Emanation Rate Research Articles

Evaluation of the atmospheric transport model NIRE-CTM-96 by using measured radon-222 concentrations

An atmospheric transport model, NIRE-CTM-96, was evaluated by using measured radon-222 concentrations. The model has 2.5×2.5 degree horizontal resolution and 15 vertical levels. Assimilated global meteorological data for 1990–1996 from the European Centre for Medium Range Weather Forecasts were used to drive the model. We used an emanation rate of radon-222 of 1 atom cm−2 s−1 over mostly ice-free land. Simulated concentrations were compared with measured concentrations for 22 sites worldwide including 10 stations in China. Simulated annual mean concentrations for Freiburg, Germany, and Socorro, New Mexico, and for four stations in northern China were consistent with the measured concentrations. Simulated daily concentrations for Ogasawara-Hahajima, Japan, correlated well with the measured concentrations. Simulated upper tropospheric concentrations for Moffet Field, California, demonstrated the cross-Pacific transport from central Eurasia and India-Indochina area. Simulated concentrations for two stations in southern China were almost half of the measured concentrations. Mixing layer depth in the model was consistent with other estimates which indicates higher emanation rate there. Simulated concentrations for the South Indian Ocean and the Antarctic during summer were significantly lower than the measured concentrations; this difference was accounted for when emanation from the ocean at a rate of 0.01 atom cm−2 s−1 was included in the model. The model failed to simulate amplitudes of high-concentration events at Mauna Loa. These high-concentration events were possibly a result of filament-like horizontal structure or laminated vertical structure. The vertical as well as horizontal resolution of the model were supposed to be insufficient to reproduce these fine structures.

Significant radioactive contamination of soil around a coal-fired thermal power plant

Soil samples were collected around a coal-fired power plant from 81 different locations. Brown coal, unusually rich in uranium, is burnt in this plant that lies inside the confines of a small industrial town and has been operational since 1943. Activity concentrations of the radionuclides 238U, 226Ra, 232Th, 137Cs and 40K were determined in the samples. Considerably elevated concentrations of 238U and 226Ra have been found in most samples collected within the inhabited area. Concentrations of 238U and 226Ra in soil decreased regularly with increasing depth at many locations, which can be explained by fly-ash fallout. Concentrations of 238U and 226Ra in the top (0–5 cm depth) layer of soil in public areas inside the town are 4.7 times higher, on average, than those in the uncontaminated deeper layers, which means there is about 108 Bq kg −1 surplus activity concentration above the geological background. A high emanation rate of 222Rn from the contaminated soil layers and significant disequilibrium between 238U and 226Ra activities in some kinds of samples have been found.

An atmospheric stability index based on radon progeny measurements for the evaluation of primary urban pollution

An atmospheric stability index for the evaluation of urban primary pollution, based on the elaboration of natural radioactivity data yielded by a stability monitor, has been developed. The instrument determines the atmospheric concentration of the short-lived decay products of radon, whose emanation rate can be assumed to be constant in the time and space scale of observation. The index gives information about the dilution properties of the lower boundary layer and allows to highlight the relevant role of the dilution factor in determining primary pollution events. The atmospheric stability indices have been calculated during a 1-yr study carried out in the urban area of Rome (October 1999–September 2000). On the basis of the index, every day of the period has been classified in terms of intensity of a potential primary pollution event. The comparison between this classification and the real concentration value of primary pollutants, measured in the background urban station of Rome, yielded very good results. This shows that the index constitutes a powerful and valuable tool for describing primary pollution events in urban areas and confirms that the role played by the mixing properties of the lower boundary layer is essential in determining primary pollution in urban areas.

Highly sensitive radon monitor and radon emanation rates for detector components

The radon emanation rates for materials were measured by using the electrostatic precipitation method as a radon monitor. It was found that a low level of radon was emanated from several material components in ELEGANT V. The radon monitor has been developed for the highly sensitive measurements of low-level radon concentration. The system was shown to have a sensitivity to radon concentrations as low as 1.6 mBq/ m 3 for one day measurement. The system was also used as the radon concentration monitor for the gas inside the airtight box of the ELEGANT V at Oto Cosmo Observatory.

222Rn emanation measurements at extremely low activities

For the solar neutrino experiment Borexino, a system has been set up to measure emanation of 222Rn in samples of up to 80 litres volume. The apparatus has been constructed to high vacuum standard and consists mainly of parts made from electropolished stainless steel. The low blank activity of the system together with a highly sensitive detection method for the extracted Rn leads to a sensitivity level in the range 70–100 μBq 222Rn. Due to the large sample capacity of the emanation chambers, it is possible to measure specific emanation rates as low as several μBq/m2. Some measurements characterizing the performance of the apparatus are discussed.

Accounting for 222Rn loss during oven drying for the immediate laboratory gamma-ray spectroscopy of collected soil samples

Drying soil samples in an oven to remove water alters the 222Rn emanation rate. Measurements of the oven drying 222Rn emanation rate from soil were made with a continuous radon monitor and the degree of 222Rn disequilibrium was quantified by laboratory gamma-ray spectroscopy. This paper presents a disequilibrium correction where the 226Ra activity in oven-dried soil samples is inferred from immediate laboratory gamma-ray spectroscopy of 214Bi before 222Rn and its decay progeny reach secular equilibrium.

Laboratory, Field and Modeling Studies of Radon‐222 as a Natural Tracer for Monitoring NAPL Contamination

The recently developed natural radon tracer method has potential as a rapid, low‐cost, nondestructive, and noninvasive method for quantifying NAPL contamination. In the subsurface, radon‐222 (radon) is produced by the decay of naturally occurring radium‐226 contained in the mineral fraction of aquifer solids. In groundwater radon occurs as a dissolved gas, with a half‐life of 3.83 days. In the absence of NAPL, the radon concentration in groundwater quickly reaches a maximum value that is determined by the mineral composition of the aquifer solids, which controls the rate of radon emanation. In the presence of NAPL, however, the radon concentration in the groundwater is substantially reduced due to the preferential partitioning of radon into the organic NAPL phase. A simple equilibrium model and supporting laboratory studies show the reduction in radon concentration can be quantitatively correlated with residual NAPL saturation. Thus, by measuring the spatial distribution in radon it may be possible to identify locations where residual NAPL is present and to quantify the NAPL saturation. When the basic processes of partitioning, radon emanation from the aquifer solids, and first‐order decay are incorporated into an advective/dispersive transport model, good agreement is obtained with the results of laboratory and field experiments. Model sensitivity analyses shows many factors can contribute to the radon concentration response, including the length of the NAPL zone, NAPL saturation, groundwater velocity, porosity, and radon emanation. Thus, care must be taken when applying the radon method to locate and quantify NAPL contamination in the subsurface.

Study of indoor radon levels in high-rise air-conditioned office buildings.

A series of measurements were conducted to study the indoor radon pollution in air-conditioned high-rise office buildings. Continuous monitoring of indoor radon levels in nine air-conditioned premises located in six office buildings in Hong Kong was conducted from August 1996 to February 1998. Each of the tests lasted for at least 48 hours. The measurement covered both day time monitoring while the air-conditioning was on and night time monitoring while the air-conditioning was off. The indoor radon level followed inversely the operation pattern of the mechanical ventilation systems in the buildings. During office hours when the mechanical ventilation was on, the indoor radon level decayed and after the mechanical ventilation was off during non-office hours, the radon level increased. The average indoor radon level during office hours on the nine premises varied from 87 Bq/m3 to 296 Bq/m3, and the indoor averaged radon levels over both day time and night time periods without mechanical ventilation were about 25 percent higher. The air infiltration rate and the radon emission characteristics from the building materials were estimated from the radon build-up curves which were observed after the mechanical ventilation was off. The radon decay curve observed after the mechanical ventilation system was turned on was used to calculate the total fresh air intake rate. Average radon emanation rates of the building materials in the six buildings varied from 0.0019 to 0.0033 Bq/m2s. It has been found that building infiltration rate accounted for about 10-30 percent of the total building ventilation rate in the buildings depending on building tightness.

Radon emanation and exhalation rates from soils measured with an electrostatic collector

The radon emanation and exhalation rates from soil samples were measured using an electrostatic collector coupled to a silicon detector. The radon progenitor activities ( 226Ra and 232Th) of soil samples were determined using γ-ray spectroscopy with a germanium detector. The radon emanation and exhalation rates have been measured accurately and rapidly. Measurements of soil samples having different physical properties were performed. Data analysis methods were developed to identify the effect of grain size and porosity of soil samples. The results confirm interesting dependencies of emanated radon as a function of two soil parameters.

Mobilization of thorium, radium and radon radionuclides in ground water by successive alpha-recoils

Abstract Alpha-recoil is an important way for the mobilization of α-decay daughters in ground water. The processes of decay and recoil from 238U to 222Rn and from 232Th to 220Rn are successive. The release rates of successive recoils in this study are quantitatively estimated under two extreme conditions: complete absorption; and no adsorption of recoil elements by the porous medium. The recoil probability, recoil embedding and effective surface area are considered during the analysis. The result of this study shows that the recoil supply rates in the porous medium decrease down the decay chains. For the isotopes within the same decay chains, recoil emanation ratios of daugthers to parent are usually greater than 1 under both end-member conditions. This is consistent with the higher daughter to parent ratios observed in the field measurements. It is also found that the emanation ratios of Th, Ra and Rn show little change with the change of porosity in general. Radon emanation is most efficient at 15–20% porosity under the condition of complete adsorption. The calculated radioactivity ratios of daughters to parents from this study can provide a partial explanation for the disequilibrium of radioactivity of daughters relative to parents observed in natural water. Because of the low solubility of thorium, radium and radon, the emanation rates calculated under the condition of complete adsorption are closer to the natural conditions.

Determination of radon emanation and back diffusion characteristics of building materials in small chamber tests

A methodology has been established to estimate the radon emanation rates and back diffusion rates of different building materials in small chamber tests. The two parameters, emanation rate and back diffusion rate, can be obtained from the growth curve of the chamber radon activity. Small impervious containers with a volume of 14900c.c. were used in the measurement and solid-state radon detectors were used to monitor the chamber radon levels continuously. In the study, the building materials were left inside the chamber for more than 400 hours in order to reach equilibrium radon concentrations. The emanation rates and the back diffusion rates of the samples were found to be fairly constant under the experimental conditions. The emanation rate was determined by the initial slope of the growth curve and the back diffusion rate was determined by the equilibrium value of the activity curve. Leakage of the chamber was estimated by checking the radon free decay rate inside the chamber before each experiment and a correction was included in the calculation. Two kinds of granite and one bare concrete were selected as test samples. The first granite sample had a mean emanation rate of 13.44 Bq · m −2 · hr −1 and a back diffusion rate of 6.89 × 10 −4 hr −1. The second granite sample had an emanation rate of 0.58 Bq · m −2 · hr −1 with a back diffusion rate of 5.40 × 10 −3 · hr −1. The bare concrete had an emanation rate of 8.69 Bq · m −2 · hr −1 and a back diffusion rate of 6.18 × 10 −3 hr −1. The methodology has been found to provide an accurate method of estimating radon emanation and back diffusion characteristics of commonly used building materials.

Radon emanation from concrete surfaces and the effect of the curing period, pulverized fuel ash (PFA) substitution and age

Radon emanation rates from 48 concrete blocks have been monitored throughout a period in excess of 1 yr. These blocks have been produced for three distinct mixes of materials, i.e. mix A with 25% by weight substitution of type A pulverized fuel ash (PFA), mix B with 25% by weight substitution of type B PFA and mix C without PFA. Each mix is represented by four sets of concrete blocks with curing periods of 1, 3, 7 and 28 days. Every set comprises of four blocks each with the same composition and curing period. From the results, rates of change of radon emanation have been identified for two distinct periods, i.e. for the first month following curing and for the period following the first month. The different emanation rates are linked to the role of the superficial and inner pores of the concrete. It is also seen that the radon emanation rate tends to decrease with the age of the blocks for curing periods of 1, 3 and 7 days, but tends to increase for a curing period of 28 days, all of which can be rationalized in terms of the gradual dehydration of concrete as it ages. The patterns are similar for the different mixes, although the exact effect of PFA upon radon emanation rates remains unresolved.

Short-term aquifer residence times estimated from 222Rn disequilibrium in artificially-recharged ground water

Radioactive disequilibrium of 222Rn permits age dating of very young ground water which has resided in the subsurface for up to 15 days. Inherent in this technique is the assumption that transported recharge flows through porous media with a relatively uniform 222Rn emanation rate. Changes in radon activity in response to rapid surface water infiltration from seasonally operated spreading basins were used to estimate subsurface residence times along ground water recharge flow paths. Radon activity was relatively constant in deep monitoring wells isolated from the immediate influence of surface recharge, suggesting that radon remained in secular equilibrium and deeper ground water residence times were in excess of the 15-day dating method limit. In contrast, radon activity in shallower ground water was highly variable during recharge and allowed estimation of ground water ages ranging from 4 to 14 days, using disequilibrium theory. Shorter residence times corresponded to wells most directly connected with the surface as indicated by seasonal changes in water temperature. Surface water recharge routes were confirmed in a related study with stable isotope fingerprinting, water level variations and agrichemical concentrations. Flow rates estimated from residence times ranged from 1.1 to 4.6 m day −1, and compare favourably with average subsurface velocities up to 4.5 m day −1 estimated from infiltration rates at the site. Residence time estimates using radon disequilibrium were in general agreement with bromide tracer breakthrough times determined in a related study. Differences between residence and breakthrough times indicate that changes in recharge flow routes, permeability and parent nuclide concentrations may produce erroneously high residence time estimates. Accumulation of fine sediment, eventually resulting in basin clogging, may increase parent isotope concentrations and radon emanation in the zone of infiltration.

Indoor Radon and Radon Emanation in the Herculane Spa (Cerna Valley) Area - Romania

Recent investigations have shown a connection between high indoor levels of radon and lung cancer risk. The main source of the radon is the emanation from soil and ground water. A study has been carried out in the Herculane Spa region, where the highest level of radioactivity (226Ra and 222Rn) from geother mal waters in Romania has been measured. This paper presents data about the radon emanation rate and indoor radon concentrations measured in the area studied. Only 20% of the measurements of indoor radon concentrations were values above 200 Bq·m-3 (average 98 Bq·m-3) when the weather was warm but in the cold season 40% of the radon levels indoors (average 185 Bq·m-3) exceeded this value. For radon emanation rates we generally found values in the range 13-70 mBq·m-2·s-1 although they were significantly higher in the vicinity of the geological faults found in the area.

A new method for studying the influence of pollution and soil nature on the radon emanation from water samples by using solid state nuclear track detectors

Radon emanation from surface water and groundwater samples has been studied by using CR-39 and LR-115 solid state nuclear track detectors (SSNTD). A calibration method for determining the thoron and radon concentrations of the water samples studied has been developed. The effect of pollution due to coal fly ashes, cement and granite dusts on the radon emanation from water samples has been investigated. The influence of the radon source on the radon emanation rates from water samples has been studied.

Atmospheric radon levels and its emanation rate in the environment of Kaiga

A systematic study of atmospheric radon levels and of radon emanation rates was conducted in the environment of Kaiga on the south-west coast of India where the installation of nuclear power reactors is in progress. 222Rn emanation rates were estimated employing the accumulating chamber method and were found to vary from 5.6 to 62.9 mBq m −2 s −1 with a geometric mean of 30.0 mBq m −2 s −1. The atmospheric concentrations were estimated using CR-39 solid state nuclear track detectors (SSNTDs) and found to vary from 4.7 to 27.9 Bq m −3 with a geometric mean of 13.4 Bq m −3. The results show a strong correlation between emanation rate and atmospheric concentration while showing a poor correlation between 226Ra concentration in surface soil and atmospheric concentration of 222Rn. The results are also compared and discussed in the light of relevant literature reported for other environments.

Evaluation of radon emanation from soil with varying moisture content in a soil chamber

The EPA chamber (2 × 2 × 4 m long) was constructed to study convective and diffusive soil gas movement under known conditions of 226Ra and 222 Rn concentration, moisture, density, soil constituent, and physical response to pressure variation. The radon emanation rates of soil are known to depend strongly on the moisture content of the soil. Because the moisture content varies greatly with depth in the EPA's soil chamber (from saturated at the bottom to nearly dry at the top), it is not possible to fully understand the radon distribution within the chamber without knowing the emanation rate as a function of moisture. Soil radon concentrations vary in the chamber from 7.4 kBq m −3, near the soil surface, to 86.2 kBq m −3, at the chamber bottom. This paper describes measurements of the emanation coefficient and diffusion of radon in soil contained in the chamber, using a wide range of moisture contents. In addition, equal amounts of well-mixed oven-dried soil were placed in 20 L aluminized gas-sampling bags, and, after approximately 1 month of in-growth, radon samples were taken, after which water was added, and another period of in-growth and sampling followed. The emanation coefficients and radon concentrations in the gas bag experiment were observed to increase with increasing moisture content and then decrease before reaching saturated conditions. The emanation and diffusion effects on the radon concentration soil gradient were identified for this sandy soil having approximately 200 Bq kg −1 radium and a soil density of 1682 kg m −3.

Effects of Enzyme Infusion on The Processing and Storage Quality of Peeled Citrus

An enzymatic peeling process is currently used to produce peeled citrus fruit that are convenient for consumption. By this process, fruit are scored and infused with pectinase or pectinase and cellulase solution and are incubated at 20 to 45C for 0.5 to 2 h. While enzyme solution apparently weakens of the albedo and thus improves separation of the fruit from its peel, we expect that enzyme infused into the flesh reduces storage quality. In these studies, fruit were vacuum- or pressure-infused with or without pectinase in water. The time required to peel white `Marsh' and `Ruby Red' grapefruit infused with solution containing enzyme were only 10% to 20% less than for fruit infused with water alone. `Hamlin' orange and `Orlando' tangelo peeling times were not improved by enzyme treatment. This suggests that water is the primary operative component of the enzyme solution and that the enzyme is an active, but nonessential, supplement. For white grapefruit and oranges stored at 5, 10, 15, or 25C, nonenzyme-treated fruit had significantly less juice leakage than enzyme-treated fruit. For example, 0.2% and 5.0% of the peeled fruit weight was lost by non-enzymatically and enzymatically peeled fruit, respectively, for vacuum-infused oranges stored at 5C for 7 days. Moreover, the enzyme treatment significantly reduced firmness, as determined by a sensory panel. Microbial levels and rates of respiration and ethylene emanation during storage were not significantly affected by enzyme treatment. Similar results were found for vacuum- and pressure-infused fruit.

Radon emanation from giant landslides of Koefels (Tyrol, Austria) and Langtang Himal (Nepal)

The identification of extremely high indoor radon concentrations in the village Umhausen (Tyrol, Austria) initiated a scientific program to get information about the source and distribution of this noble gas. The high concentrations can not be related to U anomalies or large-scale fault zones. The nearby giant landslide of Koefels, with its highly fractured and crushed orthogneisses, are the only possible source of radon, despite the fact that the U and Ra content of the rocks is by no means exceptional. The reasons for the high emanation rates from the landslide are discussed and compared to results gained from a similar examination of the giant landslide of Langtang Himal (Nepal). The exceptional geologic situation in both cases, as well as the spatial distribution of different concentration levels, indicate that both landslides must be considered as the production sites of radon. Independent of the U and Ra contents of the rocks, the most important factors producing high emanation rates are the production of a high active surface area and circulation pathways for Rn-enriched soil air by brittle deformation due to the impact of the landslidemass.

The effect of grain size on the radon emanation rate

Tailings produced during mining and milling of uranium ores represent potentially large volumes of low level radioactive materials. A typical environmental problem associated with mill tailings is radon emanation. Covering tailings is widely applied to reduce radon exhalation rate. In this paper, the fractal theories and field covering tests are used to study the fractal characters of size distribution of six types of covering materials, including waste rock, sand, laterite, kaolin, mixture of sand and laterite, and mixture of waste rock and laterite, and their influences on radon exhalation. The size distributions of uranium tailings and the six aforementioned covering materials all exhibit a good fractal structure. The contents of fine grain increase with the increasing value of fractal dimension. The results of field radon measurement show that the radon emanation rate of tailings without covers is 14.7–18.6 Bq/m2 s. Covering tests were carried out of the six abovementioned covering materials with thickness of 0.4 m, 0.8 m, 1.2 m, 1.6 m and 2.0 m. The results indicate that the application of these materials for cover layers can decrease the radon exhalation rate markedly. The effectiveness of a cover layer in reducing radon exhalation is related to its fractal texture of size distribution. Under the same thickness conditions, the attenuation coefficient of radon exhalation rate increases with the increasing fractal dimension of size distribution of covers. The empirical expressions of the attenuation coefficients in relation to fractal dimension D of size distribution and thickness x of covers is obtained for evaluating the effectiveness of final covers for uranium tailings impoundments.