CFD based simulation of thoron ((220)Rn) concentration in a delay chamber for mitigation application

Application of CFD modeling for indoor radon and thoron dispersion study: A review

This paper provides an in-depth discussion of the CFD implications to the design/study of interior environments and an overview of the most widely used CFD model for indoor radon and thoron dispersion study. For the design and analysis of indoor environments, CFD is a powerful tool that enables simulation and measurement-based validation. Simulating an indoor environment involves deliberate thought and skilful management of complicated boundary conditions. User and CFD programs can develop results through gradual effort that can be relied upon and applied to the design and study of indoor environments. Radon and thoron are natural radioactive gases and play a crucial role in accurately assessing the radioactive hazard within an indoor environment. This review comprise the work related to measurement and CFD modeling on these radioactive pollutant for indoors.Highlighting the current state of environmental radioactive pollutants and potentially identified areas that require further attention or research regarding investigating factors affecting indoor radioactive pollutants.

Development of a thoron calibration chamber based on computational fluid dynamics simulation and validation with measurements

Recently, interest in measuring the concentration of 220Rn in air has increased greatly following the development of standards and the calibration of monitoring instruments. In this study, a 220Rn calibration chamber was designed and developed at the Institute of Radiochemistry and Radioecology (RRI) based on the computational fluid dynamics (CFD) method implemented in ANSYS Fluent 2020 R1 code at the University of Pannonia in Hungary. The behavior of 220Rn and its spatial distribution inside the 220Rn calibration chamber at RRI were investigated at different flow rates. The 220Rn concentration was close to homogeneous under higher flow regimes due to thorough mixing of the gas inside the chamber. Predictions based on CFD simulations were compared with experimentally measured transmission factors (Cout/Cin). The spatial distribution of 220Rn was dependent on the flow rate and the positions of the inlet and outlet. Our results clearly demonstrate the suitability of the 220Rn calibration chamber at RRI for calibrating monitoring instruments. Furthermore, the CFD-based predictions were in good agreement with the results obtained at higher flow rates using experimental and analytical models according to the relative deviation, with a maximum of approximately 9%.

A CFD-based approach to study the deposition and distribution behaviour of 212Pb in a calibration chamber

Among the several aspects of decay products behavior, deposition is of special significance because of its prominent role in the activity removal from the environment, which eventually results in the occurrence of decay product disequilibrium with the parent gas. This point is particularly important in case of thoron dosimetry where thoron progeny ²¹²Pb accounts for the most of the radiological dose. The deposition depends on the size distribution of decay products and the structure of air turbulence at the air-surface interface. In the present work, the effect of varying air-flow (fan speed) and aerosol count median diameter (CMD) was studied on the deposition and distribution profile of ²¹²Pb using computational fluid dynamics (CFD). The simulations have been carried out in a cubical calibration chamber of volume 8 m³, facilitated at RP&AD, BARC. Simulated results showed that the increase of total depositional loss rate of attached fraction of ²¹²Pb due to increase of the fan speed was significant for CMD up to 400 nm, beyond which this effect started becoming less prominent with increasing diameter. Besides, a minimum of the total depositional loss rate curve was seen to be shifted to the higher CMD with increase of the fan speed. CFD results were found to be in good agreement with experimental observations obtained in the controlled conditions with thoron source.

A CFD-based approach to optimize operating parameters of a flow-through scintillation cell for measurement of 220Rn in indoor environments

The measurements and monitoring of ²²²Rn/²²⁰Rn have been of emerging interest in occupational environments particularly in radium/thorium handling facilities and environments with monazite deposits for the inhalation dosimetry. The performance of a flow-through Lucas scintillation cell (LSC) for long run ²²⁰Rn measurements, depends upon the exact distribution pattern of ²²⁰Rn and its decay products in the LSC which can vary with the design of inlet path and flow rates. In this work, the CFD technique has been used to study the concentration profiles of ²²⁰Rn and its decay products in LSC for varying flow rates and inlet needle lengths. The variation of alpha production efficiency (η_α) is computed and analyzed for each case; aiming to select the best operating range of parameters for the optimum performance of LSC for ²²⁰Rn measurements. It is seen that LSC can be operated in the flow rate ranging from 0.6 to 1 lpm with inlet needle length varying from 22.5 to 45 mm for improved sensitivity.

Statistical inferences from measured data on concentrations of naturally occurring radon, thoron, and decay products in Kumaun Himalayan belt

Regional averages of radon, thoron, and associated decay product concentration are reported to be higher than their respective global averages in recent studies conducted in Indian Himalayan belt. The present study explores another region in Indian Himalayan belt by conducting measurements of radon, thoron, and decay product's activity concentration in 92 dwellings of Bageshwar district. The year-long measurements were performed in all 3 seasons distinguishing dwellings as per their construction material. The average radon and thoron concentration for the study region was measured as 57 Bq/m³ and 66 Bq/m³, respectively. Analysis of the measured data in terms of seasonal effects and construction material led to well established inferences, i.e., higher concentration for mud houses and for winter season. In addition, the present study focuses on lesser probed statistical inferences. One of them is related to the appropriateness of frequency distribution function for the measured data and other dwells upon the correlation analysis of inter-related factors for high concentration cases. Three distribution functions (Lognormal, Weibull, and Gamma) were found to be following the trend of frequency distribution curve of the measured data. For mud houses in winter season, variations of radon/thoron concentration were attempted to correlate with mass/surface exhalation rate, emanation rate, and source term content. More than 80% of the dwellings of the study region were found to have gas and decay product's concentration levels, higher than the respective global average values. However, these values were mostly within the reference levels for residential environments. Nevertheless, this region requires further studies to pinpoint the causes for elevated levels and suggest simple remedial modifications if required.

Numerical simulation of 222Rn profiling in an experimental chamber using CFD technique

Measurement of indoor ²²²Rn concentration and interpretation of distribution patterns are important for inhalation dosimetry in occupational and residential areas. Experimental determination of ²²²Rn concentration distribution and estimation of inhalation doses depend on the underlying aspects such as calibration of the detectors, accuracy of the techniques etc. Therefore, ²²²Rn concentration distribution needs to be very well understood in a closed domain for the controlled studies. In the recent times, Computational fluid dynamics (CFD) technique has gained a lot of attention for the prediction and visualization of indoor ²²²Rn concentration profiles and their mixing ability in the domain. The present study aims to simulate the effect of forced mixing on the ²²²Rn concentration profile in a 22 m³ experimental chamber. This chamber is designed for carrying out the controlled experiments, calibration and inter-comparison studies of various types of ²²²Rn detectors. Effect of different parameters such as time, flow rates, fan-off and fan-on conditions have been studied on the transient response, extent of the air mixing patterns and subsequently on ²²²Rn concentration profile in the chamber. Further, Non uniformity index (NUI) is introduced as a measure of the uniformity of the distribution in the closed domain. NUI is estimated for different cases in order to efficiently interpret the effect of above mentioned parameters on ²²²Rn profile in the chamber. This study will be useful to represent the turbulent conditions in real indoor domains and occupational facilities as U-mines during calibration and inter-comparison exercises of different ²²²Rn detectors.

Indoor inhalation dose assessment for thoron-rich regions of Indian Himalayan belt

²²²Rn, ²²⁰Rn, and their decay products are significant contributors to background radiation dose. Their concentration level, pertaining exposure, and consequent dose are prime concerns in indoor environments. The present study was performed in 101 dwellings of different villages of Almora district situated in Kumaun hills of Indian Himalayan belt. Measurement of gases and decay products were made in three different types of dwellings (i.e., mud, cemented, and stone with plaster) in three seasons (winter, summer, and rainy). Concentration values for ²²²Rn and EERC were found to be varying in the order of winter > summer > rainy while obtained least in rainy season for the case of ²²⁰Rn and EETC. Concentration values for ²²²Rn and EERC were found to be lesser for cemented houses. Relative standard deviation of concentration values was found to be higher for the rainy season. Yearly averaged concentration values for ²²²Rn, EERC, ²²⁰Rn, and EETC were noted to be higher than the global averages but comparable to some Indian studies. Annual inhalation dose due to ²²²Rn, ²²⁰Rn, and their progeny was found to be 0.55-4.71 mSv/year with an average value of 2.36 ± 0.83 mSv/year. These values were measured for the first time in the study area and provide a link for future studies in the dwellings representing higher concentration values.

Thoron Mitigation System based on charcoal bed for applications in thorium fuel cycle facilities (part 2): Development, characterization, and performance evaluation

Exposure due to thoron (²²⁰Rn) gas and its decay products in a thorium fuel cycle facility handling thorium or ²³²U/²³³U mixture compounds is an important issue of radiological concern requiring control and mitigation. Adsorption in a flow-through charcoal bed offers an excellent method of alleviating the release of ²²⁰Rn into occupational and public domain. In this paper, we present the design, development, and characterization of a Thoron Mitigation System (TMS) for industrial application. Systematic experiments were conducted in the TMS for examining the ²²⁰Rn mitigation characteristics with respect to a host of parameters such as flow rate, pressure drop, charcoal grain size, charcoal mass and bed depth, water content, and heat of the carrier gas. An analysis of the experimental data shows that ²²⁰Rn attenuation in a flow through charcoal bed is not exponential with respect to the residence time, L/U_a (L: bed depth; U_a: superficial velocity), but follows a power law behaviour, which can be attributed to the occurrence of large voids due to wall channeling in a flow through bed. The study demonstrates the regeneration of charcoal adsorption capacity degraded due to moisture adsorption, by hot air blowing technique. It is found that the mitigation factor (MF), which is the ratio of the inlet ²²⁰Rn concentration (C_in) to the outlet ²²⁰Rn concentration (C_out), of more than 10⁴ for the TMS is easily achievable during continuous operation (>1000 h) at a flow rate of 40 L min^-1 with negligible (<1 cm of water column) pressure drop. The Thoron Mitigation System based on adsorption on charcoal bed offers a compact and effective device to remove ²²⁰Rn from affluent air streams in a space constrained domain. The prototype system has been installed in a thorium fuel cycle facility where it is being evaluated for its long-term performance and overall effectiveness in mitigating ²²⁰Rn levels in the workplace.

An analysis of the radioactive contamination due to radon in a granite processing plant and its decontamination by ventilation

This work focuses on studying concentration distribution of ²²²Rn radioisotope in a granite processing plant. Using Computational Fluid Dynamic Techniques (CFD), the exposure of the workers to radiation was assessed and, in order to minimise this exposure, different decontamination scenarios using ventilation were analysed. Natural ventilation showed not sufficient to maintain radon concentration below acceptable limits, so a forced ventilation was used instead. Position of the granite blocks also revealed as a determining factor in the radioactive level distribution. Thus, a correct layout of the stored material and an adequate ventilation system can guarantee free of exposure to radiation zones within the studied workshop. This leads to a drastic fall in the exposure of the workers and consequently minimises their risk of developing aggressive illness like lung cancer.

Effect of 220Rn gas concentration distribution on its transmission from a delay chamber: evolving a CFD-based uniformity index

(220)Rn mitigation can be achieved by delay chamber technique, which relies on the advantage of its short half-life. However, flow rate as well as inlet-outlet position for the delay chamber can have a significant impact on (220)Rn concentration distribution patterns and hence transmission factor. In the present study, computational fluid dynamics simulations to estimate the concentration distribution has been carried out in a chamber of 0.5 m(3) for the combination of six different inlet-outlet positions and five different flow rates. Subsequently, the transmission factor (TF) for the chamber was evaluated and found to be highly dependent on the flow rate and inlet-outlet positions. For ease of scale up, the dependency of TF on the flow rate and the inlet-outlet positions is best summarised by relative transmission factor (RTF), which is the ratio of the TFs for the case of inlet and outlet on different faces to that on the same face.