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.

CHARACTERISTICS OF INDOOR RADON AND THORON CONCENTRATIONS IN CAVE DWELLINGS IN GANSU PROVINCE, CHINA.

Short-term measurements were made in four different types of Chinese cave dwellings, 'ground', 'open-cut', 'aboveground' and 'underground', located in Gansu Province, China, in order to evaluate the diurnal and seasonal variations in indoor radon (222Rn) and thoron (220Rn) concentrations. As a result, in most of the 'ground' and 'open-cut' cave dwellings in summer, the indoor 222Rn concentrations during the nighttime were higher than those during the daytime. In the winter, such fluctuation was not observed. The daily indoor 222Rn concentrations in summer were higher than those in winter, excluding the 'aboveground' cave dwelling, and these concentrations depended on dwelling type, exhalation from the building materials and ventilation. In contrast with 222Rn, there seemed to be only very slight clear variations in 220Rn.

REDUCTION IN AMBIENT GAMMA DOSE RATE FROM RADIOCESIUM DUE TO SNOW COVER.

In the present study, variations in ambient gamma dose rate associated with snow cover were examined in a radioactive-contaminated site in Fukushima Prefecture, Japan. The ambient gamma dose rates decreased with increasing snow depth. The reduction trends were different between fresh snow (0.1-0.2 g/cm3) and granular snow (0.3-0.4 g/cm3) depending on snow density. Snow cover water content (snow water equivalent) calculated from snow depth and density was a key parameter governing the reduction in the ambient gamma dose rate. The ambient gamma dose rates reduced to 0.6 and 0.5 at 4 g/cm2 and 8 g/cm2 of snow water equivalent, respectively. Based on gamma-ray flux density distributions, the ambient gamma dose rates from the primary gamma rays decreased more compared to those from scattered gamma rays due to snow cover.

238Pu/(239+240)Pu activity ratio as an indicator of Pu originating from the FDNPP accident in the terrestrial environment of Fukushima Prefecture.

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident has caused significant radionuclide contamination. Pu isotopes at the level of GBq were released from the damaged reactors to terrestrial and marine ecosystems. In this work, 35 samples were collected at different locations of Fukushima. Samples consisted of three types, soil, forest litter and alluvial dust (road dust, sludges from drainage systems and below gutter pipe outflows). The obtained activity ratios of 238Pu/(239+240)Pu ranged from 0.030 to 1.86. 14 of our samples contained trace amounts of Pu originating from the damaged reactors (2SM verification). Our study identified a few previously unknown "hot spots" of 238Pu/(239+240)Pu activity ratio localized in an area between 15 and 30 km in the northwest direction from the FDNPP. Additionally, results obtained in this study combined with previously published data allowed us to prepare a map of spatial distribution of the Pu isotope fingerprints (238Pu/(239+240)Pu) in Fukushima Prefecture.

NATURAL RADIOACTIVITY LEVEL AND ELEMENTAL COMPOSITION OF SOIL SAMPLES FROM A HIGH BACKGROUND RADIATION AREA ON EASTERN COAST OF INDIA (ODISHA).

A comprehensive study was carried out to determine the radioactivity concentration of soil samples from different sites of a high background radiation area in the eastern coast of India, Odisha state. The dose rate measured in situ varied from 0.25 to 1.2 µSv h-1 The gamma spectrometry measurements indicated Th series elements as the main contributors to the enhanced level of radiation and allowed the authors to find the mean level of the activity concentration (±SD) for 226Ra, 228Th and 40K as 130±97, 1110±890 and 360±140 Bq kg-1, respectively. Human exposure from radionuclides occurring outdoor was estimated based on the effective dose rate, which ranged from 0.14±0.02 to 2.15±0.26 mSv and was higher than the UNSCEAR annual worldwide average value 0.07 mSv. Additionally, X-ray fluorescence analysis provided information about the content of major elements in samples and indicated the significant amount of Ti (7.4±4.9 %) in soils.

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).

Variability of radiocaesium inventory in Fukushima soil cores from one site measured at different times.

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has resulted in the deposition of radioactive contamination of soils mainly by (134)Cs and (137)Cs. The contaminated areas are of a great concern since external radiation may lead to significant long-term doses to humans and animals. External dose rate above ground will decrease with physical decay of the radionuclides and as they migrate deeper into the soil with time. In the present study, depth profiles of Cs radioisotopes have been studied at different times over a period of 1 y. Radiocaesium inventories for (134)Cs and (137)Cs down to a depth of 10 cm at the four measurement times varied from 2.09 to 4.14 MBq m(-2) with coefficient of variation (CV): 27.4 %, n = 4 and from 2.13 to 4.23 MBq m(-2) with CV: 27.3 %, n = 4, respectively. The activity ratio for (134)Cs/(137)Cs decreased from 0.62 to 0.46 during 1 y of measurement i.e. July 2012 to June 2013. However, the ratio with decay correction (15 March 2011) was found to be constant ∼1, indicating contamination from FDNPP origin. In order to understand the radiocaesium distribution in soil profile, physico-chemical characterisation of soil has been carried out.

Characteristics of indoor radon and its progeny in a Japanese dwelling while using air appliances.

Characteristics of radon and its progeny were investigated in different air conditions by turning four types of indoor air appliances on and off in a two-story concrete Japanese dwelling. The four appliances were air conditioner, air cleaner, gas heater and cooker hood. The measurements were done using two devices: (1) a Si-based semiconductor detector for continuous measurement of indoor radon concentration and (2) a ZnS(Ag) scintillation counting system for equilibrium-equivalent radon concentration. Throughout the entire experiment, the cooker hood was the most effective in decreasing indoor radon concentration over a long period of time and the less effective was the air conditioner, while the air cleaner and gas heater did not affect the concentration of radon. However, the results measured in each air condition will differ according to the lifestyles and activities of the inhabitants. In this study, indoor radon and its progeny in a Japanese dwelling will be characterised by the different air conditions.

Radioactive pollution from Fukushima Daiichi nuclear power plant in the terrestrial environment.

Major contaminants from venting and hydrogen explosions at the Fukushima Daiichi nuclear reactors between 12 and 15 March 2011 were transported northwestward and deposited on soil and plants via precipitation. Surface soils and plant leaves were sampled at 64 sites in the Fukushima Prefecture. The highest concentrations of (134)Cs (84.4 kBq kg(-1)) and (137)Cs (82.0 kBq kg(-1)) in surface soils were observed at Nagadoro in Iidate village located 32 km northwest from the Fukushima Daiichi nuclear power plant. Furthermore, (131)I, (129)Te, (129 m)Te, (110 m)Ag and (140)La were detected in the same samples. Outer surface of plant leaves, such as bamboo, cabbage and grasses were highly contaminated at the high-dose rate areas of Tsushima and Minami-Tsushima in Namie town. Mugwort leaves that grew after the pollution event had extremely low concentration of radionuclides; however, the plant/soil radiocaesium ratio was 0.023 ± 0.006. It is anticipated that decomposition of fallen leaves will promote recycling of radionuclides in the environment.

Characteristics of radon and thoron exhalation rates in Okinawa, subtropical region of Japan.

Radon and thoron exhalation rates from the ground surface were estimated in three islands of Okinawa Prefecture, a subtropical region of Japan. In situ measurements of the exhalation rates were conducted at a total of 88 points using an accumulation technique with a ZnS(Ag) scintillation detector. The radon and thoron exhalation rates were calculated to be 1-137 (arithmetic mean: 21) mBq m(-2) s(-1) and 32-6244 (1801) mBq m(-2) s(-1), respectively. In the surface soil samples collected at 53 measurement points, (238)U and (232)Th series concentrations were estimated to be 17.9-254.0 (64.0) Bq kg(-1) dry and 17.8-136.1 (58.8) Bq kg(-1) dry, respectively. The maximum rates and concentrations were observed in the dark red soil area. Recent studies strongly suggest that the base material of the soils may be the eolian dust derived from the southeastern part of China, a high background radiation area. The eolian dust is, therefore, considered to be an enhancer for the radon and thoron exhalations in Okinawa.

Distribution of terrestrial gamma radiation dose rate in the eastern coastal area of Odisha, India.

Terrestrial gamma radiation is one of the important radiation exposures on the earth's surface that results from the three primordial radionuclides (226)Ra, (232)Th and (40)K. The elemental concentration of these elements in the earth's crust could result in the anomalous variation of the terrestrial gamma radiation in the environment. The geology of the local area plays an important role in distribution of these radioactive elements. Environmental terrestrial gamma radiation dose rates were measured around the eastern coastal area of Odisha with the objective of establishing baseline data on the background radiation level. The values of the terrestrial gamma radiation dose rate vary significantly at different locations in the study area. The values of the terrestrial gamma dose rate ranged from 77 to 1651 nGy h(-1), with an average of 230 nGy h(-1). During the measurement of the terrestrial gamma dose rate, sand and soil samples were also collected for the assessment of natural radionuclides. The activities of (226)Ra, (232)Th and (40)K from these samples were measured using a gamma-ray spectrometry with a NaI(Tl) detector. Activity concentrations of (226)Ra, (232)Th and (40)K ranged from 15.6 to 69 Bq kg(-1) with an average of 46.7 Bq kg(-1), from 28.9 to 973 Bq kg(-1) with an average of 250 Bq kg(-1) and from 139 to 952 Bq kg(-1) with an average of 429, respectively. The detailed significance of these studies has been discussed from the radiation protection point of view.

Levels of thoron and progeny in high background radiation area of southeastern coast of Odisha, India.

Exposure to radon, (222)Rn, is assumed to be the most significant source of natural radiation to human beings in most cases. It is thought that radon and its progeny are major factors that cause cancer. The presence of thoron, (220)Rn, was often neglected because it was considered that the quantity of thoron in the environment is less than that of radon. However, recent studies have shown that a high thoron concentration was found in some regions and the exposure to (220)Rn and its progeny can equal or several time exceed that of (220)Rn and its progeny. The results of thoron and its progeny measurements in the houses of high background radiation area (HBRA) of the southeastern coast of Odisha, India presented here. This area is one of the high background radiation areas in India with a large deposit of monazite sand which is the probable source of thoron. Both active and passive methods were employed for the measurement of thoron and its progeny in cement, brick and mud houses in the study area. Thoron concentration was measured using RAD-7 and Raduet. A CR-39 track detector was employed for the measurement of environmental thoron progeny, both in active and passive modes. Thoron and its progeny concentrations were found to be comparatively high in the area. A comparison between the results obtained with various techniques is presented in this paper.

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.

Effects of air exchange property of passive-type radon-thoron discriminative detectors on performance of radon and thoron measurements.

Pairs of diffusion chambers with different air exchange rates are used in a large-scale survey to determine radon and thoron, separately. When they are enclosed in radon-proof bags for keeping after the exposure, since radon does not escape out immediately from the low-diffusion chamber, it leads to further exposure in the bags and disturbs the estimation of radon and thoron concentrations. In this study, the effects of the different air exchange properties of the radon-thoron discriminative detectors with CR-39 chips on the estimations of radon and thoron concentrations were investigated. The commercially available and frequently used detectors, Raduet, are examined in this study. The result shows that radon escapes out in 10 h. When degassing is not enough after the exposure in a calibration experiment or high-background radiation area, the residual radon causes the overestimation of the radon concentration and increase in the uncertainty in the thoron concentration, i.e. a low-performance quality of radon and thoron measurements.

Long-term measurements of thoron, its airborne progeny and radon in 205 dwellings in Ireland.

Long-term (circa 3 months) simultaneous measurements of indoor concentrations of thoron gas, airborne thoron progeny and radon were made using passive alpha track detectors in 205 dwellings in Ireland during the period 2007-09. Thoron progeny concentrations were measured using passive deposition monitors designed at the National Institute of Radiological Sciences (NIRS), Japan, whereas thoron gas concentrations were measured using Raduet detectors (Radosys, Budapest). Radon concentrations were measured in these dwellings by means of NRPB/SSI type alpha track radon detectors as normally used by the Radiological Protection Institute of Ireland (RPII). The concentration of thoron gas ranged from <1 to 174 Bq m(-3) with an arithmetic mean (AM) of 22 Bq m(-3). The concentration of radon gas ranged from 4 to 767 Bq m(-3) with an AM of 75 Bq m(-3). For radon, the estimated annual doses were 0.1 (min), 19.2 (max) and 1.9 (AM) mSv y(-1). The concentration of thoron progeny ranged from <0.1 to 3.8 Bq m(-3) [equilibrium equivalent thoron concentration (EETC)] with an AM of 0.47 Bq m(-3) (EETC). The corresponding estimated annual doses were 2.9 (max) and 0.35 (mean) mSv y(-1). In 14 or 7% of the dwellings, the estimated doses from thoron progeny exceeded those from radon.

Comparative dosimetry of radon and thoron.

There is a well-known discrepancy between dosimetrically derived dose conversion factor (DCF) and epidemiologically derived DCF for radon. As the latter DCFs, International Commission on Radiological Protection (ICRP) recommends a value of ∼6.4 nSv (Bq h m(-3))(-1) and 7.9 nSv (Bq h m(-3))(-1) for radon decay products (RnDP) in dwellings and workplaces, respectively. On the other hand, the dosimetric calculations based on the ICRP-66 respiratory tract model derived a DCF of 13 nSv (Bq h m(-3))(-1) and 17 nSv (Bq h m(-3))(-1) for RnDP in dwellings and workplaces, respectively, and 83 nSv (Bq h m(-3))(-1) for thoron decay products (TnDP) in dwellings. In addition, the DCFs derived from both approaches and UNSCEAR were applied to comparative dosimetry for two thoron-enhanced areas (cave dwellings in China and dwellings at a spa town in Japan), where the equilibrium equivalent concentration of radon and equilibrium equivalent concentration of thoron have been measured. In the case of the spa town dwellings, the dose from TnDP was larger than the dose from RnDP.

Long-term determination of airborne radon progeny concentrations using LR 115 detectors and the effects of thoron.

The 'proxy equilibrium factor' (F(p)) method has been developed for long-term determination of airborne radon progeny concentrations using LR 115 solid-state nuclear track detectors. In this paper, the effects of (220)Rn on the F(p) method have been studied. The correction to the track density was related to a parameter α which was the ratio of the sum of activity concentrations of alpha-particle emitting radionuclides in the (220)Rn decay chain to the activity concentration of (220)Rn alone. Under commonly encountered circumstances, α could not be smaller than 2. An attempt was made to verify this using the exposure chamber at the National Institute of Radiological Sciences (NIRS), Chiba, Japan. A most interesting observation of α < 2 for very high (220)Rn concentrations and very low equilibrium factors for (220)Rn in the exposure chambers was made. A possible explanation was the substantial deposition of (216)Po under the extreme conditions inside the exposure chambers.

Preliminary indoor thoron measurements in high radiation background area of southeastern coastal Orissa, India.

This paper presents the preliminary results of radon and thoron measurements in the houses of Chhatrapur area of southeastern coast of Orissa, India. This area is one of the high radiation background radiation areas in India, which consists of monazite sand as the source of thoron. Both active and passive methods were employed for the measurements. Radon and thoron concentrations were measured in the houses of Chhatrapur area using twin cup radon dosemeters, RAD7 and radon-thoron discriminative detector (Raduet). Thoron progeny concentration was also measured in the houses using deposition rate measurements. Radon and thoron concentrations in the houses of study area were found to vary from 8 to 47 Bq m(-3) and the below detection level to 77 Bq m(-3), respectively. While thoron progeny concentration in these houses ranges between 0.17 and 4.24 Bq m(-3), preliminary investigation shows that the thoron concentration is higher than radon concentration in the houses of the study area. The thoron progeny concentration was found to be comparatively higher, which forms a base for further study in the area. The comparison between the results of various techniques is presented in this paper.

A comparative study of thorium activity in NORM and high background radiation area.

Several industrial processes are known to enrich naturally occurring radioactive materials (NORM). To assess such processes with respect to their radiological relevance, characteristic parameters describing this enrichment will lead to interesting information useful to UNSCEAR. In case of mineral treatment plants, the high temperatures used in smelting and refining processes lead to high concentrations of (238)U and (232)Th. Also due to thermal power combustion, concentration of U and Th in the fly ash increases manifold. NORM samples were collected from a Thailand mineral treatment plant and Philippine coal-fired thermal power plants for investigation. Some studies are initiated from a high background radiation area near Gopalpur of Orissa state in India. These NORM samples were analysed by gamma-ray spectrometry as well as inductively coupled plasma mass spectrometry. The radioactivity in case of Orissa soil samples is found to be mainly contributed from thorium. This study attempts to evaluate levels of thorium activity in NORM samples.