Sorption-desorption coefficients of uranium in contaminated soils collected around Fukushima Daiichi Nuclear Power Station.

Various radionuclides including fission products and heavy nuclides were released into the environment during the Fukushima Daiichi Nuclear Power Station (FDNPS) accident. The dissolution followed by migration of deposited radionuclides of Cs, Sr and U on soils could take place to the local environment. Therefore, it is necessary to determine sorption-desorption coefficients of U in soil-water system around the FDNPS from a migration viewpoint. The determination of sorption coefficient Kd(S) as well as desorption coefficient Kd(D) for U has been carried out in the present study using a laboratory batch method. Stable U was used for sorption from simulated ground water onto contaminated soil samples collected from Okuma Town, Fukushima. Different soil parameters were measured to understand their effects on sorption and desorption processes. The obtained Kd(S) and Kd(D) values of U were compared with values of Kd(S) and Kd(D) of Cs and Sr and Kd(S)-U in known Fukushima accident contaminated soils reported in the literature for better understanding. It was observed that Kd(S)-U varied from 160 to 5100 L/kg, whereas Kd(D)-U ranged from 200 to 11000 L/kg. Kd(D) was higher than Kd(S) for U in these soils implying irreversibility of the sorption process. Pearson's correlation of Kd(S) values suggested that U sorption is affected by various soil parameters. However, desorption is decided by the nature of U species formed in sorption process and soil parameters like pH, presence of carbonates, Ca ions, clay minerals etc. to some extent. The comparison between Kd(S) and Kd(D) values for Cs, Sr and U revealed that unsorbed Sr could migrate farther than unsorbed Cs or U under the present experimental conditions. Both sorption and desorption studies are of great importance to understand migration of metal ions from contaminated sites to local uncontaminated areas.

Measurement of uranium isotope ratios in Fukushima-accident contaminated soil samples using multi collector inductively coupled plasma mass spectrometry.

In the present study, 137Cs and 238U activity concentrations, 234U/238U activity ratio, and 235U/238U isotope ratio were measured in fifteen soil samples collected from the exclusion zone around the Fukushima Daiichi Nuclear Power Station (FDNPS). The 137Cs activity concentrations of Fukushima-accident contaminated soil samples ranged from 29.9 to 4780 kBq kg-1 with a mean of 2007 kBq kg-1. On the other hand, the 238U activity concentrations of these soil samples ranged from 5.2 to 22.4 Bq kg-1 with a mean of 13.2 Bq kg-1. The activity ratios of 234U/238U ranged from 0.973 to 1.023. The 235U/238U isotope ratios of these exclusion zone soil samples varied from 0.007246 to 0.007260, and they were similar to the natural terrestrial ratio confirming the natural origin. Using isotope dilution technique, the 235U/137Cs activity ratio was theoretically estimated for highly 137Cs contaminated soil samples from Fukushima exclusion zone ranged from 5.01 × 10-8 - 6.16 × 10-7 with a mean value of 2.51 × 10-7.

Geochemical characterization of monazite sands based on rare earth elements, thorium and uranium from a natural high background radiation area in Tamil Nadu, India.

The Kanyakumari coastal area in the southernmost part of Tamil Nadu, India is a well-known natural high background radiation area due to the abundance of monazite in beach placer deposits. In the present study, the concentrations of major oxides, rare earth elements (REEs), Th and U were measured to understand geochemical characteristics of these monazite sands. Based on the ambient dose rate, 23 locations covering an area of about 60 km along the coast were selected for sample collection. The concentrations of U and Th ranged from 1.1 to 737.8 µg g-1 and 25.2-12250.6 µg g-1, respectively. The Th/U ratio ranged from 2.2 to 61.6, which clearly indicated that Th was the dominant contributing radionuclide to the enhanced natural radioactivity in this coastal region. The chondrite-normalized REEs pattern of the placer deposits showed enrichment in light REEs and depletion in heavy REEs with a negative Eu anomaly that indicated the monazite sands were derived from granite, charnockite, and granitoid rocks from the Nagercoil and the Trivandrum Blocks of the Southern Granulite Terrain.

Radiocesium and 40K distribution of river sediments and floodplain deposits in the Fukushima exclusion zone.

In this study, radiocesium and 40K analysis were accomplished for samples of riverbed sediments and floodplain deposits collected from five rivers in the vicinity (<20 km) of the damaged Fukushima Daiichi Nuclear Power Plant after seven years of the accident. Sediment particle size distribution and major oxide content were determined also for six selected samples to understand the retention and migration process of radiocesium in river environments. The radiocesium activity concentration varied from 103 ±â€¯6 Bq·kg-1 to 22,000 ±â€¯500 Bq·kg-1 in riverbed sediments and from 92 ±â€¯5 Bq·kg-1 to 117,000 ±â€¯2000 Bq·kg-1 in floodplain deposits. The 134Cs/137Cs ratio (decay corrected to 15 March 2011) was 1.02 in the both samples. Compared to monitoring results in 2011, it was proved that the radiocesium distribution pattern had been changed remarkably during seven years. The radiocesium was primarily attached to fine clay particles but its sorption on sand and coarse sand particles was also considerable. The sorption process of radiocesium was not affected by the presence of water and moreover, after seven years of the Fukushima accident, a significant radiocesium migration cannot be expected without particle migration. Consequently, radiocesium will remain for a long time in the river environments and its redistribution is mainly affected by the erosion process of the sediments. The average 40K activity concentration of riverbed sediment and floodplain deposit samples was 640 ±â€¯152 Bq·kg-1 changing from 319 ±â€¯18 Bq·kg-1 to 916 ±â€¯41 Bq·kg-1. In the river estuary zones, significant activity concentration decrements were observed for both radionuclides. This suggests that seawater intrusion has a decreasing effect on both natural and artificial radionuclides via wash-out of particulate radiocesium and 40K, and desorption of these radionuclides, but to reveal the detail of this process further investigations are required. The analysis of 40K can help in a simple and easy way to reveal the mineral composition differences of sediment samples.

Measurement of 90Sr in contaminated Fukushima soils using liquid scintillation counter.

A method based on liquid scintillation counting system has been developed for the measurement of (90)Sr in Fukushima soil samples due to contamination of (134)Cs and (137)Cs. Three soil samples were collected within 30 km radius from the Fukushima Daiichi Nuclear Power Plant (FDNPP). Activity concentration of (134)Cs and (137)Cs were measured using a gamma spectroscopy system with high-purity germanium detector. (90)Sr contamination is little elevated but comparable with the background contamination level that originated from atmospheric nuclear weapon tests, whereas radiocesium contamination has increased significantly. Activity concentration of (90)Sr in the soil samples varied in the range of 10.4±0.6-22.0±1.2 Bq kg(-1). Activity concentrations of (134)Cs and (137)Cs in the soil samples were in the range of 28.2±0.2-56.3±0.2 kBq kg(-1) and 35.2±0.1-70.2±0.2 kBq kg(-1), respectively (reference date for decay correction is 1 December 2011).

Invited article: radon and thoron intercomparison experiments for integrated monitors at NIRS, Japan.

Inhalation of radon ((222)Rn) and its short-lived decay products and of products of the thoron ((220)Rn) series accounts for more than half of the effective dose from natural radiation sources. At this time, many countries have begun large-scale radon and thoron surveys and many different measurement methods and instruments are used in these studies. Consequently, it is necessary to improve and standardize technical methods of measurements and to verify quality assurance by intercomparisons between laboratories. Four international intercomparisons for passive integrating radon and thoron monitors were conducted at the NIRS (National Institute of Radiological Sciences, Japan). Radon exercises were carried out in the 24.4 m(3) inner volume walk-in radon chamber that has systems to control radon concentration, temperature, and humidity. Moreover, the NIRS thoron chamber with a 150 dm(3) inner volume was utilized to provide three thoron intercomparisons. At present, the NIRS is the only laboratory world-wide that has carried out periodic thoron intercomparison of passive monitors. Fifty laboratories from 26 countries participated in the radon intercomparison, using six types of detectors (charcoal, CR-39, LR 115, polycarbonate film, electret plate, and silicon photodiode). Eighteen laboratories from 12 countries participated in the thoron intercomparisons, using two etch-track types (CR-39 and polycarbonate) detectors. The tests were made under one to three different exposures to radon and thoron. The data presented in this paper indicated that the performance quality of laboratories for radon measurement has been gradually increasing. Results of thoron exercises showed that the quality for thoron measurements still needs further development and additional studies are needed to improve its measuring methods. The present paper provides a summary of all radon and thoron international intercomparisons done at NIRS from 2007 to date and it describes the present status on radon and thoron passive, one-time cycle monitors.

Thoron experimental room at the National Institute of Radiological Sciences (NIRS), Japan.

A new thoron ((220)Rn) experimental room (TER) was established at National Institute of Radiological Sciences in 2011 to respond to the appearance of new passive devices for (220)Rn and its progeny ((212)Pb, (212)Bi) measurement. The room volume is 21.7 m(3) with a low air exchange rate, 0.016 h(-1). Environmental parameters, such as temperature, relative and absolute humidities, wind speed, aerosol distribution and concentration, are monitored, but not controlled. Commercially available lantern mantles (500 pcs), which can generate maximum concentrations of 6000 Bq m(-3) of (220)Rn and 2000 Bq m(-3) of its progeny, are used as a (220)Rn source. Based on preliminary results from the TER, stable and homogenised concentrations of (220)Rn and its progeny can be ensured; therefore, in the near future the room will be used for testing measuring instruments.

Radon and thoron doses in kindergartens and elementary schools.

Exposing the Raduet Rn-Tn solid-state nuclear track detectors, radon (Rn: (222)Rn) and thoron (Tn: (220)Rn) activity concentrations have been measured in 7 kindergartens and 18 elementary schools in Slovenia. Diurnal variations of both gases were monitored using a Rad7 device. The Rn concentration was in the range from 145 to 794 Bq m(-3) in kindergartens and from 70 to 770 Bq m(-3) in schools, and the Tn concentration was in the range from 21 to 73 Bq m(-3) in kindergartens and from 4 to 91 Bq m(-3) in schools. The Tn versus Rn concentration ratio varied from 0.02 to 0.83. Monthly effective doses due to radon and its decay products ranged from 109 to 600 µSv month(-1) in kindergartens and from 21 to 232 µSv month(-1) in schools, and those due to thoron and its decay products ranged from 3.8 to 13.3 µSv month(-1) in kindergartens and from 0.29 to 6.62 µSv month(-1) in schools. The contribution of thoron to the total effective dose was from 1.3 to 11 % in kindergartens and from 0.4 to 17 % in schools.

Measurement of radon/thoron exhalation rates and gamma-ray dose rate in granite areas in Japan.

Radon and thoron exhalation rates and gamma-ray dose rate in different places in Hiroshima Prefecture were measured. Exhalation rates were measured using an accumulation chamber method. The radon exhalation rate was found to vary from 3 to 37 mBq m(-2) s(-1), while the thoron exhalation rate ranged from 40 to 3330 mBq m(-2) s(-1). The highest radon exhalation rate (37 mBq m(-2) s(-1)) and gamma-ray dose rate (92 nGy h(-1)) were found in the same city (Kure City). In Kure City, indoor radon and thoron concentrations were previously measured at nine selected houses using a radon-thoron discriminative detector (Raduet). The indoor radon concentrations varied from 16 to 78 Bq m(-3), which was higher than the average value in Japan (15.5 Bq m(-3)). The indoor thoron concentration ranged from ND (not detected: below a detection limit of approximately 10 Bq m(-3)) to 314 Bq m(-3). The results suggest that radon exhalation rate from the ground is an influential factor for indoor radon concentration.

Diurnal and seasonal variations of concentration and size distribution of nano aerosols (10-1100 nm) enclosing radon decay products in the Postojna Cave, Slovenia.

At the lowest point along the tourist route in the Postojna Cave, the activity concentration of radon ((222)Rn) short-lived decay products and number concentration and size distribution of background aerosol particles in the size range of 10-1100 nm were measured. In the warm yearly season, aerosol concentration was low (52 cm(-3)) with 21 % particles smaller than 50 nm, while in the cold season, it was higher (1238 cm(-3)) with 8 % of <50 nm particles. Radon activity concentrations were 4489 and 1108 Bq m(-3), and fractions of unattached radon decay products were 0.62 and 0.13, respectively.

Preliminary study of thoron and radon levels in various indoor environments in Slovenia.

Using the Raduet discriminative radon-thoron solid-state nuclear track detectors, a limited number of measurements were recently carried out about 1 m away from any wall and 1.5 m above the floor in different environments in Slovenia. The following thoron and radon ranges were obtained, respectively (Bq m(-3)): 33-700 and 25-4900 in 2 dwellings, 11-215 and 22-422 in 5 kindergartens, 21-368 and 40-4609 in 35 elementary schools, 47-1361 and 92-3280 in 4 hospitals, 4-37 and 10-153 in 9 spas and 800-880 and 4060-6870 in 1 karst cave (2 places). In case of thoron and radon concentrations lognormal distribution was confirmed, while the statistical relationship between them was weak.

Influence of soil environmental parameters on thoron exhalation rate.

Field measurements of thoron exhalation rates have been carried out using a ZnS(Ag) scintillation detector with an accumulation chamber. The influence of soil surface temperature and moisture saturation on the thoron exhalation rate was observed. When the variation of moisture saturation was small, the soil surface temperature appeared to induce a strong effect on the thoron exhalation rate. On the other hand, when the variation of moisture saturation was large, the influence of moisture saturation appeared to be larger than the soil surface temperature. The number of data ranged over 405, and the median was estimated to be 0.79 Bq m(-2) s(-1). Dependence of geology on the thoron exhalation rate from the soil surface was obviously found, and a nationwide distribution map of the thoron exhalation rate from the soil surface was drawn by using these data. It was generally high in the southwest region than in the northeast region.

Effective dose of miners due to natural radioactivity in a manganese mine in Hungary.

In this study, short-term radon (RnDP) and thoron (TnDP) progeny measurements and dose estimation were carried out in winter and summer in a manganese mine, Hungary. Gamma-ray dose rate originating from external sources and (222)Rn and (226)Ra contents of spring-water from a mine was also measured. During working hours RnDP and TnDP concentration values changed between 12.1-175 and 0.14-0.42 Bq m(-3), respectively. The (222)Rn and (226)Ra concentration values in the karst spring-water were ∼6 Bq dm(-3) and 16 mBq dm(-3), respectively. The radiation dose resulting from the consumption of karst spring-water was negligible. The doses from the inhalation of TnDP and external gamma radiation were of the same magnitude, ∼0.1 mSv y(-1), which was rather negligible related to the estimated radiation dose of 5 mSv y(-1) from RnDP.

Radon and thoron parallel measurements in Hungary.

Hungarian detectors modified and developed at the National Institute of Radiological Sciences (NIRS), Japan were placed at different sites, including homes and underground workplaces in Hungary, in order to gain information on the average radon (222Rn) and thoron (220Rn) concentration levels. Measurements were carried out in dwellings in a village and a manganese mine in Hungary. The radon and thoron concentrations in the dwellings of the village in the summer period were found to be 154 (17-1083) and 98 (1-714) Bq m(-3), respectively. Considering the results of other radon measurements during the winter (814 Bq m(-3)) and summer (182 Bq m(-3)) periods, the thoron concentrations were also expected to be higher in winter. In the manganese mine, radon and thoron were measured at 20 points for 6 months, changing the detectors each month. The averages were 924 (308-1639) and 221 (61-510) Bq m(-3) for radon and thoron, respectively. These results showed significant variance with the date and place of the measurement.

Radiation dose from coal slag used as building material in the Transdanubian region of Hungary.

Coals mined in the Transdanubian region in Hungary have an elevated concentration of (226)Ra, which becomes enriched in the slag after burning. This slag has been used as filling and/or insulating material in building works. The aim of this study was to investigate the radiological situation in this territory in terms of the possible impact of this residual material from coal. Flats in three towns with a coal mine and a coal-fired power plant operating in their neighbourhood were examined. The radionuclide contents (including (226)Ra, (232)Th and (40)K) of the slag used for building were determined, and the slags were categorised according to the international standards and recommendations. The external gamma dose rate and the radon concentration in the sites were measured, and based on these data dose assessments were made. The (226)Ra concentration of the slag was 160-2,893 Bq kg(-1); the indoor gamma dose rates were 82-633 nGy h(-1); the radon concentration measured with a nuclear track detector varied from 29 to 1,310 Bq m(-3); the assessed dose contributions in the three towns were 0.65-1.57 mSv y(-1) due to gamma radiation and 2.2-15.2 mSv y(-1) due to radon.

Occupational and patient doses in the therapeutic cave, Tapolca (Hungary).

The radon concentration has been measured for three years in a hospital cave used for medical treatment of respiratory diseases. A mean value of the actual equilibrium factor measured in the cave in different seasons was used, different from the commonly used 0.4. The dose contribution to the patients and the staff was calculated using these data. The results of the dose assessment show that the staff in the hospital cave can receive doses up to the dose limit for occupational exposure (20 mSv y(-1)) when working 4 h per day in the cave. Patients receive 0.18-4.22 mSv committed effective dose during the treatment period depending on the exposure periods. The only solution to reduce the dose to the staff seems to be decreasing the time they spend underground, because intensive ventilation would disturb the special microclimate of the cave.

Concentration of 226Ra in Hungarian bottled mineral water.

Concentration of the radionuclide 226Ra was determined in almost every type of bottled mineral water commercially available in Hungary. Determination of the radon coming from the radium dissolved in the water was used for activity measurement. As the results show, the 226Ra concentrations exceed the level of 100 mBq l(-1) in six cases out of the 28 types of mineral water investigated. In one case 3 Bq l(-1) was measured, which provides 0.3 mSv year(-1) committed effective dose for adults in the case of a consumption rate of 1 l day(-1). In soft drinks produced from mineral water a concentration of 2.6 Bq l(-1) was determined, which means 1.4 mSv year(-1) effective dose in the age group 12-17 years in the case of permanent daily drinking of 1 l of these beverages.