Assessment of Natural Radioactivity in Irish Building Materials

E. M. Lee, G. Menezes, E.C. Finch,

Department of Physics, University of Dublin, Trinity College, Dublin 2, Ireland. Email: eric.finch@tcd.ie

Abstract. All building materials are naturally radioactive because the rocks and soils from which they are derived always contain natural radionuclides of the Uranium (238U), and Thorium (232Th) series, and the radioactive isotope of Potassium (40K). These radionuclides emit gamma rays, which can be a source of external radiation exposure. Radon gas, which is exhaled from building materials, can also be a source of internal radiation exposure. The EU regulations governing exposure to naturally occurring radioactive materials have been recently introduced, and a comprehensive study of radioactivity in building materials in Ireland has not been undertaken until now. Samples of the most common types of Irish building materials were collected from leading manufacturers and suppliers of such materials in Ireland. These samples were ground and prepared, and then analyzed using a wide-energy HPGe coaxial gamma-ray spectrometer. Samples included concrete, aggregates, sand, cement, wood, gypsum / plasterboard, bricks, natural stone, insulation materials, tiles and roofing materials. The activity concentrations of the natural radionuclides 226Ra, 232Th, and 40K were determined, along with the artificial radionuclide 137Cs (from nuclear weapons testing and Chernobyl fallout) in certain relevant samples. Results were compared with similar studies in other countries and also with relevant legislation and guidance. 1. Introduction The world is naturally radioactive, and around 90% of human radiation exposure arises from natural sources such as cosmic radiation, exposure to radon gas and terrestrial radiation. Reviewing the published literature, some studies have determined the levels of exposure due to natural radiation in Ireland [1,2] while others have examined the radioactivity of building materials in other countries [3,4,5] but there has been no comprehensive study undertaken in the Republic of Ireland on exposure to natural radiation arising from building materials. The exposure to natural sources of ionizing radiation has been controlled by statutory legislation [6] in the Republic of Ireland since May 2000 and so it was decided to conduct this study. All building materials contain various amounts of radioactivity. For example, materials derived from rock and soil contain natural radionuclides of the uranium (238U), and thorium (232Th) series and the radioactive isotope of potassium (40K). Artificial radionuclides can also be present, such as caesium-137 (137Cs), resulting from fallout from weapons testing and the Chernobyl accident. All these can be sources of both internal and external radiation exposure. Internal exposure occurs through the inhalation of radon gas, and external exposure occurs through the emission of penetrating gamma rays. This paper outlines the methodology used for determining the type and specific activity of the naturally occurring radionuclides found in commonly used building materials in the Republic of Ireland, the results obtained and the radiological health significance of such results. 2. Materials and Methods Seventeen manufacturers and suppliers of building materials in the Republic of Ireland were contacted and asked to provide one sample of each of the various types of building materials produced by that company. In total, 70 samples of building materials were collected from eight of these manufacturers / suppliers, and were analyzed by gamma spectrometry. These samples included aggregates (9 in number), sand (3), stone (2), bricks (14), cement (3), concrete (8), gypsum-based materials such as plasterboard (10), insulation materials (2), wall and floor tiles (9) and wood (10). Bricks may be made of lime and clay (called red bricks or clay bricks) or sand-lime (called white bricks). Leading manufacturers of bricks in this country have advised that only clay fired bricks are

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used in the construction of buildings in the Republic of Ireland1, and therefore only comparisons for clay bricks are referred to in this work. Most other similar studies undertaken obtained results for concrete, aggregates, sand, cement, bricks, gypsum and natural stone [7,8]. Only a few studies have obtained results for tiles, insulation materials and wood. [3,5]. Although wood and insulation materials are commonly used building products, they are unlikely to have a high radionuclide content. Tiles, on the other hand, may have a significant radionuclide content, due to the inclusion of zircon sand in the tile glaze [4,9]. Zircon sand is often used in the fine ceramics industry, where it acts as an opacifier in glazes and enamels, and it is also used as an additive in the glazing on ceramic tiles. All large samples, such as concrete blocks, clay fired bricks, certain aggregates and plasterboard samples, were firstly ground to a coarse powder, of similar densities where possible, and poured into a Marinelli beaker of approximate volume 0.5 litre. Samples of wood were sawn and the sawdust from each individual wood sample was collected for analysis. All samples were analyzed using a low background HPGe (hyperpure germanium) 'GMX' gamma ray detector manufactured by EG & G ORTEC (100 Midland Road, Oak Ridge, TN 37831-0895, U.S.A.). A sample of water was counted in the detector approximately twice a month; as most of the samples had roughly similar densities to the water, this enabled background corrections to be applied. A calibration source of known radionuclide content and activity was analyzed every month to ensure the accuracy of the results. The lowest energy peak in the calibration source was 59.5keV. This is slightly higher than the peak required to measure 210Pb at 46.5keV and could therefore be a source of error. Each sample was counted in the detector for minimum of 24 hours, i.e. 86,400 seconds. There were no observable drifts in either the energy or efficiency calibrations over periods of up to 3 days. The Marinelli beakers were sealed in order to measure the radon exhalation rate from the sample [7,10,11]. The effectiveness of the seal was tested by immediately measuring some samples for 40 sequential 10,000 second intervals and looking for an in-growth of radon by measuring the 214Pb peaks at 295 and 392 keV. There was no measurable increase in radon concentration in the sealed Marinelli beakers in any of the samples measured, either because the sealing method was not effective or because the exhalation rate was too low to be measured using this technique. This shows that further work will be necessary for this type of method to be successful. Radon exhalation rates from these building materials have therefore not been measured directly but inferred levels are noted in the discussion. As well as 226Ra, 232Th and 40K, 137Cs activities were also determined because of the fallout over Ireland from weapons testing and the Chernobyl accident [1]. 226Ra activities can be ascertained using two of the gamma ray lines in the spectrum, those at 93 keV and 186 keV and correcting for the overlap with 235U at 186 keV. The 232Th activities were determined from the gamma ray emissions at 911, 969, 338, 965, 795, and 463 keV from 228Ac, 234U from the 63.3 keV peak for 234Th, 214Pb from the peaks at 295 and 392 keV, 214Bi from the peaks at 609, 1120 and 1764 keV, and 210Pb from the peak at 46.5keV. The 40K and (where present) 137Cs activities were determined from the lines at 1461 keV and 662 keV respectively. The European Commission guidance document [12] advises on the determination of Activity Concentration Indices (I) to convert the specific activity of a building material (in Bq kg-1) into a measure of radiation dose (in mSv) that may be received by an individual occupying a 'model room’ constructed from a building material with a certain specific radioactivity:

Ra Th K300 200 3000C C CΙ = + + (1)

where CRa, CTh, and CK are the radium, thorium, and potassium activity concentrations (Bq kg-1) in the building material.

1 Power E. Ormonde Brick. Castlecomer, Co Kilkenny, Ireland, Personal Communication. (Telephone conversation)

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The values of I were calculated for all the samples studied.

3. Results

FIG. 1. Mean Specific Activities for all sample types for Uranium series, 232Th and 137Cs.

Figure 1 shows the concentration of radionuclides in the samples studied. No significant levels of disequilibrium in the Uranium series isotopes were found, as the activities for 234U, 214Bi, 214Pb and 210Pb were similar to one another. The tiles show significantly higher activities in the Uranium series but 232Th was not unusually high. As anticipated, 137Cs was undetectable in all samples except wood. This gave an average specific activity of 3.2 Bq kg-1, which is radiologically insignificant. The activity occurs because of the uptake of 137Cs from soil.

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FIG. 2. Mean specific activity (Bq kg-1) of 226Ra and 232Th in building materials in Ireland and in

other European countries. Figures 2 and 3 show the results for 226Ra 232Th, and 40K in comparison with values measured in other European Countries.

FIG. 3. Mean specific activity (Bq kg-1) of 40K in building materials in Ireland and in other European countries.

Figure 4 shows the Activity Concentration Indices for each of the 70 building materials analyzed. This figure highlights those samples likely to give rise to dose rates of greater than 0.3 mSv or 1 mSv per annum if used in bulk quantities in the construction of buildings.

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FIG. 4. Activity Concentration Indices of the Irish building materials studied.

4. Discussion 4.1. Activity Concentration Indices & Comparison of Irish Results with European Commission Guidance There are many similar methods for calculating the radiation dose that an individual may receive from the materials used in the construction of buildings. These are generally of the form of equation 1 with different constant values in the denominator. The European Commission examined the different variations and settled on equation 1 which gives values of I calculated form the specific concentrations of 226Ra, 232Th and 40K [12].

Table I. The relationship between I values and received dose per year.

< 0.3 mSv < 1mSv Bulk Materials I < 0.5 I < 1

Superficial Materials I < 2 I < 6 Regulatory control should be considered for materials that give rise to doses of between 0.3 mSv and 1 mSv per annum. Materials giving doses below 0.3 mSv should be exempt from all restrictions and those above 1 mSv must be regulated. 98% of the samples analyzed (69 of the 70 samples displayed in figure 4) had I < 1. This means that these could be used as bulk or superficial materials without regulation as occupiers of buildings manufactured from such substances would receive an annual dose of less than 1 mSv. The only sample where I > 1 was an aggregate “Limestone Dust”. This is used as an aggregate in (i.e. a component of) concrete and is utilized in much smaller quantities than materials like brick or stone. 80% of the samples analyzed (56 of the 70 samples displayed in figure 4), complied with the lower limit of I < 0.5 (i.e. annual dose would be less than 0.3). The samples that exceeded this value were: 7 tiles, 4 clay bricks, a concrete stock brick and 2 aggregate materials of limestone dust and granite dust. The concrete stock brick would be the most likely type of material to be used in bulk, and while it exceeded the lower I value which could give rise to doses of 0.3 mSv per annum, its I value was

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almost half of that which could give rise to doses of 1 mSv per annum, and therefore should be of no radiological significance. The tiles show the most consistently elevated I values due to the high uranium series activities (Figure 1) which is because of the use of zircon and mineral glazes in the firing process [4,9]. The wood samples show very low I values. This is to be expected because of the low uptake of Uranium and Thorium into plants. Equation 1 is not a function of 137Cs but the levels found in the wood were very small and would not significantly increase the received dose. Radon levels were not measured directly but an upper limit can be inferred from the European Commission guidelines [12]. If the gamma dose rate is lower than 1 mSv per annum, then the 226Ra concentrations are unlikely to be high enough to cause indoor radon concentrations in excess of the 200 Bq m-3 guidance level. 4.2. Comparison of Results with Irish Legislation Legislation for the Republic of Ireland [6] sets a limit of 400 Bq m-3 for radon exposure at work. Since this is less stringent than the Commission guidance of the previous section it will automatically be complied with if the 200 Bq m-3 limit is not exceeded. The Irish legislation also sets a limit of 1 mSv per annum, above which controls must be implemented for work activities involving significant exposure to natural radiation sources other than radon. There is nothing in this legislation which refers specifically to domestic exposure. The expression for I (equation (1)) was formulated for domestic buildings [12]. However, time spent in the workplace can often be assumed to be less than in a dwelling, and this lends support for the use of this expression to cover exposure at work. It is possible, however, that occupational exposure to doses of greater than 1 mSv may arise during the manufacture or processing of such materials. As yet, this aspect has not been investigated. 4.3. Comparison of Results with Similar Studies in Other Countries The Irish results for mean specific activities of 226Ra, 232Th and 40K in various types of building materials are compared in Figures 2 and 3 with the results for other European countries [3]. These were compiled by taking the mean specific activities for the radionuclides in question for each building material type, country by country, and calculating the average of these means. In general, the mean specific activities of building materials used in Ireland were comparable with those from other countries. The activities for each material and radioisotope shows a wide range of values, as can be seen from table 1. Results from other countries also show similar wide ranges [3]. To take the activity of 232Th in concrete as an example, the values in Ireland range between 3 and 43 Bq kg-1 while the ranges for Germany and Finland are 28-53 and 4-71 Bq kg-1 respectively. Similarly, for 226Ra in brick the ranges in these three countries are 7-139, 10-200 and 37-134 Bq kg-1 respectively. The mean values for 40K in tiles were notably higher than those reported by the only countries providing results (Bulgaria, Germany, Greece and the Netherlands). It should be noted that all of the tiles we analyzed were imported from either Spain or Italy. The activity concentrations for 232Th were lower than the average values but within the range of results found in other countries. For the majority of the bulk materials studied, the values of I were less than 0.5, which implies an exposure level of less than 0.3 mSv per annum. These results are similar to findings from other countries. I values for bulk building materials used in, for example, Canada [5] and Pakistan [13] were found in almost all cases to be less than 0.5. Also, modelled dose rates for gamma exposures from materials used in Greece were in the region of 0.3 mSv per annum [14].

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5. Conclusions The activity concentrations of 226Ra, 232Th, 40K, 137Cs, 234U, 214Bi, 214Pb and 210Pb found in Irish building materials suggest that the use of such building materials in the construction of buildings is unlikely to give rise to any significant radiation exposure (< 1 mSv per annum) to the occupants. In this respect the materials analyzed comply with the parameters outlined in the relevant national and international legislation and guidance. A significant number of building materials may give rise to doses greater than the exemption criterion of 0.3 mSv per annum. At present this does not affect practices in the Republic of Ireland but the European Commission guidance leaves the way open for legislation restricting the use of these building materials. In general, the radionuclide activity concentrations noted in the Irish building materials analyzed are comparable with the results of similar studies undertaken in other countries. Where results for Irish building materials were higher than those noted in other studies, such results were still of no radiological health significance. Acknowledgements – The work was funded by Trinity College, Dublin. The authors thank Professors Ian McAulay and Werner Blau for their advice and encouragement, and Dr Robert Goodhue, Mr Desmond Kelly and Mr Michael Reilly for their assistance in sample preparation.

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