (Directive 2013/59/Euratom, Annex XIII)(6) and are distributed as following: Natural materials (NM) - Concrete used in bulk amounts. Concretes studied were of several types: the conventional concrete (CC), the natural aggregate was substituted by 100% in the concrete made with electrical furnace slags (EFC) and in the concrete made with blast furnace slags (BFC), the self-compacting concrete (SCC) and two mortars (MOR 5) and (MOR 7.5). - Cements used in bulk amounts and superficial application. Cement of Portland Type I with <3% fly ash and Type II with fly ash of 20%. - Natural stones as marble, granite and slate, used both as bulk and superficial products. The Spanish samples of marble, granite and slate came from quarries of Extremadura region (Spain), while samples of Portugal of marble and slate came from quarries of SE region (zone of Alentejo) and granites of Centre region of Portugal (zone of Beiras). - Ceramic tiles as refractory and ceramic products to cover floors and walls, mainly. - Raw materials are of very different types and composition. Therefore, wood had been collected from Eucalyptusand Castahea Sativa trees from forest region located in Extremadura (Spain). Aggregates have also been studied (as sand, feldspars, gravel, clay) and zircon. Materials incorporating residues from industries processing naturally occurring radioactive material (PM) - Industrial by-products, typically fly, bottom ashes, gypsum and phosphogypsum resulting from the phosphates industry in Portugal. Building materials were crushed and were dried in an oven, at 105°C for 48 h. After that, they are grounded, sieved and the fraction <2 mm particle size collected for analyses. The wood dry samples were incinerated at 550°C. In Spain, the dried samples and the wood ashes were put in cylindrical plastic boxes of 190 cm3 or in 1000 cm3 Marinelli beakers. In Portugal, plastic containers with 160 ml of capacity were used to pack the samples. In both cases, the containers were hermetically sealed for at least 28 days before proceeding of the measurement in order to allow the establishment of secular equilibrium of 226Ra and 232Th with their decay products. Gamma spectrometry In Spain, the measurements were carried out with a Ge (HP) semiconductor detector, 45% relative efficiency at 1.33 MeV, coupled to a 4096-channel analyser. The detector efficiency was calibrated using certified mixed gamma standards QCY-48, supplied by Amersham, for the energy range 60–1900 keV. Spectra were recorded during 48 h. The reliability of the spectrometer was checked by measuring the activity of an IAEA reference material, IAEA-6-SOIL. The 232Th activity was determined by means of the γ-emissions of 228Ac (911 keV) and 208Tl (583.01 keV) and that of 226Ra by means of those from 214Bi (609.3 and 1764.5 keV) and 214Pb (351.9 keV) assuming that both radioactive series are left in secular equilibrium. Finally, the 40K by using the 1460.7 keV γ-emission was determined. The activity concentration (A) for 226Ra, 232Th and 40K was calculated using the relation A=Nt×P×ε (1) where N is the net counts, t is the data collection time, P is the emission probability and ε is the efficiency of the detector for the corresponding peak. The error was calculated from the combined uncertainty in the efficiency of the detector and the uncertainty in the net count rate. In the case of using several gamma-ray peaks for the calculations of the activity (i.e. 226Ra and 232Th), uncertainty in the yield is also included. In Portugal, a 50% relative efficiency broad energy HPGe detector (Canberra BEGe model BE5030), with an active volume of 150 cm3 and a carbon window was used for the gamma spectrometry measurements. A lead shield with copper and tin lining shields the detector from the environmental radioactive background. Standard nuclear electronics was used and the software Genie 2000 (version 3.0) was employed for the data acquisition and spectral analysis. The detection efficiency was determined using NIST-traceable multi-gamma radioactive standards (Eckert & Ziegler Isotope Products) with an energy range from 46.5 keV to 1836 keV and customised in a water-equivalent epoxy resin matrix (density of 1.15 g cm−3) to exactly reproduce the geometries of the samples. GESPECOR software (version 4.2) was used to correct for matrix (self-attenuation) and coincidence summing effects, as well as to calculate the efficiency transfer factors from the calibration geometry to the measurement geometry (whenever needed). The stability of the system (activity, FWHM, centroid) was checked at least once a week with a 152Eu certified point source. The acquisition time was set to 15 h and the photopeaks used for the activity determination were: 295.2 keV (214Pb), 351.9 keV (214Pb) and 609.3 keV (214Bi) for 226Ra; 238.6 keV (212Pb), 583.2 keV (208Tl) and 911.2 keV (228Ac) for 228Ra and 1460.8 keV for 40K. Figure 1 presents as an example a gamma-ray spectrum for a zircon sample. The overall quality control of the technique is guaranteed by the accreditation of the laboratory according to the ISO/IEC 17 025:2005 standards and through the participation in intercomparison exercises organised by international organisations(25, 26). Figure 1. Open in new tabDownload slide Gamma-ray spectrum of a zircon sample. Figure 1. Open in new tabDownload slide Gamma-ray spectrum of a zircon sample. RESULTS AND DISCUSSION Activity concentrations Mean and standard deviation and the range (minimum and maximum) of the activity concentration values (Bq kg−1) of 226Ra, 232Th and 40 K measured in the building materials are shown in Table 1. The activity concentrations for each material and for each radionuclide show a wide range of values ranging from one to two orders of magnitude. For example, values ranging from 47 to 258 Bq kg−1 and from 49 to 4130 Bq kg−1 for 226Ra activity concentration were measured in granite and zircon, respectively. As can be seen, the highest mean values for the activity concentration of 226Ra and 232Th are 2168 and 390 Bq kg−1, respectively, measured in zircon. For 40K, the highest mean value is 1290 Bq kg−1, measured in granite. The lowest mean values for the 226Ra activity concentration is 3.6 Bq kg−1 in gypsum. The 232Th was not detected in gypsum and phosphogypsum and the 40K in zircon samples. The average concentration of the three radionuclides in the different building materials excluding the zircon and the industrial by-products are 62, 31 and 519 Bq kg−1 for 226Ra, 232Th and 40K, respectively. These values are of the same order of magnitude of the world average (50 Bq kg−1 for 226Ra and 232Th and 500 Bq kg−1 for 40K)(19). Table 1. Activity concentration values (Bq kg−1) of 226Ra, 232Th and 40K in building materials. The numbers into parentheses correspond to the number of samples used for the mean. Building materials No of samples Activity concentration (Bq kg−1) 226Ra 232Th 40K Mean SD Range Mean SD Range Mean SD Range NM Concrete 12 28 (12) 29 8.0–87 16 (12) 13 3.9–49 290 (12) 265 23–794 Cement 13 28 (13) 16 14–77 12 (13) 6.0 3.0–27 213 (13) 120 28–447 Marble 2 6.1 (1) 1.7 — 1.9 (1) 1.0 — 34 (2) 4.5 31–37 Slate 2 31 (2) 2.9 29–33 56 (2) 28 36–75 829 (2) 17 817–841 Granite 7 116 (7) 69 47–258 53 (7) 36 16–103 1290 (7) 309 795–1746 Ceramic 27 139 (27) 84 14–362 46 (27) 16 6.0–88 653 (27) 299 43–1140 Wood 5 78 (4) 57 29−160 0.62 (2) 0.02 0.60–0.63 1.9 (3) 1.2 0.7–3.2 Aggregate 19 74 (19) 39 5.0−135 65 (18) 52 2.4–170 840 (19) 410 26–1330 Zircon 5 2168 (5) 1961 49–4130 390 (5) 300 8.8–676 — — — PM Ashes 14 75 (14) 93 8.0–171 70 (14) 61 5.0–169 157 (14) 283 27–338 Gypsum 3 3.6 (3) 2.6 2.0–6.6 — — — 16 (2) 4.1 13–19 Phosphogypsum 3 735 (3) 70 663–802 — — — 93 (1) 43 — Building materials No of samples Activity concentration (Bq kg−1) 226Ra 232Th 40K Mean SD Range Mean SD Range Mean SD Range NM Concrete 12 28 (12) 29 8.0–87 16 (12) 13 3.9–49 290 (12) 265 23–794 Cement 13 28 (13) 16 14–77 12 (13) 6.0 3.0–27 213 (13) 120 28–447 Marble 2 6.1 (1) 1.7 — 1.9 (1) 1.0 — 34 (2) 4.5 31–37 Slate 2 31 (2) 2.9 29–33 56 (2) 28 36–75 829 (2) 17 817–841 Granite 7 116 (7) 69 47–258 53 (7) 36 16–103 1290 (7) 309 795–1746 Ceramic 27 139 (27) 84 14–362 46 (27) 16 6.0–88 653 (27) 299 43–1140 Wood 5 78 (4) 57 29−160 0.62 (2) 0.02 0.60–0.63 1.9 (3) 1.2 0.7–3.2 Aggregate 19 74 (19) 39 5.0−135 65 (18) 52 2.4–170 840 (19) 410 26–1330 Zircon 5 2168 (5) 1961 49–4130 390 (5) 300 8.8–676 — — — PM Ashes 14 75 (14) 93 8.0–171 70 (14) 61 5.0–169 157 (14) 283 27–338 Gypsum 3 3.6 (3) 2.6 2.0–6.6 — — — 16 (2) 4.1 13–19 Phosphogypsum 3 735 (3) 70 663–802 — — — 93 (1) 43 — Open in new tab Table 1. Activity concentration values (Bq kg−1) of 226Ra, 232Th and 40K in building materials. The numbers into parentheses correspond to the number of samples used for the mean. Building materials No of samples Activity concentration (Bq kg−1) 226Ra 232Th 40K Mean SD Range Mean SD Range Mean SD Range NM Concrete 12 28 (12) 29 8.0–87 16 (12) 13 3.9–49 290 (12) 265 23–794 Cement 13 28 (13) 16 14–77 12 (13) 6.0 3.0–27 213 (13) 120 28–447 Marble 2 6.1 (1) 1.7 — 1.9 (1) 1.0 — 34 (2) 4.5 31–37 Slate 2 31 (2) 2.9 29–33 56 (2) 28 36–75 829 (2) 17 817–841 Granite 7 116 (7) 69 47–258 53 (7) 36 16–103 1290 (7) 309 795–1746 Ceramic 27 139 (27) 84 14–362 46 (27) 16 6.0–88 653 (27) 299 43–1140 Wood 5 78 (4) 57 29−160 0.62 (2) 0.02 0.60–0.63 1.9 (3) 1.2 0.7–3.2 Aggregate 19 74 (19) 39 5.0−135 65 (18) 52 2.4–170 840 (19) 410 26–1330 Zircon 5 2168 (5) 1961 49–4130 390 (5) 300 8.8–676 — — — PM Ashes 14 75 (14) 93 8.0–171 70 (14) 61 5.0–169 157 (14) 283 27–338 Gypsum 3 3.6 (3) 2.6 2.0–6.6 — — — 16 (2) 4.1 13–19 Phosphogypsum 3 735 (3) 70 663–802 — — — 93 (1) 43 — Building materials No of samples Activity concentration (Bq kg−1) 226Ra 232Th 40K Mean SD Range Mean SD Range Mean SD Range NM Concrete 12 28 (12) 29 8.0–87 16 (12) 13 3.9–49 290 (12) 265 23–794 Cement 13 28 (13) 16 14–77 12 (13) 6.0 3.0–27 213 (13) 120 28–447 Marble 2 6.1 (1) 1.7 — 1.9 (1) 1.0 — 34 (2) 4.5 31–37 Slate 2 31 (2) 2.9 29–33 56 (2) 28 36–75 829 (2) 17 817–841 Granite 7 116 (7) 69 47–258 53 (7) 36 16–103 1290 (7) 309 795–1746 Ceramic 27 139 (27) 84 14–362 46 (27) 16 6.0–88 653 (27) 299 43–1140 Wood 5 78 (4) 57 29−160 0.62 (2) 0.02 0.60–0.63 1.9 (3) 1.2 0.7–3.2 Aggregate 19 74 (19) 39 5.0−135 65 (18) 52 2.4–170 840 (19) 410 26–1330 Zircon 5 2168 (5) 1961 49–4130 390 (5) 300 8.8–676 — — — PM Ashes 14 75 (14) 93 8.0–171 70 (14) 61 5.0–169 157 (14) 283 27–338 Gypsum 3 3.6 (3) 2.6 2.0–6.6 — — — 16 (2) 4.1 13–19 Phosphogypsum 3 735 (3) 70 663–802 — — — 93 (1) 43 — Open in new tab The mean activity concentrations (Bq kg−1) of 226Ra, 232Th and 40K in the building materials analysed are presented in Figure 2. It can be observed that the natural materials concrete and cement presented similar values for the three radionuclides. For the natural stones, it was verified an increase in the activity concentrations from marble to slate and granite for all the radionuclides. The ceramic materials show high values for the natural radionuclides, 226Ra and 232Th, due to the use of zircon and mineral gazing in its manufacture(18). Regarding the wood the high value obtained for 226Ra is related to the high mobility of this radionuclide from the soil to the plant(24, 27). Concerning the industrial by-products, the highest 226Ra activity concentration was obtained for phosphogypsum. The phosphogypsum usually presents high-activity concentrations in 226Ra, since due to the radium solubility large amounts of this radionuclide are transferred to this material, during the phosphoric acid production(28). Values, one order of magnitude higher, for the activity concentrations in 226Ra, were reported for ashes in Hungary(29). Figure 2. Open in new tabDownload slide Mean activity concentrations (Bq kg−1) of 226Ra, 232Th and 40K in the studied building materials. Figure 2. Open in new tabDownload slide Mean activity concentrations (Bq kg−1) of 226Ra, 232Th and 40K in the studied building materials. Radium equivalent activity The non-uniformity distribution of natural radionuclides in the building materials lead to the definition of an index, the radium equivalent activity (Raeq) used to represent the specific activity of 226Ra, 232Th and 40K by a single quantity that takes into account the radiological health hazards of these radionuclides. The Raeq is a weighted sum of the activity concentrations of the above three radionuclides based on the estimation that 370 Bq kg−1 of 226Ra, 259 Bq kg−1 of 232Th or 4810 Bq kg−1 of 40K produce the same gamma dose due to both, internal dose due to radon and external gamma dose. It is defined as:(30) Raeq=ARa+1.43ATh+0.077AK (2) where ARa, ATh and AK (Bq kg−1) are the activity concentrations of 226Ra, 232Th and 40K, respectively. For radioactive safe use of the building materials, the maximum value of Raeq must be lower than 370 Bq kg−1 to keep the external dose below 1.5 mGy per year. The most conservative Raeq values of all samples (NM and PM) under study were calculated taking into account the maximum activity concentration values for each radionuclide (226Ra, 232Th and 40K). The Raeq values in increasing order of magnitude are presented in Figure 3. It can be noticed that in the natural materials, the aggregates, granites, ceramics and zircon present radium equivalent activity values higher than the recommended value (370 Bq kg−1). However, it must be emphasised that these maximum values correspond only to a small percentage of the materials analysed. The zircon can be considered within the acceptable limits since it is used in small amounts (about 5–10% wt) as an additive for vitrification in ceramic tiles industry. Regarding the natural stones, the lowest value was obtained for marble (11.6 Bq kg−1), followed by slate being the highest values for granite (540 Bq kg−1). Values of the same order of magnitude were determined for cement (150 Bq kg−1), concrete (218 Bq kg−1) and wood (161 Bq kg−1). The aggregates mainly sand, gravel, clay and the ceramic materials present higher values (which exceed the Raeq value), in the last case mainly due to the presence of same amount of zircon, as already referred. Figure 3. Open in new tabDownload slide Maximum radium equivalent activity (Bq kg−1) values for the studied building materials. The horizontal line corresponds to the maximum value of Raeq (370 Bq kg−1). Figure 3. Open in new tabDownload slide Maximum radium equivalent activity (Bq kg−1) values for the studied building materials. The horizontal line corresponds to the maximum value of Raeq (370 Bq kg−1). Regarding the materials incorporating residues from industries processing naturally occurring radioactive material, the values obtained for ashes (439 Bq kg−1) and for phosphogypsum (809 Bq kg−1) are higher than the recommended value (370 Bq kg−1). It can be emphasized that these raw materials can be used in small amounts in cement and concrete production(31). Nevertheless, it is important to note that the recommended value was calculated for materials used in bulk amounts, and it is not appropriate for decorative building materials. Therefore, the precautions to be taken with the materials from the radiological point of view depend on the proportions in which they are in the final material and varying amounts of material used in a particular case. Specifically, the use of zircon is very small since it is a part of some materials but in very low proportions and its radiological impact will be low despite their high values of Raeq. From the results, it is evident that there are considerable variations in the Raeq of the different materials. Large variation in Raeq activities were reported in many studies on natural radioactivity in building materials. In Australian building materials values ranging between 15 and 883 Bq kg−1 were reported(32). Other authors(33) reported for Algerian building materials values ranging from 28 to 190 Bq kg−1. For Egyptians materials the lowest values (about 100 Bq kg−1) were found in mud and clay bricks and the highest ones (about 400 Bq kg−1) in granites and marbles(8). In general, this fact is important from the point of view of selecting suitable materials for use in building and construction especially concerning those that have large variations in their activities. Large variation in radium equivalent activities may suggest that it is advisable to monitor the radioactivity levels of materials from a new source before adopting it for use as a building material(34). Activity concentration index Similar approaches were proposed for the assessment of the gamma radiation dose, in excess of typical outdoor exposure, that an individual may receive from building materials used in construction. Based on these approaches the European Commission set on equation 3 which gives the activity concentration index (I) based on the 226Ra, 232Th and 40K activity concentrations(6, 35). The activity concentration index (I) is defined as: I=C226Ra300+C232Th200+C40K3000 (3) where C226Ra, C232Th and C40K are the activity concentrations in Bq kg−1 of the corresponding radionuclides in the building material. In 1999, the European Commission(35) established criteria to examine the relationship between the I values and the gamma radiation dose for an individual received from building materials and recommends that regulatory control should be considered for materials that give rise to doses in the range 0.3–1 mSv y−1. Building materials given doses below 0.3 mSv y−1 should be exempt from all the restrictions and those above 1 mSv y−1 must be regulated. Currently, the European Directive 2013/59/Euratom (Annex VII)(6) recommends the use of the activity concentration index (I) as a screening tool for identifying materials that may be exempted or subjected to restrictions. For this purpose, I may be used for the classification of the materials into four classes, leading to two categories of building materials, A and B, (Table 2). However, any decision on restricting the use of a material should be based on a separate dose assessment, where other factors such as density, thickness of the material as well as the type of building and the intended use of the material (bulk or superficial) should be considered(6). Table 2. Relationship between activity concentrations index (I) and received dose from the material used in a building construction(6). Use Category (corresponding default dose) A (≤1 mSv y−1) B (>1 mSv y−1) Materials used in bulk amounts A1 B1 I ≤ 1 I > 1 Superficial and other materials with restricted use A2 B2 I ≤ 6 I > 6 Use Category (corresponding default dose) A (≤1 mSv y−1) B (>1 mSv y−1) Materials used in bulk amounts A1 B1 I ≤ 1 I > 1 Superficial and other materials with restricted use A2 B2 I ≤ 6 I > 6 Open in new tab Table 2. Relationship between activity concentrations index (I) and received dose from the material used in a building construction(6). Use Category (corresponding default dose) A (≤1 mSv y−1) B (>1 mSv y−1) Materials used in bulk amounts A1 B1 I ≤ 1 I > 1 Superficial and other materials with restricted use A2 B2 I ≤ 6 I > 6 Use Category (corresponding default dose) A (≤1 mSv y−1) B (>1 mSv y−1) Materials used in bulk amounts A1 B1 I ≤ 1 I > 1 Superficial and other materials with restricted use A2 B2 I ≤ 6 I > 6 Open in new tab The activity concentration indexes calculated for the building materials analysed are presented in Table 3. The index (I) has been found to be varying from 0.03 to 17.1 for gypsum and zircon, respectively. Table 3. Maximum values of activity concentration index, absorbed dose rate and effective dose (indoor) for the building materials. Building materials Activity concentration Index (I) Absorbed dose rate nGy h−1 Effective dose (indoor) mSv y−1 Gypsum 0.03 3.8 0.02 Marble 0.04 5.5 0.03 Cement 0.54 70.5 0.35 Wood 0.54 74.4 0.37 Slate 0.77 95.6 0.47 Concrete 0.80 102.9 0.50 Ashes 1.53 195.2 0.96 Aggregate 1.74 220.5 1.08 Granite 1.96 254.2 1.25 Ceramic 2.03 267.9 1.31 Phosphogypsum 2.70 374.4 1.84 Zircon 17.1 2316.4 11.4 Building materials Activity concentration Index (I) Absorbed dose rate nGy h−1 Effective dose (indoor) mSv y−1 Gypsum 0.03 3.8 0.02 Marble 0.04 5.5 0.03 Cement 0.54 70.5 0.35 Wood 0.54 74.4 0.37 Slate 0.77 95.6 0.47 Concrete 0.80 102.9 0.50 Ashes 1.53 195.2 0.96 Aggregate 1.74 220.5 1.08 Granite 1.96 254.2 1.25 Ceramic 2.03 267.9 1.31 Phosphogypsum 2.70 374.4 1.84 Zircon 17.1 2316.4 11.4 Open in new tab Table 3. Maximum values of activity concentration index, absorbed dose rate and effective dose (indoor) for the building materials. Building materials Activity concentration Index (I) Absorbed dose rate nGy h−1 Effective dose (indoor) mSv y−1 Gypsum 0.03 3.8 0.02 Marble 0.04 5.5 0.03 Cement 0.54 70.5 0.35 Wood 0.54 74.4 0.37 Slate 0.77 95.6 0.47 Concrete 0.80 102.9 0.50 Ashes 1.53 195.2 0.96 Aggregate 1.74 220.5 1.08 Granite 1.96 254.2 1.25 Ceramic 2.03 267.9 1.31 Phosphogypsum 2.70 374.4 1.84 Zircon 17.1 2316.4 11.4 Building materials Activity concentration Index (I) Absorbed dose rate nGy h−1 Effective dose (indoor) mSv y−1 Gypsum 0.03 3.8 0.02 Marble 0.04 5.5 0.03 Cement 0.54 70.5 0.35 Wood 0.54 74.4 0.37 Slate 0.77 95.6 0.47 Concrete 0.80 102.9 0.50 Ashes 1.53 195.2 0.96 Aggregate 1.74 220.5 1.08 Granite 1.96 254.2 1.25 Ceramic 2.03 267.9 1.31 Phosphogypsum 2.70 374.4 1.84 Zircon 17.1 2316.4 11.4 Open in new tab Of all samples analysed, the materials that can clearly be classified as materials used in bulk amounts are concrete, gypsum and cement. All these samples present values of I ≤ 1 (ranging from 0.03 to 0.80), therefore these materials can causes an increase in the annual effective dose received by an individual below to 1 mSv. Materials such as marble, slate, granite, wood and ceramic can be considered as a surface material. The provided increase doses should be below 1 mSv y−1 since for all I ≤6 (Table 3). Aggregate, ashes, phosphogypsum and zircon can be considered materials of restrictive use, as they are a part, in small proportions, of other building materials. Aggregate, ashes and phosphogypsum having values of I ≤6 also provide increases lower than 1 mSv y−1. Activity concentration indexes ranging from 0.2 to 2.6 were reported for phosphogypsum samples from Brazil(36). It can be emphasized that the majority of the materials analysed (≈99%) could be used as bulk or superficial materials without restrictions since the occupants of the buildings constructed with these materials would receive an annual dose of <1 mSv. The only exception is for zircon which I value exceeding 6 (I = 17.1) could provide increased doses higher than 1 mSv y−1. So this material should be used with caution in terms of radiation protection. Generally, the index I in the building materials studied is of the same order of magnitude of the European average(13). The contribution of each radionuclide (226Ra, 232Th and 40 K) to the maximum total activity concentration index for all the building materials was analysed and the results are presented in Figure 4. It can be noticed that the major contribution is due to 226Ra, being almost 100% for wood and phosphogypsum, followed by the zircon (80%), gypsum (78%), ceramic (60%) and marble (49%). For the natural materials, cement, concrete, aggregates and granite the contribution of the three radionuclides is approximately equal, ranging from 26 to 47% for 226Ra, from 25 to 49% for 232Th and from 25 to 33% for 40K. Figure 4. Open in new tabDownload slide Relative contribution (%) of 226Ra, 232Th and 40K activity concentrations for the maximum activity concentration index (I) in the studied building materials. Figure 4. Open in new tabDownload slide Relative contribution (%) of 226Ra, 232Th and 40K activity concentrations for the maximum activity concentration index (I) in the studied building materials. Effective dose rate Conversion factors to transform activity concentrations of ARa, ATh and AK in absorbed gamma dose rate at 1 m above the ground (in nGy h−1 by Bq kg−1) are calculated by Monte Carlo method and the values are as following(19): D(nGyh−1)=0.462ARa+0.604ATh+0.0417AK (4) The absorbed dose rate (D) is converted into annual effective dose indoors (E) using equation (5). The conversion value Q is 0.7 Sv Gy−1 for environmental exposure to gamma-rays of moderate energy; T corresponds to 1 year, i.e. 8760 h; and, 0.8 is the indoor occupancy factor implying that 80% of time is spent indoors, as proposed in UNSCEAR (1993)(37). E(mSvy−1)=T×Q×D×0.8×10−6 (5) The maximum estimated values of the absorbed dose rate and corresponding annual effective dose for all samples under study are presented in Table 3. The average absorbed dose rate from all samples (151 nGy h−1), excluding the zircon, is higher than the world average indoor absorbed dose rate, 84 nGy h−1 (population weighted), as reported in UNSCEAR (Annex B)(19). Regarding the maximum annual effective dose some of the analysed materials exceed the dose criterium of 1 mSv for public exposure(6). Annual effective dose values of 1.08, 1.25, 1.31, 1.84 and 11.4 mSv y−1 were found for aggregates, granites, ceramics, phosphogypsum and zircon, respectively (Table 3). Other authors(38) reported for zircon values of 3000 nGy h−1 and 3.7 mSv y−1 for absorbed dose rate and effective dose, respectively. CONCLUSIONS The determination of the natural radionuclides’ activity concentrations in some building materials from Iberian Peninsula was performed by gamma spectrometry. The maximum value of the mean activity concentration for 226Ra and 232Th are 2168 and 390 Bq kg−1, respectively, measured in zircon, and for 40K is 1290 Bq kg−1, measured in granite. The minimum values of the 226Ra are 3.6 Bq kg−1 measured in gypsum. The 232Th was not detected in gypsum and phosphogypsum and the 40K in the zircon samples. The mean activity concentration values of the three radionuclides in the different building materials, excluding the zircon and the industrial by-products are of the same order of magnitude of the world average. The calculated maximum Raeq values for same of the building materials examined (aggregates, granite, ceramic, zircon, ashes and phosphogypsum) are higher than the recommended maximum level (370 Bq kg−1), providing an excess effective dose higher than 1 mSv y−1, the dose criteria recommended by the European Directive(6). From a radiological point of view, it can be concluded that the use of these materials in construction of dwellings, with exception of same of the materials referred before, could be considered safe for inhabitants. The results obtained in this study are very important as they can contribute to the safe management of these materials by stakeholders and to the development of international standards and guidelines since these materials are exported and used all over the world. FUNDING In Portugal, this work was supported by the Portuguese Foundation for Science and Technology (FCT) [UID/Multi/04349/2013 project] and in Spain, by the Junta de Extremadura (Proyecto Regional de Investigación-Investigación Básica [PRI IB16114] and Ayuda a Grupos de investigación [GR18068], both projects partially funded by European Regional Development Fund). 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TI - RADIATION EXPOSURE FROM NATURAL RADIONUCLIDES IN BUILDING MATERIALS
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncy256
DA - 2019-11-30
UR - https://www.deepdyve.com/lp/oxford-university-press/radiation-exposure-from-natural-radionuclides-in-building-materials-ctM40CUckT
SP - 49
VL - 185
IS - 1
DP - DeepDyve
ER -