Natural radionuclides in building materials available in Israel

Natural radionuclides in building materials available in Israel

Building and Environment 37 (2002) 531 – 537 www.elsevier.com/locate/buildenv Natural radionuclides in building materials available in Israel K. Kov...

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Building and Environment 37 (2002) 531 – 537

www.elsevier.com/locate/buildenv

Natural radionuclides in building materials available in Israel K. Kovlera; ∗ , G. Haquinb; 1 , V. Manasherova , E. Ne’emanb , N. Lavib a Faculty

of Civil Engineering, National Building Research Institute, Technion - Israel Institute of Technology, Haifa 32000, Israel b Institute for Environmental Research, Sackler Medical School, Tel Aviv University, Tel Aviv 69978, Israel Received 18 August 2000; received in revised form 5 April 2001; accepted 24 April 2001

Abstract Most building materials contain naturally occurring radioactive elements, the most important of which are potassium 40 K and the members of two natural radioactive series, which can be represented by the isotopes of thorium 232 Th and radium 226 Ra. The presence of these radioisotopes in the materials causes external exposure to the people who live in the building. In addition, the disintegration of radium 226 Ra increases the concentration of radon gas 222 Rn and of its daughters in the house. Concentrations of natural radionuclides (226 Ra, 232 Th, 40 K) in the samples of building products, building binders, in lightweight aggregates, normal-weight aggregates and in industrial by-products used in the construction industry in Israel were determined by a gamma-ray spectrometer with a Ge-detector. A methodology was introduced to regulate the use of building materials that causes an increase in indoor radiation exposure. The results indicate that a radium equivalent varies within the range of 158.8–18:8 Bq=kg in the samples of building products, 74.3–17:5 Bq=kg in the samples of building binders, 164.5 –17.7 Bq=kg in the samples of aggregates, and 761.4 –241:6 Bq=kg in the samples c 2002 Elsevier Science Ltd. All rights reserved. of industrial by-products used in construction.  Keywords: Building materials; Indoor gamma radiation; Radioactivity concentration; Radium equivalent; Natural radionuclides; Standardization

1. Introduction Knowledge of the concentration of natural radionuclides in building materials is important in the assessment of population exposures, as most individuals spend 80% of their time indoors and natural radioactivity in building materials is a source of indoor radiation exposure [1,2]. Indoor elevated external dose rates may arise from high activities of radionuclides in building materials. The building industry uses large amounts of waste products from other industries. In recent years, there is a growing tendency to use new construction materials with naturally or technologically enhanced levels of radioactivity (e.g. phosphogypsum, coal >y ash, oil shale ash, some rare minerals, etc.) [3–5]. Therefore, most building materials contain naturally occurring radioactive elements, the most important of which are 40 K and members of two natural radioactive series, which can be represented by the isotopes 232 Th and 226 Ra. The presence of these radioisotopes in materials causes external exposure to the people who live in the building. 226 Ra ∗ Corresponding author. Tel.: +972-4-8293038; fax: +972-4-8324534. E-mail address: [email protected] (K. Kovler). 1 Soreq Nuclear Research Center, Yavne 81800, Israel.

and 232 Th can also increase the concentration of 222 Rn and 220 Rn and of its daughters in the building. 40 K and part of the above-mentioned radionuclides cause external exposure while the inhalation of 222 Rn, 220 Rn and their short lived progeny leads to internal exposure of the respiratory tract to alpha particles [6,7]. Chronic exposure of human beings to low doses of ionizing radiation can cause health damage which may occur 5 –30 years after the exposure [8]. The most critical damage which can result from such an exposure is an increase in the probability of contracting malignant diseases by the person and his oDspring from exposure. The risk increases with the dose, and the probability of the appearance of the damage is greater when the exposure starts at a younger age. It appears that the large scale use of by-products with enhanced levels of radioactivity as a raw material in building products can considerably increase the exposure of the population and therefore constitutes a real potential risk. To minimize the exposure of the population to ionizing radiation, there is a need to control and to limit the content of radioactive materials in constructions. The concentration limits are derived from the radiation dose limits, which are set by national authorities. These limits are calculated from the expected exposure rates in standard constructions. The

c 2002 Elsevier Science Ltd. All rights reserved. 0360-1323/02/$ - see front matter  PII: S 0 3 6 0 - 1 3 2 3 ( 0 1 ) 0 0 0 4 8 - 8

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K. Kovler et al. / Building and Environment 37 (2002) 531–537

exposure rate D (R=h) at a distance of 1 m from a volume source in the shape of a disk with a radius r (m), density  (kg=m3 ) and thickness d (m) that contains a concentration of the radioactive element C (Ci=m3 ), can expressed as  ki (Ei )Cpi D (R=h) = i

 ×

0

d



r 2 + (1 + x)2 exp(− i x) ln (1 + x)2

 d x;

(1)

where ki (R m2 =Ci h) is the speciHc gamma exposure rate factor for gamma rays with energy Ei , pi the gamma ray yield and i the linear attenuation coeIcient in the building material for photons of energy Ei . The summation is over all the photons emitted by the radioactive element. To compare the speciHc radioactivity of materials containing diDerent radionuclides, a common index is often used, which is called radium equivalent activity [9]. The radium equivalent activity is calculated from the following equation: Ra-eq = CRa + 1:43CTh + 0:077CK ;

(2)

where CRa ; CTh , and CK are activity concentrations of 226 Ra, Th, and 40 K, respectively. This radium equivalent definition is based on the assumption that a concentration of 4810 Bq=kg of 40 K or 370 Bq=kg of 226 Ra or 259 Bq=kg of 232 Th in the structural elements of a standard room will give the same exposure to the habitants of the room. Building materials with high concentrations of naturally occurring radioactive materials (NORM) have been used in several countries. In some cases, these are the materials of natural origin (i.e. granite or alum shale concrete), and in other cases they are by-products from diDerent industries (by-product gypsum, waste rock from mining, etc.). Local authorities can limit the use of building materials that cause a signiHcant increase in radiation exposure due to higher levels of indoor radon, and external gamma exposure. According to international recommendations quoted in the Basic Safety Series No. 115 from the IAEA, the use of building materials containing enhanced concentrations of NORM should be controlled and restricted under the application of the radiation safety standards. As a practical consequence, the use of natural origin and industrial by-products in building materials should be treated as a practice, therefore must be justiHed, optimized and dose limited. The Radiation Safety Division of the Israeli Ministry of the Environment after the justiHcation and optimization of this practice proposed to limit the dose to the general public to 0:3 mSv=yr for the practice. Assuming an average current exposure of 0:4 mSv=yr in the existing buildings in Israel, the total public exposure will not exceed 0:3 + 0:4 = 0:7 mSv=yr, including both internal and external exposures. Following this criteria a methodology which links activity concentration of the building material and annual exposure doses was proposed by experts from the Soreq Nuclear Re232

search Center [10]. The activity concentrations must fulHll the following expression: 226

40 Ra 232 Th K + + ¡ 1: (3) 150 185 3500 This criterion assumes an annual dose limit of 0:7 mSv and includes the dose due to external exposure from gamma-emitters and internal exposure due to radon emanation from the building material, although an action level for radon gas concentration already exists in dwellings (regardless of the source of the radon either from ground or building materials). Expression (3) was calculated using default values for the material density 0 = 1500 kg=m3 , wall thickness d = 0:24 m and typical Israeli ventilation rates, occupancy, etc.; for diDerent values of the above-mentioned properties the coeIcients of expression (3) will change accordingly. The proposal of Ministry of Environment will be published as guidelines, which will include standardization for the methodology of measurements, sampling of the required parameters as radionuclides activity concentration, radon emanation coeIcient, and radon exhalation rate. In the present study, the following concentration limits are used: CRa CTh CK + + ¡ 1; (4) 110 135 2500 CRa CTh CK ¡ 1; (5) + + 220 270 5000

where CRa ; CTh , and CK are activity concentrations of 226 Ra, Th, and 40 K, respectively, in Bq=kg. These limits are calculated from the expected exposure rates in a standard construction, the elements of which contain radioactive materials. They are derived from the radiation dose limits (0:5 mSv=yr) set by national authorities and based on ICRP recommendations [8,11,12]. The limit condition (2) is 2 times stronger than (1) and can be applied only in the case where the use of the speciHc building material is justiHed from the point of view of the economy and minimum damage to the environment. The examples of such materials are some local industrial by-products, which can be beneHcially utilized in the building industry.

232

2. Materials and methods Concentrations of natural radionuclides (226 Ra, 232 Th, K) were determined in the samples specially prepared from the diDerent groups of building materials (Table 1). The radioactivity of samples was measured using a gamma-ray spectrometer based on Ge-detector.

40

3. Results and discussion 40

SpeciHc radioactivity (concentrations) of 226 Ra, 232 Th, K, measured in the diDerent groups of building materials

K. Kovler et al. / Building and Environment 37 (2002) 531–537

533

Table 1 Building materials used in research Building products

Building binders

Ordinary concrete with and without coal >y ash as replacement of sand Pumice block Aerated concrete block Coal >y ash block Oil shale ash block Ordinary concrete block Ceramics

Portland cement grade 300 Portland cement grade 250 Gypsum plaster

Table 2 SpeciHc radioactivity (in Bq=kg) of

226 Ra, 232 Th, 40 K

Building materials

Aggregates

Industrial by-products used in construction

Light-weight

Normal-weight

LECA (Norwegian product, made of coal >y ash) Experimental aggregate (Israeli product, made of coal >y ash) Pumice aggregate (Greek product) TuD

Dolomite coarse aggregate Limestone coarse aggregate Basalt coarse aggregate Gravel Limestone sand Quartz sand

Coal >y ash Oil shale ash Phosphogypsum

in the building materials available in Israel SpeciHc radioactivity (Bq=kg)

Radium equivalent (Bq=kg)

226 Ra

232 Th

40 K

Ceramics (Italy) Pumice block (on Portland cement grade 250) Pumice block (on Portland cement grade 300) Ceramics (Israel) Coal >y ash block Normal-weight concrete with >y ash as part of sand Light-weight concrete block Normal-weight concrete Aerated concrete block

242.9

76.1

1131.0

438.8

60.2

65.5

801.1

215.5

42.9 45.7 146.5

47.7 48.2 9.3

870.1 776.3 107.3

178.1 174.4 168.1

24.9 24.5 18.1 9.5

11.8 8.2 5.2 6.2

63.9 73.5 51.3 96.5

46.7 41.9 29.5 25.8

Building binders

Portland cement grade 250 Portland cement grade 300 Gypsum plaster

66.3 48.2 10.5

39.2 19.6 5.9

138.1 139.6 51.4

133.0 87.0 22.9

Lightweight aggregates

LECA (Norwegian product, made of expanded clay) Pumice aggregate (Greek product) Experimental aggregate (Israeli product, made of coal >y ash) TuD

66.1

58.3

1149.0

237.9

53.2

65.9

1155.0

236.4

61.2 33.1

55.1 41.1

1015.0 534.3

218.1 133.0

12.0 28.0 18.3 15.0 12.1 3.1

13.7 3.1 7.4 3.0 4.1 3.7

308.5 33.6 77.1 50.4 51.1 90.9

55.3 35.0 34.8 23.2 21.9 15.4

747.0 202.0 246.3

14.4 62.9 17.3

63.1 377.7 181.7

772.5 321.0 285.0

Building products

Normalweight aggregates

Basalt coarse aggregate Dolomite coarse aggregate Limestone coarse aggregate Gravel Limestone sand Quartz sand

Industrial by-products

Phosphogypsum Coal >y ash Oil shale ash

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K. Kovler et al. / Building and Environment 37 (2002) 531–537

Fig. 1. EDective speciHc radioactivity for building binders and industrial by-products.

chosen for the study, and radium equivalent calculated from Eq. (2), are shown in Table 2. The values of eDective speciHc radioactivity for 226 Ra, 232 Th, and 40 K are shown in Figs. 1–3. It can be seen from Table 1 that isotope 40 K contributes the most speciHc activity, compared with other isotopes. However, Figs. 1–3 demonstrate that recalculation of radioactivity in terms of eDective speciHc activity, which is determined taking into account the biologic action of each isotope on a human being, leads to the conclusion that the most contribution comes from 226 Ra, and to a lesser extent from 232 Th. The isotope of 226 Ra is only responsible for the eDective radioactivity of phosphogypsum. This fact can be explained by changing the natural balance between radionuclides in the raw material (phosphate) due to the digestion of phosphate rock with sulfuric acid in the phosphoric acid

Fig. 2. EDective speciHc radioactivity for light- and normal-weight aggregates.

plants. This chemical process results in accumulating large amounts of by-product phosphogypsum in many countries. The digestion of phosphate rock with sulfuric acid redistributes radioactive isotopes so that radium sulfate, which is close to gypsum chemically, remains mostly in phosphogypsum. At the same time, if no preliminary chemical or thermal treatment is applied, eDective speciHc radioactivity can be distributed diDerently which is the same as in the raw material. For example, approximately equal in>uence of isotopes of 40 K, 226 Ra, and 232 Th was observed in the samples of pumice and pumice blocks, samples of building ceramics, and basalt and light-weight aggregates made of coal >y ash and tuD.

K. Kovler et al. / Building and Environment 37 (2002) 531–537

Fig. 3. EDective speciHc radioactivity for building products.

The coeIcient of radioactivity determined by Eq. (1) or (2) sets the maximum allowable concentration of radionuclides in building materials. The criterion (1) is “absolute”, and any material which meets this criterion, can be used in construction without any limitations. The criterion (2) can be applied only when the use of the tested material is “justiHed”. The radioactivity coeIcients for the local building materials, calculated from Eqs. (1) and (2), are given in Table 3. It can be seen that the aerated concrete blocks, gypsum plaster produced from natural gypsum stone and the normal-weight aggregates made of the sedimentary rocks have the smallest content of radionuclides. Light-weight aggregates have higher radioactivity. Not every light-weight aggregate meets the criteria suggested.

535

For example, tuD aggregate, which is of volcanic origin like basalt stone, but with a porous structure, has twice as much radionuclides as its basalt “brother”. At the same time, its use is still safe. It can be seen that the most problematic is the use of pumice and artiHcial light-weight aggregates made of coal >y ash. It has to be emphasized that aggregates made of >y ash are neither industrially produced in Israel, nor are they imported from other countries. The pumice aggregate, which is presently imported from the Greek island Yali, is used only in the production of light-weight blocks having a lower radioactivity coeIcient than the raw aggregate. The latest regulations of the Israeli Ministry of Environment permit the use of pumice aggregate for the production of light-weight wall blocks in dwelling construction. Coal >y ash seems to be problematic from the radionuclides content point of view. However, this by-product is used in the local building industry mostly in two applications: as an addition to Portland cement in the amount not exceeding 10% (according to the Israeli Standard No. 1) and as a substitution of Hne fractions of sand in normal-weight concrete production. In both these cases, the use of coal >y ash increases the radioactivity coeIcient insigniHcantly, because the total content of >y ash in such concrete mixes usually does not exceed 150 –200 kg=m3 . The in>uence of >y ash on the radionuclides content of concretes is presented in detail in another paper [13]. The use of coal ash (both >y and bottom ash) in the production of light-weight aggregates, where the speciHc radioactivity is expected to be signiHcantly elevated because the ash constituent becomes dominant, should be investigated separately, as well as taking into account the emission of radon. The problem of emanation and exhalation of radon from building materials was not considered in the present study, but is the subject of the authors’ future investigations. The radioactivity coeIcient of both Italian and Israeli building ceramics is rather high, and the tested ceramic tiles do not meet the requirements of the local authorities at the present time. At the same time, the total mass of these products in dwellings is small, in comparison with other building elements of the building skeleton, walls, >oors and foundations. Obviously, the contribution of building materials to the total radiation dose should also be evaluated from the point of view of location, size and mass of related structural elements in the given type of building. These factors should be taken into account in the future regulations of the allowable radioactivity level in dwellings, as well as radon emission. Finally, the maximum value of radioactivity coeIcient was obtained for phosphogypsum samples. As the latest laboratory investigations show, it is technically possible to produce environmentally friendly phosphogypsum from phosphate rock [14]; however, such processes should be evaluated economically and proved on the industrial scale. In the meantime, the elevated radioactivity of phosphogypsum does not allow its direct utilization in construction.

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K. Kovler et al. / Building and Environment 37 (2002) 531–537

Table 3 Radioactivity coeIcient for building materials Building materials

Radioactivity coeIcient according to Eq. (1)

Eq. (2)

Building products

Ceramics (Italy) Pumice block (on Portland cement grade 250) Pumice block (on Portland cement grade 300) Ceramics (Israel) Coal >y ash block Normal-weight concrete with >y ash as part of sand Light-weight concrete block Normal-weight concrete Aerated concrete block

3.21 1.34 1.08 1.07 1.44 0.34 0.31 0.22 0.17

2.19 0.92 0.74 0.73 0.98 0.23 0.21 0.15 0.12

Building binders

Portland cement grade 250 Portland cement grade 300 Gypsum plaster

0.95 0.64 0.16

0.64 0.43 0.11

Light-weight aggregates

LECA (Norwegian product, made of expanded clay) Pumice aggregate (Greek product) Experimental aggregate (Israeli product, made of coal >y ash) TuD

1.48 1.42 1.36 0.81

1.01 0.97 0.93 0.55

Normal-weight aggregates

Basalt coarse aggregate Dolomite coarse aggregate Limestone coarse aggregate Gravel Limestone sand Quartz sand

0.33 0.29 0.25 0.18 0.16 0.09

0.23 0.20 0.17 0.12 0.11 0.06

Industrial by-products

Phosphogypsum Coal >y ash Oil shale ash

6.92 2.45 2.44

4.72 1.67 1.66

4. Conclusions Samples of typical raw materials, industrial by-products and building products and their components were collected from diDerent productive quarries and building factories in Israel. Concentrations of natural radionuclides (226 Ra, 232 Th, 40 K) in these samples were determined using a gamma-ray spectrometer based on Ge-detector. The results indicate that radium equivalent varies within the range of 25.8–438:8 Bq=kg in the samples of building products, 22.9 –133:0 Bq=kg in the samples of building binders, 15.4 –237:9 Bq=kg in the samples of aggregates and 285.0 –772:5 Bq=kg in the samples of industrial by-products used in construction. The authors hope that the measurements of the concentrations of natural radioisotopes of 226 Ra, 232 Th and 40 K, performed in the present study, will help to create a primary data base on radionuclides content in typical building materials and their components available in Israel. These data will make it possible to calculate the expected levels of the radioactivity in the material, the composition of which is

known, and to develop the national standard on the radioactivity of building materials.

References [1] Zikovsky L, Kennedy G. Radioactivity of building materials available in Canada. Health Physics 1992;63(4):449–52. [2] Othman I, Mahrouka M. Radionuclide content in some building materials in Syria and their indoor gamma dose rate. Radiation Protection Dosimetry 1994;55(4):299–304. [3] O’Brien RS. Gamma doses from phosphogypsum plaster-board. Health Physics 1997;72(1):6–96. [4] Beretka J, CioI R, Marroccoli M, Valenti GL. Energy-saving cements obtained from chemical gypsum and other industrial wastes. Waste Management 1996;16(1–3):231–5. [5] Dinelli G, Belz G, Majorana CE, Schre>er BA. Experimental investigation on the use of >y ash for lightweight precast structural elements. Materials and Structures=Materiaux et Constructions 1996;29:632–8. [6] Keller G, Folkerts KH, Muth H. Discussing possible standards of natural radioactivity in building materials. Radiation and Environmental Biophysics 1987;26:143–50.

K. Kovler et al. / Building and Environment 37 (2002) 531–537 [7] Savidou A, Raptis C, Kritidis P. Study of natural radionuclides and radon emanation in brick used in the Attica region, Greece. Environmental Radioactivity 1996;31(1):21–8. [8] Annals of the ICRP. 1990. Recommendations of the international commission on radiological protection. ICRP Publication 60, Oxford: Pergamon Press, 1991. [9] Hamilton EI. The relative radioactivity of building materials. American Industrial Hygiene Association Journal 1971;32: 398. [10] Hareubeni R, Margaliot M, Shlesinger T. Limitations to the contents of radioactive materials in building materials: proposal for an Israeli Standard, Soreq Nuclear Research Center 1996.

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[11] Recommendations of the International Commission on Radiological Protection. ICRP Publication 26, Oxford: Pergamon Press, 1977. [12] International Commission on Radiological Protection. Principles for limiting exposure of the public to natural sources of radiation. ICRP Publication 39, Oxford: Pergamon Press, 1984. [13] Kovler K, Manasherov V, Haquin G, Ne’eman E, Rogozhin V, Lavi N. Radionuclides content of concrete with and without coal >y ash, in press. [14] Kovler K, Somin M. Direct production of sodium ortho- and=or pyrophosphates and gypsum binders from phosphate rock. Abstracts of papers of the American Chemical Society, 214: 5-FERT, Part 1, September 7, 1997.