8.1.1 Radiation basics

Radioactivity is a process by which certain naturally occurring or artificial nuclides undergo spontaneous decay releasing a new energy. This decay process is accompanied by the emission of one or more types of radiation, ionizing or non-ionizing, and/or particles. This decay, or loss of energy, results in an atom of one type, called the parent nuclide, transforming to an atom of a different type, named the daughter nuclide. The SI derived unit of radioactivity is the becquerel (symbol Bq), which is defined as the activity of a quantity of radioactive material in which one nucleus decays per second; in other words, Bq is equivalent to s⁻¹.

Ionizing radiation is electromagnetic (in the form of waves with a wavelength of 100 nm or less, i.e. a frequency of 3 × 10^− 15 Hz or more) or corpuscular radiation that has sufficient energy to ionize certain atoms of the matter in its path by stripping electrons from them. This process can be direct (as with alpha particles) or indirect (gamma rays and neutrons).

Gamma radiation composed of high-energy photons, which are weakly ionizing but have high penetrating power (more than the X-ray photons used in radiodiagnosis), can travel through hundreds of meters of air. Thick concrete shielding or lead helps to protect personnel. Gamma radiation is primarily responsible for external exposure. As far as internal radiation exposure hazard is concerned, the high penetrating power means that the energy released by gamma rays and taken up by a small volume of tissue is comparatively small. Hence the harm to the organ is also smaller. Therefore, the internal radiation exposure hazard caused by gamma rays is not as severe as that induced by other types of radiation (alpha and beta).

Alpha radiation consists of ⁴He nuclei and has low penetrating power. Its path in biological tissues is no longer than a few tens of micrometers. This radiation is strongly ionizing, i.e. it easily strips electrons from the atoms in the matter it travels through, because the particles shed all their energy over a short distance. Alpha emitters are primarily responsible for internal exposure, which includes inhalation, ingestion and skin contact.

Beta radiation is made up of electrons and has moderate penetrating power. Hence, exposure to beta particles presents greater external irradiation hazard and less internal radiation hazard than exposure to alpha particles. However, as the external irradiation brought by beta particles is mostly confined to the epidermis and outer skin tissue, such external irradiation hazard is not too severe.

Exposures are not limited to the intake of large amounts at one time (acute exposure). Chronic exposure may arise from an accumulation of small amounts of radioactive materials over a long period of time.

8.1.2 Radiation dosimetry

Radiation dosimetry deals with the calculation of the absorbed dose in matter and tissue resulting from the exposure to indirectly and directly ionizing radiation. The absorbed dose is the mean energy imparted by ionizing radiation to the matter per unit mass. To remind readers, dose is reported in grays (Gy) for the matter or sieverts (Sv) for biological tissue, where 1 Gy or 1 Sv is equal to 1 joule per kilogram. The distinction between absorbed dose (Gy) and dose equivalent (Sv) is based upon the biological effects of the weighting factor (denoted w_r); tissue/organ weighting factors (WT) have been established, which compare the relative biological effects of various types of radiation and the susceptibility of different organs.

X-rays and gamma rays have a weighting factor of unity, such that 1 Gy = 1 Sv (for whole-body irradiation). Values of w_r are as high as 20 for alpha particles and neutrons, i.e. for the same absorbed dose in Gy, alpha particles are 20 times as biologically potent as X- or gamma rays.

The weighting factor for the whole body is 1, such that 1 Gy of radiation delivered to the whole body (i.e. an evenly distributed 1 joule of energy deposited per kilogram of body) is equal to one sievert (for photons with a radiation weighting factor of 1). Therefore, the weighting factors for each organ must sum to 1 as the unit gray is defined per kilogram and is therefore a local effect. Organ dose weighting factors W_T recommended in IAEA (2011) are given in Table 8.1. For example, for the lungs a tissue-weighting factor of 0.12 is recommended.

The interaction of ionizing radiation with biological material results in ionizations and excitations of molecules and atoms, which may cause molecular changes in the DNA in the cell nucleus. Building inhabitants are exposed mainly to alpha and gamma radiation, which may result in chromosomal abnormalities and gene mutations, if the doses are high.

The induction of a radiation-induced cancer is assumed to be probabilistic in nature and proportional to the radiation dose, with no threshold. Such a model is known as the linear, non-threshold theory (LNT). According to LNT, doubling the exposure will double the number of cells struck, and so doubles the chances of developing a cancer, yielding a linear dose–response relationship. However, several studies reported a hormetic dip in the low dose range suggesting a beneficial, protecting effect at low exposures. On the basis of the current literature ICRP (2007) judged that the knowledge of these phenomena is insufficient to be incorporated in a meaningful way into the modeling of epidemiological data. Therefore, the LNT model is considered as the most appropriate in radiological protection at present. The legislation in the field of radiological protection is based on a fundamental concept called ALARA (as low as reasonably achievable), which is a direct derivative of the LNT model.

To convert the absorbed dose rate in air due to exposure to gamma radiation into an effective dose, various coefficients are available, which depend on radiation geometry and gamma-ray energy. A conversion coefficient of 0.7 Sv Gy^− 1 can be adopted from ICRP publication 74 (ICRP, 1996). Cosmic radiation contributes to the absorbed dose rate indoors. At the same time, the building materials of the dwelling partly shield this component. Shielding factors range from 1 for wooden houses down to 0.3 for lower floors of concrete buildings (Miller and Beck, 1984). Julius and Van Dongen (1985) have determined an average value of 0.6 for the Netherlands.

8.1.3 Radiation sources

Radiation sources can be natural or artificial (man-made). People are now exposed to both types of radiation, and have been exposed to radiation from the natural environment throughout history. The predominant part of the natural radiation in the environment and in humans is caused by cosmic radiation and telluric radiation. In addition to the natural exposure, human activities involving the use of radiation and radioactive substances cause radiation exposure. Man-made activities include the fallout from atmospheric testing of nuclear weapons and radiological events like the Chernobyl accident. Deposition studies of these activities indicate that radioactive particles travel around the world on streams of air.

This chapter deals mainly with natural radioactivity from building materials, although a few cases related to the presence of artificial radionuclides in building materials have been described in the literature. Natural radiation, which accounts for 85% of total radioactivity (natural plus artificial), is made up of 14% cosmic radiation and 71% telluric radiation.

Cosmic radiation is caused by radionuclides, which are formed by the interaction of cosmic rays arriving from stars, and especially the Sun, with the nuclei of elements present in the atmosphere (oxygen and nitrogen). Cosmogenic radionuclides include carbon ¹⁴C, tritium ³H and beryllium ⁷Be. Cosmic radiation increases with altitude above sea level, and is higher in high floors of buildings.

Telluric radiation is caused by so-called primordial radionuclides – naturally occurring radioactive materials, or briefly NORM. The primordial radionuclides are present in bedrock, soil, building materials, water, air, and the human body. Primordial radionuclides are left over from the creation of the earth. They typically have half-lives of hundreds of millions of years. The telluric radiation of primordial radionuclides is a key source of external gamma radiation, both inside and outside buildings.

Examples of primordial radionuclides are uranium (²³⁵U and ²³⁸U), thorium ²³²Th, and potassium ⁴⁰ K. The contents of the natural radioactive substances vary widely between different rocks and soil types, due to the different ways in which they were formed. The presence of these isotopes in building materials can cause the levels of radiation indoors to be even greater than those found outside. Whereas buildings made from wood do not have this problem, the downside is that wood acts as a poor shield from the gamma radiation coming from the soil.

8.2.1 Potassium-40

Potassium-40 (⁴⁰ K) is a naturally occurring radioactive isotope of the common element potassium (potassium represents about 2.4% by weight of the earth’s crust). Two stable (non-radioactive) isotopes of potassium exist, ³⁹ K and ⁴¹ K. Potassium-39 comprises most (about 93%) of naturally occurring potassium, and potassium-41 accounts for basically all the rest. The half-life of ⁴⁰ K is 1.27 × 10⁹ years. Radioactive ⁴⁰ K comprises a very small fraction (about 0.012%) of naturally occurring potassium. Because potassium-40 represents 0.012% of naturally occurring potassium, its concentration in the Earth’s crust is about 1.8 mg/kg, or 480 Bq/kg. The report of United Nations Scientific Committee on the Effects of Atomic Radiation gives the mean ⁴⁰ K concentration of 400 Bq/kg in soil (UNSCEAR, 2000).

Potassium-40 is an important radionuclide in terms of the dose associated with naturally occurring radionuclides. The health hazard of ⁴⁰ K is associated with cell damage caused by the ionizing radiation that results from radioactive decay, with the general potential for subsequent cancer induction.

8.2.2 Decay chain of radium-226 (uranium-238)

Uranium was isolated by Martin Heinrich Klaproth in 1789 from the mineral pitchblende. At that time uranium was not considered as particularly dangerous and was used for coloring pottery and glass. In 1896 Henri Becquerel observed that uranium was emitting invisible rays that fogged a photographic plate as if it was exposed to daylight (Emsley, 2003). In honor of his discovery of radioactivity, the unit for radioactivity is given the name becquerel (Bq), corresponding to one disintegration per second. Natural uranium mainly contains ²³⁸U, which is the parent of a decay series schematically presented in Fig. 8.1. The ²³⁸U series is often called the ²²⁶Ra series. As shown in this figure, each member of this series is unstable and decays by either alpha or beta emission until stable ²⁰⁶Pb has been formed. Besides ²³⁸U natural uranium contains 0.73% ²³⁵U. This isotope is also the parent of a decay series ending at ²⁰⁷Pb. Uranium-238 is the most common isotope of uranium found in nature. Around 99.284% of natural uranium is uranium-238, which has a half-life of 4.468 × 10⁹ years. Uranium is not as rare as it was once thought. It is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as molybdenum or arsenic (Hammond, 2000). It occurs in numerous minerals such as pitchblende, uraninite, carnotite, autunite, uranophane, davidite, and tobernite. It is also found in phosphate rock, lignite and monazite sands, and can be recovered commercially from these sources. Traces of uranium (as well as thorium ²³²Th, which is described later) can be found in practically all the mineral raw materials used for production of concrete and other construction cementitious products.

8.2.3 Decay chain of thorium-232

Thorium, ²³²Th, occurs naturally and has a half-life of 14.1 × 10⁹ years. Thorium occurs in thorite and thorianite (Hammond, 2000). Its trace amounts are also found in the majority of building materials of mineral origin, including concrete. The decay chain of ²³²Th is presented in Fig. 8.2. One of the isotopes of this chain is radon ²²⁰Rn, called thoron. However, of the two main radon isotopes, ²²⁰Rn and ²²²Ra, which will be discussed in the next section, thoron has a shorter lifetime and accounts for a minority of radiation exposure.

8.2.4 Radon-222

Radon (²²²Rn) is the most stable radon isotope, with a half-life of 3.8 days. It is created as part of the normal radioactive decay chain of uranium ²³⁸U (radium ²²⁶Ra) (see Fig. 8.1). Uranium, radium, and thus radon, will continue to occur for millions of years at about the same concentrations as they do now. Chemically, radon is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of radium ²²⁶Ra. It is one of the densest substances that remains a gas under normal conditions and is considered to be a health hazard due to its radioactivity.

Radon ²²²Rn, like ²²⁰Rn, decays to isotopes of solid elements, the atoms of which attach themselves to the dust particles present in air. When the radon equilibrium factor (the ratio between the activity of all short-period radon progenies, which are responsible for most of radon’s biological effects, and the activity that would be at equilibrium with the radon parent) is 1, it means that the decay products have stayed close to the radon parent long enough for the equilibrium to be reached. These conditions are usually not met in most buildings. Because of their electrostatic charge, radon progenies adhere to surfaces or dust particles, whereas gaseous radon does not. Attachment removes them from the air, usually causing the equilibrium factor in the atmosphere to be less than 1. The typical value of the equilibrium fraction in dwellings is 0.4 (UNSCEAR, 2000). The equilibrium factor is also lowered by air circulation or air filtration devices, and is increased by airborne dust particles, including cigarette smoke. In high concentrations, airborne radon isotopes contribute significantly to human health risk.

Radon ²²²Rn is responsible for the majority of the mean public exposure to ionizing radiation. It is often the single largest contributor to an individual’s background radiation dose and is the most variable from place to place. Radon gas from natural sources can collect in buildings, especially in limited areas such as attics and foundations. It can also be found in some spring waters and hot springs. Epidemiological proof shows a connection between breathing high concentrations of radon and incidence of lung cancer. Therefore, radon is considered a significant contaminant that affects indoor air quality worldwide. Radon is the second most frequent cause of lung cancer after cigarette smoking, causing 21,000 lung cancer deaths per year in the United States (Agency for Toxic Substances and Disease Registry, 1990).

Radon emanates naturally from the ground and from mineral building materials, wherever traces of uranium or thorium can be found, and particularly in regions with soils containing granite or shale, which have a higher concentration of uranium. In fact, every square mile of surface soil, to a depth of 6 inches (every 2.6 km² to a depth of 15 cm), contains approximately 1 gram of ²²⁶Ra, which releases radon in small amounts to the atmosphere (Agency for Toxic Substances and Disease Registry, 1990). Due to its very short half-life (3.8 days for ²²²Rn), its concentration decreases very quickly when the distance from the production area increases. Its atmospheric concentration varies greatly depending on the season and conditions. For instance, it has been shown to accumulate in the air if there is a meteorological inversion and little wind (Steck et al., 1999).

As a frame of reference, typical domestic exposures are about 10–20 Bq m^− 3 outdoors (Office of Radiation and Indoor Air, 2003) and about 100 Bq m^− 3 indoors (Agency for Toxic Substances and Disease Registry, 1990). Depending on how houses are built and ventilated, radon may accumulate in foundations and dwellings. Radon concentrations in the same location may differ by a factor of 2 over a period of 1 hour. Also, the concentration in one room of a building may be significantly different from the concentration in an adjoining room.

Typical excess indoor radon concentrations due to building materials are low: about 10–20 Bq m^− 3, which is only 5–10% of the design value of 200 Bq m^− 3 introduced by the European Commission (RP-112, 1999; European Commission, 2010). However, in some cases the building materials may be an important source also, and occasionally the concentration may rise to 1000 Bq m^− 3 or more (European Commission, 2010). For example, in Sweden, the radon emanating from building materials is a major problem in some areas, where many dwellings with walls made of lightweight concrete based on alum shale – so-called ‘blue concrete’ – have been built (The Radiation Protection Authorities in Denmark, Finland, Iceland, Norway and Sweden, 2000). The problem of enhanced radon exhalation from building materials will be discussed in more detail in further sections.

Radon ²²²Rn has been classified as being carcinogenic to humans (UNSCEAR, 2000). The primary route of exposure to radon and its progeny is inhalation. Radiation exposure from radon is indirect. The health hazard from radon does not come primarily from radon itself, but rather from the radioactive products formed in the decay of radon. The general effects of radon on the human body are caused by its radioactivity and consequent risk of radiation-induced cancer. Lung cancer is the only observed consequence of high-concentration radon exposures; both human and animal studies indicate that the lung and respiratory system are the primary targets of radon daughter-induced toxicity (Agency for Toxic Substances and Disease Registry, 1990).

The range of dose conversion factors for radon, derived from epidemio-logical studies and physical dosimetry, varies from 6 to 15 nSv (Bq h m^− 3)^− 1 (UNSCEAR, 2000; Chen, 2005). Assuming 7000 hours per year indoors (an occupancy factor of 80%), and an equilibrium factor of 0.4, and using the UNSCEAR (2000) recommendation for a radon conversion factor of 9 nSv per (Bq h m^− 3), exposure to radon at 100 Bq m^− 3 will be equivalent to an annual effective dose of 100 Bq m^− 3× 0.4 × 7000 h × 9 nSv (Bq h m^− 3)^− 1 = 2.5 mSv.

Recently the ICRP (2009) published a statement that it indeed intends to publish dose coefficients that result in an increase of the effective dose per unit exposure.

• 200 Bq m^− 3 for new buildings

• 400 Bq m^− 3 for existing dwellings and buildings with a high occupancy of the public (such as nursing homes, schools and prisons)

• 1000 Bq m^− 3 for existing workplaces and other public buildings.

8.4.1 Gamma-ray spectrometry

Spectrometric measurements indicate that the three components of the external radiation field, particularly from the gamma-emitting radionu-clides in the ²³⁸U and ²³²Th series and ⁴⁰ K, make approximately equal contributions to the outwardly occurring gamma radiation dose to individuals in typical conditions both outdoors and indoors. Each material may have a unique radiation spectrum, which can serve as its ‘fingerprint’ in quality control operations.

Gamma-ray spectrometers are typical instruments used in a wide variety of scientific and industrial applications. In order to measure the radioactivity concentration and estimate the radiation hazards from building materials, gamma-spectroscopy is usually applied using different detectors. Two main types of gamma-spectrometers are presently in use, high-pure germanium (HPGe) crystal spectrometers and scintillation NaI(Tl) spectrometers. Their pros and cons will be discussed below.

The main advantages of gamma-ray spectrometry are:

These advantages, allow using gamma-spectrometry not only for controlling the radioactivity of building products available on the local market, but also for controlling the radionuclide and phase composition of various building materials, such as hardened concrete.

At the same time, the method has the following limitations and difficulties:

If the first limitation is an intrinsic property of the method, in our opinion the second problem can be technically solved.

A literature review shows that gamma-spectrometry could be effectively applied for controlling concrete composition; however, a number of limitations exist. In the publications of Kovler and Manasherov (1999), Rowbottom et al. (1997) and Pakou and Assimakopolous (1994) the application of gamma-spectrometry for determination of concrete mix composition in hardened specimens is described; however, there was no precedent to test fresh concrete mixes. One of the problems in measuring specimens immediately after placing them in the container is a possible disequilibrium in the decay chains of ²²⁶Ra. Hence, testing fresh concrete mixes is usually impossible; the limitation to the test is related to the need to seal the probe and wait until equilibrium occurs between radium, radon and radon progeny. Usually the standards require sealing the probe and waiting three weeks before the measurement, which is about five times longer than the half-life of ²²²Rn. It should be noted that both of the long decay chains described previously (of radium and thorium) pass through a step involving isotopes of radon, which is a noble gas. If radon escapes from a sample to any appreciable extent, radioactive equilibrium is disturbed. This alters the ratio of the number of gamma rays emitted from radionuclides located before and after radon ²²²Rn. The extent of radon escape and its effect on the gamma-ray spectrum can be gaged by comparison with a sealed source, in which radioactive equilibrium is maintained.

Another source of uncertainty in gamma-ray spectrometry is a possible diffusion of radon gas through the walls of the container, made of plastic material. It is known that plastics are generally permeable to gases, including radon.

8.4.2 High-pure germanium (HPGe) vs scintillation NaI(Tl) spectrometers

The activity concentrations of natural radionuclides ²²⁶Ra, ²³²Th and ⁴⁰ K can be determined by y-ray spectrometry. Practice shows that NaI(Tl)-based detectors could be successfully used for quantitative determination of activity concentrations of compounds of few and known radionuclide composition, such as those containing natural radionuclides.

Most common detectors include sodium iodide doped with thallium, or NaI(Tl), scintillation counters and high-purity germanium (HPGe) detectors. HPGe detectors show the best energy resolution, while NaI(Tl) detectors have the highest efficiency and the lowest minimum detectable activity (Perez-Andujar and Pibida, 2004).

NaI(Tl) detectors yield good efficiency but have poor resolution. For this reason they are often considered as not suitable for the identification of complicated mixtures of γ-ray-producing materials and quantitative determination of their radionuclide composition. Figure 8.5 shows spectra from the same calibrated source of ²²⁶Ra obtained by means of HPGe and NaI(Tl) detectors. It can be seen that the HPGe spectrum demonstrates clear and sharp peaks characteristic of ²²⁶Ra (located at energies of 295, 392 and 609 keV), while the peaks obtained by the NaI(Tl) detector are wide. This makes the identification of radionuclides and the quantitative determination of activity concentration of radionuclides by means of scintillation detectors rather difficult, especially in cases when the peaks are close and overlap. Finally, when radionuclide composition of the tested material is complex and unknown, scintillation spectrometers become practically useless for quantitative determination of activity concentrations.

The energy resolution of a scintillation spectrometer is significantly worse than that predicted by photon-statistics alone. The additional degrading effects are a consequence of several factors. A first effect is the variance in the scintillation efficiency of the crystal itself, which is energy dependent and cannot be corrected for in a simple way. A second effect is the non-uniformity of the response of the photo-cathode. A third effect is the variance in the light-collection efficiency of the crystal and photo-multiplier assembly for events that occur in different locations within the detector crystal.

One of the methods to compensate for the lower spectral resolution of NaI(Tl) detectors is to apply spectral deconvolution to the raw energy-loss data collected by the spectrometer. Deconvolution is a technique used in spectroscopy and other diverse fields, in which a raw data spectrum obtained with a detection system is deconvolved with a response function representing the response of the detection system to known input signals (Meng and Ramsden, 2000).

HPGe detectors provide significantly improved energy resolution in comparison to NaI(Tl) detectors; however, cryogenic temperatures are vital to their operation, which makes the maintenance of the spectrometric system more complicated and more costly. One of the disadvantages of the HPGe detector is that it can only function as a spectrometer if cooled to liquid-nitrogen temperatures, otherwise electrons can be thermally excited into the conduction band and so generate a high level of noise. This means that an HPGe detector is neither compact nor rugged. The second disadvantage is that in order to provide a stopping power equivalent to a commonly available size of scintillation spectrometer, the germanium crystal becomes very expensive to fabricate (Perez-Andujar and Pibida, 2004).

At the same time, the practice shows that NaI(Tl)-based detectors can be successfully used for quantitative determination of activity concentrations of mixtures of few and known radionuclide composition, such as those containing natural radionuclides (NORM) only. In many applications, such as the mass control of radioactive contaminants in building materials, it is not practical to use a high-purity germanium spectrometer cooled to liquid-nitrogen temperatures. The improved efficiency, for the same size detector, and the lower cost of a NaI(Tl) detector must be traded off against the better resolution of an HPGe detector. However, for performing a quantitative analysis, the NaI(Tl)-based system should have special software capable of distinguishing between different radionuclides in the mixture and accurately determining their activity concentrations (Kovler et al., 2010).

• Closed-chamber methods (CCM), which, in their turn, can be divided into two categories:

• Open-chamber methods (OCM).

(a) there is no leakage of radon out of the container; and

(b) the activity concentration in the container air is low compared to the activity concentration in the pore air of the sample.

1. Natural materials:

2. Materials incorporating residues from industries processing naturally occurring radioactive materials, such as:

8.7.1 Granite

Granite is a common, coarse-grained, hard igneous rock consisting chiefly of quartz, orthoclase or microcline, and mica. Granite has been used as a building material since ancient times. It is one of the oldest and most durable building products available, and will far outlast the building in which it is installed. It has become the material of choice for today’s luxury homes and offices because of its enduring beauty, and because no synthetic material can yet compare to its elegance and performance. Granite is a popular choice for kitchen and bathroom counter tops. Granite tiles of 20 to 50 mm thickness are widely used as covering and building materials for counters, cashier desks, shelves, benches and tables. These surfaces are often referred to as granite, but in fact they can consist of different stone types that include granite and marble.

The concern of using granite products available in the market is the external radiation dose from them. Activity concentrations of the natural radionuclides (Bq kg^− 1) of a large number of granite samples from different countries measured by Chen and Lin (1996) and by Pavlidou et al. (2006) are shown in Table 8.5.

8.7.2 Phosphogypsum

Phosphogypsum is a waste by-product from the processing of phosphate rock in plants producing phosphoric acid and phosphate fertilizers, such as superphosphate. The wet chemical phosphoric acid treatment process, or ‘wet process’, in which phosphate ore is digested with sulfuric acid, is widely used to produce phosphoric acid and calcium sulfate, mainly in dihydrate form (CaSO₄ · 2H₂O):

[8.10]

Annual world production of phosphogypsum is estimated to be ~300 Mt (Yang et al., 2009). This by-product is contaminated by various impurities, both chemical and radioactive, and is usually stockpiled within special areas. The problem of contaminated phosphogypsum has already become an international ecological problem. For example, a huge amount of phospho-gypsum has accumulated in Florida (more than 1 billion (!) tons), in Europe (where the contaminated phosphogypsum is discharged into the River Rhine close to the North Sea), in Canada, Morocco, Togo, India, China, Korea, Israel, Jordan, Syria, Russia, and other parts of the world.

The building materials industry seems to be the largest among all the industries which is able to reprocess the greatest amount of this industrial by-product and benefit man. However, because of the contamination, only 15% of world phosphogypsum production is recycled as building products and asset retarder in the manufacture of Portland cement (a small amount is recycled as agricultural fertilizer or for soil stabilization amendment), while the remaining 85% is disposed of without any treatment (Tayibi et al., 2009). Disposed phosphogypsum is usually dumped in large stockpiles, occupying considerable land areas and causing serious environmental damage due to both chemical and radioactive contamination.

Typical concentrations of radium (²²⁶Ra) in phosphogypsum are 2003000 Bq kg^− 1(US Environmental Protection Agency, 1990). They are similar to those in phosphate ores. Digestion with sulfuric acid causes the selective separation and concentration of naturally occurring radium (²²⁶Ra), uranium (²³⁸U) and thorium (²³²Th): about 80% of ²²⁶Ra is concentrated in phospho-gypsum, while nearly 86% of ²³⁸U and 70% of ²³²Th end up in phosphoric acid (Tayibi et al., 2009). In other words, most of the ²²⁶Ra follows phospho-gypsum, which is responsible for its enhanced radioactivity, and most of the ²³⁸U and ²³²Th remain in the phosphoric acid product.

In addition to radionuclides, phosphogypsum contains some trace contaminants which may pose health and environmental hazards, such as arsenic, lead, cadmium, chromium, fluorine, zinc, antimony, and copper (US Environmental Protection Agency, 1990). These trace elements may be leached from phosphogypsum, as radionuclides, migrate to the nearby surface and ground water, and cause fluorescence on the surface of building elements.

The key problem restraining the utilization of phosphogypsum in construction is its radiological effect on the human population, and it is not solved yet. Unfortunately, no effective technologies are known for processing phosphogypsum and for its utilization in the construction industry. The main problem is the slightly elevated radioactivity of phosphogypsum, which is due to the high activity concentration of ²²⁶Ra, while the remaining impurities can be extracted relatively easily, for example by using phase transformations between different kinds of calcium sulfate hydrate and filtering the obtained solution. Traditional technologies of purification of phosphogypsum from radium are not effective, because of the similarity of chemical properties of radium sulfate and calcium sulfate salts, when the radioactive salt is isomorphously included in the gypsum crystal lattice (Kovler, 2004).

There have been several attempts to manufacture building materials from phosphogypsum in different countries. For example, phosphogypsum was used some time ago by a New Jersey company for the manufacture of wallboard, partition blocks, and plaster for distribution in the northeastern United States (Fitzgerald and Sensintappar, 1978). Due to the absence of low-cost natural gypsum and the lack of long-term storage place, phospho-gypsum has been used extensively for wallboard and other building materials and also as a cement retarder in Japan and South Korea.

Among European countries phosphogypsum is used in limited amounts (or was formerly used) in Austria, Belgium, Germany, the Netherlands, the United Kingdom, Finland, Greece and some other countries that are not members of the EU (RP-96, 1997). However, the modern environmental norms, which are getting stricter year by year in different countries, leave almost no chance for commercial application of phosphogypsum in construction without previously solving the awkward problem of its elevated radioactivity. No wallboard containing phosphogypsum is commercially manufactured now in the USA, and the situation is not going to change in the near future.

8.7.3 Building products containing coal fly ash

The building industry uses large amounts of by-products from other industries. In recent years there is a growing tendency to use new recycled materials with technologically enhanced levels of radioactivity. Coal fly ash is one of the best known examples.

Large quantities of coal fly ash are expelled from coal-fired thermal power plants and these may contain enhanced levels of radionuclides along with other toxic elements. More than 280 Mt of coal ash (fly ash and bottom ash combined) are produced annually. About 40 Mt of these are used in the production of bricks and cement (IAEA, 2003). Since most of the process residues further processed into building materials do not meet the required technical specifications, they are typically mixed with pristine raw materials. The net effect is a dilution of the NORM (naturally occurring radioactive material) content relative to the process residues.

Recycling and utilization of coal fly ash (FA) in concrete construction has clear environmental, technological and economic advantages. Fly ash, a by-product of coal combustion, is widely used as a cementitious and pozzolanic ingredient in Portland-cement concrete. It may be introduced either as a separately batched material, or as a component of blended cement. The use of coal fly ash in concrete construction is increasing because it often results in lower-cost concrete and improves some properties of concrete. Among the positive technological effects are workability improvement, bleeding reduction, lowering heat of hydration, refinement of pore structure and decrease of permeability. The continued pozzolanic activity of fly ash contributes to increased strength gain at later ages if the concrete is kept moist.

The use of coal fly ash in concrete is a well-recognized source of gamma exposure that is due to the presence of activity concentrations of ²²⁶Ra, ²³²Th and, to a lesser extent, ⁴⁰ K, while the effect of coal fly ash via radon exhalation is controversial, in particular due to the low emanation coefficient from the ash (Kovler et al., 2004b). This effect will be discussed in a following section. Most of the coal fly ash is reused by cement or concrete producers. Concrete is the most popular building material in the world: annual production of concrete is about 1 m³ per capita. The radionuclide composition of concrete depends on its constituents: cement, aggregates and mineral additives, and their dosages.

As can be seen from Table 8.4, radioactivity concentrations found in ordinary concrete are rather close to the worldwide average concentrations of radium, thorium and potassium in the earth’s crust, which are about 40 Bq kg^− 1, 40 Bq kg^− 1 and 400 Bq kg^− 1, respectively, and lower than the values found in the most common building materials and industrial byproducts used for construction.

Concrete aggregate consisting of crushed stone often has the greatest significance for the total radioactivity of the material, because its total mass content is usually the highest among concrete constituents. If radium-rich and thorium-rich granites are included as aggregates in concrete, the indoor gamma radiation from the walls and floors may be appreciably higher than the average outdoors. In buildings with walls and floors made of concrete containing aggregate of granite or basic gneiss origin with high contents of ²³⁸U, the indoor gamma radiation level can reach 0.3 μSv h^− 1. Such radiation levels also occur in Swedish buildings with walls of certain types of bricks made of glacial clays with enhanced ²³⁸U and ²³²Th concentrations (The Radiation Protection Authorities in Denmark, Finland, Iceland, Norway and Sweden, 2000).

The highest radium contents, from 42 to 62 Bq kg^− 1, have been measured in Finnish concrete (Mustonen, 1984). The measured mean rates of radon exhalation rate (E) were 20 to 32 Bq m^− 2 h^− 1. The corresponding airborne radon concentration was 46.2 Bq m^− 3 with a mean air exchange rate of 0.64 h^− 1 when calculated for radon exhalation rate of 20 Bq m^− 2 h^− 1. The enhanced radium concentrations can be explained by the fact that granite aggregates are popular in Finland.

Commonly used aggregates manufactured from natural stone of sedimentary origin (such as limestone or dolomite) do not normally enhance the radionuclide content of concrete mix. However, some mineral additives, such as blast furnace slag or ash (either coal fly ash, or peat ash, which is often used in Finland, or oil shale ash), although not introduced in high dosages, can cause enhanced activity concentrations of concrete. Phosphogypsum, when used as a set retarder in cement in small amounts not exceeding 4–5% by cement mass, usually has almost no influence on the ²²⁶Ra activity concentration of concrete, because the mass fraction of phosphogypsum in concrete does not exceed even 1%.

As an example, let us consider typical concrete compositions, with and without coal fly ash, which can be introduced into the mix as a partial replacement for cement and fine aggregates (sand). Activity concentrations of coal fly ash, which has been subject to a kind of enrichment as a result of the coal combustion process in the thermal power plant, are usually higher than of those of cement and aggregates, which can be seen from Table 8.6.

The simple calculation of the activity concentrations expected in these concrete mixes, taking into account the mass contents of the raw materials they contain, shows that NORM activity concentrations in the concrete with fly ash are higher than in the reference concrete (Table 8.7). For example, radium equivalent activity calculated by equation 8.9 is higher by 44% in the mix containing fly ash.

Similarly to coal fly ash, some other materials, like blast furnace slag and peat ash (the latter has been widely used in Finland instead of coal fly ash), can be used as an aggregate material replacing virgin natural stone aggregate, or as an additive in cement manufacturing. These industrial byproducts may cause also enhanced concentrations of natural radionuclides.

In general, the tendency to reuse industrial by-products and wastes as raw materials in construction is growing, for both economic and environmental reasons.

8.8.1 Granite

Measurements of radon exhalation for a total of 205 selected samples of building materials used in Saudi Arabia (Al-Jarallah and Fazal-ur-Rehman, 2005) showed that granite samples were the main source of radon exhalation. The radon exhalation rates per unit area from these granite samples varied from below MDA (minimum detectable activity, or minimum detection limit) up to 13.1 Bq m^− 2 h^− 1. The ²²⁶Ra contents were measured in 27 granite samples and varied from below MDL up to 297 Bq kg^− 1. A linear correlation coefficient between exhaled radon and radium content was found.

Emanation of radon (²²²Rn) from granite used for counter tops and mantels was measured on 24 granite samples. Measured radon flux ranged from 0.6 to 86 Bq m^− 2 h^− 1, with most granites emitting < 5.6 Bq m^− 2 h^− 1 (Kitto et al., 2009). Granite counter tops, in particular, have received recent media attention regarding their potential to emit radon. Allen et al. (2010) measured radon flux on 39 full slabs of granite from 27 different series. Flux was measured at up to six pre-selected locations on each slab and also at areas identified as potentially enriched after a full-slab scan using a Geiger–Müller detector. Whole-slab average emissions ranged from less than MDA to 79.4 Bq m^− 2 h^− 1, similar to the range reported in the literature for convenience samples of small granite pieces. Modeled indoor radon concentrations were less than the average outdoor radon concentration (15 Bq m^− 3) and average indoor radon concentrations (50 Bq m^− 3) found in the United States. Finally, significant within-slab variability was observed for stones on the higher end of whole slab radon emissions, underscoring the limitations of drawing conclusions from discrete samples.

Chen et al. (2010) determined radon exhalation rates for 53 different samples of drywall, tile and granite available on the Canadian market for interior home decoration. The radon exhalation rates ranged from non-detectable to 13 Bq m^− 2 h^− 1. It was found that slate tiles and granite slabs had relatively higher radon exhalation rates than other decorative materials, such as ceramic or porcelain tiles. The average radon exhalation rates were 1.2 Bq m^− 2 h^− 1 for slate tiles and 1.8 Bq m^− 2 h^− 1 for granite slabs of various types and origins. Analysis showed that even if an entire floor was covered with a material having a radon exhalation rate of 12.5 Bq m^− 2 h^− 1, it would contribute only 18 Bq m^− 3 to a relatively tightly sealed house with a ventilation rate of 0.3 h^− 1. It was concluded that the local building materials for interior home decoration studied in the research project make no significant contribution to indoor radon for a house with adequate air exchange.

8.8.2 Phosphogypsum

Ulbak et al. (1984) found that samples of ordinary Danish concrete, where phosphogypsum has been used as an additive in cement, showed a significant increase of radon flux.

The porous microstructure of gypsum in building elements, such as wallboards, explains the relatively high values of the radon emanation factor, which can reach 30–50% (Bossew, 2003; Stoulos et al., 2004; Kovler et al., 2004b). Taking this into account, together with the elevated concentrations of ²²⁶Ra in phosphogypsum, the radon concentration in the living areas of buildings should be high as well. This was proved recently by Máduara et al. (2011), who measured radon concentrations of ~ 100 Bq m^− 3 during long-term radon tests (three months) conducted in a special model room built of wallboards with 10–15 mm thickness produced from phosphogypsum having ²²⁶Ra activity concentration of ~ 400 Bq kg^− 1. Because of elevated levels of ²²⁶Ra and high values of the radon exhalation rate, building materials containing phosphogypsum could result in elevated radiation exposures (from both gamma radiation and inhalation of radon) to building occupants.

8.8.3 Concrete

The results obtained in Finland, Greece, Germany and the Netherlands (RP-96, 1997) show that concrete has the highest normalized radon exhalation rate (exhalation rate per unit area and per unit content of ²²⁶Ra), about 0.2 to 0.5 Bq m^− 2 h^− 1/Bq kg^− 1. Therefore, concrete in buildings can contribute to indoor radon levels more than other building materials with the same ²²⁶Ra content. This fact can be explained by the relatively thick building elements (walls, floors and ceilings) made of concrete, and the high specific surface area of cement hydrates, which facilitates the release of radon atoms into the porous microstructure and their transport to the surface. In addition, concrete usually contains some moisture in its capillary pores. Radon can be easily trapped and transported to the surface of concrete elements with water and water vapor flow. As a result, moisture of concrete significantly influences its radon exhalation rate.

It is known that the recoil distance of the radon atoms in air is about 900 times higher than in water. Thus, water traps the recoiled radon atoms more easily than air. A thin water film, which continuously engulfs cement particles and hydration products, would be sufficient to stop the recoiled atoms in the water. Radon is being produced in soil and building materials as a result of the presence of trace amounts of ²³⁸U. Depending on the properties of the materials such as porosity, tortuosity, permeability and the presence of cracks, and the conditions such as moisture content, rate of saturation and pressure gradients, surface coating, radon diffusion length in the material and thickness of the building element, radon can be transported through the soil and building materials and enter the dwelling. The amount of radon from the soil and building materials entering the indoor air can range over several orders of magnitude due to changes in these properties.

It was shown by Stranden et al. (1984) and Yu et al. (1996) that the radon-trapping ability of pore water and the probability of radon emanation from the pores decreased when concrete dried. It has been reported that high moisture content increases the radon emanation power of materials but reduces the radon diffusion coefficient. Tanner (1964) gave the following explanation of the effect of high moisture content: if the pores are filled with water, radon must diffuse through the water, but the radon diffusion coefficient for water is very low, only 10^− 5 cm² s^− 1, compared to the diffusion coefficient of about 10^− 2 cm² s^− 1 for air. Stranden et al. (1984) found that the increase in radon emanation due to moistening will more than compensate the reduction in radon diffusion coefficient up to a certain level of moisture content, at which the pores have some liquid water but are not completely filled with water. When the pores are completely filled with water, the reduced diffusion will reduce the radon exhalation dramatically. They discovered that the effects of moisture are rather dramatic and a factor of about 20 between the highest and lowest exhalation rate could be found in several cases.

An attempt has been made recently to study the relationship between the moisture content and the radon exhalation rate of hardening cementitious systems (Kovler, 2008). The radon flux from the hardening cement paste observed in these experiments reached unusually high values of 0.6–1.0 mBq kg^− 1 s^− 1 when the cement set. Such values significantly exceed those known before about normal building materials (including those made of Portland cement). The most reasonable explanation for the sharp increase of radon exhalation rate while the cement sets seems to be a synergy of the following main mechanisms:

It has to be underscored that the elevated radon concentrations which develop in a few hours after mixing cement with water are very short-lived. So, comparing these findings with an action level that is meant to be applied to average long-term radon concentrations is fraught with danger. In addition, most construction sites are drafty and have much activity going that will increase air exchange, so the true radon exposure is likely to be on the lower side. Let us not forget that workers often move from one construction site to another, so it is unlikely that they are always exposed to the low ventilation. Finally, the minimum air exchange rate usually accepted in design practice (0.5 h^− 1) guarantees that the concentrations in most cases will not exceed the action level and that they are not of any radiological concern for construction workers.

As was mentioned before, the use of coal fly ash in concrete and other building materials is a well-recognized source of gamma exposure that is due to the presence of activity concentrations of the three primordial radio-nuclides, ²²⁶Ra, ²³²Th and, to a lesser extent, ⁴⁰ K, while the effect of coal fly ash via radon exhalation is controversial, in particular due to the low emanation coefficient of the ash particles. The radon exhalation rate of concrete does not change significantly with its introduction into building materials. Although the radon exhalation rate of concrete containing coal fly ash sometimes can be slightly higher than that of concrete without fly ash, the radon emanation coefficient of such concrete is usually lower (Kovler, 2011). This phenomenon can be explained by the low radon emanation power of fly ash (Kovler et al., 2004b). Van Dijk and de Jong (1991) studied the radon emanation of concrete with ordinary Portland and Portland–fly ash cements and found that the radon emanation coefficient of concrete made with Portland–fly ash cement was lower by ~ 20%. Roelofs and Scholten (1994) concluded that under certain conditions, the addition of coal fly ash to the concrete mix could even reduce the radon exhalation rate.

Alum shale was formerly used in both ordinary and lightweight (aerated) concrete mainly in Sweden, but also to a very limited extent in Denmark. About 300,000 dwellings with walls made of lightweight concrete based on alum shale were built in Sweden, where elevated radon exhalation rates became a major problem (The Radiation Protection Authorities in Denmark, Finland, Iceland, Norway and Sweden, 2000). This can be explained by the extremely high ²²⁶Ra content (up to 4500 Bq kg^− 1, see Table 8.3) and porous structure of lightweight concrete, which has a long radon diffusion length and high emanation power. As a result, all the concrete production based on alum shale was shut down in 1975 because of concerns about radioactivity (Mjönes, 1986).

8.9.1 Existing regulations

The convenient parameter used in the norms to limit the overall content of radionuclides in concrete and other building materials of mineral origin is the so-called activity concentration index. The activity index in the EC document (RP-112, 1999) and in many other national standards regulating radioactivity of building materials is calculated on the basis of the activity concentrations of radium (²²⁶Ra) in the uranium (²³⁸U) decay series, thorium (²³²Th) in the thorium (²³²Th) decay series, and potassium (⁴°K). Other nuclides are sometimes taken into consideration as well: for example, the activity concentration of cesium (¹³⁷Cs) from fallout is regulated in the Finnish guidelines (ST12.2, 2005).

If the activity index exceeds 1, the responsible party is required to show specifically that the relevant action level is not exceeded. If the activity index does not exceed 1, the material can be used, so far as the radioactivity is concerned, without restriction. The criterion of meeting the standard is the non-dimensional value of the so-called activity concentration index taking into account the total effect of the three main natural radionuclides, which can be present in building materials:

[8.11]

where A_Ra, A_Th and A_K are the activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰ K, respectively, in Bq kg^− 1 (RP-112, 1999). According to RP-112 (1999), the activity concentration index I shall not exceed the values listed in Table 8.10, depending on the dose criterion and the way and the amount in which the material is used in a building.

The EC guidelines allow for controls to be based on a lower dose criterion, if it is judged that this is desirable and will not lead to impractical controls. It is recommended to exempt building materials from all restrictions concerning their radioactivity if the excess gamma radiation originating from them increases the annual effective dose of a member of the public by 0.3 mSv at the most.

Similarly to the guidelines of the European Commission (RP-112, 1999), the draft of the new European Basic Safety Standard Directive (European Commission, 2010) refers to two types of building products: those used in bulk amounts and those used as superficial and other materials with restricted use. In addition, two categories of building products are defined:

According to this directive, type A building products and raw materials will be exempted at national level (except activity concentration surveillance), while type B building products and raw materials should be controlled by the national authorities.

The Member States will also be required to insert in their legislation a list of the different types of building materials which need to be controlled with regard to their emitted gamma radiation. Materials incorporating byproducts or residues from NORM industries, such as fly ash, are in this list.

In spite of the fact that the overall radiation hazard due to ionizing radiation from building materials includes both the gamma radiation component, which depends on their radionuclide content, and the component caused by their radon exhalation, most of the standards in the world that regulate the radioactivity of building materials, including concrete, address the gamma radiation only (Kovler, 2011). Indeed, the evaluation of the excess dose caused by building materials for the radon pathway is rather complicated (Markkanen, 2001). One of the reasons is that the actual correlation between the radon exhalation rate measured in the laboratory and the excess indoor radon concentration on site might be rather poor. Numerous factors, such as temperature (both indoors and outdoors), air pressure and humidity fluctuations, total porosity, pore distribution and pore type (open or closed), surface treatment at the building site and type of coating material applied, influence significantly radon exhalation in dwellings. Finally, it is extremely difficult to take into account the effect of the behavior of the occupier, particularly concerning ventilation. That is why most of the standards regulating radioactivity of building materials address the radon exhalation in a very simplified form, through the limitation of ²²⁶Ra, the precursor of ²²²Rn in the ²³⁸U radioactivity chain.

For example, the amount of radium in building materials is recommended to be restricted to a level where it is unlikely to be a major cause for exceeding the design level for indoor radon. In most of Europe the design level for indoor radon is 200 Bq m^− 3 (RP-112, 1999). Taking into account that the most important source of indoor radon is the underlying soil, the majority of indoor radon on the upper floors of a building is expected to originate from building materials. Typical excess indoor radon concentration due to building materials is usually very low: about 10–20 Bq m^− 3, which is only 5–10% of the design value introduced in the European Commission Recommendation (RP-112, 1999). In a practical sense, taking into account that within the European Union, gamma doses due to building materials exceeding 1 mSv yr^− 1 are very exceptional and can almost be disregarded from the radiation protection point of view, the limitation on ²²⁶Ra concentrations is reduced to the assumption that when gamma doses are limited to levels below 1 mSv yr^− 1, the ²²⁶Ra concentrations in the materials are below the design level (200 Bq m^− 3). This recommendation is, of course, too general.

Some countries apply separate restrictions for ²²⁶Ra content in building materials. For example, Nordic countries recommend 100 and 200 Bq kg^− 1, respectively, as an exemption level and an upper allowable level for the activity concentration of ²²⁶Ra in building materials for new constructions as a source of indoor radon (The Radiation Protection Authorities in Denmark, Finland, Iceland, Norway and Sweden, 2000). The maximum allowable concentration of ²²⁶Ra in building materials used for dwellings is limited at 200 Bq kg^− 1 in Poland (Regulation of the Council of Ministers, Republic of Poland, 2007, January 2) and China (GB-6566, 2001).

The norms in the Czech Republic are slightly more complicated (Hůlka, 2008). Radon emanation from building materials is taken into account by limiting the ²²⁶Ra content in premises, depending on their character (inhabited or uninhabited) and on the type of building products (products used in bulk amounts, or products with restricted use, such as superficial products) (Table 8.11). Raw materials, such as aggregates of coal fly ash, are included in the same category as products with ‘restricted use’. In addition to ²²⁶Ra concentrations, Czech regulations take into account radon emanation coefficient, radon diffusion length in building materials and air exchange rate (Hůlka et al., 2008).

Two recently published national standards, Austrian Standard ÖNORM S 5200 ‘Radioactivity in Construction Materials’ (2009) and Israeli Standard SI 5098 ‘Content of Natural Radionuclides in Building Products’ (2009), address radon emanation from building materials as a part of their controls criteria. Both standards include the dose from radon inhalation in the total dose excess for the inhabitants.

ÖNORM S 5200 sets the following controls criterion for natural radio-nuclides:

[8.12]

where ε is the emanation coefficient, ρ is the density and d is the wall thickness.

According to ÖNORM S 5200 (2009), if the emanation factor is not known, the activity concentration index I can be calculated using the precondition value ε = 10%. The real emanation factor can be also determined in the direct experiment, but its value should not be higher than the precondition value. The coefficients A_Ra, A_Th and A_K consider external radiation exposure, and the part including the radon emanation coefficient is responsible for radon inhalation in the final dose criterion. The precondition values for the wall thickness and density are d = 0.3 and ρ = 2000 kg m^− 3, respectively. It can be seen that the part of this combined dose criterion responsible for gamma exposure does not depend on the density and geometry of the building element, but the part responsible for radon inhalation does. SI 5098 (2009) uses a similar approach and allows using the following values of emanation coefficient, if the measurement data are not available: 6% for masonry blocks made with lightweight aggregate (such as pumice), 7% for masonry blocks made of normal-weight aggregates, and 12% for other building products, including concrete. This standard sets the following expression for the activity concentration index I:

[8.13]

where the first, third and fourth components are responsible for the direct gamma-radiation exposure of inhabitants, and the second for the radon inhalation dose.

It can be seen that the first component of this criterion takes into account the reduction of gamma radiation dose caused by ²²⁶Ra, because of its disintegration and emanation of ²²²Rn. The coefficients A₁, A₂, A₃ and A₄ depend on the specific surface mass, i.e. mass per unit surface area of the building product (ρd). For example, for building products made of normal-weight concrete with ρd = 450 kg m^− 2 the coefficients A₁, A₂, A₃ and A₄ are equal to 421, 11.6, 298 and 4150 Bq kg^− 1, respectively.

8.9.2 Challenges in regulation of radioactivity of building materials

The legislation in the field of radiological protection is based on a fundamental concept called ALARA (as low as reasonably achievable). At the same time the question remains how to define the term ‘reasonably achievable’. The second important related principle of radiological protection provides that no level of radiation exposure is acceptable without justification. Restricting the use of certain building materials might have significant economic, environmental or social consequences, which should be assessed and considered when establishing binding regulations. Coal fly ash is an example of an industrial by-product for which recycling and utilization in construction is technologically and economically beneficial. In view of the tendency observed in recent years to make environmental norms stricter, the only alternative left would be making a cost–benefit analysis. Indeed, the stricter are the restrictions, the more expensive would be their implementation. The environmental protection, including the radiological protection of populations exposed to ionizing radiation, is under the financial and juridical responsibility of governmental authorities. The expenses are usually shared between governments, the private sector and the public. However, the budget resources are usually limited, and implementation of strict regulations is often impossible and remains as a declaration only. Two different approaches available for such cost–benefit analysis, based on estimating the cost of a person-mSv, have been described by Kovler (2009), but more approaches are available.

Such cost–benefit analysis should serve as an instrument for deciding how strict the regulations should be. The cost-benefit analysis should also address the aspects of recycling of industrial by-products and wastes in manufacture of building materials. When considering the justification for recycling, the radiation exposure of the public and workers is the negative aspect, and saving the natural resources and protection of the environment are the positive aspects. In addition, different social–economic and political aspects have to be taken into account. For example, when deciding to exclude a certain class of building products or industrial by-products from being manufactured or imported, alternative sources of mineral resources or building products for the construction market should be provided, otherwise the additional expenses are laid on the public. In cases where the import of raw materials, building items or industrial by-products and wastes is restricted, there is also a need to ensure that the international laws related to free trade, import and export are not violated.

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