The ambient gamma dose-rate and the inventory of fission products estimations with the soil samples collected at Canadian embassy in Tokyo during Fukushima nuclear accident
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- Zhang, W., Friese, J. & Ungar, K. J Radioanal Nucl Chem (2013) 296: 69. doi:10.1007/s10967-012-2040-3
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In this study, soil samples were collected at Canadian embassy in Tokyo (about 300 km from Fukushima) on 23 March and 23 May of 2011 for purposes of estimating concentrations of radionuclides in fallout, the total fallout inventory, the depth distribution of radionuclide of interest and the elevated ambient gamma dose-rate at this limited location. Some fission products and actinides were analyzed using gamma-ray spectrometry, alpha spectrometry and liquid scintillation counting. The elevated activity concentration levels of 131I, 132I, 134Cs, 137Cs, 136Cs, 132Te, 129mTe, 129Te, 140Ba and 140La were measured by the gamma-ray spectrometer in the first sample collected on 23 March. Two months after the accident, the 134Cs and 137Cs became only detectable nuclides. A mass relaxation depth of 3.0 g/cm2 was determined by the activities on the depth distribution of 137Cs in a soil core. The total fallout inventory was thus calculated as 225 kBq/m2 on March sampling date and 25 kBq/m2 on May sampling date. The ambient gamma dose-rates in the sampling area estimated by the fallout fission products inventory and 137Cs depth distribution ranged from 184 to 38 nGy/h. There was no detectable americium or plutonium in the soil samples by alpha spectrometry. Although 90Sr or 89Sr were detected supposedly as a result of this accident, it was less than the detection limit, which was about 0.4 Bq/kg in the soil samples.
KeywordsRadionuclide inventory of soilFukushima nuclear accidentAlpha spectrometryGamma spectrometryFission products and actinide
The Fukushima nuclear accident on March 11th, 2011 released large amounts of radioactive material in the environment, which raises great concerns over the ambient gamma dose-rate and isotope concentrations in residential soils. For ensuring the safety of Canadians at abroad, Health Canada deployed two NaI(Tl) detectors to the Canadian embassy in Tokyo. The detectors have been calibrated for ambient dose-rate following the procedures described by Grasty et al. . In the procedures, the calibration coefficients in a particular energy region of the spectrum were experimentally determined using certified standard radionuclide sources with known activities and gamma-ray emission rates. The total ambient gamma dose-rate can then be calculated by integrating the product of the energy deposited in each specific energy region and the calibration coefficient corresponding to that energy region over the spectrum. It is of interest to apply the calibrated detector for an on-site ambient gamma dose-rate measurement within the fission product contaminated landscape. The atmospheric fallout fission products provide an opportunity to establish on-site cross-calibration to further verify these coefficients by the location specifically measured soil inventories of fallout fission products and the depth distributions of 137Cs.
Materials and methods
The soil sampling and gamma spectrometry analysis
In the laboratory, all samples were air dried about 3 days in a fume hood, then crushed and homogenized as well as possible. Evidence suggests that such air-dry procedure cannot result in any significant iodine loss in the sample . The large stones were removed before the samples were weighed (about 150 g) and sealed in a plastic jar for analysis. The jar is cylinder-shaped (60 mm-diam × 55 mm-height) PVC vial. These vials were then counted on the top of a calibrated HPGe detector for the gamma-spectrum acquisition. The HPGe detector used in this study was an Ortec n-type GMX coaxial detector with a crystal diameter of 6.62 cm and a length of 6.90 cm. This detector had a relative efficiency of 25 % with respect to a standard NaI(Tl) detector at 1,332.5 keV of 60Co, and a resolution (FWHM) of 0.85 keV at 122.1 keV peak of 57Co. The detector endcap was made entirely of carbon fiber to meet the high radiation transmission requirement. A semi-empirical full energy peak (FEP) efficiency calibration protocol was developed for this counting geometry to properly quantify radionuclide activity in samples . This model combines both experimental measurements and Monte Carlo (MC) simulations, taking into consideration the soil bulk density and its chemical composition. The mathematical calibration software tool for MC simulation is Virtual Gamma Spectroscopy Laboratory (VGSL) . With the empirically validated detector model and soil sample geometry as inputs, the FEP efficiencies for photons between 30 and 2,000 keV were simulated by VGSL. An IAEA soil reference material (IAEA-375), with known amounts of 40K, 134Cs, 137Cs activity concentration and elemental composition, was selected to test the FEP simulation. In the case of nuclides decay by cascading photons, for example 134Cs, losses by sum coincidences were taken into account. The results obtained for the reference material all fall within the 95 % confidence interval of the certified mean values.
The sample processing and alpha spectrometry and liquid scintillation counting
One gram sub-samples were taken and dissolved with mineral acids. No filtering or sieving was done prior to dissolution. The actinides were separated from the resulting solution by the use of Eichrom resins using 242Pu, 233U and 243Am as chemical yield tracers. The isolated actinides were then electrodeposited and counted for 3 days. Strontium was isolated from the dissolved soil by the use of Eichrom Sr resin. The separation was done three times to remove all other beta activity from the sample. A small fraction of the isolated strontium fraction was taken for chemical yielding by ICP/OES, and the remaining fraction was counted by LSC for 100 min. In all cases, the samples were run in quadruplicate.
Results and discussions
Summary of soil sample gamma spectrometry analysis results
Ambient dose-rate (nGy/h)
Ambient dose-rate (nGy/h)
(7.13 ± 0.11) × 103
1.14 × 102
(6.09 ± 0.10) × 102
(4.57 ± 0.08) × 102
(8.22 ± 0.16) × 101
(4.68 ± 0.07) × 102
(9.28 ± 0.46) × 101
(6.10 ± 0.31) × 101
9.76 × 10−1
(8.24 ± 0.12) × 102
(6.01 ± 0.22) × 102
7.92 × 10−1
(3.81 ± 0.21) × 102
6.15 × 10−1
(4.68 ± 0.84)
7.49 × 10−3
2.83 × 10−3
(1.77 ± 0.07) × 102
(6.45 ± 0.11) × 102
(7.05 ± 0.35) × 102
Total dose rate
Average dose rate by NaI(Tl)
A summary of ambient dose-rate calculated from the May 23rd soil samples is also shown in Table 1. It is clear from the table that the 134Cs and 137Cs became only detectable nuclides in the soil 2 months after the accident. The total ambient dose-rate was reduced by factor of 5 compared to the dose-rate estimated with March 23rd soil sample inventory. The ambient dose-rate due to radiation from radiocesium isotopes to ambient dose-rate is almost at the natural gamma radiation level measured in the Tokyo metropolitan area . The results, listed in Table 1, show that the activity mass concentrations of caesium isotopes in vegetation are eight times higher than those in the soil. The radiation from vegetation in the top soil layer became the major ambient dose-rate contributors (about 83 % of total). After the early phase of direct deposit on soil, some radiocesium migrate downwards the deep layer of the soil. Their activities are only attributable to 17 % of the total ambient dose-rate.
Summary of investigated alpha- and beta-emitters in the soil sample
60 ± 25 % (Activity concentration)
Detected with 5.7 ± 0.8 counts per minute
The results have demonstrated that the significant volatile fission products carried with aerosol were deposited on the ground by wet or dry precipitation within 300 km around Fukushima plant. The actinide data suggests that there was not a measureable amount of nuclear fuel from the Fukushima reactors present in this distance. In addition, the lack of strontium isotopes above detection indicates that there was significant fractionation of the more refractory elements like strontium from the more volatile elements like cesium.