Radon Diffusion Coefficient Research Articles

Determination of the radon diffusion coefficient and radon exhalation rate in Moroccan quaternary samples using the SSNTD technique

Measurements have been made of radon (222Rn), release from diverse quaternary samples collected from different sediment deposits in the Errachidia and Beni-Mellal areas (Morocco). The radon diffusion coefficient as one of some important parameters of radon transport in the soil has been measured using solid state nuclear track detectors (SSNTD). Radon α-activity, uranium content and radon exhalation rate have been determined in the studied samples. Uranium concentrations were found to vary from 0.14 to 9.52 ppm whereas the radon exhalation rate varied from 0.003 to 0.145 Bq.m-2.h-1. A positive correlation has been found between radon exhalation rate and uranium content in the studied samples. The average radon diffusion coefficients were found to vary from (1.26±0.09).10-6 m2.s-1 to (4.3±0.36).10-6 m2.s-1. Furthermore, the correlation between 222Rn diffusion coefficient and porosity are also discussed.

Estimates of Parameter and Scenario Uncertainties in Shallow-Land Disposal of Uranium Wastes Using Deterministic and Probabilistic Safety Assessment Models

Safety and uncertainty analyses for the shallow-land disposal of uranium wastes were performed using the deterministic and probabilistic safety assessment models. The analyses for uranium accumulated with 4.5% enrichment show that the doses in residence scenario are of great importance in the safety assessment owing to the influence of daughters built up by uranium decay chain. The dose in residence scenario is sensitive to the release condition of radionuclides from the facilities over long-term period. The parameter uncertainties for the important pathways in residence scenario are estimated from the probabilistic analyses using the statistical methodology. The uncertainty analysis indicates that the influence of parameter uncertainty is the most remarkable in the estimation for the inhalation of radon gas with residence. The parameter importance in each exposure pathway is estimated from using the partial rank correlation coefficients (PRCCs) between variable parameters and the evaluated doses. The important parameters identified by the PRCCs are depth of intrusion, infiltration rate, thickness of covered soil, diffusion coefficient of radon in soil etc. for the inhalation exposure of radon.

Applicability of various insulating materials for radon barriers

The effectiveness of various insulating materials for limiting radon entry into houses has been investigated under laboratory conditions. Results for the radon diffusion coefficient measurements in more than 80 insulating materials are summarized. We have discovered that great differences exist in diffusion properties, because the diffusion coefficient varies within four orders from 10 −13 m 2/s to 10 −10 m 2/s. A methodological approach is proposed in order to identify the minimal thickness of radon-proof membranes, depending on building and soil characteristics. General guidelines for the selection of radon-proof insulation are presented.

Radon diffusion through polymer membranes used in the solar neutrino experiment Borexino

We have measured the radon diffusion, solubility and permeability coefficients of various foils being of interest for the Borexino experiment. The applied methods and instrumentation are described and the mathematical model for diffusion of Rn, including its decay, is outlined. A very strong influence of water on the Rn diffusion through nylon foils was found. The rather low Rn permeability for dry nylon foil (mean value is equal to 3.35×10 −12 cm 2/ s) increases by more than two orders of magnitude if the foil is immersed in water. No significant influence compared to dry nylon has been observed if the foil was surrounded by pseudocumene.

A new method for studying the transport of radon and thoron in various building materials using CR-39 and LR-115 solid state nuclear track detectors

Radon ( 222Rn) and thoron ( 220Rn) α-activities per unit volume were measured inside and outside different building materials by using two types of solid state nuclear track detectors (SSNTD) (CR-39 and LR-115 type II). In addition, the radon and thoron emanation coefficients of the studied materials were evaluated. Based on these data, the transport of radon and thoron across parallelepipedic blocks of the building materials could be investigated and radon and thoron global α-activities per unit volume outside different building material blocks were determined. Moreover, the diffusion length and the effective diffusion coefficient of radon in the building materials were evaluated and the total alpha activity due to radon in the atmospheres of different rooms consisting of different building materials was studied.

Characterisation of the gas transport properties of porous materials by determining the radon diffusion coefficient

A method for the determination of the diffusion coefficient of materials by using the radioactive gas radon is presented. This gas is inert and extremely low concentrations can be detected because of the radioactive decay. The method was applied to mortars and concretes in order to optimise their composition with respect to a low radon diffusion rate. These optimised materials are to be used for concrete slabs in buildings. Compared to regular concrete, a reduction of the radon diffusion rate by more than 90% could be achieved.

Diffusive transport of radon in a column of moisturized sand.

Diffusive transport of radon in a moisturized sand column was investigated under well-defined and controlled conditions using a laboratory facility that consists of a cylindrical vessel (height and diameter 2 m) filled with sand. Equilibrium pore-gas radon-concentration profiles in the sand column were measured as a function of depth for several different levels of the ground-water table. The measurements were performed in the framework of a systematic validation study that also covered experiments with dry sand. The experimental data were compared with results of model calculations. Most parameters in the transport model were obtained from separate experiments on small sand samples. In this manner, the radon diffusion coefficient and emanation coefficient were determined as functions of pore-water content, as well as the water-retention characteristics of the sand. In general, the experimental data are reasonably well reproduced by the calculations with differences of 10%-40%. However, with respect to earlier results with dry sand indicating differences of generally smaller than 10%, the agreement between experiment and model for moisturized sand is clearly worse. The main part of the disagreement probably results from inhomegeneities in the sand column that are not accounted for in the transport model.

Radon emanation and soil moisture effects on airborne gamma‐ray measurements

A theoretical model is developed to explain variations in airborne gamma‐ray measurements over a calibration range near Ottawa, Ontario. The gamma‐ray flux from potassium and the thorium decay series showed an expected decrease with increasing soil moisture. However, the gamma‐ray flux from the uranium decay series was highest in the spring when the ground was water‐saturated and even covered with snow. These results are explained through the build‐up of radon and its associated gamma‐ray—emitting decay products in the clay soil of the calibration range with increasing soil moisture. Similar results were found from airborne measurements over other clay soils. However, measurements over sandy soils showed that the count rates from all three radio elements increased with decreasing soil moisture. This difference between soil types was attributed to the lower radon emanation of the more coarse‐grained sandy soils compared to finer‐grained clay soils. The theoretical and experimental results demonstrate that any estimate of the natural gamma‐ray field caused by radium in the ground must take into consideration the radon emanation coefficient of the soil. The radon diffusion coefficient of the soil must also be considered since it depends strongly on soil moisture. This has significant implications for the assessment of outdoor radiation doses using laboratory analyses of soil samples and the use of ground and airborne gamma‐ray measurements for radon potential mapping.

Diffusion of Randon in Porous Media Saturated with Gels and Emulsions

The effective diffusion coefficient of radon was determined in polymer/silicate gels and clay suspension used as sealing materials in environmental protection. On the basis of the experimental findings, it was concluded that both materials drastically decrease the convective mass transport in porous media. Simultaneously, the effective diffusion coefficient was reduced significantly. Thus, the radon flux might be decreased by 5 to 6 orders of magnitude in porous systems originally having gas or low water saturation by injection of gel-forming materials or placement of clay suspensions. At high water saturation, however, the diffusion transport of radon can be slightly restricted in consolidated and unconsolidated porous media. The laboratory studies may firmly allow us to conclude that hydrogels and clay suspensions are prospective candidates in an integrated environmental technology to be used for restriction of radon migration in subsurface regions.

Effect of soil and sand moisture content on radon diffusion using plastic track etched detector

Radon diffusion from soil and sand has been studied as a function of moisture content of soil and sand using plastic track etched detectors (LR-115 type II). It is observed that in soil and sand as the percentage of water is increased from dry state to 21% and 26%, respectively, a slow decrease in radon concentration (295 to 250 and 315 to 240 Bq/l) takes place; thereafter it reduces sharply to 27 and 50 Bq/l when soil and sand become saturated (25% and 40% water). The water-saturated soil and sand give the lowest concentration because of the much lower diffusion coefficient of radon through water.

Radon migration in the soils of the Irno Valley (Southern Italy) inferred from radioactive disequilibrium

Radon migration along vertical profiles in the soils of Irno River alluvial Valley (Southern Italy) was studied using radioactive disequilibrium between 226 Ra and 210 Pb. Fractional Radon loss, migration length, diffusion and emanation coefficient and Radon flux density were determined. Our results are in agreement with a migra- tion model by simple diffusion. The migration parameters are within typical values, except the Radon flux density, which is about one order of magnitude higher than the values reported in literature. The values of fractional Radon loss are sensitive to changes in the physical properties of the soil.

The effect of soil parameters on the radon concentration values in the environment

A study was made of the effect of soil moisture content and porosity on the diffusion of radon through soil. It was found that the diffused radon concentration decreases exponentially with the increase of soil thickness. The diffusion coefficient of radon through soil was found to decrease with the increase of the moisture content and to increase with the increase of porosity. A field study was also conducted to investigate the effect the above parameters on radon concentration values as measured at a depth of ∼50 cm below the earth's surface.

Measurement of effective diffusion coefficient of radon in porous media with etched track radon monitors

We have developed two methods to measure the effective diffusion coefficient of radon in porous media. We used etch track type radon monitors to measure the radon exposure on both the exposure and the measuring sides. We have used a computer code to model the transport of radon through porous media. We found the effective diffusion coefficient by changing its value in the code to find the best fit of model calculation to experimental data.

Problems of Radon Screening in the Upper Layer of the Soil

Abstract Soil is known to be the main source of high radon concentrations indoors which contribute about half of the mean natural annual radiation exposure of the German population. It is an aim of precautionary radiation protection to collect information about regions with elevated radon concentrations. To screen the radon potential of the regions of interest, a simple measurement of radon at the surface is desirable. However, the upper layer of the soil is mostly influenced by metrological factors such as solar heating and rainfall, e.g. in the upper layer of the soil, as a consequence of a sudden rainfall, there occur short-term increases in the radon concentration by a factor of 3 and more. Conversely, a slow and long-term increase of the water content of the soil may reduce the diffusion coefficient of radon by several orders of magnitude. The daily heating of the atmosphere results in an enhanced gas exchange between the upper layer of porous and dry soils and the air above. Consequently, variations of the radon concentration by a factor of 5 arise according to the intensity of the heating and the porosity of the soil. Test measurements on radon rich soils show both diurnal and seasonal variations of the radon concentration in the upper layers. This result excludes the use of radon measuring devices with integration times less than one day for the radon screening of any area. A combination of alpha spectroscopy and solid-state nuclear track detectors is suggested to scan the radon potential of a soil. This method is neither expensive in equipment, time or costs, and nevertheless sufficiently accurate.

Radon Diffusion Coefficients for Residential Concretes

Radon gas diffusion through concrete can be a significant mechanism for radon entry into dwellings. Measurements of radon diffusion coefficients in the pores of residential concretes ranged from 2.1 x 10(-8) m2 s-1 to 5.2 x 10(-7) m2 s-1. The pore diffusion coefficients generally increased with the water-cement ratio of the concrete and decreased with its density. A least-squares regression of the diffusion coefficients on concrete density gave an r value of -0.73.

A numerical method for characterizing the time‐dependent diffusion of radon in a porous material

The diffusion coefficient of radioactive radon (Rn) in a porous medium was determined in this study by measuring the time‐dependent diffusion of radon through a thin slab of the material. The diffusion process was required to reach a steady state so as to obtain a good estimate of the diffusion coefficient. A numerical method using the nonlinear least‐squares and Marquardt's method (NLSM) was proposed for characterizing the diffusion coefficient of Rn so as to determine whether the process has reached steady‐state or not. Zeolite 4A was selected as testing porous material. The NLSM method was shown by the results to be able to agree well with the results by the conventional method while the process had already reached steady‐state. The results show that the apparent diffusion coefficient of zeolite 4A of porosity 0.62 is 8.31×10−6m²s−1 from NLSM calculation. The Rn adsorption coefficient for this porous matrix is about 6.9×10−4m³kg−1 derived on the basis of linear adsorption isotherm assumption. The method could be applied toward other porous materials, i.e., soils and building materials that are the primary sources of indoor radon.

Diffusion of radon through nuclear track microfilters

Nuclear track microfilters prepared by irradiating CN-85 cellulose nitrate detectors with 252Cf in 2 π geometry were given post-irradiation thermal annealing and ultraviolet treatments followed by etching. These filters were employed to investigate the diffusion of radon. Alpha particles produced due to the decay of radon and its daughters were registered and etched electrochemically in lexan polymeric detectors. Experimental findings have led to determine diffusion coefficient and activation energy for diffusion through various filters/membranes. Diffusion coefficient of radon through the filter treated with UV from the laser is (1.8±0.05) 10-4 cm2 sec-1 , being a maximum value as compared to other filters and membranes. Corresponding activation energy for diffusion bears a minimum value of (0.018 ± 0.001) eV for this filter. It can be speculated that the post-irradiation laser exposure modifies the profiles of the pores which reduce the resistance to the flow of radon.

Correlation between the concentration of 222-Rn at the earth's surface and in waters with seismic activities

The effects of moisture content, grain size, temperature, major elemental composition, and the pH of soils on the radon emanation and diffusion coefficients were evaluated in this study. The emanation and diffusion coefficients are strongly influenced by moisture content and grain size. The radon emanation coefficient increased and the diffusion coefficient decreased with decreasing particle size. However, for soils with large particle sizes, the radon emanation and diffusion coefficient remain almost unchanged with variation in grain size. Comparing five different sized soil particles, the emanation coefficient increased and the diffusion coefficient decreased with moisture content. The radon emanation coefficient reached a constant value with different moisture contents depending on the range of grain sizes. The saturation emanation coefficient for less than 0.1, 0.1–0.2, 0.2–0.3, 0.3–0.5, and more than 0.5 mm sized soil grain ranges are 0.47, 0.42, 0.35, 0.26 and 0.23, respectively, with saturation moisture contents of 16%, 14%, 10%, 6% and 4%, respectively. A drastic increase in radon emanation is found at smaller grain sizes with increasing moisture content. Based on the content of major elements and pH of the soils, the multiple regression indicates that the radon emanation coefficient appears to be significantly dependent on iron content and pH. Effective diffusion coefficient values calculated in our study agree with the results calculated by a previous model. Experimental values show that the temperature dependence of the radon diffusion coefficient follows Arrhenius behavior.

Determination of radon and thoron permeability through some plastics by track technique

Experiments have been performed to study the usefulness of several types of plastic foils as filter to separate radon and thoron. Time-integrated alpha-activity measurements have been carried out by using the so-called “can-technique” equipped with both LR-115 and CR-39 track detectors. The track density observed on the detectors, taken as a measure of radon activity concentration, has been determined as a function of the thickness of filter foils. The radon permeability and the thoron separation factors have been determined. It is shown that various plastic foils exhibit considerable differences in radon diffusion coefficient owing to their different chemical structures. Among the plastic foils investigated the polyethylene proved to have the highest gas diffusion coefficient.

Radon diffusion modelling

A mathematical model has been developed that examines the ingress of radon into houses, through a vertical crack in an otherwise impervious concrete floor. Initially, the model considered the diffusive flow of radon from its soil source and this simulation has highlighted the dependency of the flux of radon into the house on the magnitude of various parameters, such as the diffusion coefficient of radon in soil. A preliminary investigation of the modelling of pressure-driven flow into a building is presented, and the potential of this type of analysis is discussed.