Keiko Tagami

Isotopic evidence of plutonium release into the environment from the Fukushima DNPP accident

The Fukushima Daiichi nuclear power plant (DNPP) accident caused massive releases of radioactivity into the environment. The released highly volatile fission products, such as 129mTe, 131I, 134Cs, 136Cs and 137Cs were found to be widely distributed in Fukushima and its adjacent prefectures in eastern Japan. However, the release of non-volatile actinides, in particular, Pu isotopes remains uncertain almost one year after the accident. Here we report the isotopic evidence for the release of Pu into the atmosphere and deposition on the ground in northwest and south of the Fukushima DNPP in the 20–30 km zones. The high activity ratio of 241Pu/239+240Pu (> 100) from the Fukushima DNPP accident highlights the need for long-term 241Pu dose assessment, and the ingrowth of 241Am. The results are important for the estimation of reactor damage and have significant implication in the strategy of decontamination.

Measurement of 240Pu/239Pu isotopic ratios in soils from the Marshall Islands using ICP-MS.

Nuclear weapons tests conducted by the United States in the Marshall Islands produced significant quantities of regional or tropospheric fallout contamination. Here we report on some preliminary inductively coupled plasma-mass spectrometry (ICP-MS) measurements of plutonium isolated from seven composite soil samples collected from Bikini, Enewetak and Rongelap Atolls in the northern Marshall Islands. These data show that 240Pu/239Pu isotopic signatures in surface soils from the Marshall Island vary significantly and could potentially be used to help quantify the range and extent of fallout deposition (and associated impacts) from specific weapons tests. 137Cs and 60Co were also determined on the same set of soil samples for comparative purposes.

Determination of plutonium concentration and its isotopic ratio in environmental materials by ICP-MS after separation using and extraction chromatography

An analysis method for 239 Pu and 240 Pu in environmental samples (including four certified reference materials) by ICP-MS was studied. Two types of chromatographic resin, Dowex 1X8 and TEVA, were examined for their applicability to the separation of Pu from the matrix elements. Sufficient decontamination factors (10 4 -10 5 ) for many matrix elements including U, which interferes with the detection of mass 239, were obtained with both resins. The detection limit of Pu by ICP-MS was about 0.02 pg ml –1 (0.05 mBq ml –1 for 239 Pu; 0.17 mBq ml –1 for 240 Pu) in the sample solution or 0.1 pg in the sample. Analytical results of 239+240 Pu in certified reference materials (IAEA-135, -SOIL-6, -368 and -134) indicated that the accuracy of the method was satisfactory. Data on the 240 Pu/ 239 Pu atom ratios in these reference materials, which are scarce in the literature, were also obtained, i.e., 0.211 for IAEA-135, 0.191 for IAEA-SOIL-6, 0.043 for IAEA-368 and 0.200 for IAEA-134. Compared with alpha-spectrometry, the ICP-MS method has significant advantages in terms of its simple analytical procedures, prompt measurement time and capability of determining the 240 Pu/ 239 Pu ratio.

Release of plutonium isotopes into the environment from the Fukushima Daiichi Nuclear Power Plant accident: what is known and what needs to be known.

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has caused serious contamination in the environment. The release of Pu isotopes renewed considerable public concern because they present a large risk for internal radiation exposure. In this Critical Review, we summarize and analyze published studies related to the release of Pu from the FDNPP accident based on environmental sample analyses and the ORIGEN model simulations. Our analysis emphasizes the environmental distribution of released Pu isotopes, information on Pu isotopic composition for source identification of Pu releases in the FDNPP-damaged reactors or spent fuel pools, and estimation of the amounts of Pu isotopes released from the FDNPP accident. Our analysis indicates that a trace amount of Pu isotopes (∼2 × 10(-5)% of core inventory) was released into the environment from the damaged reactors but not from the spent fuel pools located in the reactor buildings. Regarding the possible Pu contamination in the marine environment, limited studies suggest that no extra Pu input from the FDNPP accident could be detected in the western North Pacific 30 km off the Fukushima coast. Finally, we identified knowledge gaps remained on the release of Pu into the environment and recommended issues for future studies.

Specific activity and activity ratios of radionuclides in soil collected about 20km from the Fukushima Daiichi Nuclear Power Plant: Radionuclide release to the south and southwest

Soil samples at different depths (0-2, 5-7 and 10-12cm) were collected from J Village, about 20km south of Fukushima Daiichi Nuclear Power Plant (FNPP) to determine their radionuclide specific activities and activity ratios. The concentrations and activity ratios of (131)I, (134, 136, 137)Cs and (129m)Te were obtained, but only trace amounts of (95)Nb, (110m)Ag and (140)La were detected which were too low to provide accurate concentrations. Radionuclides such as (95)Zr, (103, 106)Ru and (140)Ba that were found in Chernobyl fallout, were not found in these soil samples. This suggests that noble gasses and volatile radionuclides predominated in the releases from FNPP to the terrestrial environment. The average activity ratios of (131)I/(137)Cs, (134)Cs/(137)Cs, (136)Cs/(137)Cs and (129m)Te/(137)Cs were 55, 0.90, 0.22 and 4.0 (corrected to March 11, 2011) in the 0-2cm soil samples of April 20 and 28, 2011.

Concentrations of lanthanide elements, Th, and U in 77 Japanese surface soils

Abstract Inductively coupled plasma-mass spectrometry (ICP-MS) was used to measure lanthanide elements, Th, and U in 77 surface soil samples collected from upland fields, paddies, forests, and open areas throughout Japan. The measurements provide an understanding of the background levels of these elements in Japanese soils. The mean concentrations in μg/g for the 77 soils were 18 (La), 40 (Ce), 4.5 (Pr), 18 (Nd), 3.7 (Sm), 0.96 (Eu), 3.7 (Gd), 0.56 (Tb), 3.3 (Dy), 0.68 (Ho), 2.0 (Er), 0.29 (Tm), 2.0 (Yb), 0.29 (Lu), 7.0 (Th), and 2.3 (U). Differences between four classifications of soils based on their utilization were not significant for any measured elements except U. The concentration of U in agricultural soils (upland fields and paddies) tended to be higher than that in open areas and forests, suggesting the possible accumulation of U derived from phosphate fertilizer. Red and yellow soils, which are mainly found in western Japan, have high concentrations of lanthanides and Th. Their high concentrations were mainly attributable to the high concentrations of these elements in granitic rocks, which are the main source materials of these soils. However, red and yellow soils derived from another source material also had high concentrations of lanthanides and Th, indicating that the elements can be enriched during the soil formation process of red and yellow soils even if the source material is not granitic rocks. Andosols, which are derived from volcanic ash, have relatively low concentrations of light lanthanides, Th and U. Concentrations of the elements in peat soils and sand-dune regosols were low. Each lanthanide element in soils had a reasonable correlation with its neighboring lanthanide elements. Thorium and U had correlations with light lanthanide elements. These good correlations might be due to the similarity of their ionic radii.

Soil-to-Plant Transfer Factors of Stable Elements and Naturally Occurring Radionuclides : (1) Upland Field Crops Collected in Japan

In long-term dose assessment models for radioactive waste disposal, an important exposure pathway to humans is via ingestion of contaminated foods. In order to obtain soil-to-plant transfer factors (TFs) of radionuclides under equilibrium conditions, naturally existing elements were measured as analogues of radionuclides. Crops grown in upland fields and associated soil samples were collected from 62 sampling sites throughout Japan. The total concentrations of 52 elements in the crops and 54 elements in the soil samples were measured. The TFs of 40 elements (Li, Na, Mg, Al, Si, P, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Y, Mo, Cd, Sn, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Pb, Th and U) were calculated on a dry weight basis. Among all the TF data, K showed the highest TF with a geometric mean (GM) of 2.1, followed by P. The GMs of TFs for rare earth elements, Th and U were on the order of 10−4. Most of the TF-GMs for green vegetables were higher than GMs of all crops for the elements. The obtained TFs of some elements for green vegetables and potatoes were compared with those in the technical report series-364 (TRS-364) compiled by IAEA in 1994. The TF-GMs were usually lower than the best estimates (expected values) listed in TRS-364; however, the GMs of TF for La and TF for Th observed for potatoes were slightly higher than the expected values.

Translocation of radiocesium from stems and leaves of plants and the effect on radiocesium concentrations in newly emerged plant tissues

An accident occurred at the Fukushima Dai-ichi Nuclear Power Plant in March 2011 at which time large amounts of radionuclides were released into the atmosphere and the sea. In early May 2011, it was found that newly emerged tea (Camellia sinensis) leaves contained radiocesium, both (134)Cs and (137)Cs in some areas more than 300 km away from the Fukushima plant. To understand the mechanisms of radiocesium transfer to newly emerged tissues (shoots, leaves and fruits) of other plants in the future, radiocesium concentrations in newly emerged leaves of 14 plant species collected from the sampling areas in and near National Institute of Radiological Sciences in Chiba, Japan. The studied plant types were: (1) herbaceous plants, (2) woody plants with no old leaves at the time of the March accident, and (3) woody plants with old leaves out before the accident. About 40-50 d after the start of the accident, newly emerged leaves from woody plant with old leaves tended to show higher values than other woody or herbaceous plants. Concentrations of radiocesium in newly emerged tissues of trees decreased with time, but they did not decrease to the level of herbaceous plants. The type of the plant and presence of old leaves at the time of the heavy deposition period affected the radiocesium concentrations in newly emerged tissues.

Soil-to-Plant Transfer Factors of Stable Elements and Naturally Occurring Radionuclides: (2) Rice Collected in Japan

The critical paths of radionuclides and the critical foods in Japan are different from those in European and North American countries because agricultural products and food customs are different. Consequently, safety assessment in Japan is required to consider rice and vegetables as the critical foods. In this study, we measured soil-to-plant transfer factors (TFs) for rice using naturally existing elements as analogues of radionuclides under equilibrium conditions. Rice and associated soil samples were collected from 50 sampling sites throughout Japan and TFs of 36 and 34 elements for brown rice and white rice, respectively, were calculated on dry weight basis. Probability distributions of TFs of elements for brown rice and white rice were a log normal type. Except for As and Mo, the TFs for brown rice were usually lower than those for wheat and barley especially for K, Fe, Sr (t-test: p < 0:05) and slightly lower than those for Ba, rare earth elements, Th and U. When the TFs of brown rice were compared with those of white rice, TFs of Th and U were higher in white rice, but, for many other elements, the TFs were lower in white rice than those in brown rice. Comparison of the data with previously reported values for Sr and Cs showed that the TFs of stable Sr and Cs were close to the respective values of fallout 90Sr and 137Cs observed in field experiments. Thus, the TFs of naturally existing elements can be used as TFs of those radionuclides which have been in contact with the environment for a long time and have reached equilibrium conditions.

135Cs/137Cs Isotopic Ratio as a New Tracer of Radiocesium Released from the Fukushima Nuclear Accident

Since the Fukushima Daiichi nuclear power plant (FDNPP) accident in 2011, intensive studies of the distribution of released fission products, in particular (134)Cs and (137)Cs, in the environment have been conducted. However, the release sources, that is, the damaged reactors or the spent fuel pools, have not been identified, which resulted in great variation in the estimated amounts of (137)Cs released. Here, we investigated heavily contaminated environmental samples (litter, lichen, and soil) collected from Fukushima forests for the long-lived (135)Cs (half-life of 2 × 10(6) years), which is usually difficult to measure using decay-counting techniques. Using a newly developed triple-quadrupole inductively coupled plasma tandem mass spectrometry method, we analyzed the (135)Cs/(137)Cs isotopic ratio of the FDNPP-released radiocesium in environmental samples. We demonstrated that radiocesium was mainly released from the Unit 2 reactor. Considering the fact that the widely used tracer for the released Fukushima accident-sourced radiocesium in the environment, the (134)Cs/(137)Cs activity ratio, will become unavailable in the near future because of the short half-life of (134)Cs (2.06 years), the (135)Cs/(137)Cs isotopic ratio can be considered as a new tracer for source identification and long-term estimation of the mobility of released radiocesium in the environment.