Abstract

The tin industry in Indonesia is a major global producer, and mining and smelting processes can mobilize and concentrate Naturally Occurring Radioactive Materials (NORM), particularly radionuclides from the 238U and 232Th decay series, potentially leading to occupational radiation exposure. We conducted a comprehensive assessment of annual effective doses to workers by integrating external and internal exposure pathways at two tin smelters using raw materials with different tin contents. External exposure was evaluated using ambient dose equivalent rate measurements, while internal exposure was assessed through simultaneous measurements of radon (222Rn), thoron (220Rn), and thoron progeny concentrations with passive discriminative detectors. Activity concentrations of 226Ra, 232Th, and 40K in raw materials and by-products were determined by HPGe gamma spectrometry to identify dominant contributors to gamma radiation fields. The total annual effective dose for most workers was <2 mSv, but reached 20.72 mSv for slag handling at one site, exceeding the recommended IAEA occupational limit. Radionuclides from the 232Th decay chain were the dominant contributors to external gamma exposure, particularly in slag, with activity concentrations increasing from raw materials to slag by approximately 28-fold for 226Ra and 79-fold for 232Th at Smelter A. Internal exposure was mainly influenced by thoron progeny in smelting areas, whereas radon contributed more in office and laboratory environments. These results identify slag storage as a critical radiological hotspot and highlight the need to evaluate both exposure pathways to support ALARA-based protection strategies and regulatory frameworks for NORM management in industrial settings.

1 Introduction

Natural resource developments, including mining, refining, and waste management, are the basis of a substantial portion of the global economy and provide the raw materials for much of the infrastructure and tools used in daily life (1, 2). One industrial activity involving naturally occurring radioactive sources is tin mining and smelting on Bangka Island, Indonesia. Tin ores contain multiple chemical constituents, including rare earth elements (REEs) and may contain naturally occurring radioactive material (NORM), such as uranium, thorium, and their radioactive progeny, as by-products (3, 4). This major industry employs a large workforce in an environment where NORM can be mobilized and concentrated within process streams (5).

The tin industry in Indonesia has a history spanning more than 200 years and is currently the second-largest tin producer in the world, with an annual production of approximately 50,000 tons (4, 6, 7). As with many metallic ores, tin deposits commonly contain multiple chemical elements. Consequently, mining and smelting activities can result in the release and concentration of radionuclides and heavy metals, leading to radiation exposure of workers, the public, and the surrounding environment.

The tin smelting industry on Bangka Island, Indonesia, represents a significant NORM-related activity, processing ores that contain primordial radionuclides from the 238U and 232th decay series, as well as 40K. Previous studies in Bangka Island have shown that ambient radiation levels are approximately twice the global average (2.4 mSv y−1), with the maximum background dose rates reaching 596 nSv h−1 in Muntok City, the location of the Pemali tin deposit (6). Other studies have reported potential environmental risks associated with the storage of tin-smelting by-product (6, 8–10). In addition to environmental concerns, tin smelting poses potential occupational radiation exposure. Workers may be exposed to external gamma radiation from radionuclides in the ores, intermediate materials, and slag. Internal exposure may occur through inhalation of radon (222Rn) and thoron (220Rn) released into the workplace air, as well as through airborne particulates caused during high-temperature processing and from stored materials. Given the large workforce employed in this industry, where NORM can be released and concentrated during processing, both external and internal radiation exposures, particularly from radon and thoron progeny, are of relevance for workers directly involved in smelting operations.

Issues related with NORM-related industries are particularly pronounced in tin-producing regions such as Bangka Island, where economic dependence on tin mining coexists with high levels of environmental radiation exposure and complex waste management challenges. To date, no published studies have comprehensively evaluated occupational radiological exposure among workers in the tin-smelting industry in Indonesia. This study addresses this gap by providing the first integrated assessment of annual radiation doses received by tin-smelting workers, including a range of occupational roles from administrative staff to personnel directly involved in production processes. The assessment integrates external gamma doses with internal exposure from thoron progeny in high-temperature smelting environments and evaluates the distribution of radiation doses across multiple exposure pathways for distinct worker categories in the NORM industry. Measurements were conducted across key operational areas, including raw-material storage, smelting, and post-processing zones, as well as slag and waste-handling facilities, using two representative tin sources. By identifying and characterizing radiation-exposure points associated with NORM enrichment during high-temperature processing, this study clarifies potential impacts on both workers and the surrounding environment.

This study provides important baseline data to support improved occupational radiation protection, application of the As Low As Reasonably Achievable (ALARA) principle, and the development of national policies and regulatory frameworks for NORM and industrial-waste management. In this context, the study is aligned with the United Nations Sustainable Development Goals (SDGs) to advance sustainable development and benefit humanity. This research focuses on good health and well-being (SDG 3), clean water and air (SDG 6), industrial innovation and infrastructure (SDG 9), responsible consumption and production (SDG 12), and life on land (SDGs 14 and 15).

2 Materials and methods2.1 Study area

Bangka Belitung Province contributes approximately 90–95% of Indonesia’s tin production, equivalent to 65,000–70,000 tons per year (11, 12). Approximately 76% of workers in the tin mining industry between 2004 and 2013 were local residents (13). This study categorized workers into several occupational types to determine the spread of radionuclides caused by this industry and to clarify its impact on the environment and surrounding communities.

The study was conducted at two tin smelter industries located in two major cities on the Bangka island, Sungailiat and Pangkalpinang. Sungailiat city has Smelter A located at S 1.8680388, and E 106.1069149, while Pangkalpinang city has Smelter B located at S2.1410465, and E106.1133337 (Figure 1). Smelter A is located in a rural area with a population of 95,427 people in 2023. Meanwhile, Smelter B is located near a city with a population of 236,267 people (14, 15). Maps were created by using the software ArcGIS version 10.8, derived from Geological Agency.

Geological map of Bangka Island, Indonesia and the study locations in Sungailiat (Smelter A) and Pangkalpinang (Smelter B).

These two smelters represent the tin-processing industry in the region, which handles significant volumes of tin ore and produces various by-products. Each smelter uses a different type of raw material: in Smelter A, tin ore is used, while Smelter B uses byproduct slag, which still contains 20% tin. Measurement locations at each smelter were carefully selected to represent the potential areas for highest radiation levels. These areas include raw material storage, processing areas, slag storage areas, product storage areas, and temporary waste disposal sites.

Smelter A consists of an administration building and a laboratory on the second floor, with the smelting area in front of the building (Figure 2a). Meanwhile, Smelter B consists of an administration building and a laboratory on the ground floor near the main entrance, and the smelting location is far behind the building (Figure 2b).

Location of facilities at (a) Smelter A and (b) Smelter B.

In the tin smelting process, the raw material is melted at high temperatures several times in a furnace to maximize tin recovery. The tin smelting process involves adding other materials, such as anthracite and flux, in specific ratios. Anthracite acts as a high carbon reducing agent to convert tin oxide into molten tin, while limestone acts as a flux to combine with impurities to form a slag that floats on top of the tin. Fuel, air, and oxygen injection are also regulated to maintain controlled conditions. This smelting process takes place at 1150 °C and produces crude tin with a content of > 80% in the bottom layer (16). The next stage is the reduction process, where the slag from the previous smelting process is then reduced again with anthracite and a high number of additional reductants. Remelting is then carried out at a slightly higher temperature of 1,150–1,250 °C, producing a Sn/Fe alloy and a slag with a tin concentration of around 3% (17, 18).

2.2 Comprehensive radiation measurement

In this study, external radiation exposure and its contribution from different operational activities within the NORM industry were systematically evaluated. Measurements were conducted across three representative occupational categories: office, laboratory, and field work, which are most likely to be influenced by nearby mineral-processing operations. The external exposure was conducted ambient dose equivalent rate measurement using a CsI(Tl) scintillator detector survey meter with dimensions approximately (Fuji Electric, Japan). This survey meter detects X-ray gamma radiation with a measurement energy range from 40 keV to 6 MeV. Measurements were taken at a height of 1 meter above ground level around the tin smelter area and in the smelter area at each stage of the industrial process at both smelters (Figure 3).

An external and internal exposure measurement scheme to identify the exposure received by workers in the tin smelting industry.

In this study, radon and thoron concentrations were measured simultaneously using a passive detector, namely the RADUET passive radon-thoron discrimination monitor (Radosys, Hungary) (19–21). The lower detection limits (DLs) of the RADUETs were determined to be 3 Bq m−3 for radon and 4 Bq m−3 for thoron (22). The RADUET was calibrated at Institute of Radiation Emergency Medicine (IREM), Hirosaki University (23, 24). Additionally, direct measurement of thoron progeny concentration is needed to accurately estimate the radiation dose caused by their radionuclides using a deposition rate monitor (passive monitors). To obtain tracks of alpha particles from thoron progeny, especially 212Po, it is necessary to construct an appropriate method. The method was the same method as previously reported thoron progeny measurement in public dwellings in Bangka (6).

The total 24 passive detectors and monitors were placed inside the smelter to measure outdoor 222Rn and 220Rn progeny after obtaining prior permits. A total of 14 passive detectors and monitors were installed in Smelter A, and 11 in Smelter B. In Smelter A, the installation was carried out in the smelter area (10 detectors), the office room (2 detectors), and the laboratory room (2 detectors). While in Smelter B, the installation was carried out in the smelter area (7 detectors), the office room (2 detectors), and the laboratory (2 detectors). The equipment was installed by hanging it at a height of 1 to 2 m above the ground and positioned in the middle of the room at least 50 cm from the wall surface according to the location of the workers. Meanwhile, installation in the smelting place is done by hanging it on the top of a pole or pillar. The monitor was installed two times in a year: September 2023 to May 2024 and May 2024 to November 2024. This is to obtain representative data in calculating the average annual dose, which is more stable and less influenced by short-term fluctuations. In both smelters, the storage area for the materials to be used is kept in an open space together near the smelter. The building studied was constructed of sand bricks held together with a mixture of cement, water, and sand, then coated with wall paint, and the floor was covered with cement tiles. Meanwhile, the foundations of the smelting plants and floor are typically a mixture of gravel and cement, with steel walls used on some sides. Inside the smelter, there are several pillars to support the roof to keep it sturdy.

After sampling for the passive detector and monitor is completed, the CR-39 chip is analyzed by etching to determine the number of alpha tracks. The etching process is carried out according to each manufacturer’s specifications. In the Radosys product, Hungary, etching is carried out using a 6.25 M NaOH solution at 90 °C for 6 h, while in the Nagase Landauer, Ltd. Japan product etching is carried out using a 6 M NaOH solution at 60 °C for 24 h. The etching process increases the size of the tracks caused by alpha particles in CR-39, so they can be read and quantified under a microscope. Calculation of radon () and thoron () concentrations from the track in RADUET using Equations 1, 2 (6, 21, 22, 25, 26):

Where and are the alpha track densities (track cm−2) in the CR-39 from the low and high air exchange rate chambers, respectively. and are the 222Rn and 220Rn calibration coefficients (tracks m−2 kBq−1 m3 h−1) in the low air exchange rate chamber, respectively. and are the 222Rn and 220Rn calibration coefficients in the high air exchange rate chamber (tracks m−2 kBq−1 m3 h−1), respectively. is the exposure time in hours, and is the background track density of the CR-39 detector. The concentration calculation for Equivalent Equilibrium Thoron Concentration () is based on the track counted using the Equations 3, 4:

Where and is the track density in the thoron progeny deposition detector that was installed and from the monitor that was isolated as background, is the equilibrium equivalent thoron concentration, and is the conversion factor for thoron progeny which was (tracks m−2 kBq−1 m3 h−1). is the duration of the monitor installation (6, 21). Calculation of uncertainty in radon and thoron concentrations in RADUET and , using the equations previously reported by Pradana et al. (6). Furthermore, for quality control, the 222Rn and 220Rn monitor has been calibrated in the Institute of Radiation Emergency Medicine, Hirosaki University (23).

Inhalation of short-lived solid decay products will deliver most of the radiation dose to the human lung. So, the estimated dose using the is based on direct measurements of thoron progeny. The equilibrium factor between radon, thoron, and their progeny determines the radioactive equilibrium level. The thoron equilibrium factor () was derived from field data in this study using the Equation 5 (27).

Where is the thoron gas concentration in Bq m−3.

The activity concentration of samples was measured using an HPGe detector (GEM 60–5, ORTEC, USA). This detector has a resolution of 1.81 keV at 1.33 MeV and a relative efficiency of 60%. The detector also has ultra-low background shielding using an aged lead-containing resist (ISuS, Sweden) used to measure 226Ra, 232th, 228th, 40K and enclosed in a compact 10 cm thick lead shield (28, 29). The counting system must be calibrated using a standard source in the same geometry as the sample. To prepare such a standard, a known amount of standard reference material (SRM) was filled into vials identical to the sample vials (30, 31).

Sample calculations were obtained by analyzing spectra from Multi Channel Analyzer (MCA) on a PC using Gamma Vision software (ORTEC, United States). The sample calculation time is about 80,000 counting times to obtain adequate uncertainty statistics. An estimate of the potassium concentration was obtained by detecting 1,460 keV gamma rays emitted by 40K. An Uranium estimation was performed through the detection of 609 keV gamma rays from 214Bi and 351 keV gamma rays from 214Pb, a product of the 238U decay series. Similarly, thorium estimation was performed through the detection of 581 keV gamma rays from 208Tl, 238 keV from 212Pb, and 911 keV from 228Ac, a product of the 232th decay series (4, 28, 32). The response of a gamma-ray instrument to radiation from K, U, or Th progeny source depends on the source concentration, the detector volume and efficiency, and the instrument’s energy threshold (33). We used Equations 6–8 to calculate the activity concentrations (), uncertainty (), and detection limit () from these measurements, respectively (4, 28, 32, 34, 35).

Where is the number of counts per second, is the efficiency, is the energy yield of the radionuclide, is the sample weight (kg), and is the correction factor (including summing in, summing out, decay factor, recovery factor, attenuation factor, branching ratio, and growth factor). is the coverage factor in k = 1.96 for a 95% confidence interval, is the combined uncertainty of the measurements, and is the absolute uncertainty of each relevant standard uncertainty. is the detection limit, is the critical level below which no signal can be detected, is the standard deviation, and is the error probability. The detection limits for 40K, 226Ra, and 232Th are 0.21 Bq kg−1, 0.08 Bq kg−1, and 0.08 Bq kg−1, respectively.

The absorbed dose rate in air can be evaluated from the activity concentration of radionuclides in a homogeneously distributed sample. The activity concentrations of radionuclides from the U and Th decay series will be the same. From this, the absorbed dose rate in air () can be estimated based on calculations of the conversion factors for each radionuclide by using Equation 9 (4, 26).

Where is the absorbed dose rate in the air (nGy h−1), , , and are the activity concentrations of 226Ra, 228Ra and 40K, respectively, in Bq kg−1. The coefficients of 0.462, 0.604 and 0.0417 are conversion factors (absorbed dose rate in air per unit activity per unit of sample mass, in units of nGy−1 per Bq kg−1) evaluated for 238U-series, 232Th series and 40K, respectively (4, 26).

The annual effective dose calculation is performed to evaluate the risk of radiation exposure in the workplace. These effective dose estimates are used to determine the highest dose exposure in the workplace. This calculation is performed by accumulating the effective doses from internal exposure and external exposure. Estimation of the annual effective dose (mSv) from indoor radon () and thoron () in workplace using Equations 10, 11, respectively:

Where and are the activity concentrations of 222Rn and 220Rn progeny in Bq m−3 from passive detector measurements. is the radon equilibrium factor in indoor environments with a value of 0.4 according to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and International Commission on Radiological Protection (ICRP) (36, 37). T is the working time, with 2080 h year−1. and are inhalation dose conversion factors with values 1.7 × 10−5 mSv (Bq h m3) and 1.07 × 10−4 mSv (Bq h m−3)−1 for 222Rn and 220Rn, respectively (37).

Meanwhile, the estimation of the annual effective dose (mSv) from external gamma radiation ( is based on the measurement of the ambient dose equivalent rate. Calculation of using Equation 12:

Where is the external gamma dose rate (nSv h−1), is the working time, with 2080 h year−1. This is calculated by multiplying the 52 weeks in a year by 40 working hours per week based on Labour laws No 13 (38). is conversion factor 0.6 (6). The total annual effective dose (from internal and external exposure is calculated per the Equation 13:

3 Result and discussion3.1 Characterization of NORM in materials

Gamma radiation exposure is a significant pathway for workers in mineral processing environments, especially tin smelting. This gamma radiation exposure comes from 40K and the 238U and 232Th, decay chains, where these radionuclides emit alpha, beta, and gamma rays. Workers can receive external and internal exposure; the closer to the source, the greater the gamma exposure received. For this reason, measurements were carried out on raw materials and by-products to determine the gamma-emitting source’s activity concentration. The dominant gamma emitters are short-lived decay daughters in the 238U decay chain; 214Pb and 214Bi from 226Ra, while in the 232Th decay chain are 228Ac, 212Pb, and 208Tl.

The activity concentration values for each dry material are shown in Table 1. The results of the calculation of the highest absorbed dose rate from the activity concentration are 16,943 nGy h−1 and 11,072 nGy h−1 in both slags. In addition, it is also found in the raw material at smelter B at 8,672 nGy h−1, which includes slag material (Slag 1) originating from other industries. The absorbed dose rate is within the range reported for other Tin slag measurements on Bangka Island, around 8,631–21,104 nGy h−1. This shows that the average absorbed dose rate in tin slag falls within that range, depending on the geological conditions used for raw material extraction.

AreaActivity concentration (Bq kg−1 dry)Absorbed dose rate226Ra232Th40K(nGy h−1)Smelter ARaw material334±8265±717±3315Anthracite4.8±0.411±141±210Flux10±1≤ LLD1.7±0.25Dust95±3170±5120±5152Crude tin1.4±0.15±1≤ LLD3.44Slag9,187±20520,930±4541,382±4216,943Smelter BRaw material4,949±11010,515±227817±248,672Anthracite22±241±359±537Flux65±2112±416±198Dust379±9846±19163±7693Crude tin150±5348±925±2280Slag5,784±13113,845±302887±3211,072

Distribution of absorbed dose rate from the calculation of activity concentration.

Based on the activity concentration reported in Table 1, substantial enrichment was observed from raw materials to the final slag-storage stage at Smelter A, with activity concentrations increasing by approximately 28-fold for 226Ra and 79-fold for 232Th. This enrichment reflects differences in feedstock characteristics: at Smelter A, the raw material consists of tin ore concentrate, whereas at Smelter B the feedstock comprises slag from other smelters that still contains approximately 20% residual tin.

The relative contributions of 232th, 226Ra, and 40K to the absorbed dose rate are shown in Figure 4. In slag samples, 232Th was the dominant contributor, accounting for74.6 and 75.5% of the absorbed dose rate at Smelter A and B, respectively.

Percent contribution of radionuclides to dose equivalent rate at smelters.

3.2 External exposure field

The tin smelting process has the potential expose workers to ionizing radiation from gamma-emitting radionuclides present in the chimney, intermediate products, and slag. Results showed that radiation exposure levels in smelting areas were consistently higher than those measured in offices and laboratories, with ambient dose equivalent rates ranging from 120 to 570 nSv h−1. In Smelter A, the mean ambient dose equivalent rate was approximately twice that observed at Smelter B of 338 nSv h−1. This situation is caused by two smelters handling different material concentrations, where smelter A uses 60–75% tin raw material, while smelter B uses 20% tin raw material from the previous tin smelter. Smelter A extracts more tin metal than smelter B, so the NORM enrichment in the Smelter A area is higher than in the Smelter B area. Meanwhile, radiation exposure in offices and laboratories varies depending on their location relative to the smelter. The ambient dose equivalent rate measurements for both smelters during operation are shown in Table 2.

AreaNAmbient dose equivalent rate (nSv h−1)Average (Value ±SD)RangeSmelter ASmelter10338±141120–570Office room2165±5160–170Laboratory2175±5170–180Storage2206±30176–2362166±15151–1812124±10114–1342797±30767–8272178±20158–19823,150±903,060-3,240Smelter BSmelter7160±31120–210Office room275±2550–100Laboratory2175±45130–220Storage2218±40178–258