Katsumi Shozugawa

Concentration of strontium-90 at selected hot spots in Japan.

This study is dedicated to the environmental monitoring of radionuclides released in the course of the Fukushima nuclear accident. The activity concentrations of β− -emitting 90Sr and β−/γ-emitting 134Cs and 137Cs from several hot spots in Japan were determined in soil and vegetation samples. The 90Sr contamination levels of the samples were relatively low and did not exceed the Bq⋅g−1 range. They were up four orders of magnitude lower than the respective 137Cs levels. This study, therefore, experimentally confirms previous predictions indicating a low release of 90Sr from the Fukushima reactors, due to its low volatility. The radiocesium contamination could be clearly attributed to the Fukushima nuclear accident via its activity ratio fingerprint (134Cs/137Cs). Although the correlation between 90Sr and 137Cs is relatively weak, the data set suggests an intrinsic coexistence of both radionuclides in the contaminations caused by the Fukushima nuclear accident. This observation is of great importance not only for remediation campaigns but also for the current food monitoring campaigns, which currently rely on the assumption that the activity concentrations of β−-emitting 90Sr (which is relatively laborious to determine) is not higher than 10% of the level of γ-emitting 137Cs (which can be measured quickly). This assumption could be confirmed for the samples investigated herein.

Analysis of Japanese Radionuclide Monitoring Data of Food Before and After the Fukushima Nuclear Accident

In an unprecedented food monitoring campaign for radionuclides, the Japanese government took action to secure food safety after the Fukushima nuclear accident (Mar. 11, 2011). In this work we analyze a part of the immense data set, in particular radiocesium contaminations in food from the first year after the accident. Activity concentrations in vegetables peaked immediately after the campaign had commenced, but they decreased quickly, so that by early summer 2011 only a few samples exceeded the regulatory limits. Later, accumulating mushrooms and dried produce led to several exceedances of the limits again. Monitoring of meat started with significant delay, especially outside Fukushima prefecture. After a buildup period, contamination levels of meat peaked by July 2011 (beef). Levels then decreased quickly, but peaked again in September 2011, which was primarily due to boar meat (a known accumulator of radiocesium). Tap water was less contaminated; any restrictions for tap water were canceled by April 1, 2011. Pre-Fukushima 137Cs and 90Sr levels (resulting from atmospheric nuclear explosions) in food were typically lower than 0.5 Bq/kg, whereby meat was typically higher in 137Cs and vegetarian produce was usually higher in 90Sr. The correlation of background radiostrontium and radiocesium indicated that the regulatory assumption after the Fukushima accident of a maximum activity of 90Sr being 10% of the respective 137Cs concentrations may soon be at risk, as the 90Sr/137Cs ratio increases with time. This should be taken into account for the current Japanese food policy as the current regulation will soon underestimate the 90Sr content of Japanese foods.

Plutonium release from Fukushima Daiichi fosters the need for more detailed investigations

The contamination of Japan after the Fukushima accident has been investigated mainly for volatile fission products, but only sparsely for actinides such as plutonium. Only small releases of actinides were estimated in Fukushima. Plutonium is still omnipresent in the environment from previous atmospheric nuclear weapons tests. We investigated soil and plants sampled at different hot spots in Japan, searching for reactor-borne plutonium using its isotopic ratio 240Pu/239Pu. By using accelerator mass spectrometry, we clearly demonstrated the release of Pu from the Fukushima Daiichi power plant: While most samples contained only the radionuclide signature of fallout plutonium, there is at least one vegetation sample whose isotope ratio (0.381 ± 0.046) evidences that the Pu originates from a nuclear reactor (239+240Pu activity concentration 0.49 Bq/kg). Plutonium content and isotope ratios differ considerably even for very close sampling locations, e.g. the soil and the plants growing on it. This strong localization indicates a particulate Pu release, which is of high radiological risk if incorporated.

Post-Accident Sporadic Releases of Airborne Radionuclides from the Fukushima Daiichi Nuclear Power Plant Site

The Fukushima nuclear accident (March 11, 2011) caused the widespread contamination of Japan by direct deposition of airborne radionuclides. Analysis of weekly air filters has revealed sporadic releases of radionuclides long after the Fukushima Daiichi reactors were stabilized. One major discharge was observed in August 2013 in monitoring stations north of the Fukushima Daiichi nuclear power plant (FDNPP). During this event, an air monitoring station in this previously scarcely contaminated area suddenly reported (137)Cs activity levels that were 30-fold above the background. Together with atmospheric dispersion and deposition simulation, radionuclide analysis in soil indicated that debris removal operations conducted on the FDNPP site on August 19, 2013 are likely to be responsible for this late release of radionuclides. One soil sample in the center of the simulated plume exhibited a high (90)Sr contamination (78 ± 8 Bq kg(-1)) as well as a high (90)Sr/(137)Cs ratio (0.04); both phenomena have usually been observed only in very close vicinity around the FDNPP. We estimate that through the resuspension of highly contaminated particles in the course of these earthmoving operations, gross (137)Cs activity of ca. 2.8 × 10(11) Bq has been released.

Reduction process of Cr(VI) by Fe(II) and humic acid analyzed using high time resolution XAFS analysis.

The initial reduction behavior of Cr(VI) to Cr(III) has not been clearly understood due to its rapid reduction reaction. In order to study the reduction process of Cr(VI) in detail, we applied quick X-ray absorption fine structure (QXAFS) analysis to observe how Cr(VI) was reduced to Cr(III) by Fe(II) and humic acid (HA) with time. The Cr(VI) concentration was analyzed every 60s, and the plots of ln(Cr(VI)/Cr(VI)0) versus time were used to evaluate the reduction process based on their linearity. Reduction by Fe(II) showed a linear relation, whereas reduction by HA showed a nonlinear relation. With combined Fe(II) and HA, the linearity was unlike those of Fe(II) and HA individually. The reduction rate was not constant. The structure of Fe(II) produced by HA during the Cr(VI) reduction was investigated by using Mössbauer spectroscopy, which showed that Fe(II) produced by HA reduction of Fe(III) had the same structure as the initial Fe(II). HA can reduce Fe(III) back to Fe(II), and reproduced Fe(II) reduces Cr(VI). For Cr(VI) reduction by combined Fe(II) and HA, each reductant contribute differently: Fe(II) directly contributes to the Cr(VI) reaction, whereas HA reduces both Cr(VI) and Fe(III).

Iron and sulfur speciation in some sedimentary-transformation- type of lead-zinc deposits in West Kunlun lead-zinc ore deposit zone, Northwest China

Redox conditions are very important for almost all geological processes including ore deposit formation. However, the direct measurement of redox potential (Eh) values for solid materials is difficult. The chemical speciation of redox sensitive elements is one factor which determines the redox conditions. In this study, four groups of ore samples and their host rocks were collected from four sites along the west Kunlun lead–zinc ore deposit zone in northwest China and the iron- and sulfur- species analyzed using Mössbauer spectroscopy and X-ray absorption near edge fine structure (XANES). The results showed systematical change in redox conditions between the ore rocks and their associated host rocks. All ore samples contained more reducing sulfur species than their corresponding host-rocks indicating significantly enhanced reducing conditions in the ore bodies than their host rocks. Such reducing conditions should be more favorable for the sulfidization of metal during the ore formation. In addition very small concentrations of ferrous iron were found in only some of the small samples. This may indicate the removal of iron during metal sulfide ore deposit formation.

Detection of Fuel Release in a Nuclear Accident: A Method for Preconcentration and Isolation of Reactor-Borne 239Np Using Ion-Specific Extraction Chromatography

Although actinides are the most informative elements with respect to the nature of a nuclear accident, plutonium analysis is complicated by the background created by fallout from atmospheric nuclear explosions. Therefore, we propose (239)Np, a short-lived actinide that emits several γ rays, as a preferred proxy. The aim of this study was to screen ion specific extraction chromatography resins (RE-, TEVA-, UTEVA-, TRU-, and Actinide-Resin) for the highest possible recovery and separation of trace amounts of (239)Np from samples with large activities of fission products such as radiocesium, radioiodine, and, most importantly, radiotellurium, the latter of which causes spectral interference in gamma spectrometry through overlapping peaks with (239)Np. The investigated environmental media for these separations were aqueous solutions simulating rainwater and soil. Spiked samples containing (239)Np and the aforementioned volatile radionuclides were separated through extraction chromatographic columns to ascertain the most effective means of separating (239)Np from other fission products for detection by gamma spectroscopy. We propose a method for nuclear accident preparedness based on the use of Eichroms RE-Resin. The proposed method was found most effective for isolating (239)Np from interfering radionuclides in both aqueous solution and soil using 8 M HNO3 as the loading solution and H2O as the eluent. The RE-Resin outperforms the more commonly used TEVA-Resin because the TEVA-Resin showed a higher affinity for interfering radiotellurium and radioiodine.

Low-Temperature Clay Mineral Dehydration Contributes to Porewater Dilution in Bering Sea Slope Subseafloor

Widespread diagenesis of clay minerals occurs in deeply buried marine sediments under high-temperature and high-pressure conditions. For example, the smectite-to-illite (S-I) transformation has been often observed in sediments at in situ temperatures above ~60°C. However, it remains largely unknown whether such diagenetic processes naturally occur in relatively shallow and low-temperature sediments and, if so, what the consequences are of any related chemical reactions to the geochemical characteristics in the deep biosphere. We evaluated the possibility of naturally occurring S-I transformation at temperatures below 40°C in continental slope sediments of the Bering Sea by examining porewater chemistry, clay mineralogy, and chemical composition of clay minerals measured to ~800 m beneath the seafloor (mbsf) in core samples acquired during Integrated Ocean Drilling Program Expedition 323. In porewater from these cores, chloride concentrations decreased with increasing depth from 560 mM near the seafloor to 500 mM at ~800 mbsf; δ18O increased from 0‰ to 1.5‰; and δD decreased from –1‰ to –9‰. These trends are consistent with the addition of water derived from S-I transformation. The discrete low Cl– spikes observed between ~200 mbsf and ~450 mbsf could be attributed to the dissociation of methane hydrate. X-ray diffraction analysis of the clay-size fraction (<2 µm) showed an increase of illite content in the I/S mixed layer with increasing depth to 150 mbsf. This increase may imply the occurrence of S-I transformation. The decrease of Fe3+/Fe2+ in the clay-size fraction with increasing depth strongly suggests microbial reduction of Fe(III) in clay minerals with burial, which also has the potential to promote the S-I transformation. Our results imply the significant ecological roles on the diagenesis of siliciclastic clay minerals underlying the high-productivity surface seawater at continental margins.

A study on redox sensitive elements in the sediments at dredged trenches in Tokyo Bay by instrumental neutron activation analysis

To evaluate redox conditions at dredged trenches in Tokyo Bay, the concentrations of U, Th, Ce, Fe, and Mn in sediments were measured. In the Th/U–Ce/U plots, the values of Th/U and Ce/U ratios at each site gathered in different places on a same line. Therefore, it is considered that Th/U and Ce/U ratios are reflecting the redox states in different sites. The concentrations of Fe and Mn in August 2011 decreased near the surface at dredged trenches. It is presumed that their concentrations at dredged trenches are influenced by the redox conditions of the surface of sediments.

Oxidation of antimony (III) in soil by manganese (IV) oxide using X-ray absorption fine structure

The oxidation of antimony (III) in soils was studied using X-ray absorption fine structure (XAFS) spectra. An andosol soil sample and artificial soil samples (SiO2 blended with iron (III) hydroxide and manganese (IV) oxide) were used herein. After adding antimony (III) oxide to all soil samples, the oxidation process was observed by recording the XAFS spectra of Sb K-edge, Fe K-edge, and Mn K-edge. The results indicated that manganese (IV) oxide played an important role in the oxidation of Sb(III); however iron (III) hydroxide was not directly related to the reaction. During a 2-hr continuous Sb K-edge X-ray absorption near edge structure (XANES) measurement with an interval of 1min of one of the artificial soil samples (SiO2+MnO2+Sb2O3), a pseudo-first-order reaction was determined with an average estimated rate of 0.52±0.04hr-1. Compared to the lower oxidation rate of andosol, it is suggested that because of the low concentration of Mn(IV) in natural soils, the oxidation process of Sb(III) might be relatively slow and require more time to convert Sb(III) to Sb(V).