Takumi Kubota

Analysis of cesium isotope compositions in environmental samples by thermal ionization mass spectrometry – 1. A preliminary study for source analysis of radioactive contamination in Fukushima prefecture

Cesium was recovered from plant samples obtained from Fukushima prefecture. The isotopic ratios of 134Cs/137Cs and 135Cs/137Cs were analyzed by thermal ionization mass spectrometry with a single filament method using a TaO activator. Samples containing 5 Bq of 137Cs were analyzed with typical analytical errors of approximately 0.5% for 134Cs/137Cs and approximately 0.1% for 135Cs/137Cs. Measurements of both ratios showed profiles that were characteristic of the measurements of among other environmental samples reported in the literature. The results showed the isotopic ratios of 134Cs/137Cs and 135Cs/137Cs were applicable for the source analysis of radioactive Cs in Fukushima prefecture.

Removal of radioactive cesium, strontium, and iodine from natural waters using bentonite, zeolite, and activated carbon

Cs-134, Sr-85, and I-131 were produced by neutron irradiation of CsCl, SrCl2, and K2TeO3, respectively, using the Kyoto University Reactor. These radioactive nuclides were added to river water and seawater to prepare artificially contaminated samples, and the removal of these nuclides using bentonite, zeolite, and activated carbon was then investigated. In the river water samples, Cs-134 and Sr-85 were successfully removed using bentonite and zeolite, and I-131 was removed using activated carbon. In the seawater samples, Cs-134 was removed using bentonite and zeolite, whereas Sr-85 and I-131 were hardly removed at all by these adsorbents.

Prediction of groundwater contamination with 137Cs and 131I from the Fukushima nuclear accident in the Kanto district.

We measured the concentrations of (131)I, (134)Cs, and (137)Cs released from the Fukushima nuclear accident in soil and rainwater samples collected March 30-31, 2011, in Ibaraki Prefecture, Kanto district, bordering Fukushima Prefecture to the south. Column experiments revealed that all (131)I in rainwater samples was adsorbed onto an anion-exchange resin. However, 30% of (131)I was not retained by the resin after it passed through a soil layer, suggesting that a portion of (131)I became bound to organic matter from the soil. The (137)Cs migration rate was estimated to be approximately 0.6 mm/y in the Kanto area, which indicates that contamination of groundwater by (137)Cs is not likely to occur in rainwater infiltrating into the surface soil after the Fukushima accident.

Modeling of the complex formation of metal ions with humic acids

Summary Based on the conceptual model of cation-humic interactions considering the heterogeneous composition and polyelectrolyte nature of humic acids, the description of the interaction by the expression, log Kapp= log K - m log [M]+ a log α - b log [Na+], is proposed, where Kapp=[M··(zR)]/([M][R]), [M··(zR)] is the concentration of the cation bound to the humic acid, [M] is the concentration of the free cation with charge z, and [R] is the concentration of the free anionic sites of the humic acid, [R]=(CR-z[M··(zR)])α (CR is a total concentration of proton exchanging sites and α is a degree of dissociation). log K, m, a and b are the constant parameters depending on the cation and the humic acid. The expression is successfully applied to the experimental results on the complex formation (or protonation) of humate and polyacrylate (a homogeneous polymeric weak acid) with H+, NpO2+, Eu3+, Ca2+, Fe2+ and Fe3+ obtained under the condition where hydrolysis of cations and saturation of the reacting sites can be neglected.

235U/238U Isotopic ratio in plant samples from Fukushima Prefecture

Abstract Uranium was recovered from plant samples obtained from Fukushima Prefecture, and the 235U/238U isotopic ratio was analyzed using thermal ionization mass spectrometry. The possibility of soil and biosphere contamination with U by the nuclear accident at Fukushima Daiichi nuclear power plant was evaluated. The isotopic ratio of U in the plant samples was the same as the natural abundance, which suggests that no significant contamination with U occurred.

Determination of isotopic ratios of plutonium and uranium in soil samples by thermal ionization mass spectrometry

Isotopic ratios of plutonium and uranium in soil samples were investigated, in order to understand the level of contamination with plutonium and uranium released at the accident of Fukushima Daiichi nuclear power plant. At first, plutonium and uranium were recovered from the standard reference soil/sediment materials by extraction/ion-exchange chromatography, and their isotope compositions were analyzed by TIMS. By applying our analytical technique to soil samples obtained from Fukushima prefecture, a possibility of atmospheric release of nuclear fuel material during the accident of FDNPP was discussed. The contamination with uranium was under detection limit, while that with plutonium was observed.

Analysis of cesium isotope compositions in environmental samples by thermal ionization mass spectrometry-3

ABSTRACT Cesium was recovered from soil samples obtained in Fukushima prefecture. Isotopic analysis of Cs was performed by γ-spectrometry to determine the activity ratio of 134Cs/137Cs and thermal ionization mass spectrometry was used to determine the isotopic ratios of 133Cs/137Cs and 135Cs/137Cs. The analytical results showed that both the activity ratio of 134Cs/137Cs and the isotopic ratio of 135Cs/137Cs were within the expected values for the Fukushima Daiichi Nuclear Power Plant estimated using the ORIGEN-II code, suggesting that most of the radioactive Cs in the soil sample originated from the Fukushima Daiichi Nuclear Power Plant. The concentration of 137Cs and the contribution of radioactive Cs from global fallout were correlated to the distance from the Fukushima Daiichi Nuclear Power Plant, while the contribution of radioactive Cs from each reactor did not show any similar distance dependence.

Application of a CZT detector to in situ environmental radioactivity measurement in the Fukushima area

Instead of conventional Ge semiconductor detectors and NaI(Tl) scintillation spectrometers, an application of a CdZnTe semiconductor (CZT) whose crystal has the dimension of 1 cm cubic to the in situ environmental radioactivity measurement was attempted in deeply affected areas in Fukushima region. Results of deposition density on soil for (134)Cs/(137)Cs obtained seemed consistent, comparing obtained results with those measured by the Japanese government.

Isotopic investigations for the origin of relic from the Matsusaki site, Japan

We measured the environmental radioactivity and isotope ratio, (87)Sr/(86)Sr ratio, (234)U/(238)U, delta(13)C, and (228)Ra/(226)Ra activity ratio, of adhesion that adhered to a wooden tubing vessel obtained from the Matsusaki site, one of the salt-production sites where Moshio is manufactured, in Aichi Prefecture, Japan. The environmental radioactivity and isotope ratio verified that the adhesion originated from the seaweed and/or sea grass.

In Situ Environmental Radioactivity Measurement in High–Dose Rate Areas Using a CdZnTe Semiconductor Detector

For the purpose of determining a surface deposition density on soil for radio-cesiums, a CdZnTe (CZT) semiconductor detector whose crystal has dimensions of 1 cm cubic was applied to the in situ environmental radioactivity measurement in deeply contaminated areas in Fukushima region. Even in high–dose rate areas where pulse height spectra weren’t able to be properly obtained by the conventional high-purity Ge (Hp-Ge) semiconductor detector, proper pulse height spectra were obtained by the CZT detector with certain accuracy. Results of deposition density on soil for 134Cs and 137Cs derived from net peak areas by the CZT detector seemed consistent, comparing with those measured by the Japanese government. Air kerma rates were estimated by the same pulse height spectra for determining surface deposition density on soil for radio-cesiums. Estimated results showed almost the same values as obtained by the NaI(Tl) scintillation survey meter. The results indicate that the CZT detector can be applied to rapid and simple in situ gamma ray radioactivity measurement in higher–dose rate areas whose dose rates exceed several tenth μSv h−1. The study also strongly supports that the CZT detector is one promising candidate for the detector to be used for checking the effect of decontamination works and for long-term monitoring in heavily contaminated areas.