Influence of humidity on radon and thoron exhalation rates from building materials

Exploring statistical and machine learning techniques to identify factors influencing indoor radon concentration

Radon is a radioactive gas with a carcinogenic effect. The malign effect on human health is, however, mostly influenced by the level of exposure. Dangerous exposure occurs predominantly indoors where the level of indoor radon concentration (IRC) is, in its turn, influenced by several factors. The current study aims to investigate the combined effects of geology, pedology, and house characteristics on the IRC based on 3132 passive radon measurements conducted in Romania. Several techniques for evaluating the impact of predictors on the dependent variable were used, from univariate statistics to artificial neural network and random forest regressor (RFR). The RFR model outperformed the other investigated models in terms of R2 (0.14) and RMSE (0.83) for the radon concentration, as a dependent continuous variable. Using IRC discretized into two classes, based on the median (115 Bq/m3), an AUC-ROC value of 0.61 was obtained for logistic regression and 0.62 for the random forest classifier. The presence of cellar beneath the investigated room, the construction period, the height above the sea level or the floor type are the main predictors determined by the models used.

The Determination of Radon/Thoron Exhalation Rate in an Underground Coal Mine-Preliminary Results

The objective of this work was to perform a series of measurements of radon and thoron exhalation in the underground workings of an experimental coal mine. In the years 2012–2015, experiments on underground coal gasification were carried out in a coal mine, which caused, among other effects, damage to rock mass. Afterward, periodic increases in the concentration of potential alpha energy (PAEC) of radon decay products in the air were found, which could pose a hazard to miners. The question posed was whether the gasification experiment resulted in the increased migration of radon and thoron. If so, did it increase the radiation hazard to miners? The adaptation of the existing instrumentation to the specific conditions was conducted, and a series of measurements were made. It was found that the measured values of radon and thoron exhalation rates ranged from 3.0 up to 38 Bq·m⁻²·h⁻¹ for radon and from 500 up to 2000 Bq·m⁻²·h⁻¹ for thoron.

Development of 222Rn Emanation Sources with Integrated Quasi 2π Active Monitoring

In this work, a novel approach for the standardization of low-level 222Rn emanation is presented. The technique is based on the integration of a 222Rn source, directly, with an α-particle detector, which allows the residual 222Rn to be continuously monitored. Preparation of the device entails thermal physical vapor deposition of 226RaCl2 directly onto the surface of a commercially available ion implanted Si-diode detector, resulting in a thin-layer geometry. This enables continuous collection of well resolved α-particle spectra of the nuclei, decaying within the deposited layer, with a detection efficiency of approximately 0.5 in a quasi 2π geometry. The continuously sampled α-particle spectra are used to derive the emanation by statistical inversion. It is possible to achieve this with high temporal resolution due to the small background and the high counting efficiency of the presented technique. The emanation derived in this way exhibits a dependence on the relative humidity of up to 15% in the range from 20% rH to 90% rH. Traceability to the SI is provided by employing defined solid-angle α-particle spectrometry to characterize the counting efficiency of the modified detectors. The presented technique is demonstrated to apply to a range covering the release of at least 1 to 210 222Rn atoms per second, and it results in SI-traceable emanation values with a combined standard uncertainty not exceeding 2%. This provides a pathway for the realization of reference atmospheres covering typical environmental 222Rn levels and thus drastically improves the realization and the dissemination of the derived unit of the activity concentration concerning 222Rn in air.

Ion implantation of 226Ra for a primary 222Rn emanation standard

Laser resonance ionization at the RISIKO 30 kV mass separator has been used to produce isotopically and isobarically pure and well quantified ²²²Rn emanation standards. Based upon laser-spectroscopic preparation studies, ion implantation into aluminum and tungsten targets has been carried out, providing overall implantation efficiencies of 40% up to 60%. The absolute implanted activity of ²²⁶Ra was determined by the technique of defined solid-angle α-particle spectrometry, where excellent energy resolution was observed. The ²²²Rn emanation coefficient of the produced targets was studied using α-particle and γ-ray spectrometry, and yielded results between 0.23 and 0.34, with relative uncertainty on the order of 1%. No dependence exceeding a 1% change of the emanation on humidity could be identified in the range of 15 %rH to 75 %rH, whereas there were hints of a slight correlation between the emanation and temperature. Additionally, and as expected, the emanation coefficient was found to be dependent on the target material as well as the implanted dose.

The possibility of the phosphogypsum use in the production of brick: Radiological and structural characterization

In this paper, phosphogypsum (PG) with the content of ²²⁶Ra of about 500 Bq kg^-1 was used as a clay additive in mass ratios of (0-40) % and its influence on the radiological and mineralogical characteristics of the obtained brick samples was monitored. After sintering the samples at 1000 ℃, the formation of the mineral phase gehlenite was observed, and its share increased with the share of PG in the samples. The Monte Carlo method was used to determine the gamma dose rates, and consequently annual effective dose, for a standard room, with dimensions 4 × 5 × 2.8 m, whose walls were built of brick with PG. The obtained values were in the range (0.22-0.35) mSv y^-1. In addition, the active device RAD7 was used to determine the radon surface exhalation rates from the samples, which were found to be in the range (63-150) mBq m^-2 h^-1. The estimated indoor radon concentrations were found to be drastically lower than 100 Bq m^-3, leading to low radon inhalation doses. However, estimated annual effective doses from external gamma exposure were found not to be insignificant.

Indoor Radon Measurements Using Radon Track Detectors and Electret Ionization Chambers in the Bauxite-Bearing Areas of Southern Adamawa, Cameroon

The current work deals with indoor radon (222Rn) concentrations and ambient dose-equivalent rate measurements in the bauxite-bearing areas of the Adamawa region in Cameroon before mining from 2022. In total, 90 Electret Ionization Chambers (EIC) (commercially, EPERM) and 175 Radon Track Detectors (commercially, RADTRAK2) were used to measure 222Rn concentrations in dwellings of four localities of the above region. A pocket survey meter (RadEye PRD-ER, Thermo Scientific, Waltham, MA, USA) was used for the ambient dose-equivalent rate measurements. These measurements were followed by calculations of annual doses from inhalation and external exposure. 222Rn concentrations were found to vary between 36 ± 8-687 ± 35 Bq m-3 with a geometric mean (GM) of 175 ± 16 Bq m-3 and 43 ± 12-270 ± 40 Bq m-3 with a geometric mean of 101 ± 21 Bq m-3 by using EPERM and RADTRAK, respectively. According to RADTRAK data, 51% of dwellings have radon concentrations above the reference level of 100 Bq m-3 recommended by the World Health Organization (WHO). The ambient dose equivalent rate ranged between 0.04-0.17 µSv h-1 with the average value of 0.08 µSv h-1. The inhalation dose and annual external effective dose to the public were assessed and found to vary between 0.8-5 mSv with an average value of 2 mSv and 0.3-1.8 mSv with an average value of 0.7 mSv, respectively. Most of the average values in terms of concentration and radiation dose were found to be above the corresponding world averages given by the United Nations Scientific Commission on the Effects of Atomic Radiation (UNSCEAR). Even though the current exposure of members of the public to natural radiation is not critical, the situation could change abruptly when mining starts.

Heat-air-moisture coupled model for radon migration in a porous media

Radon is one of the main causes of environmental pollution and lung cancer. The precipitation of radon from porous media is affected by the coupling of heat and moisture, which has not been considered in the existing knowledge. We present a model for predicting radon migration in porous media. This model combines the heat-air-moisture (HAM) coupling model of porous media with a radon migration model to establish three-dimensional partial differential equations for steady-state radon migration under HAM coupling conditions. The finite element method (FEM) was used to obtain a numerical solution. Experimental verification showed that the model had high calculation accuracy; the calculated maximum relative error did not exceed 15%. The results of the model were compared with the results of a conventional model that does consider the coupling of heat and humidity; the results showed significant differences in the radon concentrations and radon flux distribution curves for the two models. The newly developed model revealed that there is a significant coupling effect between migration and the distribution of the temperature field, the humidity field, and radon flux in unsaturated porous media. The radon exhalation rate on the surface of porous media increases linearly with the increase of permeability. The exhalation rate decreased exponentially with the increase in relative humidity. When the trend of the temperature gradient was consistent with the concentration gradient, the radon exhalation rate decreased linearly with the increase in temperature gradient. We establish a new model to study the radon migration in porous media under the coupling of heat and moisture. The model provides a theoretical basis for an effective and accurate analysis of the impact of radon exhalation on the environment.

QUANTIFICATION OF BACK DIFFUSION IN RADON AND THORON EXHALATION RATE MEASUREMENTS

The radon (222Rn) and thoron (220Rn) fluxes from the soil and building materials are the major contributors to their indoor levels. Hence, the measurement of radon and thoron exhalation rates from the source matrix becomes the foremost step in controlling the indoor radon and thoron exposure. It is a challenge to measure the exhalation rates without disturbing the natural conditions. The back-diffusion phenomenon modifies the exhalation rate. The work presented here is to measure the back-diffusion coefficient and takes it into consideration while estimating the exhalation rate. For radon measurements, the back-diffusion coefficient and the free exhalation rates were simultaneously estimated by adopting a novel methodology. The leak rate of the experimental setup measured by this methodology was agreeable with the value measured by adopting the standard technique. In the case of thoron, the back-diffusion effect was found to be negligible for the present experimental conditions and it is duly explained. The above results were obtained by analyzing two soil samples with high 238U and 232Th content collected from monazite-rich coastal area.

THE SCREENING INDOOR RADON AND PRELIMINARY STUDY OF INDOOR THORON CONCENTRATION LEVELS IN KUWAIT

Indoor measurements of radon and thoron in Kuwait were conducted during the years 2015 and 2016. In this study, 65 dwellings were selected for the long-term radon-thoron survey using passive nuclear track monitors. The monitors (at least one) were used at various locations in the dwellings for 83-306 days. Some measurements were also repeated at the same locations in different seasons. This current study is a preliminary thoron survey with relatively small sample size. The results showed that the range of thoron concentration was from below the lower limit of detection to 35 Bq m-3, whereas the range of radon concentration was within 10-202 Bq m-3. Furthermore, 22% of the radon results exceeded the WHO radon reference level of 100 Bq m-3. The analysis of variance showed a correlation between indoor radon concentration and the season. However, the thoron measurements were rather limited and the values were low. In addition, the relationship was investigated between radon and thoron concentrations involving the floor levels and the type of ventilation systems used.

Numerical modeling of the sources and behaviors of 222Rn, 220Rn and their progenies in the indoor environment-A review

²²²Rn, ²²⁰Rn and their short-lived progenies are well known radioactive indoor pollutants, identified as the leading environmental cause of lung cancer next to smoking. Apart from the conventional measurement methods, numerical modeling methods are developed to simulate their physical and decay processes in ²²²Rn and ²²⁰Rn's life cycle, estimate their levels, concentration distributions, as well as effects of control strategies in the indoor environment. In this article, we summarized the numerical models used to illustrate the physical processes of each source of ²²²Rn and ²²⁰Rn entry into the indoor environment, and the application of Jacobi room models and CFD (Computational Fluid Dynamic) models used to present the behaviors of indoor ²²²Rn, ²²⁰Rn and their progenies. Furthermore, we consider that the development of numerical modeling of ²²²Rn and ²²⁰Rn would have a bright prospect in the directions of stochastic methods based on a steady-state model, the fine simulation of the time-dependent model as well as the multi-dimension model.

Potential factors that impact the radon level and the prediction of ambient dose equivalent rates of indoor microenvironments

This study aimed to measure the equilibrium equivalent radon (EEC_Rn) concentration in an old building (Building-1) and a new building (Building-2) with mechanical ventilation and a natural ventilation system, respectively. Both buildings were located at the campus of University Kebangsaan Malaysia. The concentration of indoor radon was measured at 25 sampling stations using a radon detector model DOSEman PRO. The sampling was conducted for 8 h to represent daily working hours. A correlation of the radon concentration was made with the annual inhalation dose of the occupants at the indoor stations. The equilibrium factor and the annual effective dose on the lung cancer risks of each occupant were calculated at each sampling station. The average equilibrium equivalent radon measured in Building-1 and Building-2 was 2.33 ± 0.99 and 3.17 ± 1.74 Bqm^-3, respectively. The equilibrium factor for Building 1 ranged from 0.1053 to 0.2273, and it ranged from 0.1031 to 0.16 for Building 2. The average annual inhalation doses recorded at Building-1 and Building-2 were 0.014 ± 0.005 mSv y^-1 and 0.020 ± 0.013 mSv y^-1, respectively. The annual effective dose for Building-1 was 0.034 ± 0.012 mSv y^-1, and it was 0.048 ± 0.031 mSv y^-1 for Building-2. The values of equilibrium equivalent radon concentration for both buildings were below the standard recommended by the International Commission on Radiological Protection (ICRP). However, people may have different radon tolerance levels. Therefore, the inhalation of the radon concentration can pose a deleterious health effect for people in an indoor environment.

Extremely high radon activity concentration in two adits of the abandoned uranium mine 'Podgórze' in Kowary (Sudety Mts., Poland)

Measurements of radon activity concentration were conducted for a period of 6 months, from April to September 2011, in the air of two adits constituting part of the disused uranium mine 'Podgórze' in Kowary. Adits no. 19 and 19a in Kowary had been chosen owing to the occurrence within them of the highest documented radon concentrations in Poland, With levels higher than a million Bq m^-3. The main goal of this study was to characterize the level of ²²²Rn activity concentration registered in selected workings of this underground space, investigate ²²²Rn changes and their characteristics over selected periods of time (an hour, a day, a month, six months) and determine the effective doses, which provided the basis for estimating the risk of exposure to increased ionizing radiation for employees and visitors to the mine. The highest values of ²²²Rn activity concentration inside the adits occurred at the time when visitors, guides and other members of the staff were present there. The recorded values of radon activity concentration, regardless of the time and the month when the measurement was performed, remained at an average level of 350-400 kBq m^-3. These values were far above the limit of 1.5 kBq·m^-3 recommended by international guidelines. The maximum values ranged from 800 to more than 1000 kBq·m^-3. Radon activity concentration changes occurred only in periods determined by 7-h cycles of connecting and disconnecting the mechanical ventilation. For about 7 h after activating the ventilation system, between 7 a. m. and 2 p. m., and after closing the adit, between 7 p. m. and 2 a. m., ²²²Rn activity concentrations decreased to levels even as low as 100 kBq·m-3. However, as early as 3-4 h after disconnecting the ventilation system, there was a sharp rise in the values of ²²²Rn activity concentration, to the level higher than 800 kBq·m-3. The risk of receiving a radiation dose higher than the national standard of 1 mSv/year by members of the public occurred as soon as after spending 1 h inside the workings. The minimum monthly effective radiation dose received by every employee in the tourist adit no. 19 in Kowary was higher than 1/5 (4 mSv) of the annual effective dose allowed by Polish law (20 mSv/year). In the non-tourist adit no. 19, the minimum monthly radiation dose was more than 3 times as high as the allowed value of 4 mSv. Due to the highly disturbing and unfavourable, from a radiological protection point of view, conditions inside the disused uranium mine 'Podgórze' in Kowary, the mine manager decided to increase the efficiency of the designed mechanical ventilation system and launch measurements of radon activity concentration in the workplace.

Indoor radon risk associated to post-tectonic biotite granites from Vila Pouca de Aguiar pluton, northern Portugal

At Vila Pouca de Aguiar area, northern Portugal, crops out a post-tectonic Variscan granite pluton, related with the Régua-Vila Real-Verín fault zone, comprising three types of biotite granites. Among these granites, PSG granite yield the highest average contents of U, probably due to its enrichment in accessory U-bearing minerals such as zircon. In the proximity of faults and joints, these granites are often affected by different degrees of hydrothermal alteration, forming reddish altered rocks, commonly known as "episyenites". These altered rocks are probably associated to the occurrence of hydrothermal processes, which led to uranium enrichment in the most advanced stages of episyenitization. In these granites, both average gamma absorbed dose rates in outdoor and indoor air are higher than those of the world average. Furthermore, even in the worst usage scenario, all these granites can be used as a building material, since their annual effective doses are similar to the limit defined by the European Commission. The geometric mean of radon activity of 91 dwellings located at the Vila Pouca de Aguiar pluton is 568Bqm(-3), exceeding that of other northern Portuguese granites. Measurements carried out during a winter season, indicate that 62.6% of the analysed dwellings yield higher indoor radon average values than the Portuguese legislation limit (400Bqm(-3)), and annual effective doses due higher than the world's average value (1.2mSvy(-1)). The interaction of geogenic, architectural and anthropogenic features is crucial to explain the variance in the geometric mean of radon activity of dwellings from Vila Pouca de Aguiar pluton, but the role of geologic faults is probably the most important decisive factor to increase the indoor radon concentration in dwellings. Hence, the development of awareness campaigns in order to inform population about the incurred radiological risks to radon exposure are highly recommended for this specific area.