Hirofumi Tazoe

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.

A method of measurement of (239)Pu, (240)Pu, (241)Pu in high U content marine sediments by sector field ICP-MS and its application to Fukushima sediment samples.

An accurate and precise analytical method is highly needed for the determination of Pu isotopes in marine sediments for the long-term marine environment monitoring that is being done since the Fukushima Dai-ichi Nuclear Power Plant accident. The elimination of uranium from the sediment samples needs to be carefully checked. We established an analytical method based on anion-exchange chromatography and SF-ICP-MS in this work. A uranium decontamination factor of 2 × 10(6) was achieved, and the U concentrations in the final sample solutions were typically below 4 pg mL(-1), thus no extra correction of (238)U interferences from the Pu spectra was needed. The method was suitable for the analysis of (241)Pu in marine sediments using large sample amounts (>10 g). We validated the method by measuring marine sediment reference materials and our results agreed well with the certified and the literature values. Surface sediments and one sediment core sample collected after the nuclear accident were analyzed. The characterization of (241)Pu/(239)Pu atom ratios in the surface sediments and the vertical distribution of Pu isotopes showed that there was no detectable Pu contamination from the nuclear accident in the marine sediments collected 30 km off the plant site.

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.

Ultra-trace plutonium determination in small volume seawater by sector field inductively coupled plasma mass spectrometry with application to Fukushima seawater samples

Long-term monitoring of Pu isotopes in seawater is required for assessing Pu contamination in the marine environment from the Fukushima Dai-ichi Nuclear Power Plant accident. In this study, we established an accurate and precise analytical method based on anion-exchange chromatography and SF-ICP-MS. This method was able to determine Pu isotopes in seawater samples with small volumes (20-60L). The U decontamination factor was 3×10(7)-1×10(8), which provided sufficient removal of interfering U from the seawater samples. The estimated limits of detection for (239)Pu and (240)Pu were 0.11fgmL(-1) and 0.08fgmL(-1), respectively, which corresponded to 0.01mBqm(-3) for (239)Pu and 0.03mBqm(-3) for (240)Pu when a 20L volume of seawater was measured. We achieved good precision (2.9%) and accuracy (0.8%) for measurement of the (240)Pu/(239)Pu atom ratio in the standard Pu solution with a (239)Pu concentration of 11fgmL(-1) and (240)Pu concentration of 2.7fgmL(-1). Seawater reference materials were used for the method validation and both the (239+240)Pu activities and (240)Pu/(239)Pu atom ratios agreed well with the expected values. Surface and bottom seawater samples collected off Fukushima in the western North Pacific since March 2011 were analyzed. Our results suggested that there was no significant variation of the Pu distribution in seawater in the investigated areas compared to the distribution before the accident.

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).

Rapid determination of 135Cs and precise 135Cs/137Cs atomic ratio in environmental samples by single-column chromatography coupled to triple-quadrupole inductively coupled plasma-mass spectrometry

For source identification, measurement of (135)Cs/(137)Cs atomic ratio not only provides information apart from the detection of (134)Cs and (137)Cs, but it can also overcome the application limit that measurement of the (134)Cs/(137)Cs ratio has due to the short half-life of (134)Cs (2.06 y). With the recent advancement of ICP-MS, it is necessary to improve the corresponding separation method for rapid and precise (135)Cs/(137)Cs atomic ratio analysis. A novel separation and purification technique was developed for the new generation of triple-quadrupole inductively coupled plasma-mass spectrometry (ICP-MS/MS). The simple chemical separation, incorporating ammonium molybdophosphate selective adsorption of Cs and subsequent single cation-exchange chromatography, removes the majority of isobaric and polyatomic interference elements. Subsequently, the ICP-MS/MS removes residual interference elements and eliminates the peak tailing effect of stable (133)Cs, at m/z 134, 135, and 137. The developed analytical method was successfully applied to measure (135)Cs/(137)Cs atomic ratios and (135)Cs activities in environmental samples (soil and sediment) for radiocesium source identification.

Determination of strontium-90 from direct separation of yttrium-90 by solid phase extraction using DGA Resin for seawater monitoring

It is important for public safety to monitor strontium-90 in aquatic environments in the vicinity of nuclear related facilities. Strontium-90 concentrations in seawater exceeding the background level have been observed in accidents of nuclear facilities. However, the analytical procedure for measuring strontium-90 in seawater is highly demanding. Here we show a simple and high throughput analytical technique for the determination of strontium-90 in seawater samples using a direct yttrium-90 separation. The DGA Resin is used to determine the abundance of strontium-90 by detecting yttrium-90 decay (beta-emission) in secular equilibrium. The DGA Resin can selectively collect yttrium-90 and remove naturally occurring radionuclides such as (40)K, (210)Pb, (214)Bi, (238)U, and (232)Th and anthropogenic radionuclides such as (140)Ba, and (140)La. Through a sample separation procedure, a high chemical yield of yttrium-90 was achieved at 95.5±2.3%. The result of IAEA-443 certified seawater analysis (107.7±3.4 mBq kg(-1)) was in good agreement with the certified value (110±5 mBq kg(-1)). By developed method, we can finish analyzing 8 samples per day after achieving secular equilibrium, which is a reasonably fast throughput in actual seawater monitoring. By processing 3 L of seawater sample and applying a counting time of 20 h, minimum detectable activity can be as low as 1.5 mBq kg(-1), which could be applied to monitoring for the contaminated marine environment. Reproducibility was found to be 3.4% according to 10 independent analyses of natural seawater samples from the vicinity of the Fukushima Daiichi Nuclear Power Plant in September 2013.

135 Cs activity and 135 Cs/ 137 Cs atom ratio in environmental samples before and after the Fukushima Daiichi Nuclear Power Plant accident

135Cs/137Cs is a potential tracer for radiocesium source identification. However, due to the challenge to measure 135Cs, there were no 135Cs data available for Japanese environmental samples before the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. It was only 3 years after the accident that limited 135Cs values could be measured in heavily contaminated environmental samples. In the present study, activities of 134Cs, 135Cs, and 137Cs, along with their ratios in 67 soil and plant samples heavily and lightly contaminated by the FDNPP accident were measured by combining γ spectrometry with ICP-MS/MS. The arithmetic means of the 134Cs/137Cs activity ratio (1.033 ± 0.006) and 135Cs/137Cs atom ratio (0.334 ± 0.005) (decay corrected to March 11, 2011), from old leaves of plants collected immediately after the FDNPP accident, were confirmed to represent the FDNPP derived radiocesium signature. Subsequently, for the first time, trace 135Cs amounts before the FDNPP accident were deduced according to the contribution of global and FDNPP accident-derived fallout. Apart from two soil samples with a tiny global fallout contribution, contributions of global fallout radiocesium in other soil samples were observed to be 0.338%–52.6%. The obtained 135Cs/137Cs database will be useful for its application as a geochemical tracer in the future.

Plutonium Isotopes (239–241Pu) Dissolved in Pacific Ocean Waters Detected by Accelerator Mass Spectrometry: No Effects of the Fukushima Accident Observed

The concentration of plutonium (Pu) and the isotopic ratios of 240Pu to 239Pu and 241Pu to 239Pu were determined by accelerator mass spectrometry (AMS) in Pacific Ocean water samples (20 L each) collected in late 2012. The isotopic Pu ratios are important indicators of different contamination sources and were used to identify a possible release of Pu into the ocean by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. In particular, 241Pu is a well-suited indicator for a recent entry of Pu because 241Pu from fallout of nuclear weapon testings has already significantly decayed. A total of 10 ocean water samples were prepared at the Radiochemie München of the TUM and analyzed at the Vienna Environmental Research Laboratory (VERA). Several samples showed a slightly elevated 240Pu/239Pu ratio of up to 0.22 ± 0.02 compared to global fallout (240Pu/239Pu = 0.180 ± 0.007), whereas all measured 241Pu-to-239Pu ratios were consistent with nuclear weapon fallout (241Pu/239Pu < 2.4 × 10-3), which means that no impact from the Fukushima accident was detected. From the average 241Pu-to-239Pu ratio of 8-2+3 ×10-4 at a sampling station located at a distance of 39.6 km to FDNPP, the 1-σ upper limit for the FDNPP contribution to the 239Pu inventory in the water column was estimated to be 0.2%. Pu, with the signature of weapon-grade Pu was found in a single sample collected around 770 km off the west coast of the United States.

Isotopic compositions of 236U, 239Pu, and 240Pu in soil contaminated by the Fukushima Daiichi Nuclear Power Plant accident

Six years after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, data for 236U and 236U/238U have remained limited to a few heavily contaminated samples. In the present study, activities of 236U, 239Pu, and 240Pu, along with other U isotopes in 46 soil samples both heavily and lightly contaminated by this accident were measured by inductively coupled plasma–mass spectrometry (ICP-MS) and triple-quadrupole ICP-MS. The 236U activities and 236U/238U atom ratios in these soil samples were in the range of (0.469–24.4) × 10−5 Bq kg−1 and ((0.099–1.35) × 10−7), respectively. Higher 240Pu/239Pu atom ratios (0.245–0.312) and 238Pu/239+240Pu activity ratios (0.859–1.62) indicated Pu contamination originated from this accident and global fallout in some samples. For those soil samples along with black substances collected along roads in Fukushima Prefecture, high linear correlations were presented between 236U activities and 239+240Pu activities (Pearson’s r = 0.755, p < 0.01), and between 236U activities and 238Pu activities (Pearson’s r = 0.844, p < 0.01). The analysis of these soil samples confirmed the release of 236U, although in trace amounts, during the FDNPP accident.