Naofumi Akata

Activity concentrations of environmental samples collected in Fukushima Prefecture immediately after the Fukushima nuclear accident

Radionuclide concentrations in environmental samples such as surface soils, plants and water were evaluated by high purity germanium detector measurements. The contribution rate of short half-life radionuclides such as 132I to the exposure dose to residents was discussed from the measured values. The highest values of the 131I/137Cs activity ratio ranged from 49 to 70 in the environmental samples collected at Iwaki City which is located to the south of the F1-NPS. On the other hand, the 132I/131I activity ratio in the same environmental samples had the lowest values, ranging from 0.01 to 0.02. By assuming that the 132I/131I activity ratio in the atmosphere was equal to the ratio in the environmental samples, the percent contribution to the thyroid equivalent dose by 132I was estimated to be less than 2%. Moreover, the contribution to the thyroid exposure by 132I might be negligible if 132I contamination was restricted to Iwaki City.

Plant induced changes in concentrations of caesium, strontium and uranium in soil solution with reference to major ions and dissolved organic matter

For a better understanding of the soil-to-plant transfer of radionuclides, their behavior in the soil solution should be elucidated, especially at the interface between plant roots and soil particles, where conditions differ greatly from the bulk soil because of plant activity. This study determined the concentration of stable Cs and Sr, and U in the soil solution, under plant growing conditions. The leafy vegetable komatsuna (Brassica rapa L.) was cultivated for 26 days in pots, where the rhizosphere soil was separated from the non-rhizosphere soil by a nylon net screen. The concentrations of Cs and Sr in the rhizosphere soil solution decreased with time, and were controlled by K+NH(4)(+) and Ca, respectively. On the other hand, the concentration of U in the rhizosphere soil solution increased with time, and was related to the changes of DOC; however, this relationship was different between the rhizosphere and non-rhizosphere soil.

Concentration of 3 H in plants around Fukushima Dai-ichi Nuclear Power Station

A large amount of radionuclides was released from the Fukushima Dai-ichi Nuclear Power Station (FDNPS) following the damage caused by the tsunami due to the Great East Japan Earthquake on 11 March 2011. Although many radionuclides in various environmental samples around the FDNPS have been measured, 3H in the terrestrial environment has not yet been reported. We present here the first survey results of 3H concentrations in plant samples collected around the FDNPS in 2011 from shortly after the accident. The free-water 3H concentrations in herbaceous plant shoots and evergreen tree leaves were considerably higher than the previous background concentration, and diminished with distance from the FDNPS. Although reconstruction of atmospheric 3H concentrations after the accident is difficult, a rough estimate of the radiation dose due to 3H inhalation about 20 km from the FDNPS is on the order of a few microsieverts (μSv).

Estimation of External Dose by Car-Borne Survey in Kerala, India

A car-borne survey was carried out in Kerala, India to estimate external dose. Measurements were made with a 3-in × 3-in NaI(Tl) scintillation spectrometer from September 23 to 27, 2013. The routes were selected from 12 Panchayats in Karunagappally Taluk which were classified into high level, mid-level and low level high background radiation (HBR) areas. A heterogeneous distribution of air kerma rates was seen in the dose rate distribution map. The maximum air kerma rate, 2.1 μGy/h, was observed on a beach sand surface. 232Th activity concentration for the beach sand was higher than that for soil and grass surfaces, and the range of activity concentration was estimated to be 0.7–2.3 kBq/kg. The contribution of 232Th to air kerma rate was over 70% at the measurement points with values larger than 0.34 μGy/h. The maximum value of the annual effective dose in Karunagappally Taluk was observed around coastal areas, and it was estimated to be 13 mSv/y. More than 30% of all the annual effective doses obtained in this survey exceeded 1 mSv/y.

Effects of radiocesium inventory on 137Cs concentrations in river waters of Fukushima, Japan, under base-flow conditions

To investigate the behavior of nuclear accident-derived (137)Cs in river water under base-flow conditions, concentrations of dissolved and particulate (137)Cs were measured at 16 sampling points in seven rivers of Fukushima Prefecture, Japan, in 2012 and 2013. The concentration of dissolved (137)Cs was significantly correlated with the mean (137)Cs inventory in the catchment area above each sampling point in both sampling years. These results suggest that the concentration of dissolved (137)Cs under base-flow conditions is primarily determined by the (137)Cs inventory of the catchment area above the sampling point. However, the concentration of particulate (137)Cs did not show a clear relationship with either the mean (137)Cs inventory or the dissolved (137)Cs concentration, thus indicating that particulate and dissolved forms do not effectively interact in rivers. To evaluate the contribution of the (137)Cs inventory within catchment areas, we analyzed relations between the (137)Cs concentration and the mean (137)Cs inventory over the area within certain flow path lengths that were traced along the river and slope above the sampling point. Coefficients of determination for dissolved (137)Cs concentrations were highest for the longest flow path, i.e., the whole catchment area, and lower for shorter flow paths. Coefficients of determination for particulate (137)Cs concentrations were only moderately high for the shortest flow path in 2012, whereas the values were quite low for all flow paths in 2013. These results suggest that dissolved (137)Cs can originate from a larger area of the catchment even under base-flow conditions; however, particulate (137)Cs did not show such behavior. The results also show that under base-flow conditions, dissolved and particulate (137)Cs behave independently during their transport from river catchments to the ocean.

Comparative dosimetry for radon and thoron in high background radiation areas in China

The present study focuses on internal exposure caused by the inhalation of radon and thoron progenies because the internal exposures have not yet been clarified. For their dose assessment, radon, thoron and thoron progeny concentrations were measured by passive monitors over a long period (for 6 months). Consequently, radon, thoron and equilibrium equivalent thoron concentrations were given as 124 ± 78, 1247 ± 1189 and 7.8 ± 9.1 Bq m(-3), respectively. Annual effective doses are estimated to be 3.1 ± 2.0 mSv for radon and 2.2 ± 2.5 mSv for thoron. Total dose are estimated to be 5.3 ± 3.5 mSv a(-1). The present study has revealed that the radon dose was comparable with the thoron dose, and the total dose was ∼2 times higher than the worldwide average.

Low-Level Measurement with a Noble Gas Mass Spectrometer for Organically Bound Tritium in Environmental Samples

Abstract We developed an analytical method for organically bound 3H (OBT) in biological environmental samples by using noble gas mass spectrometry of 3He produced from 3H. Three environmental samples with background level OBT concentrations were analyzed, and the results agreed well with those by the conventional liquid scintillation counting of electrolyzed combustion water of the samples. This showed that the method is practical and effective. We also developed an analytical method for non-exchangeable OBT as a combination of pre-treatment of dried samples with free water 3H and our newly developed analytical method for OBT. The repeated analysis of a grass sample with moderate 3H concentration had smaller variance of results for non-exchangeable OBT than for OBT. The sum of non-exchangeable and exchangeable OBT agreed well with OBT measured in the samples. The developed method was successfully applied to terrestrial and marine environmental samples with background 3H levels.

Distribution and dynamics of radionuclides and stable elements in the coastal waters off Rokkasho Village, Japan, prior to the opening of a nuclear reprocessing facility

A nuclear fuel reprocessing facility is currently under construction in Rokkasho Village, Aomori, Japan. After completion and start-up, this facility will discharge radionuclides into the Pacific Ocean through an outlet pipe set on the seafloor offshore. For future assessments of the stability of these radionuclides in the environment, a sufficient understanding of the behavior of radionuclides in this ocean ecosystem before the start-up of the facility is necessary. To understand the processes by which radionuclides and various other types of elements are eliminated from seawater, we measured the sedimentation flux of suspended particles in the coastal waters off Rokkasho Village where the sea emissions pipes will be placed.

Iodine-129 in water samples collected adjacent to a spent nuclear fuel reprocessing plant in Rokkasho, Japan

The spent nuclear fuel reprocessing plant in Rokkasho, Japan, has been undergoing final testing since March 2006. The concentrations of 129I in environmental water samples collected around the reprocessing plant were measured from 2005 to 2012. The 129I concentrations in water samples of the brackish lake adjacent to a plant from 2006 to 2008 which the spent fuel rods were cut and chemically processed, was higher than the background level. The major source of 129I in the lake was dominated by the direct deposition of 129I from the atmosphere than that from the ocean.

Radiocarbon Concentrations in Environmental Samples Collected Near the Spent Nuclear Fuel Reprocessing Plant at Rokkasho, Aomori, Japan, During Test Operation Using Spent Nuclear Fuel

Abstract The contribution of atmospheric discharged 14C to local 14C concentrations was investigated by analysis of 14C in environmental samples collected around the spent nuclear fuel reprocessing plant in Rokkasho, Aomori, Japan. From June 2006 to October 2011, the range of monthly averaged specific activities in atmospheric CO2 collected 2.6 km from the plant was 0.226–0.279 Bq g C−1; at several sampling times, the observed concentrations were higher than the background value. Specific activities of atmospheric 14C simulated with an atmospheric dispersion model coupled with a mesoscale weather model reproduced the measured values fairly well, supporting the idea that the higher measured specific activities were due to 14C discharged from the reprocessing plant. For investigation of the 14C distribution around the facility, samples of Cyperus microiria, wild annual sedge, were collected from nine locations. Plant samples collected east and west of the facility in 2008 showed slightly higher specific activities than samples collected at the other locations, reflecting the prevailing wind direction. The measured 14C specific activities in vegetable, polished rice, pasture, and milk samples collected around the facility did not differ significantly from the background value (except for one polished rice sample, which had a value slightly higher than background). The annual committed effective radiation dose to humans resulting from consumption of the polished rice was estimated as 3.6 × 10−4 mSv, which is negligibly small compared with 1 mSv, an index of the dose limit for the general public.