Yuji Shibahara

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.

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.

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.

Application of Mass Spectrometry for Analysis of Cesium and Strontium in Environmental Samples Obtained in Fukushima Prefecture

For the assessment of Fukushima Daiichi Nuclear Power Plant accident, the applicability of the thermal ionization mass spectrometry (TIMS), which is a type of mass spectrometry, was studied. For the study of the recovery/analysis method of cesium and strontium, at first, the radioactive cesium and strontium were generated by the irradiation of natural uranium at KUR. After this study, the applicability of this method to the environmental samples obtained in Fukushima prefecture was verified.

Speciation of 137Cs and 129I in Soil After the Fukushima NPP Accident

We evaluated the migration of radionuclides (131I, 129I, 134Cs, 136Cs, 137Cs, and 132Te) in the surface soil after the Fukushima nuclear accident. The radionuclides in the soil collected late March in 2011 were barely leached with ultrapure water, indicating that these are insoluble. We observed the chemical behavior of 137Cs and 129I in soil: (1) 137Cs was predominantly adsorbed within a depth of 2.5 cm from the ground surface; (2) 137Cs was hardly released from soil by the water leaching experiments that lasted for 270 days; (3) approximately, more than 90 % of 137Cs was adsorbed on organic matters and the residual fractions, while 129I was mainly fixed on the Fe-Mn oxide and organically bounded fraction. Therefore, we conclude that 137Cs and 129I in soil seldom leach into the soil water and migrate downward because of the irreversible adsorption. The shallow groundwater which residence time is short.

Isotopic Ratio of 135Cs/137Cs in Fukushima Environmental Samples Collected in 2011

The isotopic ratios of radioactive cesium derived from the Fukushima accident were determined by γ-spectrometry and thermal ionization mass spectrometry (TIMS). In order to ascertain the initial ratios at the time of the accident, environmental samples collected during 2011 were used for the analysis. Soil, litter, and seaweed were incinerated, and the cesium contained therein was adsorbed into ammonium phosphomolybdate powder. The cesium in the seawater was adsorbed into AMP-PAN resin (Eichrom Technologies, LLC); its recovery ratio was almost one without the carrier being added. Incinerated samples and the AMP-PAN resin were then measured by γ-spectrometry. The cesium solution recovered from adsorbers was subjected to TIMS measurements. The isotopic ratios of 134Cs/137Cs and 135Cs/137Cs were found to be independent of the type of sample in question, as well as the sampling location; the ratios were 0.07 and 0.36 (decay correction: 11 March 2011), respectively, which differ from the results of atmospheric nuclear tests (i.e., 0 and 2.7, respectively). This difference in the ratio of 135Cs/137Cs will contribute to estimations of the origin of radioactive contamination in the future.