Full length article

Naturally occurring radioactive materials (NORM) in energy production
sectors: exposure, effective doses and regulatory challenges

Jelena Mrdakovic Popic<sup>a, *</sup>, Nathalie Vanhoudt<sup>b, c</sup>, Gennaro Venoso<sup>c, d</sup>, Federica Leonardi<sup>d, e</sup>, Hallvard Haanes<sup>a, d</sup>, Alla Dvorzhak<sup>e, d</sup>, Cristina Nuccetelli<sup>c, d</sup>, Rosabianca Trevisi<sup>d</sup>, Raffaella Ugolini<sup>f</sup>, Flavio Trotti<sup>f</sup>, Almudena Real<sup>e</sup>, Danyl Pérez-Sánchez<sup>e, d</sup>, Alicia Escribano<sup>e, d</sup>, Joana Lourenco<sup>g, d</sup>, Ruth Pereira<sup>h</sup>, Laureline Fevrier<sup>i, d</sup>, Boguslaw Michalik<sup>j, d</sup>

ARTICLE INFO

Keywords:

Natural radioactivity

Hazards

Oil and gas industry

Coal-fired power plants

Geothermal energy production

Radiation protection

A B S T R A C T

The oil and gas industry, coal mining, and coal-fired power plants, as well as geothermal energy plants, have been investigated in the past with respect to various aspects of naturally occurring radioactive materials (NORM). However, existing assessments are usually highly specific and often inconclusive due to differing perspectives, and an overview of NORM issues across these energy sectors is still missing.

The European data collected within the international RadoNorm project, and presented in this paper, have made it possible to identify common denominators related to the primordial source of radiation risk in these industrial sectors. Depending on the technological processes, radionuclides of natural origin (e.g., ²²⁶,²²⁸Ra, ²²⁸Th, ²²²Rn, ²¹⁰Pb, ²¹⁰Po, from ²³⁸U and ²³²Th decay chains) were found in a range of materials, such as scales, sludges, produced water, and liquid and gaseous discharges. Both radionuclides and materials may follow different pathways, leading to radiation exposure doses in various exposure scenarios for workers, the public, and the environment.

The novelty of this paper lies in the comparative qualitative and quantitative evaluation of radiation pro-
tection aspects across the entire sequence of energy generating processes, which includes the sourcing of raw
materials, NORM generating processes, their fate and potential risks to humans and the environment, as well as a
comparison of management, regulatory control, and existing challenges. The presented findings will support
future international harmonization of decision-making processes during industrial operations, decommissioning
activities or at legacy sites, which are often encountered in these energy generation sectors. Moreover, this in-
formation will be particularly valuable for countries worldwide that are still in the initial phase of NORM in-
ventory characterization and/or legislation development.

<footnote><sup>a</sup> Norwegian Radiation and Nuclear Safety Authority (DSA), Grini Næringspark 13, Østerås, Norway</footnote>

<footnote><sup>b</sup> Belgian Nuclear Research Centre (SCK CEN), Biosphere Impact Studies, Boeretang 200, 2400 Mol, Belgium</footnote>

<footnote><sup>c</sup> National Institute of Health (ISS), National Centre for Radiation Protection and Computational Physics, Rome, Italy</footnote>

<footnote><sup>d</sup> National Institute for Insurance Against Accidents at Work (INAIL), DiMEILA, Monteporzio Catone, Rome, Italy</footnote>

<footnote><sup>e</sup> Research Centre on Energy, Environment and Technology (CIEMAT), Av. Complutense 40, Madrid 28040, Spain</footnote>

<footnote><sup>f</sup> Environmental Protection Agency of Veneto (ARPAV), Verona, Italy</footnote>

<footnote><sup>g</sup> Department of Biology and CESAM, University of Aveiro, Campus Universitario de Santiago, 3810-193 Aveiro, Portugal</footnote>

<footnote><sup>h</sup> GreenUPorto – Sustainable Agrifood Production Research Centre/Inov4Agro, Department of Biology, Faculty of Sciences of the University of Porto, Campus de Vairão, Rua de Agrária, 747 Vila do Conde, Portugal</footnote>

<footnote><sup>i</sup> Autorité de Sûreté Nucléaire et de Radioprotection (ASNR), DREE/SPDR/LT2S, F-13115 Saint-Paul-lez-Durance, France</footnote>

<footnote><sup>j</sup> Central Mining Institute-National Research Institute, Silesian Centre for Environmental Radioactivity (GIG-PIB), Plac Gwarków, 1, 40-166 Katowice, Poland</footnote>

<footnote>* Corresponding author.
E-mail address: Jelena.Popic@dsa.no (J. Mrdakovic Popic).</footnote>
# 1. Introduction

Naturally occurring radionuclides (NOR), uranium ($^{238}$U), thorium ($^{232}$Th), their progeny, and potassium ($^{40}$K), have been ubiquitously present in the environment since Earth's origin. Exposure to NOR is commonly considered background radiation, which is of no concern for radiation protection (RP), although the activity concentrations of these radionuclides may sometimes be naturally elevated (e.g., in certain minerals, or due to weathering, transport and accumulation favoured by edaphoclimatic conditions). Moreover, raw materials containing NOR are used in different industries, where activity concentrations of radio-nuclides may be enhanced in products, by-products, residues, and dis-charges. Such enhancement occurs due to technological processes that commonly involve mass reduction and high temperature or pressure conditions, leading to the creation of naturally occurring radioactive materials (NORM) (IAEA, 2022).

The first concerns regarding radiation risks related to industrial activities leading to NORM emerged several decades ago (Gesell, 1975; Testa et al., 1994; Underhill, 1996). Soon after, NORM related radiation exposure was recognized by international organizations, and consequently, efforts were made to regulate the associated hazards (e.g., EC, 1996, 2014; IAEA 2014; IAEA 2024; ICRP, 2019). Following the European Union (EU) Directive 2013/59/Euratom (EU BSS) (EC, 2014), renewed interest and general awareness arose regarding improved management, regulation and investigation of research gaps related to NORM (Gilbin et al., 2021; Mrdakovic Popic et al., 2023a; Turcanu et al., 2022).

Energy production sectors based on fossil fuels (such as oil, gas and coal) and geothermal energy, involve processes generating externalities that contain certain forms of NORM. The combustion of coal for electricity production, where NOR initially present in coal are accumulated in coal combustion products (CCP), is an illustrative example. As coal-fired power plants still account for approximately 35 % of the worldwide electricity production (IEA, 2024), this phenomenon contributes to over half of the total radiation dose to the global population from all forms of electricity generation (including nuclear power plants) (UNSCEAR, 2016). However, despite each energy production sector being characterized by specific exposure scenarios, NORM related issues are rarely included in the overall evaluation and comparison of these energy sources.

The main aims of the European RadoNorm project were to manage hazards from radon and NORM exposure situations to ensure effective radiation protection based on improved scientific evidence and social considerations (Kulka et al., 2022). During the initial project activities, it was identified that despite the Green Deal efforts (Fetting, 2020; The European Green Deal - European Commission (europa.eu)), most countries still rely on non-renewable energy resources; for instance, energy production sectors based on fossil fuels remain active in many European countries (37 %) (IEA, 2021). Moreover, coal mines and coal-fired power plants are still operational in several parts of Europe (Mrdakovic Popic et al., 2023a). On the other hand, it was also observed that an increasing number of countries have started using geothermal energy (Mrdakovic Popic et al., 2023a); however, its contribution to the total energy sources is still minimal, generating only 0.2 % of it in the EU in 2022 (IEA, 2022).

Based on previously gathered information across European countries concerning NORM involving non-nuclear energy production sectors (Mrdakovic Popic et al., 2023a) and considering that oil and gas, coal-fired and geothermal energy production sectors are explicitly listed in the EU BSS and IAEA BSS as of potential RP concern, these both non-renewable and renewable energy sources involving NORM were assessed in the current study.

Different aspects of NORM have, in general, been underrepresented in the international literature in comparison with artificial, man-made radionuclides and sources of ionizing radiation. International reports exist for some types of energy production, such as oil and gas, but these

are more than twenty years old (IAEA, 2003a), and although still a valuable source of information, require updating. Moreover, oil and gas, coal-fired and geothermal forms of energy production and their associated NORM issues have previously been investigated sporadically and in a separate manner with partial focus, only on certain aspects (IAEA, 2003a; Omar et al., 2004; Leonardi et al., 2024; Suhana and Rashid, 2016). It is, therefore, of great novelty and value to provide a comparative qualitative and quantitative assessment of these three different energy-producing industries regarding fuel sourcing, NORM-generating processes, NORM final fates, and potential risks to humans and the environment, as well as to compare their management, regulatory control, and existing challenges. It should be, however, mentioned that other energy production-related activities that may also involve NORM, such as battery production, electricity generation by solar panels and wind turbines, were outside the scope of the paper. These will, however, be considered in future investigations to improve the overall understanding of NORM issues in emerging energy technologies expected to expand in Europe and worldwide.

The main aim of the current study was to provide an overview of natural radioactivity occurrences and associated exposures in the most common energy producing sectors – oil and gas, coal-fired and geothermal energy – to support and facilitate future legislation, decision-making, and the development of good practices on an international level, introducing certain pros and cons into the discussion on the European Green Deal. This work is particularly intending to assist countries worldwide that may still be in the initial phase of legislation, inventory and regulatory control development for NORM in energy production sectors. To achieve this, an analysis of NORM related issues for these energy production sectors was performed and concluding remarks, which may serve as guidance, were formulated, while identified challenges are also presented to highlight the need for future research addressing the optimisation of industrial activities and regulatory work.

The present paper contains an introduction and outlines the scope and objectives in Section 1, the applied methodology in Section 2, while a general explanation and information about the processes and stages in the investigated industries are given in Section 3. Section 4 provides details on NORM in different matrices within the investigated energy production sectors, while radiation exposures of workers, the public and the environment, as well as the handling of NORM, are discussed in Sections 5 and 6, respectively. Finally, the main conclusions are provided in Section 7. Additionally, information collected entirely in this project, across European countries, regarding legislative frameworks, regulatory approaches as well as on presence of non-radioactive hazards, and detailed, identified challenges faced by researchers, regulators and industry operators related to oil and gas, coal-fired and geothermal energy production are provided as Supplementary Material (Annexes A and B, respectively).

# 2. Methodology

The methodological approach adopted in this work was based primarily on data collected through dedicated tools and surveys developed within the RadoNorm project. These data were analysed and subsequently compared with relevant international literature sources to ensure broader validation and contextualisation. A detailed description of each methodological step is provided in the following subsections.

## 2.1. Information and data collection using RadoNorm developed tools

The primary data, forming the basis for the current study, were obtained by applying an identification methodology for NORM exposure situations (Michalik et al., 2023), and data collection tools (Mrdakovic Popic et al., 2023b). These tools, developed in the form of Excel registers and specifically adapted for NORM involving industries listed in the EU BSS (Annex 6), enabled harmonized collection of general information and NORM-specific data relevant to these industries. Depending on the
type of information required, both open and closed questions—with multiple-choice options—were included in the systematised tools to ensure standardised and comparable data across participating countries.

The information collected covered the following aspects:

* Legislative requirements and applied regulatory approaches,

* Site characterization - facility type, occurrence of NORM matrices, operations of relevance for NOR accumulation, mobilisation, and transport into the environment,

* Quantitative data on NOR activity concentrations before and after industrial processing,

* Information on monitoring programs, presence of other, non-radioactive hazards,

* Exposure scenarios, corresponding exposure pathways, and magnitude of radiation exposure doses for workers, public and biota,

* Type of treatment processes, handling and disposal for identified NOR residues,

* Identified research, regulatory and management challenges.

More details on the applied tools are available in Mrdakovic Popic et al. (2023b). An extensive collection of information and data was conducted during the period 2022–2024 in European countries participating in the RadoNorm project (https://www.radonorm.eu). Data were obtained either directly from industry operators, researchers or national regulators, through RadoNorm national contacts. Only data derived using reliable measurement methodologies for NOR ($^{238}$U and $^{232}$Th and progeny $^{226, 228}$Ra, $^{220,222}$Rn, $^{210}$Pb, $^{210}$Po, $^{228}$Th and $^{40}$K) were included. All data sets met national quality-control requirements and had sufficient sample sizes for the relevant material matrices in the three energy sectors investigated. Although measurement methodologies differed among countries according to national guidelines, they were validated through established quality-assurance procedures. The data included in this work covered the last ten years and originated from European countries Belgium, Germany, France, Italy, Norway, Poland, and Spain.

## 2.2. Electronic NORM survey across European countries

Additional information and data were collected through the RadoNorm electronic NORM survey (e-NORM survey), which was sent to countries that were not participating in the RadoNorm project, to obtain an updated European perspective on the issues related to NORM. The main outcomes from this survey are provided in Mrdakovic Popic et al. (2023a), while unpublished data on energy sectors were analysed and used as a complementary source in the current study. The criteria for inclusion/exclusion of data were the same as for data collected by the developed tools (as described above in Section 2.1.). Responses from the following countries were included in the study: Austria, Bulgaria, the Czech Republic, Denmark, Estonia, Greece, Ireland, the Slovak Republic, Slovenia, Sweden, and Portugal.

## 2.3. Supporting literature and contextual references

While the primary results presented in this study are based on the original data collected by the RadoNorm project developed tools and electronic NORM survey, relevant literature was additionally consulted in order to support interpretation and ensure consistency with existing international knowledge. Key sources included publications and reports from international organizations (European Commission (EC), International Atomic Energy Agency (IAEA), International Committee on Radiological Protection (ICRP), United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)) and selected peer-reviewed papers, which were carefully chosen for relevance and reliability. For that, a pre-defined transparent process was used, which included (a) targeted keyword search (e.g., NORM, occupational exposure, energy sector, oil and gas industry, coal-fired power plants, geothermal energy

production, dose assessment, NORM exposure doses to workers and public) in scientific databases and organizational repositories, (b) choice of the scientifically robust studies properly tailored to NORM in energy sectors, findings from soundly designed studies published as peer-reviewed papers, which provided relevant and reliable details about measurement or data collection methodology for radionuclides $^{232}$U, $^{232}$Th and their progeny.

These literature sources were used to:

* support explanation and illustration of the sequence of processes in energy generating sectors – from raw material and energy carriers sourcing, their pre-treatment and transport, energy generation to residues and waste occurrence (Section 3),

* illustrate examples from other studies that either confirmed or differed from our findings related to NORM occurrence in different matrices, exposure doses to workers, public and environmental biota, and residue and waste handling in investigated energy sectors (Sections 4, 5 and 6),

* benchmark exposure scenarios and workers' dose magnitudes, allowing comparison with the collected data in this study (Section 5).

However, the authors emphasize that this paper is not a review article in the formal publishing sense; therefore, a comprehensive literature review process commonly applied in review papers was not conducted.

# 3. General information on energy production for three sectors

According to recent data (EUROSTAT, 2022; IEA, 2021), Europe has an energy production industry dominated by natural gas power plants (22 %), nuclear energy (21 %), coal-fired power plants (16 %), while other energy sources contribute a total of 41 % (with geothermal energy production accounting for 0.2 %).

To compare the energy sectors of concern in terms of NORM issues, the following sequence of activities and corresponding waste streams was considered:

1. Raw materials and energy carriers sourcing,

2. Energy carriers' pre-treatment and transport,

3. Energy generation,

4. Emerging residues and waste streams.

These processes are summarised in Fig. 1, which presents a kind of conceptual model of NORM occurrence in the investigated energy generation sectors. Depending on the sector, the effective dose to workers in conservative scenarios can be below the maximum allowable dose of 1 mSv per year, as given in EU BSS (EC, 2014), or elevated up to 6 mSv y⁻¹ or even higher. In such cases, categorization of exposed workers, implementation of RP measures, and notification of the responsible regulatory body are required.

Furthermore, the content of radionuclides in NORM can be below, or show varying degrees of enhancement relative to the EU BSS (EC, 2014)-defined Exemption Level (EL) and Clearance Level (CL) values. When NOR levels in materials are below these values, such materials are usually of no RP concern, and can commonly be reused wherever or whenever possible. However, if NOR are enhanced, the materials are classified as residues or wastes, i.e., may be subject to either reuse/recovery or disposal (Gellermann et al., 2023). Such decisions, however, depend on prevailing national conditions – i.e., adopted legislation – since flexibility is given in the EU BSS for countries to adopt higher CLs for certain NOR, materials, and specific conditions or processes, depending on available infrastructure, quantity of NORM, reuse options etc.
<figure><img src="image_1.png" /><figcaption>Fig. 1. Conceptual model of NORM occurrence in energy production processes of the investigated energy production sectors.</figcaption></figure>

### 3.1. Raw materials and energy carriers sourcing

Oil and gas production facilities are commonly associated with offshore oil and gas reservoirs in the deep sea environments. Usually, oil and gas bedrock sources and crude oil contain only low concentrations of $^{238}$U and $^{232}$Th and their progeny, excluding them from any RP concern at this stage (IAEA, 2003a; this study). From a given source, the main production process starts with well drilling, followed by the extraction of oil and gas from the wellhead manifolds. Subsequently, the separation and stabilization of crude oil or condensates and gas are performed before delivery to a refinery plant (Devold, 2013). This results in the creation of a liquid waste stream consisting of produced water, derived sediments, and scales, which often contain elevated concentrations of NOR.

Coal-fired power plants produce electricity by burning coal in a boiler. Underground coal mining is an extraction industry that represents a preliminary step in obtaining coal. However, in the EU, this industry is limited, as approximately 71 % of hard coal used in the EU is imported from outside the EU, while brown coal is mostly consumed domestically (EUROSTAT, 2022). Generally, coal, as well as associated waste rock, similarly to crude oil, is characterized by a low content of NOR. On the other hand, similar to the oil and gas industry, liquid waste streams from mines – consisting of formation water released due to excavation processes and derived sediments – may exhibit elevated concentrations of NOR (Chalupnik et al., 2017; Jirasek et al., 2020).

Geothermal energy production exploits the continuous flow of heat from the subsoil to the Earth's surface, providing a reliable and stable energy source. A geothermal power plant uses geothermal fluid, which can be water (brine), steam, or a mixture of both. Geothermal fluid acts as the carrier that transports heat from the geothermal reservoir to the surface through mechanical drilling (wells). Similar to oil, gas, and coal sources, these fluids generally do not exhibit elevated levels of NOR, although higher activity concentrations of radionuclides

have been measured at certain sites.

### 3.2. Energy carriers' pre-treatment and transport

In oil and gas production, pumped oil and gas are treated to remove produced water, sand, and mud. The oil stream is then transported for fractionation in oil refinery plants, while gas is introduced into a pipeline system and delivered to the end user directly, or is subjected to liquefaction. Besides the common offshore oil and gas facilities, onshore facilities also exist in Europe, connected to fields via pipelines to meet the needs of transport, storage and processing of oil and gas (Devold, 2013). Transportation of oil leads to the formation of scales rich in $^{226}$Ra, while transportation of natural gas through pipelines leads to the creation of waste composed of so-called 'black sand', which contains elevated levels of $^{210}$Pb and $^{210}$Po. It has recently been shown that transportation using Liquefied Natural Gas (LNG) carriers and gasification lead to the accumulation of $^{222}$Rn, and, subsequently, its decay product $^{210}$Pb in LNG. Eventually, particulate matter with enhanced $^{210}$Pb and $^{210}$Po levels is collected on filters during LNG gasification. All residues occurring in the processes listed above, identified to be of RP concern, are commonly classified as wastes, entering both solid and liquid waste streams.

In coal mining, when further considering coal extraction and beneficiation processes, the production of gangue and process water is of importance, as they enter the respective solid and liquid waste streams. However, NORM is of minor relevance regarding both waste types.

Geothermal fluids contain water, Total Dissolved Solids (TDS), and Non-Condensable Gases (NCG), which can be sources of NORM (Finster et al., 2015). Similar to the oil and gas industry, radionuclides can accumulate in geothermal energy production during fluid transportation, forming scales in pipes, and resulting in solid NORM waste streams (Lafortune and Pinte, 2010). However, as geothermal fluids are usually used in installations located close to their sources, no significant
problems have been identified during their transport.

### 3.3. Energy generation

Considering oil and gas, most of the oil and natural gas industries undergo refining to obtain fuels for transportation (petrol, diesel, and jet fuel for internal combustion engines), heating, industrial production, and electricity generation (mainly from natural gas, while oil to a lesser extent). During refining and final combustion, the radioactivity of oil and gas is negligible. After the combustion of oil shale, the NOR accumulation rate in ash is relatively small, typically no more than a factor of 3, due to the small proportion of oil in the fuel (Vaasma et al., 2014a). Notwithstanding this, the ash is still assessed with respect to NOR occurrence (Vaasma et al., 2014b; Tsamos et al., 2022). Furthermore, in the process of energy generation, combustion of fossil fuels emits various pollutants, including particulate matter (PM) and a range of volatile organic compounds (VOC) that may impact air quality. However, regarding NOR and radiation risks, only the discharge of $^{222}$Rn may be of certain RP concern (Keith et al., 2012). In general, when considering combustion alone, there are negligible direct gaseous NOR discharges from this type of energy production.

In coal-fired power plants, to produce electricity, coal is first crushed and then burned in a boiler to generate steam, which is used in turbines to produce electricity. In active coal-fired power plants, NOR accumulate during the combustion process in a range of materials, unlike in oil and gas combustion and geothermal energy production (Kardos et al., 2014; Walencik-Lata and Smolka-Danielowska, 2020). When coal is combusted, most of the incombustible mineral matter remains in coal combustion products (CCP), as bottom ash, boiler slag, and fly ash, the finest fraction of CCP. As the radionuclides are present in the coal's mineral constituents, they tend to remain in the CCP, leading to enhanced NOR concentrations. It has been observed that the NOR accumulation ratio in CCP is proportional to the ash content of coal, although deviations may occur depending on the combustion method and the CCP fraction considered. However, the total NOR activity contained in coal is balanced in CCP, apart from a small proportion of volatile radionuclides escaping through the chimney with the finest fractions of PM, depending on the efficiency of the filtering system applied. The polonium isotope $^{210}$Po, as the most volatile radionuclide, is accumulated in the finest fractions of CCP, and then emitted with PM regardless of the filtration systems applied. Furthermore, CCP show a lack of secular equilibrium in the $^{238}$U and $^{232}$Th decay series. The waste stream created after combustion that might be of RP concern depends on boiler type, combustion temperature and the filtering system used, and is classified as: (1) spent fluidized bed material, bottom ash and boiler slag residues from coal burning, (2) residues from the filtering systems, often called fly ash, (3) atmospheric emissions, which can occur in the form of PM of different sizes, and, (4) residues from boilers and pipes cleaning, such as scales, sludge and water. Waste from flue gas desulfurization mainly consists of gypsum and is not considered in terms of radioactivity due to its low activity concentration, usually an order of magnitude lower than that of CCP (Roper et al., 2013).

For many countries that provided data within RadoNorm project on coal-fired power plants, the contribution of coal-based electricity production to the total energy mix is small, e.g., 2 % in Belgium, 4 % in France, 5 % in Italy and 3 % in Spain, and no coal-based energy production in Norway. However, there are still countries where coal contributes to electricity generation to a greater extent (e.g., Poland > 40 %, Germany approximately 30 %). Additionally, some countries reported decommissioned or inactive coal-fired power plants (e.g., Austria, Belgium, Spain). In this context, it was shown that during the dismantling of coal-fired power plants, activities led to the generation of residues with enhanced NOR, which may still be present at such sites. Therefore, the information and data collected regarding closed power plants were also analysed in this study.

**Geothermal power generation** in Europe currently occurs only in a

few countries (Iceland, Italy and Turkey, but notable growth has also been observed in Belgium and the Netherlands) in terms of electricity production (Geysmans et al., 2024; Sanner et al., 2022). Electricity can be produced using either vapor-dominated or liquid-dominated geothermal systems. Dry steam power plants use steam from a geothermal fluid that is sent through a turbine, which drives a generator to produce electricity. When the fluid consists mostly of hot water, electricity is produced by either a flash process or a binary process. The processes of steam generation and heat exchange in geothermal energy production, considered independently of water pumping and transportation described in the previous section, do not produce any waste, nor significant atmospheric or liquid discharges of NOR, similar to observations in the oil and gas industry.

## 4. NORM occurrence in energy production sectors

Values for the activity concentrations of NOR in different identified NORM per energy production sector are provided in this section. Numerical results are given as ranges or maximum values (Tables 1 and 2). No statistical analysis was performed, as the data were obtained from 19 European countries in varying formats (as maximum values, averages, ranges). The data are therefore pooled and given as ranges or, where not possible, as maximum values. The authors emphasize that, given the heterogeneity of data formats and the lack of statistical harmonization, the reported activity concentrations should be interpreted with caution, as they may not always accurately reflect central tendencies or variability within or across sectors.

### 4.1. NOR in raw materials

The oil and gas production data were collected as average national values from over 400 facilities in Europe, including oil and gas exploration and production plants, conventional offshore and onshore drilling rigs, transport installations, oil storage facilities, as well as oil refinery facilities.

Raw materials, source rock, crude oil, and natural gas do not have elevated concentrations of $^{238}$U and $^{232}$Th or their progeny (Table 1). These findings are in accordance with the values reported by international organizations (IAEA, 2003a; ICRP, 2019). Additionally, based on older national data, significantly high NOR activity concentrations, especially of $^{222}$Rn, $^{210}$Pb and $^{210}$Po measured in natural gas were reported.

Regarding coal-fired power plants, the content of NOR in coal is highly dependent on the geological formation of the coal seams (IAEA, 2003b; Lauer et al., 2015; UNSCEAR, 2016). Although data in this study indicated that the majority of the coal used in European countries was imported from other regions of the world, activity concentrations for $^{238}$U, $^{226}$Ra, $^{210}$Pb, $^{210}$Po, $^{232}$Th, and $^{228}$Ra (Table 1) were within similarly low ranges for different coal samples, confirming secular equilibrium and the fact that coal with high NOR concentrations is relatively rare (IAEA, 2003b; UNSCEAR, 2016).

For geothermal energy production, the geothermal fluid is the only raw material present. Generally, due to the reducing conditions in the geothermal reservoir, which limit the mobility of $^{238}$U and $^{232}$Th (Vasile et al., 2017; Feige and Roloff, 2012), the geothermal fluid does not contain significant concentrations of these isotopes (Eggeling et al., 2013; Pauwels et al., 1993). At such low concentrations, $^{238}$U and $^{232}$Th are not directly of concern for radiation or environmental protection. Still, the fluid contains different anions and cations at variable concentrations (e.g., Ba$^{2+}$, Sr$^{2+}$ SO$_4$ $^2-$, HCO$_3$ $^-$, CO$_3$ $^{2-}$). Under these conditions, $^{226,228}$Ra isotopes are commonly present in the geothermal fluid, either as TDS or in its ionic form (Ra$^{2+}$), and as such are available for transport processes in the environment (UNSCEAR, 2016). Collected European data showed that radionuclides with the maximum activity concentrations, $^{226}$Ra, $^{228}$Ra and $^{210}$Pb, up to 109 Bq L$^{-1}$, 51 Bq L$^{-1}$ and 160 Bq L$^{-1}$, respectively, also recently reported by Leonardi et al. (2024), were in
<caption>**Table 1**</caption>

Activity concentration of NOR in raw materials, energy carriers of investigated energy sectors, data collected in the RadoNorm project.

<table><thead><tr><th rowspan="2">Energy sector, material</th><th colspan="2">Oil and gas production</th><th rowspan="2">Oil shale (kBq kg<sup>-1</sup>)</th><th colspan="2">Coal mining and imported material</th><th rowspan="2">Geothermal energy production Geothermal fluid (Bq L<sup>-1</sup>)</th></tr><tr><th>Crude oil (kBq kg<sup>-1</sup>)</th><th>Natural gas, well fluid<sup>a</sup> (Bq m<sup>-3</sup>)</th><th>Coal - coke - lignite (kBq kg<sup>-1</sup>)</th></tr></thead><tbody><tr><td colspan="7">Radionuclides</td></tr><tr><td><sup>238</sup>U</td><td>0.001-0.060</td><td>n.a.</td><td>0.022-0.025</td><td>0.001-0.060</td><td></td><td>n.a.</td></tr><tr><td><sup>226</sup>Ra</td><td>0.0001-0.038</td><td>n.a.</td><td>0.024-0.025</td><td>0.005-0.030</td><td></td><td>1.5-109</td></tr><tr><td><sup>232</sup>Th</td><td>0.001-0.006</td><td>n.a.</td><td>Up to 0.012</td><td>0.004-0.040</td><td></td><td>n.a.</td></tr><tr><td><sup>40</sup>K</td><td>n.a.</td><td>n.a.</td><td>0.360-0.393</td><td>0.004-0.200</td><td></td><td>32</td></tr><tr><td><sup>210</sup>Pb</td><td>n.a.</td><td>n.a.</td><td>0.0023-0.026</td><td>0.004-0.040</td><td></td><td>160</td></tr><tr><td><sup>210</sup>Po</td><td>0.005-0.01</td><td>n.a.</td><td>n.a.</td><td>Up to 0.030</td><td></td><td>0.036</td></tr><tr><td><sup>228</sup>Th</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td></td><td>2.40</td></tr><tr><td><sup>228</sup>Ra</td><td>n.a.</td><td>n.a.</td><td>Up to 0.012</td><td>0.020-0.030</td><td></td><td>51</td></tr></tbody></table>

accordance with previous studies (Eggeling et al., 2013; Pauwels et al., 1993). Results confirmed the reducing conditions but also showed a disequilibrium with respect to the parent radionuclides $^{238}$U and $^{232}$Th.

In summary, comparing the raw materials and energy carriers, no obvious differences were observed in terms of low NOR activity concentrations in oil source rocks, crude oil and coal, including coke and lignite. Somewhat higher values were, however, reported for $^{226}$Ra, $^{210}$Pb, $^{228}$Th, and $^{228}$Ra in geothermal energy production. Geothermal fluids, which often originate from deep geological reservoirs, come into contact with various rock formations and typically pass through multiple layers that, what under the aforementioned favourable conditions, can mobilize certain radionuclides, resulting in slightly elevated activity levels in the fluid.

## 4.2. Solid NORM residues

For each energy sector, data on activity concentrations of radionuclides in NORM residues are reported in Table 2. Solid waste streams that are of RP concern in the **oil and gas industry** are precipitates, scales, and sludges, which is consistent with previous studies (IAEA, 2003a; Liland et al., 2012; Veguería et al., 2002). These wastes are mostly rich in $^{226,228}$Ra and $^{228}$Th isotopes, while over time, depending on the prevailing conditions in the system, $^{222}$Rn decay products - $^{210}$Pb and $^{210}$Po - can occur and accumulate. Isotopes $^{226}$Ra and $^{228}$Ra, which co-precipitate in produced water with $Ba^{2+}$ and $Sr^{2+}$ salts, settle inside transportation facilities, where changes in system pressure, temperature, and salinity result in radioactive scales being formed on contact surfaces (in wells, storage tanks, separators, pipes, and tubes). These findings are consistent with studies by Garner et al. (2015) and Koppel et al. (2021). Further, collected data showed activity concentrations of Ra-isotopes, $^{210}$Po and $^{210}$Pb in scales exceeding the CL and EL defined in EU BSS, reaching up to 20 kBq kg$^{-1}$, 22 kBq kg$^{-1}$, 2 kBq kg$^{-1}$ and 2 kBq kg$^{-1}$, for $^{226}$Ra, $^{228}$Ra, $^{210}$Pb and $^{210}$Po, respectively (Table 2). The observed NOR activity concentrations are similar at European level, but it should be highlighted that several orders of magnitude higher values have been published in other parts of the world (Ali et al., 2019; MacIntosh et al., 2022).

The NOR levels in sludges were similar to those in scales, with the exception of $^{210}$Pb and $^{210}$Po, whose average activity concentrations were approximately five times higher in sludges than in scales, while the activity concentration of $^{228}$Ra was an order of magnitude lower in sludges than in scales.

Natural gas usually contains moderate to high activity concentrations of $^{222}$Rn. Subsequently, $^{210}$Pb, a longer-lived decay product of $^{222}$Rn, may accumulate as thin deposits (films) within handling equipment, pipes and storage tanks (Faria and Moreira, 2018; IAEA, 2003a). This is especially important in natural gas transportation, including gas liquefaction/volatilization, filtering, and considering the possible reactivity of $^{210}$Pb particles. Available data for $^{210}$Pb and $^{210}$Po in filters and deposited thin film materials from oil and gas facilities showed

maximum values of 7 kBq kg$^{-1}$ and 10 kBq kg$^{-1}$, respectively, also exceeding the EL and CL for solid NORM material. However, data from the current study are much lower than $^{210}$Pb activity concentrations (up to 3000 kBq kg$^{-1}$) reported in dust filters from LNG regasification (Dilling and van Bochove, 2024).

Regarding solid NORM waste from **coal energy production**, NOR activity concentrations in CCP were reported to be roughly an order of magnitude higher compared to coal (Table 2). This factor of 10 can be explained by the combustion of organics and the subsequent residual inorganic fraction, usually around 10 % in coal composition, which remains as residue after combustion. The enrichment factor is specific to the coal type and the applied combustion process. Usually, the finest fractions of CCP are characterized by the highest content of NOR, which is especially noticeable in the case of $^{210}$Pb as well as $^{210}$Po (Dlugosz et al., 2010). This trend has been confirmed in our data (Table 2) for NOR present at the highest activity concentrations in fly ash, 1.191 kBq kg$^{-1}$ and 0.344 kBq kg$^{-1}$ of $^{210}$Pb and $^{210}$Po, respectively, versus coal values of 0.040 kBq kg$^{-1}$ and 0.030 kBq kg$^{-1}$ for $^{210}$Pb and $^{210}$Po, respectively. The highest NOR value was reported for flue gas desulfurization gypsum filter cake, 5 kBq kg$^{-1}$ of $^{226}$Ra, which is significantly elevated compared to values typically reported in the literature and may warrant further investigation regarding RP and waste management.

In the commitment to the energy transition and green economy, many coal-based electricity plants were closed during the last decade, and residues such as the ashe, slag, and gypsum are either reused in many countries or transferred to landfills as waste. The values of NOR in material from the considered closed facilities fall within the same ranges as those of the active plants in the current study and those previously published (Baeza et al., 2012; Font et al., 1993).

In **geothermal energy production**, depending on the nature of the radionuclides, their solubility, mobility, and ability to precipitate as a mineral phase, several situations may occur. The formation of scales and sludges in the geothermal energy production process (Table 2) happens under similar conditions as observed in oil and gas production, as well as in coal mining and combustion (Lafortune and Pinte, 2010). Scales with elevated levels of radionuclides can also be found on the internal surfaces of drilling and production equipment (e.g., steam turbines, heat exchangers, valves, fluid handling equipment, etc.). $^{226}$Ra-rich barite has frequently been observed in natural systems such as groundwater (Martin and Akber, 1999), thermal water (Sturchio et al., 1993) and scales in oil wells (Phillips et al., 2001). As for sludge, it is generally collected in the tank of the cooling tower and in NCGs emissions. In the current study, wide ranges of dominant radionuclides were found in scales, 0.01-27 kBq kg$^{-1}$ for $^{226}$Ra, 0.01-250 kBq kg$^{-1}$ for $^{210}$Pb, up to highly elevated 9160 kBq kg$^{-1}$ for $^{210}$Po. In addition, for the filter material from geothermal energy facilities, activity concentrations of $^{226}$Ra reached up to 7 kBq kg$^{-1}$, and of $^{210}$Pb up to 19 kBq kg$^{-1}$ (Table 2).

In summary, comparison of NOR activity concentrations for residue NORM in investigated sectors showed similarly elevated $^{226}$Ra in scales and sludges from oil and gas as well as geothermal energy production,

<footnote><sup>a</sup> Information was provided that high NOR was found in well fluid during operations in oil and gas industry, n.a. = quantitative data not available.</footnote>
<table><caption>**Table 2**
Activity concentrations of NOR (kBq kg⁻¹) in solid NORM residues in investigated energy sectors, data collected in the RadoNorm project.</caption><thead><tr><th rowspan="2">Energy sector NOR</th><th colspan="2">Oil and gas production</th><th colspan="4">Coal-fired power plants</th><th colspan="2">Geothermal energy production</th></tr><tr><th>Sludge</th><th>Scale</th><th>Filter material</th><th>Fly ash</th><th>Bottom ash</th><th>Slag</th><th>Flue gas desulfurization gypsum filter</th><th>Fluidized bed combustion gypsum filter</th><th>Scale Sludge</th><th>Filter material</th></tr></thead><tbody><tr><td colspan="11"><strong>Radionuclides</strong></td></tr><tr><td>238U</td><td>0.01-0.2</td><td>0.01-0.2</td><td>n.a.</td><td>0.005-0.256</td><td>0.055-0.210</td><td>0.021-0.132</td><td>0.047</td><td>n.a.</td><td>0.005-0.010</td><td>0.003-0.016</td></tr><tr><td>226Ra</td><td>0.01-20</td><td>0.01-20</td><td>n.a.</td><td>0.044-0.288</td><td>0.123-0.210</td><td>0.100-0.154</td><td>0.018</td><td>0.015</td><td>0.01-27</td><td>0.02-7.2</td></tr><tr><td>232Th</td><td>0.01-0.2</td><td>0.0001-0.1</td><td>n.a.</td><td>0.066-0.195</td><td>0.090-0.172</td><td>0.073-0.114</td><td>0.003</td><td>0.005</td><td>n.a.</td><td>n.a.</td></tr><tr><td>40K</td><td>0.01-0.3</td><td>0.01-0.3</td><td>n.a.</td><td>0.170-1.701</td><td>0.143-0.510</td><td>0.310-0.423</td><td>0.022</td><td>0.023</td><td>0.04-0.62</td><td>0.007-0.45</td></tr><tr><td>210Pb</td><td>0.01-10</td><td>0.04-2</td><td>0.1-7</td><td>0.029-1.191</td><td>0.028-0.162</td><td>0.043-0.063</td><td>0.049</td><td>0.008</td><td>0.01-250</td><td>0.01-19</td></tr><tr><td>210Po</td><td>0.01-10</td><td>0.02-2</td><td>n.a.</td><td>0.085-0.344</td><td>0.032</td><td>0.009-0.064</td><td>0.051</td><td>0.007</td><td>9160</td><td>n.a.</td></tr><tr><td>228Th</td><td>0.001-0.2</td><td>0.001-0.1</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>0.054-0.071</td><td>n.a.</td><td>n.a.</td><td>0.004-0.36</td><td>0.007-0.038</td></tr><tr><td>226Ra</td><td>0.01-1.8</td><td>0.1-22</td><td>n.a.</td><td>0.066-0.197</td><td>0.011-0.114</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>0.005-2.65</td><td>0.013-0.042</td></tr></tbody></table>

while ²¹⁰Pb and ²¹⁰Po levels in scales from geothermal sources were particularly elevated. The accumulations of these NOR in the inner parts of equipment highlight the importance of process conditions (e.g., salinity, presence of cations and anions contributing to precipitation). Residue materials from coal combustion contained, however, low NOR levels, with the exception of elevated ²²⁶Ra activity concentrations in flue gas desulfurization gypsum filters, which was inconsistent with the findings of Roper et al. (2013).

### 4.3. NORM discharges

In oil and gas energy production, liquid waste, known as 'produced water' typically contains ²²⁶, ²²⁸Ra, ²²²Rn, ²¹⁰Pb and ²¹⁰Po. These radionuclides are more soluble than their progenitors ²³⁸U and ²³²Th and, as observed in the current study, represent the primary radioactivity issue of liquid wastes. When discharged from production facilities, NOR may lead to environmental contamination through different processes of radionuclides mobilization, transport, and incorporation into food chains (Eriksen et al., 2006; Mrdakovic Popic et al., 2023b; Veguería et al., 2002). For example, in the case of offshore platforms, produced water is commonly discharged into seawater rich in SO₂. Most of the dissolved ²²⁶,²²⁸Ra isotopes precipitate when mixed with seawater, forming fine PM that disperses with water currents before settling into marine sediments. Crucially, the co-occurrence of Ba, Sr, or Ca is required, as the concentration of Ra alone rarely exceeds its solubility level. Similar processes are relevant for NOR in geothermal fluids. Onshore facilities are more complex, as freshwater bodies have limited capacity to receive high effluents volumes without environmental harm.

Quantitative data from this study regarding NOR in produced water, from both off- and onshore facilities, are lower than those obtained by the e-NORM survey (Mrdakovic Popic et al., 2023a) and other global studies (Bamber et al., 2011; IAEA, 2003a; ICRP, 2019; NRPA, 2010). The lower activity concentrations of the most dominant radionuclides, ²²⁶Ra up to 20 Bq L⁻¹ and ²²⁸Ra up to 18 Bq L⁻¹ found in the present study, likely reflect more frequent modern, improved treatment of produced water to protect the environment (Liland et al., 2012).

In coal mining process, formation waters in coal basins, containing elevated activity concentrations of ²²⁶Ra and ²²⁸Ra isotopes, are the main RP concern. In this study, ²²⁶Ra and ²²⁸Ra isotopes activity concentrations reached up to 390 and 130 Bq L⁻¹, respectively. Previously, investigations conducted by Tomza and Lebecka (1981) showed that the Ra concentration in formation water correlates with its salinity. Approximately 40 % of Ra-isotopes in formation water remain in underground coal mining installations, while the rest is discharged into surface settling ponds. Depending on the local conditions and water chemistry, a fraction of Ra-isotopes may enter inland watercourses (Chalupnik et al., 2020; Tomza and Lebecka, 1981).

In coal-fired power plants, coal combustion results in a direct release of radionuclides through the flue-gas stack and the production of CCP containing NOR. Only a small fraction of the produced ash is released into the atmosphere as fine PMs (Długosz-Lisiecka and Perka, 2020; Mora et al., 2011), and modern power plants have efficient particulate collection/filtration systems, as given in UNSCEAR (2016).

In some geothermal plants, during maintenance work, part of the geothermal fluid is collected in a temporary storage basin. This water is commonly later discharged into the sewer system after physicochemical treatment (Kunze et al., 2018). Moreover, the geothermal fluid can also contain ²²²Rn, as non-condensable gas. If the plant is not based on a binary-cycle process, ²²²Rn is normally extracted from the condenser and emitted by the gas compressor into the atmosphere through stacks or cooling towers, and, is therefore, of no concern (Ciolini and Mazed, 2010).

In summary, NORM released from oil and gas, coal, and geothermal energy sectors in liquid and gaseous forms primarily involves radionuclides ²²⁶Ra, ²²⁸Ra, and ²²²Rn. These can contaminate water, air, and sediments under favourable environmental conditions. Offshore oil

<footnote>a Sludge from water treatment of the desulfurization system and sludge from water treatment of the plant (water used for washing boilers, pipes etc.); Values in bold are maximum values; n.a. = not available.</footnote>
operations lead to marine sediment deposition, coal activities impact both water and air, while geothermal plants mainly emit ²²²Rn. While European data indicated that improved, modern treatment and filtration technologies have reduced overall impact, site-specific factors continue to influence the environmental risk.

## 5. Radiation exposure of the workers, the public, and the environment due to NORM

Requirements and recommendations to control radiation exposure of the workers, the public and the environment from NORM in planned and existing exposure situations, including the investigated energy sectors, are given in the EU BSS (EC, 2014) and the IAEA BSS (IAEA, 2014), respectively. These include numerous activities, such as NORM mapping, compliance with ELs and CLs, graded control, dose limitations, inspections, assessment of discharges and any solid NORM disposals, monitoring programs and reporting, and other measures for proper management and control. Additional specific legislative, international, and national frameworks, such as the Marine Strategy Framework Directive 2008/56/EC (EC, 2008) in case of discharge from oil rigs, or the Groundwater Directive (2006/118/EC) (EC, 2006) in case of coal mining as well as national regulations are in use to ensure appropriate NORM management and to reduce potential risks to human and environmental health. Details on the legislative framework and regulatory control approaches across European countries are given in Annex A of the Supplementary Material.

### 5.1. Radiation exposure of the workers

The main processes that should be mapped and controlled in terms of possible radiation exposure doses to the workers in the energy production sectors may be grouped as:

* mining (in the case of coal), mechanical or chemical processing and transport of raw materials,

* facility operations (production processes, maintenance, decontamination, etc.),

* facility decommissioning (equipment decommissioning, handling, transport and storage),

* waste disposal operations (transport to disposal site and waste storage).

This study showed that the scenarios provided in the report Radiation Protection 122, part 2 (EC, 2002) are still used in European countries, but in many situations these are adapted as more specific or tailor-made exposure scenarios, such as: (a) miners working in underground mines, (b) wagon drivers and unloading operators of imported coal, (c) oil and gas, coal, ash and slag, geothermal power plant operators, (d) harbour controller and bulldozer drivers, (e) operators responsible for cleaning installations, (f) plant maintenance and repair operators, (g) truck drivers transporting NORM residues, and (h) waste dump workers.

The main exposure scenarios, relevant pathways considered for workers in different European countries, and related exposure doses reported as summaries for all potential activities/tasks in each of the considered sectors are presented in Table 3. Both internal exposure pathways, through inhalation (In) and ingestion (Ig) of contaminated dust, and external ones (Ext) through exposure to gamma radiation are considered when calculating the yearly doses in energy production sectors. For some scenarios, radon inhalation (Rn) was also considered, and then, explicitly reported.

The highest doses to workers in the oil and gas industry are received during cleaning operations of installations, caused by scales with high ²²⁶Ra activity concentrations and exposure to gamma doses coming from ²²⁶Ra and ²²²Rn progenies. External gamma radiation exposure may give (in the exposure scenarios that include working with scales and sludges in limited amount of time, rotation of workers) an annual exposure radiation dose of 0.2 mSv, which is within the lower part of the range of 0.1 to 2.5 mSv given in previously published studies (Elghawi and Elammari, 2024; Fall et al., 2023). Moreover, although the total average annual effective dose, given in the recent UNSCEAR (2021) report, based on global data, was up to 1.26 mSv y⁻¹, lower values for total effective doses to workers in oil and gas production facilities were reported in this work, 0.10–0.35 mSv y⁻¹, also well below the dose constraint of 1 mSv y⁻¹ (EC, 2014; IAEA, 2003a).

In coal mining, due to the low content of NOR in coal and associated rocks, there is no significant exposure in open pit mining. The presence of Rn and its short-lived decay products has been identified as the most important contributor to radiation exposure for miners in underground

<caption>**Table 3**</caption>

Workers exposures to NORM: scenarios, pathways and annual effective doses, responses obtained by the RadoNorm tools are provided together with exposure scenarios and pathways published by the RP documents of European Commission (EC, 1999; 2002; 2003) and the annual exposure dose published by UNSCEAR (2021) (for comparison purposes).

<table><thead><tr><th>Energy production sector</th><th>Exposure scenarios identified in this study</th><th>Exposure scenario defined in RP documents</th><th>Exposure pathways identified in this study</th><th>RadoNorm Total exposure dose<sup>a</sup> (mSv y<sup>-1</sup>)</th><th>UNSCEAR (2021) Total exposure dose<sup>b</sup> (mSv y<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Oil and gas industry</td><td>Maintenance: cleaning of equipment, removing residues<br/>Waste management<br/>Decommissioning<br/>Remediation of contaminated areas</td><td>RP 107 (EC, 1999)</td><td>Ext, In, Ig, Rn</td><td>0.10–0.35</td><td>0.21–1.26</td></tr><tr><td>Coal mining</td><td>Mining<br/>Radon in mine atmosphere, Mine dewatering System maintenance (water galleries, settling ponds, pipelines)</td><td></td><td>Ext, In, Rn</td><td>0.5–1.7</td><td>0.59–2.33</td></tr><tr><td>Coal fired power plants</td><td>Transport (coal/ash) Maintenance</td><td>RP 122, Part II (EC, 2002)</td><td>Ext, In</td><td>0.007–0.17</td><td>—</td></tr><tr><td>Geothermal energy production</td><td>Storage, Transport<br/>Maintenance: removing residues inspection of process vessels and pipes<br/>Maintenance: inspection of process vessels and pipes</td><td>RP 122, Part II (EC, 2002)<br/>RP 107 (EC, 1999)</td><td>Ext, In, Ig, Rn</td><td>&lt; 0.1</td><td>—</td></tr></tbody></table>

<footnote><sup>a</sup> Most relevant exposure scenarios are given per energy production sectors, but Pathways and Exposure doses are not provided per each scenario, but as summary for each energy sector; Ex: external exposure; In: inhalation; Ig: ingestion; Rn: radon inhalation; <sup>b</sup>Average annual effective doses over the period 1990–2014 for all workers subject to individual or workplace monitoring.</footnote>
mines (Chalupnik et al., 2017; Skubacz et al., 2019; Skowronek et al., 1997). In the current study, it was reported that the applied exploitation methods and the ventilation system determine the level of this hazard in the underground workings. Effective doses reported for coal mines in EU countries varied from 0.5 to 1.7 mSv y⁻¹. This is in accordance with findings of Vearrier et al. (2009), and with the average annual effective doses in the range of 0.59–2.33 mSv y⁻¹ given by UNSCEAR (2021, and references therein).

In coal-fired power plants, doses to workers are mainly due to cleaning operations, and inhalation of dust containing NOR, ²²²Rn or its progeny ²¹⁰Pb. Still, in the current study, the obtained national data indicated that occupational exposures were in the range of 0.007–0.17 mSv y⁻¹, below the legislative requirement of 1 mSv y⁻¹.

As for the oil and gas industry and coal-fired power plants, in geothermal energy production, doses from external irradiation may be received during maintenance of plants, but these are also found to be below 1 mSv y⁻¹. In most cases, exposure to ²²²Rn for workers is negligible due to a low fraction of ²²²Rn emanation from scales (typically around 5 %).

In summary, depending on the considered exposure scenario, the presence of scales, sludges, and sediments with the enhanced NOR activity concentrations in operational facility systems and during maintenance activities may expose workers to localized radiation hotspots. However, during these operations in all investigated sectors, RP but also occupational, health and safety (OHS) measures for other hazardous substances are applied. Workers are commonly equipped with standard personal protection equipment (PPE), and RP equipment, including personal gamma or radon monitors. These measures greatly reduce the risk of ingestion and inhalation, which is sufficient to protect against radiation exposure as well (Gellermann et al., 2018; this investigation).

In this study the highest annual exposure doses of workers, up to 1.7 mSv was reported for coal mining, while doses of workers in the oil and gas industry, coal-fired power plants and geothermal energy production were below the dose criterion of 1 mSv y⁻¹ (EU BSS), approximately one to two orders of magnitude lower, with estimated doses in geothermal energy production being the lowest.

## 5.2. Radiation exposure of the public and the environment

### 5.2.1. Radiation exposure of the public

In Table 4, for each energy production sector, exposure scenarios and the corresponding exposure pathways are reported, along with the collected values of public effective exposure doses.

For the oil and gas industry, considering both offshore and onshore installations and facilities, the relevant exposure scenarios for the possible radiation exposure of the public and the environment are the following: (a) accidental or deliberate releases of liquid NORM from onshore installations to land or surface water bodies, where NOR could enter groundwater or the food chain and affect the environment, biota, and the public, (b) living near the oil / gas facilities or disposal sites, where solid NORM (scale, sludge), when not properly managed or disposed of, could lead to contamination of the surrounding environment, such as agricultural land, groundwater and drinking water supplies, and (c) discharge of liquid effluents from offshore installations into seawater and NOR uptake by marine organisms in nearby areas. These scenarios and pathways are illustrated in Fig. 2.

Internal exposure of humans through ingestion of food containing accumulated NOR, ingestion of accidentally contaminated water, and external exposure to solid NORM are reported as the possible exposure pathways for the public regarding oil and gas production (Table 4). However, numerical data on public exposure doses for European countries were relatively scarce and site-specific, reporting low dose values of up to 0.010 mSv y⁻¹ for all summarized exposure pathways. Previously, the study by Eriksen et al. (2006) investigated potential links between radioactivity in produced water discharged from the oil and gas industry on the Norwegian continental shelf, accumulation of NOR in marine

<caption>**Table 4**</caption>

Public exposure to NORM – scenarios, pathways and yearly effective doses, data collected in the RadoNorm project.

<table><thead><tr><th>Energy production sector</th><th>Exposure scenarios RadoNorm project</th><th>Exposure scenarios Defined in RP documents</th><th>Exposure pathway RadoNorm project</th><th>Total exposure dose<sup>a</sup> (mSv y<sup>-1</sup>) RadoNorm project</th></tr></thead><tbody><tr><td>Oil and gas industry</td><td>Discharge of NORM liquid effluents Improper handling and/or disposal of solid NORM waste – living in vicinity of facilities or disposal sites</td><td>RP 135 (EC, 2003)<br>RP 122, Part II (EC, 2002)</td><td>Ex, In, Ig</td><td>0–0.010</td></tr><tr><td>Coal-fired power plant</td><td>Atmospheric discharges</td><td>RP 135 (EC, 2003)<br>RP122, Part II (EC, 2002)</td><td>Ex, In, Ig, Rn</td><td>0–0.013</td></tr><tr><td>Geothermal energy production</td><td>Radon discharge Reuse or disposal of residues</td><td>UNSCEAR approach (2016)</td><td>Ex, In, Ig, Rn</td><td>n.a.</td></tr></tbody></table>

a Most relevant exposure scenarios are given per energy production sectors, but Pathways and Exposure doses are not provided per each scenario, but as summary for each energy sector; Ex: external exposure; In: inhalation; Ig: ingestion; Rn: radon inhalation;

organisms, and potential doses to humans via the food chain and ingestion. The findings of this study also indicated negligible dose rates to marine biota, and consequently low exposure doses to humans.

Radon emissions from coal mining may potentially contribute to the exposure of the public from the coal cycle (UNSCEAR, 2016), but due to the applied modern ventilation and monitoring systems no such European data were reported. Furthermore, the other NOR content of coal-mine waste rock was reported not to differ significantly from background concentrations of natural radionuclides in soil, and according to UNSCEAR (2016), this exposure route does not need to be considered for public exposure.

Data showing low public exposure doses in European countries (Table 4) from coal-fired power plants across all considered exposure pathways (Fig. 2), were up to a maximum of 0.013 mSv y⁻¹, which is lower than, but still comparable to the results of Kunze et al. (2019) and to public doses for other types of industries as reported in Mrdakovic Popic et al. (2023a).

Regarding geothermal energy production, the ²²²Rn emission scenario has been considered in dose assessments for the public, but no numerical data were available in this study. However, the dispersion of ²²²Rn and its progeny from facilities was modelled in Germany by different authors, leading to the conclusion that ²²²Rn does not contribute to elevated dose levels around the plant (Kunze et al., 2018; Feige and Roloff, 2012). Regarding the ²²²Rn progeny, measurements of the ²¹⁰Pb deposition rate were performed in Italy, showing that radioactivity levels in geothermal fields are similar to those in areas not subject to geothermal exploitation (Lattanzi et al., 2020). However, the recent development of the geothermal sector, together with its site-specific features, highlights the need for continuous attention to effective risk management in this sector (Geysmans et al., 2024).

### 5.2.2. Radiation exposure of biota

The results collected in this RadoNorm project study, based on only a few European national monitoring programs, which included measurements of both seawater and sediments at different distances from oil and gas installations as well as modelling of potential dose rates in non-
<figure><img src="image_2.png" /><figcaption>**Fig. 2.** Exposure scenarios, with transport directions and exposure pathways, for atmospheric NORM discharge from coal-fired power plants, and NORM solid and liquid waste from oil and gas facilities, to be considered when calculating the dose for the representative person.</figcaption></figure>

human biota, confirmed a negligible dose rate, i.e., lower than 10 µGy h⁻¹ for marine organisms. It should, however, be highlighted that there are substantial uncertainties (e.g., in the design of monitoring programs, sampling sites and frequency, types of organisms, measurement methods, and modelling approaches) in the reported estimates of impact. Further research is still needed, regarding both offshore and onshore installations as well as during decommissioning activities (Gilbin et al., 2021; Hosseini et al., 2012; McIntosh et al., 2022).

Regarding the environmental impacts of **coal mining**, NORM should be considered only in terms of surface contamination from wastewater. While coal itself generally contains NOR concentrations below ELs and CLs, and thus, is not subject to radiological control, the wastewater produced during mining and processing may still pose a contamination risk. Reported data showed that monitoring programs have been developed for these areas with potential contamination, while inspections are another control mechanism for environmental protection. However, limited data were available in our study regarding both possible exposure doses or effects on biota and regarding impacts on the environment themselves, again highlighting the need for more research in cases of evident environmental contamination caused by coal mining.

Aquatic discharges from coal-fired power plants and the consequent uptake by aquatic organisms are considered small and site-specific, and therefore, do not need to be considered, according to replies obtained through the e-NORM survey (Mrdakovic Popic et al., 2023a) and UNSCEAR (2016). Despite an increase in the terrestrial dose rate to approximately twice the local background levels, being observed at sites around coal-fired power plants (Marovic et al., 2004; Papp and Dezsö, 2003), national monitoring data concerning the potential impact of coal ash dumps on the environment showed that there was no significant migration of radionuclides into neighbouring environmental compartments, even in cases of ash with a high content of radionuclides. Additionally, there was no significant increase in NOR levels in plants growing next to or even on ash dumps, confirming that NOR in ash are less bioavailable than NOR naturally present in soil (Skoko et al., 2017).

No data on possible radiation exposure of biota due to geothermal energy production were available in the current study.

In general, the collected data on radiation exposure from this study support negligible to low doses to the public (Table 4) and dose rates to biota and the environment (< 10 µGy h⁻¹). There are still obvious knowledge gaps that indicate the need for more research regarding biota and environmental exposures, and effects on ecosystem functions and services in general, possibly also taking climate change and other aspects into consideration.

## 6. Handling and disposal of NORM

There have been observed inconsistencies among European countries in their categorisation of NORM residues and wastes with respect to radioactivity, as well as in their handling and disposal practices. More details on European national considerations of ‘NORM waste’ versus ‘NORM radioactive waste’ can be found in HERCA (2021) and Mrdakovic Popic et al. (2023a). Regarding the energy production sectors investigated in this study, treatment and disposal practices depend on the energy sector from which the production residues originate, the type of residue or waste, the availability of disposal sites, other infrastructural aspects, and, finally, on the extent to which a circular economy action plan has been developed in each country. In general, residues in the form of scale, sludge and filter material containing elevated NOR activity concentrations are considered radioactive waste and are properly disposed of at dedicated disposal sites. For other NORM waste types, according to their chemical forms and activity concentrations, different disposal sites are identified, e.g., conventional landfills (hazardous and non-hazardous), low-level radioactive waste disposal sites, and NORM-dedicated disposal sites.

In most cases, produced water from the **oil and gas industry**, formation water from mining and geothermal fluid are reinjected back into geological formations or released into the sea (the latter valid only for oil and gas produced water). In a few European countries, physical,
chemical, and biological methods have also been identified as treatment options. Recently, research efforts have focused on the development of innovative technologies with lower costs and long-term environmental benefits. For example, electrochemical technologies have been investigated as a treatment alternative (Dos Santos et al., 2014). Waste disposal strategies for the onshore disposal of NORM waste are developed nationally and may include proper waste characterization, methods for containment, storage and safe disposal in conventional landfills or specifically designed facilities (e.g., using containers with shielding materials such as concrete, where necessary), deep-well injection, and environmental monitoring at disposal sites.

Residues from **coal-fired power plants**, generally referred to as CCP, should be considered valuable raw materials in light of the global 'Zero waste economy' tendencies and EU Green Deal efforts. Multiple potential uses have been identified, such as in the construction industry, in cement and building materials manufacturing, in the mining industry as a backfilling material, in industries manufacturing valuable materials (e.g., artificial zeolites (Behin et al., 2017)) or in sourcing of rare earth elements, as well as in road construction and as an underlying barrier in disposal sites construction. The commonly observed NOR activity concentration in CCP do not create significant problems when reused as described above. In cases where CCP are not reused, they are commonly disposed of at hazardous or non-hazardous disposal sites. The current study results indicate that all residues are either recycled within coal-fired power plants (e.g., France) or transferred to non-hazardous landfills (e.g., Spain).

No specific details were available regarding waste from geothermal energy production; depending on NOR content, the waste was either disposed of at specific dedicated landfills, or at conventional landfills with RP requirements for handling and disposal.

## 7. Concluding remarks

Naturally occurring radioactive material (NORM) in the oil and gas industry, coal mining and coal-fired power plants, and geothermal energy production facilities was investigated and compared, regarding main RP aspects within the consecutive segments of energy production lines. The main outcomes of the study showed that:

* Raw materials (oil formation rocks, crude oil, coal) commonly contain low activity concentrations of NOR. Most of radionuclides are in secular equilibrium with their parent radionuclides $^{238}$U and $^{232}$Th and are, therefore, of no concern for RP. Nevertheless, in certain cases geothermal fluids may have elevated levels of $^{226}$Ra, $^{210}$Pb and $^{210}$Po depending on system conditions. This information calls for a cautious approach with proper protection measures since such fluids may be mobile and have a long-term radiotoxic impact on the environment, including risks for the public and non-human biota due to environmental contamination and for workers in geothermal facilities.

* Energy carriers' pre-treatment and transportation in all sectors lead to the accumulation of NOR in the residues, scales, sludges and filter deposits with enhanced concentration of $^{226,228}$Ra, $^{210}$Pb and $^{210}$Po. These radionuclides, along with $^{222}$Rn in certain situations, were identified as the main dose contributors in all energy sectors. Consequently, caution and protective measures for workers are necessary, along with proper handling and disposal of such materials to prevent eventual NOR mobilisation and transport from disposal sites into the environment. This is particularly important when considering the long-term radiotoxicity of NOR and the prospective future land use of current disposal sites.

* The combustion process and energy generation itself produce residues containing NOR mainly in coal-fired power plants (fly and bottom ash).

* Maintenance and cleaning processes were found to be the most concerning exposure scenarios for workers from an RP standpoint,

with external radiation as the main exposure pathway for all investigated sectors.

* The oil and gas industry is mainly responsible for the release of significant volumes of liquid NORM, produced water containing $^{226}$Ra. Energy generation from coal is the source of the largest waste stream, when considering solid waste from coal mining, and CCP from coal fired power plants. However, the average NOR activity concentration is only slightly enhanced, and a high proportion of solid waste is reused. Geothermal energy production processes are responsible for the creation of scales and sediments due to similar processes to those observed in the case of formation water released from coal mining and oil and gas exploitation.

* The application of occupational, health and safety and radiation protection measures reduces the potential exposure doses at most of the facilities and exposure scenarios in Europe, leading to estimated effective doses for workers below 1 mSv y$^{-1}$. Furthermore, no significant doses were recorded for the public or biota due to NORM releases from investigated energy sectors.

* The release of liquid NORM from the oil and gas industry, as well as liquid and solid NORM waste from coal mining and geothermal fluids enriched in NOR, can have a radiological impact of varying degrees, depending on each specific case and all related uncertainties (site characteristics, risk awareness and existence of protective and monitoring measures, type and magnitude of recipient for effluent and/or waste, sensitivity of the environment, radionuclides of concern, volume of released effluents/waste etc.). Therefore, the long-term behaviour of these NORM — both large-volume effluents and wastes with lower NOR activity concentrations, but also low-volume residues with high NOR activity concentrations, such as scale and sediment— highlights the need for continuous source and environmental monitoring (with specific requirements for radionuclides of concern, organisms of concern, type and time frequency of analysis, record keeping) inspections, and proper disposal practices to prevent environmental mobilization and ensure overall protection.

* According to national information collected, common standards to protect the environment from pollutants have also been applied in the case of NORM, in a graded approach, when screening of exposure gives a dose estimation greater than 1 mSv y$^{-1}$ (what has been shown to be rare). There is a consensus among investigated countries that environmental impact assessment can be useful in terms of the justification of PR actions regarding both human and biota, but this is not fully developed in practice.

* It should be highlighted that the work of international organizations and research societies has contributed to societal acceptance that the long-term preservation of the environment, with humans and all other species, is a global concern both today and in the future.

* Treatment and disposal practices for NORM residues and wastes depend on their type, NOR content, the availability of disposal sites and other infrastructural aspects, and finally on the extent to which a circular economy action plan has been developed in each country. In general, residues in the form of scales, sludges and filter materials containing high NOR activity concentrations are considered as radioactive waste and are properly disposed of at dedicated disposal sites. For other NORM waste types, different disposal sites are identified, e.g., conventional landfill, both hazardous and non-hazardous ones. The issues of highest importance when disposing of NORM waste are related to landfill stability, sediment migration, and land use changes.

* With the generally low impact that NORM from the investigated sectors may have both on workers, the public, and the environment, it should also be emphasized that NORM with even low activity levels, but present in large volumes and having potential for cumulative exposure, always requires compliance with specific regulatory frameworks (e.g., EU BSS Directive 2013/59/Euratom).
Furthermore, the main identified challenges across the herein
investigated energy sectors included:

* Need for more efficient collaboration between regulators, industry operators and researchers to identify knowledge gaps and provide research, information and data for scientifically based regulatory control and management of NORM.

* Optimization of processes with regards to efficiency and RP measures.

* Use of NORM residues to support circular economy and European Green Deal initiatives.

* Development and application of a holistic approach to NORM management and control.

* Waste management – improvement of mitigation methods and disposal options; sharing of best practices.

To address these, we propose several future research directions such as:

* Characterization of NORM in different exposure sites, such as NORM affected multiple contaminants legacy sites, but also in thermal energy processes.

* Development of NORM reference materials for the most common matrices and inter-laboratory measurements.

* Development of harmonized criteria and risk assessment for the reuse of NORM-containing materials in a transparent way for better social acceptance within a circular economy framework.

* Environmental impact assessment for different NORM exposure scenarios and non-human biota (e.g. in marine ecosystems, polluted freshwater ecosystems with liquid NORM, at NORM affected legacy sites),

* Development of improved and internationally harmonized monitoring programs,

* Improved dose modelling in different NORM exposure scenarios,

* Minimising risks through the development of cost-effective mitigation, treatment, and remediation technologies.

CRediT authorship contribution statement

**Jelena Mrdakovic Popic:** Writing – review & editing, Writing – original draft, Supervision, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Nathalie Vanhoudt:** Writing – review & editing, Writing – original draft, Resources, Methodology, Investigation, Formal analysis.
**Gennaro Venoso:** Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis.
**Federica Leonardi:** Writing – review & editing, Writing – original draft, Validation, Investigation, Formal analysis.
**Hallvard Haanes:** Writing – review & editing, Validation, Methodology, Formal analysis.
**Alla Dvorzhak:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization.
**Cristina Nuccetelli:** Writing – review & editing, Validation, Methodology, Conceptualization.
**Rosa-bianca Trevisi:** Writing – review & editing, Validation, Investigation, Formal analysis.
**Raffaella Ugolini:** Writing – review & editing, Validation, Methodology, Investigation, Data curation.
**Flavio Trotti:** Validation, Resources, Methodology, Conceptualization.
**Almudena Real:** Supervision, Methodology, Data curation, Conceptualization.
**Danyl Pérez-Sánchez:** Writing – review & editing, Validation, Methodology, Investigation.
**Alicia Escribano:** Investigation, Formal analysis.
**Joana Lourenço:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation.
**Ruth Pereira:** Writing – review & editing, Visualization, Validation, Conceptualization.
**Laureline Fevrier:** Writing – review & editing, Methodology, Investigation, Funding acquisition.
**Boguslaw Michalik:** Writing – review & editing, Writing – original draft, Validation, Resources, Methodology, Investigation, Formal analysis, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work has been carried out within the RadoNorm project and received funding from the Euratom research and training programme 2019-2020 under grant agreement No 900009. Authors additionally acknowledge funding from: the Norwegian Research Council (NFR) under grant agreement for project 313057; the Polish Ministry of Science and Higher Education programme "PMW" (international co-fund projects) under grant agreement No 5156/H2020- Euratom/2020/2021/2; the POPH and national funds (OE), through FCT—Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the Article 23, of the Decree Law 57/2016, of August 29, changed by Law 57/2017, of July 19. We acknowledge financial support to CESAM by FCT/MCTES (UIDP/50017/2020 + UIDB/50017/2020 + LA/P/0094/2020), through national funds. Joana Lourenço was funded by national funds (OE), through FCT—Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen in the numbers 4, 5 and 6 of the Article 23, of the Decree Law 57/2016, of August 29, changed by Law 57/2017, of July 19. CESAM is a research unit financially supported by FCT/MCTES (UIDP/50017/2020 and UIDB/50017/2020), through national funds.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.envint.2025.109958.

Data availability

The data that has been used is confidential.

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