A practical approach to limit the radiation dose from building materials applied in dwellings, in compliance with the Euratom Basic Safety Standards

Journal of Environmental Radioactivity 196 (2019) 40–49

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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad

A practical approach to limit the radiation dose from building materials applied in dwellings, in compliance with the Euratom Basic Safety Standards

T

Ronald C.G.M. Smetsers∗, Jasper M. Tomas National Institute for Public Health and the Environment (RIVM), Antonie van Leeuwenhoeklaan 9, NL-3721 MA, Bilthoven, the Netherlands

ARTICLE INFO

ABSTRACT

Keywords: Natural radioactivity Building materials NORM Basic Safety Standards Gamma radiation Thoron

Individuals receive a significant part of their radiation exposure indoors. We anticipate that this exposure is likely to increase in the near future, due to a growing use in the building industry of recycled materials and materials previously regarded as waste. Such materials often contain elevated levels of natural radionuclides. Directive 2013/59/Euratom (‘Basic Safety Standards’, BSS) pays comprehensive attention to indoor exposure from natural radionuclides, but proper implementation of all corresponding BSS regulations is not straightforward, especially when regarding the regulation of building materials containing so-called Annex XIII materials. In this paper, we discuss the most relevant deficiencies in the BSS and present a practical approach to cope with these. Our most important observation is that adequate methods for assessing the annual dose due to gamma radiation from building materials are not provided by the BSS. This is in particular difficult because compliance of single building materials has to be tested, but the corresponding BSS reference level refers to gamma radiation emitted by all building materials present in a room. Based on a simple model of three layers of building materials, we present a set of operational conditions for building materials, either used for construction purposes (‘bulk layers’) or for the finishing of walls, floors and ceilings (‘superficial layers’). Any customary combination of building materials meeting these conditions will stay below the BSS reference level for gamma radiation. This statement holds for the middle of a reference room, but is not always the case close to the walls, especially when low density materials with a relatively high content of natural radionuclides are present at the inner side of the room. This can be avoided by applying more strict conditions for those kind of materials than presented in this paper. We further focus on the indoor exposure to thoron progeny. Building materials that pass the test for gamma radiation can still be a significant source for indoor air concentrations of thoron progeny. When the average annual thoron inhalation dose were to be restricted to 1 mSv a−1 − a level comparable to the BSS reference level for gamma radiation − the activity concentration of Ra-224 in (especially porous) building materials used for wall finishing purposes should be limited to a value of typically 50 Bq kg−1. Even if our suggested approach of the BSS regulations is fully implemented, it still allows for a significant increase in the average radiation exposure in dwellings due to external radiation and thoron progeny. However, the situation will be worse if a less strict interpretation of the BSS regulations will be applied.

1. Introduction In many, if not most western countries, three categories of sources are responsible for almost 100% of the radiation dose received by the population: (1) medical diagnostics, (2) uncontrollable natural sources, such as cosmic radiation and K-40, and (3) soil and soil-based building materials responsible for the radiation dose received indoors. All other sources of ionizing radiation, including the nuclear industry, the process industry, consumer products with radioactive content and the fallout from previous accidents together account for only one or two per cent of the total dose received by the public. Fig. 1 shows, on the left ∗

side, the distribution of the annual averaged radiation dose received by the Dutch population. The dose from medical diagnostics and the dose received indoors contribute to 40%, respectively 38% of the total dose of 2.7 mSv a−1 in the Netherlands (reference year: 2015). Uncontrollable natural sources are responsible for 21% and all remaining sources contribute to only 1% of the total annual dose. Although the absolute annual averaged radiation dose received by the population is generally different in other western countries, the relative importance of the categories of radiation sources mentioned above is roughly similar. Examples of the situation in the USA, the UK, Germany and Belgium are shown on the right side of Fig. 1 (BfS, 2017; NCRP, 2009;

Corresponding author. E-mail address: [email protected] (R.C.G.M. Smetsers).

https://doi.org/10.1016/j.jenvrad.2018.10.007 Received 8 August 2018; Received in revised form 16 October 2018; Accepted 17 October 2018 0265-931X/ © 2018 Published by Elsevier Ltd.

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Fig. 1. Relative importance of sources contributing to the average radiation dose received by the general public in the Netherlands, Germany (DE), United Kingdom (UK), Belgium (BE) and the United States of America (US) (BfS, 2017; NCRP, 2009; PHE, 2016; Willems, 2018). The category ‘Indoor’ refers to the sum of the inhalation doses due to radon and thoron and the external irradiation dose from gamma radiation emitted by building materials (excluding shielding effects).

dwellings and a demand for national radon action plans (Euratom, 2014). The paper is set up as follows. In Section 2 the Euratom directive 2013/59 is reviewed regarding the topic of radiation exposure in dwellings in relation to the use of building materials. One of the findings of the review is that the proper implementation of the Euratom directive 2013/59 requires a generalized test method for the use of multiple construction layers. In Section 3 the criteria for such a generalized test method are derived. Based on these criteria, a simple threelayer model is presented that can be used to derive practical conditions to comply with the BSS reference level. Section 4 is devoted to the exposure to thoron progeny in dwellings. A regulation is proposed based on a thoron inhalation dose reference level comparable to the BSS reference level for gamma radiation. Furthermore, Section 5 puts the topic of radiation exposure in dwellings in a broader perspective through a discussion on several topics: the balance between sustainability and radiation protection, points of particular interest for regulators, the activity concentration index I, and the regulation approach for thoron versus radon. Finally, Section 6 summarizes the conclusions of this study.

PHE, 2016; Willems and Vanaudenhove, 2018). Exposure to pure natural radiation sources, such as K-40 in food and cosmic radiation, is inevitable, and as such impossible to regulate. Medical diagnostics using ionizing radiation are intentionally applied to benefit the patient. In this area, dose control is primarily a matter of justification awareness by medical staff and the development of dose reduction techniques for both equipment and medical (imaging) procedures. The radiation dose received indoors, however, is the unwanted result of the nearby presence of materials containing (enhanced) levels of naturally occurring radionuclides. Indoor exposure to ionizing radiation bears no benefit to the population at all and should therefore be avoided as far as reasonably achievable. The following exposure pathways are dominant here: (1) inhalation of radon progeny, where the radon emerges from the soil beneath the building and from construction materials with a certain amount of Ra-226, such as concrete and ceramic brick; (2) inhalation of thoron progeny, where the thoron may exhale from the superficial layer on walls, ceilings and floors, especially when a porous material with a high Th-232 content, such as phosphogypsum, lightweight concrete with alum shale, Italian tuff or mud is applied (Chambers, 2010; Meisenberg et al., 2013); (3) external irradiation by gamma radiation emitted by building materials with a mineral component, containing natural radionuclides from the U-238 series, the Th-232 series and K-40.

2. Indoor exposure as treated by Euratom directive 2013/59 The basis of the protection from radiation exposure in dwellings, as formulated in the BSS, is given by Article 100 on ‘Existing exposure situations’: “Member States shall ensure that measures are taken, upon indication or evidence of exposures that cannot be disregarded from a radiation protection point of view, to identify and evaluate existing exposure situations taking into account the types of existing exposure situations listed in Annex XVII, and to determine the corresponding … public exposures”. Annex XVII explicitly mentions indoor exposure to radon and thoron and external exposure from building materials as situations to be treated as existing situations in the sense of Article 100 (Euratom, 2014). To regulate the indoor external exposure to gamma radiation emitted by building materials, the BSS has established a reference level with a value of 1 mSv a−1 “in addition to outdoor external exposure” (Article 75). This applies especially to building materials containing (raw) materials of a kind as mentioned in Annex XIII of the BSS (Euratom, 2014). A so-called activity concentration index I is introduced that can be used as a conservative screening tool to identify materials that may cause the reference level to be exceeded:

From the viewpoint of radiation protection, proper control of these exposure pathways is required. Directive 2013/59/Euratom, further referred to as the Basic Safety Standards (BSS), is the first Euratom directive that pays comprehensive attention to the radiation dose received indoors. The BSS treats the exposure to natural radiation in dwellings, workplaces and other buildings as an existing exposure situation, as further explained in Article 100 (Euratom, 2014). All Euratom member states should have had implemented this directive by 6 February 2018. It appears, however, that proper implementation of these regulations is not straightforward since there are a few issues still to be solved. In this paper, we will discuss the most relevant deficiencies in the present regulation and we will present a practical approach to cope with these. We will restrict ourselves to indoor radiation in dwellings in relation to the use of building materials with (enhanced) levels of natural radionuclides from the U-238 series, the Th-232 series and K-40. Our primary focus will be on the compliance of building materials with the BSS reference level for gamma radiation, assuming the presence of multiple construction layers. Secondly, we will pay attention to the potentially large inhalation dose, resulting from exhalation of thoron out of materials that comply with the BSS reference level for gamma radiation. We will not consider the exposure to radon, for the following two reasons: (1) in areas where it matters, the soil under a building is a much more important source for indoor radon than the building materials applied in the dwelling, and (2) indoor radon has already been regulated quite satisfactory, by the establishment of a reference level for the annual averaged concentration of radon in

I=

CRa 226 C C + Th 232 + K 40 300 200 3,000

(1) −1

where Cnuclide is the activity concentration (in Bq kg ) of the radionuclide under consideration. When the index I exceeds the value of 1 for a certain material, a more precise dose assessment is needed that should take into account “other factors such as density, thickness of the material as well as factors relating to the type of building and the intended use of the material (bulk or superficial)” (Annex VIII). The indoor exposure to thoron, which is also listed in Article 100 as 41

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5. The exhalation of thoron from building materials is not regulated. As we will show later on, a building material that passes the test for gamma radiation can still be a significant source for indoor concentrations of thoron and progeny and the associated effective doses.

Table 1 Average annual effective dose [mSv a−1] for members of the Dutch population, exposed to various natural sources of ionizing radiation, for several ratios of the time being spent indoors and outdoors. On average, members of the Dutch population spend approximately 90% of their time indoors. Annual effective dose [mSv a−1] Due to external irradiation Terrestrial radiation Secondary cosmic radiationa Gross contribution from building materials Total effective dose from external radiation Shielding from building materials Net contribution from building materials Due to inhalationb Radon and progeny Thoron and progeny Total effective dose from inhalation

100% outdoors

100% indoors

90% indoors/ 10% outdoors

0.21 0.35 0.00

0.02 0.21 0.39

0.04 0.22 0.35

0.56

0.62

0.61

0.00

−0.33

−0.30

0.00

0.06

0.05

0.16 0.01 0.17

0.50 0.22 0.72

0.47 0.20 0.67

2.2. Shielding of outdoor radiation by building materials Building materials with a mineral content are a source of gamma radiation, but on the other hand, they shield external radiation from the outdoor environment (as well as from nearby rooms). This shielding can be quite effective, depending on the kind of material applied. When establishing the gross and net contribution of building materials to the gamma dose rate indoors, it is necessary to know (1) the gamma dose rate indoors, (2) the gamma dose rate outdoors due to terrestrial and secondary cosmic radiation, and (3) the shielding factors for terrestrial and secondary cosmic radiation. Indoor measurements of the ambient dose equivalent rate were performed in the Netherlands in a sample of approximately 300 dwellings (built between 1994 and 2003), yielding an average value of 0.89 mSv a−1 for the living room. Slightly lower values were found for the bedroom (0.87 mSv a−1) and the entrance hall (0.86 mSv a−1) (De Jong and van Dijk, 2009). The outdoor radiation environment in the Netherlands was thoroughly investigated in the early nineties of the previous century (Smetsers and Blaauboer, 1996, 1997). The ambient dose equivalent rate from terrestrial radiation varies in the Netherlands between 15 and 75 nSv.h−1. Averaging over space and time yields an average annual ambient dose equivalent rate of 0.35 mSv a−1. The average annual ambient dose equivalent rate outdoors due to secondary cosmic radiation in the Netherlands is also 0.35 mSv a−1. Combining these data yields a gross value for the indoor ambient dose equivalent rate from building materials of 0.64 mSv a−1, where we assume that, on average, 90% of the outdoor terrestrial radiation is shielded by building materials and 40% of the secondary cosmic radiation. The latter value was derived from a series of measurements of the shielding factor for secondary cosmic radiation in four categories of houses in the Netherlands (Julius and van Dongen, 1985a, b). To calculate the average annual effective dose received by the public, we have to convert the ambient dose equivalent to effective dose by applying a factor of 0.6 for terrestrial radiation as well as for radiation from building materials. For secondary cosmic radiation, we use a conservative conversion factor of 1. Finally, we must take into account the time being spent in- and outdoors. On average, the Dutch population spends approximately 90% of the time indoors and 10% of the time outdoors. The results of this analysis are shown in Table 1. From these data, we derive a gross contribution from building materials to the external effective dose indoors of 0.35 mSv a−1 in the Netherlands and a net contribution of 0.05 mSv a−1, the latter with an estimated uncertainty of the same order of magnitude, due to the various assumptions in the calculation. This results in a gross value of the BSS reference level for gamma radiation from building materials for the Netherlands of 1.0 (= net value of the reference level) + 0.3 (= dose reduction due to shielding of outdoor external radiation) = 1.3 mSv a−1. In Table 1, we also show the average annual inhalation doses received from radon and thoron progenies. The fundamentals of the BSS approach concerning the exposure to natural radioactivity of building materials were laid down in (EC, 1999). That document presents calculations of the annual effective dose received in a reference room, without doors and windows and made of 20 cm thick concrete walls. The average outdoor kerma rate from terrestrial radiation equals about 50 nGy h−1. It was assumed that this terrestrial radiation would be completely blocked by the concrete. Using a dose conversion coefficient for the conversion of kerma to effective dose of 0.7 Sv Gy−1 and an occupancy rate of 7,000 h per year (80%) yielded a dose reduction value due to shielding of outdoor external radiation of 0.25 mSv a−1. Any shielding of secondary cosmic

a

The radiation dose due to air traffic has been excluded here. Based on dose conversion coefficients from (UN, 2009). Radon and thoron data are taken from Smetsers and Blaauboer (1996); Smetsers et al. (2018). b

an existing exposure, is not further regulated in the BSS, in large contrast to the way indoor exposure to radon is treated (a reference level for the indoor annual averaged radon concentration, a radon action plan, etc.). 2.1. General observations To transform the BSS directive with respect to building materials to a practical regulatory system is not a straightforward process. When examining ways to do so, the following observations can be made: 1. The BSS-phrase “in addition to outdoor external exposure” is not clearly defined. The use of different values by different member states, which is possible because the average outdoor radiation levels vary from country to country, may lead to different requirements for the same type of building materials, depending on the country where the requirements have been issued; 2. The current annual dose due to gamma radiation from building materials is, at least in the Netherlands but also in many other Euratom member states, much lower than the BSS reference level. The introduction of a relatively high reference level, together with growing demands for sustainability, may easily lead to a gradual increase of the average radioactive content in building materials and therefore to an increase of public exposure to ionizing radiation in dwellings. Such a development would give rise to the opposite effect of the radiation protection goals as aimed for by this directive; 3. When a given building material does not pass the index I test, a more precise dose assessment is required. But how should this assessment take place? Which party, for instance, takes the so-called other factors into account, i.e. density, thickness of the material and factors relating to the type of building and the intended use of the material? Will that be the manufacturer of the building material under investigation? Or the local regulator? Note that different choices of parameters yield very different outcomes. 4. The BSS Reference Level is defined for indoor external exposure to gamma radiation emitted by building materials and refers, as such, to the combined dose from all building materials present in a room. In practice, however, compliance of a single building material has to be tested. This makes the challenge of adequate dose assessment of single building materials even more complex; 42

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Fig. 2. Spatial distribution of the annual effective dose in the CEN reference room computed with the Markkanen model; a) The contribution of the concrete walls with a thickness of 20 cm; b) The contribution of a gypsum layer with a thickness of 3 cm; c) Total effective dose due to both construction layers (including the attenuation of the gamma radiation emitted by the concrete layer due to the gypsum layer).

radiation by building materials was not taken into account. A more recent elaboration of the indoor annual dose rate due to gamma radiation is described in detail in a technical report from the European Committee for Standardization, CEN (CEN, 2017). CEN follows the convention of (EC, 1999) but assumes an outdoor kerma rate from terrestrial radiation of 60 nGy h−1, yielding a dose reduction value due to shielding of outdoor external radiation of 0.29 mSv a−1. It shows that different assumptions yield different values for the dose reduction due to shielding of outdoor external radiation by building materials. For reasons of clarity, it might therefore be better to use a fixed value for the gross reference level, which can be directly linked to straightforward calculations of the gamma dose emerging from building materials. As we will show later on, this is especially convenient when the presence of more layers of building materials is considered. Therefore, throughout this paper, we assume a fixed gross reference level for the annual effective dose due to gamma radiation from building materials of 1.3 mSv a−1.

presented in the next sections, on detailed assessment of the spatial distribution of the annual effective dose in a reference room, are calculated with this computer code. Based on results acquired with the Markkanen model CEN has derived a formula that estimates the annual effective dose for gamma radiation in the middle of a reference room with dimensions 3.0 m × 4.0 m × 2.5 m and no doors or windows (CEN, 2017). The room is assumed to shield all outdoor exposure. A conversion factor of 0.7 Sv Gy−1 is used for converting the absorbed dose in air to the effective dose. In addition, the average occupancy time indoors is assumed to be 7,000 h per year. The CEN-formula, with parameters ρ (mass density, in kg m−3), d (thickness, in m) and Cnuclide (activity concentration, in Bq kg−1):

2.3. Dose assessment due to gamma radiation from building materials

turns out to be a convenient tool to estimate the annual effective dose from gamma radiation in the middle of a reference room constructed with one kind of building material. Note that, since we compare all calculation results with a gross reference level of 1.3 mSv a−1, we do not subtract the background correction factor of 0.29 mSv a−1 in this formula, as is done by CEN (CEN, 2017). When a building material does not fulfil the index I test (Eq. [1]), it is obvious to use the CEN-formula for a more precise dose assessment. Parameter values for the specific mass and activity concentrations follow directly from the material under investigation, but a choice has to be made regarding the “thickness of the material …. relating to the type of building and the intended use of the material” (Euratom, 2014), as well as the appropriate reference value to compare the result with. This part of the procedure is not clearly described in the BSS, possibly leading to different interpretations. This can easily lead to situations where combinations of approved materials together exceed the reference level of the annual effective dose from gamma radiation from building materials.

[281 + 16.3 d 0.0161( d ) 2] × CRa 226 D = + [319 + 18.5 d 0.0178( d )2] × CTh 232 × 10 + [22.3 + 1.28 d 0.00114( d )2] × CK 40

In 1995, Markkanen introduced a dose assessment method for building materials based on the geometry of the room of interest, the thickness of the construction layer(s) and the material properties, such as activity concentrations of radionuclides and mass density (Markkanen, 1995). The model is suitable for considering two material layers, thereby taking into account the effect of shielding of gamma radiation from the underlying construction layer by the upper construction layer. The radionuclides Ra-226 (and progeny), Th-232 (and progeny), and K-40 are considered, since those are the main contributors to the gamma dose. In theory, the absorbed dose rate should be calculated separately for every gamma line. However, Markkanen (1995) argues that sufficiently accurate results are obtained when considering a single yield-average gamma line for each radionuclide. Only the 2615 keV gamma line of Th-232 is treated separately because it causes over 40% of the thorium series dose rate. We implemented the model described by Markkanen in a computer code. The results 43

6

[mSv a 1] (2)

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We will show this, by calculating the maximum allowed percentage of Annex XIII materials (Euratom, 2014) that can be added to building materials. The composition of the material that can be added, further referred to as sNORM (Standardized NORM), is defined in the same way as introduced by (de With, 2017): (1) the activity content of the weighted activity concentrations of sNORM equals 1, in formula:

CRa 226 C C + Th 232 + K 40 1,000 1,000 10,000

= 1

phosphogypsum) will easily pass this test (calculated annual dose using Equation (2): 0.71 mSv a−1). But when we look at the combined result (Fig. 3c), we see that the annual dose in the middle of the CEN reference room exceeds the BSS reference level by approximately 0.34 mSv a−1. This outcome is by no means surprising because the correction value of 0.48 mSv a−1 due to the presence of a bulk layer, following CEN recommendations, holds for reference concrete and not for concrete with an extra amount of sNORM. Note that the gypsum superficial layer now generates much more gamma rays than that it absorbs. We further notice that the annual dose close to the walls is significantly higher (close to 2 mSv a−1) compared to the dose in the middle of the room, which is mainly the result of the enriched activity concentrations of natural radionuclides present in the superficial layer. This example clearly shows that building materials, each seeming to satisfy the conditions of the BBS on their own, may easily exceed the BSS reference level when applied together.

(3)

and (2), the ratio of the three relevant groups of radionuclides is similar to the one in fly ash, as reported in (EC, 1997). sNORM has therefore the following composition: CRa-226 = 500 Bq kg−1, CTh-232 = 300 Bq kg−1 en CK-40 = 2,000 Bq kg−1. We assume that sNORM under consideration has the same density as the material to which it is added. Fig. 2 shows the annual effective dose due to gamma radiation from building materials that contain no additional sNORM. The results are shown on a height of 1.25 m in the CEN reference room, for an exposure time of 7,000 h. Fig. 2a shows the result in the case when walls, floor and ceiling are made of 0.2 m thick reference concrete. Fig. 2b shows the calculated result for a 0.03 m thick layer of natural gypsum. Fig. 2c shows the combination of both. Note that the gypsum layer has a net shielding effect in this case. The relatively low dose in the corner regions of the room are caused by the fact that the Markkanen model does not consider the contribution of the materials in the room corners. From Equation (2), it follows that a bulk material, consisting of 79.4% reference concrete and 20.6% sNORM, yields an annual dose from gamma radiation just below the BSS reference level. For superficial layers, i.e. up to a thickness of 0.03 m, CEN prescribes an additional dose of 0.48 mSv a−1 to take the presence into account of an underlying bulk layer made of reference concrete (CEN, 2017). To put this in a different way, the calculated dose rate of materials used for finishing purposes should thus be compared with an operational gross reference level of 0.82 mSv a−1. In spite of this subtraction, a 0.03 m thick layer of gypsum consisting of 100% sNORM (e.g.

3. Requirements for a generalized test method As shown in the previous section the proper regulation of exposure to gamma radiation emitted by building materials requires a generalized test method that takes into account multiple building layers. Such a test method should fulfil the following basic requirement: “Application in a dwelling of a building material to be tested should, together with any combination of other building materials that passes this test, lead to a dose not exceeding the BSS reference level for gamma radiation”. One way to comply with this requirement is to practice the following set of derived criteria: ● Distinguish between bulk layers (i.e. layers necessary for the construction of the building) and superficial layers (i.e. top or finishing layers that cannot exist without the presence of at least one bulk layer); ● The regulator prescribes the test conditions, which means: fixed

Fig. 3. Spatial distribution of the annual effective dose in the CEN reference room computed with the Markkanen model; The concrete and gypsum contain the maximum amount of sNORM based on the CEN formula (Equation (2)); a) The contribution of the concrete walls with a thickness of 20 cm; b) The contribution of a gypsum layer with a thickness of 3 cm; c) Total effective dose due to both construction layers (including the attenuation of the gamma radiation emitted by the concrete layer due to the gypsum layer). 44

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column shows the index I values for the building materials with maximum allowed sNORM. To test whether the operational conditions for building materials as derived with the three-layer-model are satisfactory, we performed a number of calculations, using the computer code described in Section 2.3. The results are shown in Fig. 5. The upper row shows the results for two construction layers, with combinations of building materials with properties as given on the right side of Table 2 (maximum percentage of sNORM). In all cases, the annual dose in the middle of the reference room stays below 1.3 mSv a−1. The lower row shows the results of the same combinations, but now with an extra superficial layer of gypsum with a maximum amount of sNORM, in accordance with the value as given in Table 2. The calculated annual doses in the middle of the room are now slightly higher compared to the results without a superficial gypsum layer, but they all stay below the gross BSS reference level of 1.3 mSv a−1. This is, however, not always the case close to the walls, especially not when low density materials with a maximum allowed amount of sNORM are present at the inner side of the reference room. 4. A proposal to regulate the exposure from thoron progeny in dwellings

Fig. 4. Schematic view of the three-layer-model, used for the derivation of operational reference levels for bulk layer materials and building materials used for wall finishing purposes.

In the past decades, much attention has been paid to radon and progeny in dwellings and other buildings, but indoor data for thoron (Rn-220) and especially thoron progeny are not widely available. Due to its relatively short half-life of 56 s, the radioactive noble gas thoron, if present in the living areas of a dwelling, can only be exhaled from the superficial layer of walls, ceilings or floors. The second one in the range of short-lived thoron decay products, Pb-212, has a half-life of 10.6 h, which means that thoron progeny is much longer present in the indoor environment than its parent, Rn-220. Thoron progeny is usually expressed in terms of the equilibrium-equivalent thoron progeny concentration, EETC (McLaughlin, 2010). For a long time, the general opinion was that indoor exposure to short-lived decay products of thoron was small, compared to that of radon. In the last decade, however, a growing amount of information has become available, showing that this is not always the case. Concentrations of indoor thoron are reported from several countries now, such as Hungary, Serbia, Canada and China (McLaughlin, 2010), but such data are difficult to interpret because thoron gas is not homogeneously dispersed in dwellings. Indoor data for thoron progeny, which are more appropriate for dose estimations, are still scarce, but there are a few noteworthy studies to mention. In Ireland, a measurement campaign was carried out in a sample of 205 dwellings, yielding EETC values between 0.05 en 3.8 Bq m−3, with an average value of 0.47 Bq m−3. The corresponding annual inhalation doses are 0.35 (mean value) and 2.9 (maximum value) mSv a−1. In 14 out of the 205 dwellings investigated, the annual dose from thoron progeny exceeded that from radon (McLaughlin, 2010). In Germany, measurements of average concentrations of Pb-212 were carried out in modern earthen buildings, showing results in the range 2–10 Bq m−3. These concentrations result in annual inhalation doses of up to 4 mSv, from Pb-212 alone. There are currently about two million buildings in Germany that contain earthen building materials. These materials are applied in both traditional and modern constructions, where especially the walls and in some cases also the ceiling are decorated with mud (Meisenberg et al., 2013). A nationwide survey to both radon and thoron progeny in a representative group of some 2,500 Dutch dwellings built since 1930 yielded an average EETC value of 0.64 Bq m−3. A maximum value of 13.3 Bq m−3 was recorded (Smetsers et al., 2018). Moreover, spot measurements, carried out in close to a hundred dwellings in the Netherlands, showed that actual thoron exhalation rates vary over two orders of magnitude, mainly due to differences in wall finishing manners and materials. The largest recorded value was approximately 1 Bq m−2 s−1, which is 45 times larger than the median value of 2.2 × 10−2

values have to be established for the thickness of the material to be tested and the allowed dose for that type of layer. These values are different for bulk layers and superficial layers; ● The calculated dose of any (reasonable) combination of layers should not exceed 1.3 mSv a−1. It turns out that the use of a simple three-layer-model, covering the majority of everyday situations, yields a clear and limited set of practical conditions for the two groups of building materials. Fig. 4 gives a schematic overview of the three-layer-model. Applying this model yields the following set of operational constraints for the calculated annual dose for the two categories of building materials, where we make use of the CEN-formula as given in Equation (2): [281 + 16.3 dB 0.0161( dB )2] × CRa 226 Bulk : DB = + [319 + 18.5 dB 0.0178( dB )2] × CTh 232 × 10 + [22.3 + 1.28 dB

0.00114( dB )2] × CK

6

[mSv a 1]

DBmax

40

(4a) 0.0161( dS ) 2] × CRa 226 Superficial : DS = + [319 + 18.5 dS 0.0178( dS ) 2] × CTh 232 × 10 6 [mSv a 1] + [22.3 + 1.28 dS 0.00114( dS ) 2] × CK 40 [281 + 16.3 dS

DSmax

(4b) where dB and dS are fixed, regulated values for the thickness of bulk and superficial layers, respectively, and DBmax and DSmax are operational gross reference values for bulk layers and superficial layers, respectively. The following set of parameter values seems to work out well for our purpose, although we stress once again that parameter values should be established by the regulator. For bulk layers, we applied a fixed thickness, dB, equal to 0.1 m. The maximum allowed annual dose for bulk layers, DBmax, was set to 0.75 mSv a−1. For superficial layers, we choose a fixed thickness, dS, equal to 0.03 m and a maximum annual dose, DSmax, equal to 0.25 mSv a−1. We applied these conditions to five commonly used building materials (concrete, ceramic brick, calcium silicate brick, aerated concrete and gypsum) and calculated the maximum allowed percentage of sNORM that can be added while still fulfilling the conditions as given in Equation (4). Table 2 shows, on the left, the reference values of the activity concentrations in these materials (EC, 1997). On the right, the calculated results are shown of the activity concentrations in these materials when the maximum allowed percentage of sNORM has been added. This percentage is given in the middle column. The outer right 45

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Table 2 Properties of five commonly used building materials, four applied in bulk layers (B) and one in superficial layers (S). The activity concentrations on the left half of the table refer to reference materials (EC, 1997). The activity concentrations on the right are the calculated results for these materials with a maximum allowed percentage of sNORM being added, following the conditions derived from the three-layer-model with parameter values as given in the text. The maximum allowed percentage of sNORM is listed in between. The right column shows the activity concentration index I, calculated for materials with maximum allowed sNORM. Material

Density −3

[kg m Concrete (B) Ceramic brick (B) Calcium Silicate brick (B) Aerated concrete (B) Gypsum (S)

U-238 ]

2350 1900 1750 625 900

Th-232 −1

[Bq kg

]

40 50 10 60 10

K-40

−1

[Bq kg

]

sNORM −1

[Bq kg

30 50 10 40 10

]

400 670 330 430 80

U-238

Th-232 −1

[%]

[Bq kg 14 13 25 55 34

]

104 108 132 300 174

−1

[Bq kg

K-40 ]

68 82 82 181 107

Index I −1

[Bq kg

]

624 840 744 1280 723

0.90 1.05 1.10 2.33 1.36

Fig. 5. Annual effective dose distribution for several combinations of construction layers with maximum amount of sNORM according to the three-layer model (Equation (4)). The upper row shows from left to right the combinations of two bulk layers (outer layer – inner layer); concrete - concrete, concrete - aerated concrete, ceramic bricks - concrete, ceramic bricks - calcium silicate bricks. The lower row shows the same combinations supplemented with a gypsum superficial layer. The values correspond to the annual effective dose in the middle of the room.

Bq m−2 s−1 (Blaauboer, 2012; Smetsers et al., 2018). The concentration of thoron progeny in a living room is proportional to the average activity concentration of Th-232 (or, more precisely, its daughter Ra-224) in the material applied as superficial layer. Based on (1) in situ EETC measurements in 2,500 dwellings, (2) measurements of thoron exhalation rates from various building materials, (3) approximately 175 in situ measurements of thoron exhalation rates and (4) a series of model calculations, de With et al. derived this relationship for a Dutch reference dwelling (de With et al., 2018). Their results are summarized in Table 3, where they assume the presence of a 0.01 m thick superficial layer of gypsum on all walls and ceilings with a specific Ra-224 activity of 250 Bq kg−1. The range in figures in Table 3 indicates the influence of paint and/or wallpaper, covering the inner side of the building envelope. The higher values refer to the absence of any paint or wallpaper, the lower values assume an average reduction of the thoron exhalation rate by paint or wallpaper of 40%. So it

appears that, when all walls and ceilings in a dwelling are finished with a 0.01 m thick layer of (phospho)gypsum with a specific Ra-224 activity of 250 Bq kg−1, an annual radiation dose due to the inhalation of thoron progeny will be in the order of 5 mSv a−1. We have shown in Section 2.3 that, when we follow the recommendations of CEN, phosphogypsum with a Th-232 activity concentration of approximately 300 Bq kg−1 still complies with the BSS reference level for gamma radiation. When we apply the more strict conditions derived from the three-layer-model (Section 3), gypsum with a specific Th-232 activity of approximately 100 Bq kg−1 complies. When such materials are applied as finishing layer throughout a dwelling, thoron inhalation doses are anticipated to be well above 1 mSv a−1. In other words, compliance of a building material with the BSS reference level for gamma radiation does not guarantee that applying such material will never lead to ‘exposures that cannot be disregarded from a radiation protection point of view’ (Euratom, 2014). Restricting the average annual thoron inhalation dose to 1 mSv a−1, a level comparable to the BSS reference level for gamma radiation, implies that the activity concentration of Th-232 (or Ra-224) in (especially porous) building materials used for wall finishing purposes should be limited to a value of typically 50 Bq kg−1. In this approach, we used phosphogypsum as a representative for various porous materials with a high content of Th-232 commonly used for wall finishing purposes. Other examples are lightweight concrete with alum shale, Italian tuff and mud (Chambers, 2010; Meisenberg et al., 2013). These materials are all mentioned in Annex XIII of the BSS because of their properties with respect to gamma radiation (Euratom, 2014).

Table 3 Relation between the activity concentration of Ra-224 in a 0.01 m thick superficial layer of gypsum and the annual thoron progeny inhalation dose, derived for a typical Dutch dwelling. The higher values refer to the absence of paint or wallpaper, the lower values assume an average reduction of the thoron exhalation rate by paint or wallpaper of 40% (de With et al., 2018). Activity concentration of Ra-224 Thoron exhalation rate Thoron total source term Thoron progeny concentration (EETC) Annual thoron inhalation dose

250 Bq kg−1 1.7 – 2.9 Bq m−2 s−1 200 – 330 Bq s−1 14 – 23 Bq m−3 4.4 – 7.3 mSv a−1

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5. Discussion

made from a building material with a low density and a relatively high content of natural radionuclides. This observation is relevant, because in many dwellings (child) beds and lounge suites are positioned close to the wall. Thirdly, we call for extra restrictions for the use of porous wall finishing materials with a high Ra-224 content, in order to reduce the exhalation of thoron and consequent inhalation of thoron decay products. From the last two arguments it follows that it would be appropriate to make the conditions for building materials used in superficial layers even stricter than chosen in the elaboration of the presented three-layer-model.

5.1. The balance between sustainability and radiation protection Sustainability is one of the major challenges of our time. In the coming decades the demands for a more sustainable society will have great impact on many areas and the building industry will be one of them. Recycling of products and the use of former industrial waste products as raw materials will be of growing importance in the manufacturing of building materials. This has serious advantages, above all less dependence on natural resources, reduced waste streams, less transportation of bulk materials and, at least in principle, less pollution. On the other hand, we should be careful too, because the use of new kinds of raw materials may introduce new risks as well. This is for instance the case when raw materials with relatively high levels of natural radionuclides are used in building materials, meant to be applied in dwellings or other buildings with a high occupancy factor. An indicative number of building materials or additives which may contain relatively high levels of natural radionuclides are summed up in Annex XIII of the BSS (Euratom, 2014). Examples are natural materials such as granites or pozzolana and residues from industries processing naturally occurring radioactive materials, such as fly ash, red mud and phosphogypsum. These materials are considered in Annex XIII with regard to their emitted gamma radiation, but they may also be a source of radon and thoron. Due to demands of sustainability, we anticipate that Annex XIII materials will be used more and more by the building industry. A first example of maximizing the amount of industrial waste product in building materials up to the BSS reference level is already available (Sas et al., 2015), and we foresee that more will follow. This development may result in a gradual but steady increase of the indoor average annual radiation dose received by the general public, which is, even now already, one of the major contributors to the total radiation dose received by the public. This increase may be potentially large in the end. As stated earlier, the BSS reference level for gamma radiation from building materials is typically one order of magnitude larger than the current average annual gamma dose received by the population in many western countries, so there is much regulatory room for extended use of Annex XIII materials. Apart from that, use of Annex XIII materials may also increase the average annual inhalation doses due to the exhalation of radon and thoron from building materials, depending on the physical and chemical structure of the product and its application. For radon, we did not quantify this here, but for thoron we anticipate annual dose levels well above 1 mSv a−1 when porous materials with a high but still permitted activity concentration of Ra-224 are applied on all walls and ceilings of a dwelling. In this assessment, we made use of dose conversion coefficients given by UNSCEAR (UN, 2009), but the dose conversion coefficients for radon and thoron are currently under debate. The ICRP recommends values that are nearly twice as large, compared to UNSCEAR (ICRP, 2017; UN, 2009). When we follow the ICRP recommendations, the radiation dose resulting from enhanced radon and thoron exhalation due to the presence of Annex XIII materials in building materials will thus be even more significant.

5.3. The activity concentration index I as a screening tool The activity concentration index I, as postulated by the BSS, is generally regarded as a conservative screening tool. But when the presence of multiple building layers is taken into account, the index I appears to be less conservative than expected. For materials with a density between 1,750 and 2,000 kg m−3 and a maximum percentage of sNORM as calculated with Equation (4) (using the parameter values of the three-layer-model as provided in this paper), the corresponding index I lies between 1.0 and 1.1. In this range, the index I is thus a good estimator for the allowed content of natural radionuclides. Ceramic brick and calcium silicate brick fall in this range. For materials with a much lower density, such as aerated concrete, the index I is indeed a conservative estimate for the annual effective dose from gamma radiation in the middle of a room. However, for these low-density materials the annual effective dose close to the wall is much higher than in the middle of the room. For concrete, with a density of around 2,350 kg m−3, the index I overestimates the amount of natural radionuclides that is allowed to be present, according to the conditions suggested in this paper. That is due to the fact that the index I is based on the presence of 0.2 m thick concrete walls, but without taking into account the presence of a superficial layer containing high percentages of sNORM. For gypsum, the index I is slightly conservative compared to the conditions presented in this paper, but that is without taking into account extra precautionary conditions to prevent high thoron exhalation rates to occur. The relation between the annual gamma dose and the index I, as a function of material density for both bulk layers and superficial layers, is illustrated in Fig. 6. 5.4. Differences in the regulation approach for thoron versus radon In this paper, we propose the introduction of a reference level for the activity concentration of Th-232 (or Ra-224) in (especially porous) building materials used for wall finishing purposes, in order to limit the indoor exposure to thoron progeny. This approach differs from the way the exposure to radon progeny has been regulated: the radon reference level is namely defined as a specific value (to be established by the national authority) for the annual averaged indoor radon concentration (in Bq m−3) (Euratom, 2014). This different approach has a reason. Radon enters a building in various ways and originates partly from building materials and partly from the soil underneath the building. The latter pathway dominates in so-called radon prone areas where radon concentrations are significantly enhanced compared to the worldwide average value. Appropriate countermeasures are in general directed at the way buildings are constructed and ventilated (Rahman and Tracy, 2009). The use of building materials with enhanced concentrations of Ra-226 has certainly impact on the radon concentration indoors, but this pathway is of limited importance. For thoron, this situation is quite different. Due to the short half-life of thoron, effectively all thoron progeny inside a building originates from thoron exhaled from the superficial layer of walls, ceilings and floors. It is now more reasonable to limit the exposure to thoron progeny by regulating the maximum allowed concentration of Ra-224 in (porous) building materials used for finishing purposes, instead of establishing a reference level for the annual averaged concentration of thoron progeny in

5.2. Points of particular interest for regulators For the purpose of radiation protection, we therefore call for extra caution regarding the use of Annex XIII materials. Firstly, we propose that the reference level for gamma radiation from building materials will be applied in a proper manner, i.e. by taking into account the (potential) presence of multiple building layers. This leads to more restrictive conditions, compared to the situation when building materials are compared to the BSS reference level on their own. Secondly, we ask attention for the fact that the calculated annual dose from gamma radiation near walls may be significantly higher than in the middle of the (reference) room, especially when the superficial layer is 47

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difficult because the corresponding BSS reference level refers to gamma radiation emitted by all building materials present in a room. But in practice, compliance of single building materials has to be tested. Based on a simple model of three layers of building materials, we derived a set of operational conditions for building materials, either used for construction purposes (‘bulk layers’) or for the finishing of walls, floors and ceilings (‘superficial layers’). We showed for a number of everyday situations that any combination of building materials meeting these conditions stays below the BSS reference level for gamma radiation. This statement holds for the middle of a reference room, but is not always the case close to the walls, especially when low density materials with a relatively high content of natural radionuclides are present at the inner side of the reference room. This can be avoided by applying more strict conditions for those kind of materials. So far, no international regulations exist to limit the indoor exposure to thoron progeny. We showed that building materials that pass the test for gamma radiation can still be a significant source for indoor concentrations of thoron and progeny. When the average annual thoron inhalation dose were to be restricted to 1 mSv a−1 − a level comparable to the BSS reference level for gamma radiation − the activity concentration of Th-232 (or Ra-224) in (especially porous) building materials used for wall finishing purposes should be limited to a value of typically 50 Bq kg−1. Even if our suggested approach of the BSS regulations is fully implemented, it still allows for a gradual increase in the average radiation exposure in dwellings due to external radiation and thoron (progeny) up to a few mSv a−1. The situation will, however, be worse if a less strict interpretation of the BSS regulations will be applied.

Fig. 6. Calculated (Equation (4)) annual gamma dose for building materials (ratio of natural radionuclides similar as in fly ash) with an index I equal to one, as a function of density (continuous lines). For bulk layers, we used a layer thickness of 0.1 m, and for superficial layers a thickness of 0.03 m. The horizontal dashed lines represent the proposed operational reference levels for bulk layers (upper line, 0.75 mSv a−1) and superficial layers (lower line, 0.25 mSv a−1). The markers indicate the dose in the middle of a reference room of commonly used building materials, such as concrete and gypsum. These data are calculated with a computer code based on the model described by Markkanen (1995). The vertical lines on top of each marker show the range of higher dose values when approaching the wall.

buildings. Mitigation techniques for thoron (e.g. applying a wall coating) are available (de With et al., 2016, 2018), but the main problem is that dwellings with enhanced concentrations of thoron progeny are very difficult to identify. And there are more arguments against coating existing walls. Firstly, there is a large difference in the stopping power of regular wall painting materials, ranging from virtually zero to almost 100%, and the percentage of shielding also decreases in time due to wearing (de With et al., 2016, 2018). Moreover, detailed information of the shielding characteristics of various brands of paint are – at least at the moment – not publically available, which makes it impossible for inhabitants to choose a proper product. Secondly, there is a growing interest in the use of ‘natural products’ (such as clay, loam or mud) for the finishing of walls. The natural look of these kinds of materials is a prevailing characteristic. For aesthetical reasons, house owners are not willing to cover such walls with a layer of paint. Based on all these arguments, we recommend to take appropriate actions to prevent that enhanced concentrations of thoron progeny may occur in dwellings, instead of mitigating existing situations.

Acknowledgements This study was partly funded by the Dutch Authority for Nuclear Safety and Radiation Protection (ANVS). We further thank Harry Slaper, Lars Roobol, Roelf Blaauboer, Teun van Dillen and other colleagues from RIVM for their contributions to this paper. References BfS, 2017. Umweltradioaktivität und Strahlenbelastung im Jahr 2015: Unterrichtung durch die Bundesregierung. Bundesamt für Strahlenschutz (BfS), Berlin. Blaauboer, R., 2012. Meetmethoden Thoron in Survey, VERA-onderzoek. Dutch National Institute for Public Health and the Environment (RIVM), Bilthoven. CEN, 2017. Construction Products: Assessment of Release of Dangerous Substances — Radiation from Construction Products — Dose Assessment of Emitted Gamma Radiation. CEN. Chambers, D.B., 2010. Thoron and decay products, beyond UNSCEAR 2006 Annex E. Radiat. Protect. Dosim. 141, 351–356. De Jong, P., van Dijk, J., 2009. Analyse Van Het Externe Dosistempo in Woningen. NRG. de With, G., 2017. Development of an assessment method for building materials under Euratom scope. Health Phys. 113, 392–403. de With, G., de Jong, P., Donk, J.J., 2016. Thoron mitigation from building materials with surface barriers. Health Phys. 111, 420–426. de With, G., Smetsers, R., Slaper, H., de Jong, P., 2018. Thoron exposure in Dutch dwellings - an overview. J. Environ. Radioact. 183, 73–81. EC, 1997. Enhanced Radioactivity of Building Materials, Radiation Protection 96. European Commission, Luxembourg. EC, 1999. Radiological Protection Principles Concerning the Natural Radioactivity of Building Materials, Radiation Protection 112. European Commission, Luxembourg. Euratom, 2014. Council Directive 2013/59/EURATOM of 5 December 2013 Laying Down Basic Safety Standards for Protection Against the Dangers Arising from Exposure to Ionising Radiation. Euratom. ICRP, 2017. Occupational Intakes of Radionuclides: Part 3. Julius, H.W., van Dongen, R., 1985a. Radiation doses to the population in The Netherlands, due to external natural sources. Sci. Total Environ. 45, 449–458. Julius, H.W., van Dongen, R., 1985b. Stralingsbelasting van de bevolking en stralingsniveaus in het binnenmilieu in Nederland t.g.v. natuurlijke gammabronnen. Rapportage SAWORA-projecten A1.1 en A1.2. TNO, RIVM. Markkanen, M., 1995. Radiation Dose Assessments for Materials with Elevated Natural Radioactivity. Finnish Centre for Radiation and Nuclear Safety, Helsinki. McLaughlin, J., 2010. An overview of thoron and its progeny in the indoor environment. Radiat. Protect. Dosim. 141, 316–321. Meisenberg, O., Gierl, S., Tschiersch, J., 2013. Measurement of thoron and its progeny in traditional and modern earthen buildings in germany - methodology and results. In: 2013 International Radon Symposium of the American Association of Radon

6. Conclusion A significant part of the public radiation exposure is received indoors. Due to demands of sustainability, we anticipate that raw materials containing elevated amounts of Ra-226 (and progeny), Th-232 (and progeny) and K-40 will be used more and more by the building industry, resulting in a gradual but steady increase of the indoor average annual radiation dose received by the general public. Directive 2013/59/Euratom pays comprehensive attention to this subject, but proper implementation of all corresponding BSS regulations is not straightforward, especially when regarding the regulation of building materials which contain certain amounts of so-called Annex XIII materials. In this paper, we discussed the most relevant deficiencies in the BSS and presented a practical approach to cope with these. The most important observation we made is that satisfactory methods for assessing the annual dose due to gamma radiation from building materials are not provided by the BSS. This is in particular 48

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R.C.G.M. Smetsers, J.M. Tomas Scientists and Technologists, Springfield, Illinois. NCRP, 2009. Ionizing Radiation Exposure of the Population of the United States. NCRP, Washington DC. PHE, 2016. Ionising Radiation Exposure of the UK Population: 2010 Review. Public Health England, Chilton. Rahman, N., Tracy, B., 2009. Radon control systems in existing and new construction: a review. Radiat. Protect. Dosim. 135, 12. Sas, Z., Somlai, J., Szeiler, G., Kovács, T., 2015. Usability of clay mixed red mud in Hungarian building material production industry. J. Radioanal. Nucl. Chem. 306, 271–275. Smetsers, R., Blaauboer, R., 1996. Variations in Outdoor Radiation Levels in the Netherlands (Thesis). Rijksuniveriteit Groningen, Bilthoven, pp. 264.

Smetsers, R., Blaauboer, R., 1997. Source-dependent probability densities explaining frequency distributions of ambient dose rate in The Netherlands. Radiat. Protect. Dosim. 69 (1), 33–42. Smetsers, R., Blaauboer, R., Dekkers, F., Slaper, H., 2018. Radon and thoron progeny in Dutch dwellings. Radiat. Protect. Dosim. 181, 11–14. UN, 2009. UNSCEAR 2006 REPORT Vol. II - Effects of Ionizing Radiation - ANNEX E Sources-to-effects Assessment for Radon in Homes and Workplaces. United Nations, New York. Willems, P., Vanaudenhove, T., 2018. Berekening van de jaarlijkse gemiddelde blootstelling aan ioniserende straling in België: Methodologie en Evolutie. FANC, Brussels, pp. 33.

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