ACTIVITY RATIO OF URANIUM IN FUKUSHIMA SOIL SAMPLES USING MULTI-COLLECTOR INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY

The Fukushima soils have been collected from Namie and Futaba areas for the radiocaesium and uranium isotope ratio studies. The 137Cs activity concentration of soil samples ranged from 6 ± 1 to 756 ± 14 kBq/kg. The uranium isotope ratios are measured using multi collector inductively coupled plasma mass spectrometry. The activity ratio (234U/238U) of the Fukushima soils is calculated from the measured 234U/238U isotope ratio. Activity ratio varied from 0.98 to 1.02 which indicates that 234U and 238U are in secular equilibrium. The 235U/238U atomic ratio of the Fukushima soils did not show any heterogeneity compared with the natural terrestrial ratio even with high level of 137Cs in soils.

Measurement of uranium isotope ratios in Fukushima-accident contaminated soil samples using multi collector inductively coupled plasma mass spectrometry

In the present study, ¹³⁷Cs and ²³⁸U activity concentrations, ²³⁴U/²³⁸U activity ratio, and ²³⁵U/²³⁸U isotope ratio were measured in fifteen soil samples collected from the exclusion zone around the Fukushima Daiichi Nuclear Power Station (FDNPS). The ¹³⁷Cs activity concentrations of Fukushima-accident contaminated soil samples ranged from 29.9 to 4780 kBq kg^-1 with a mean of 2007 kBq kg^-1. On the other hand, the ²³⁸U activity concentrations of these soil samples ranged from 5.2 to 22.4 Bq kg^-1 with a mean of 13.2 Bq kg^-1. The activity ratios of ²³⁴U/²³⁸U ranged from 0.973 to 1.023. The ²³⁵U/²³⁸U isotope ratios of these exclusion zone soil samples varied from 0.007246 to 0.007260, and they were similar to the natural terrestrial ratio confirming the natural origin. Using isotope dilution technique, the ²³⁵U/¹³⁷Cs activity ratio was theoretically estimated for highly ¹³⁷Cs contaminated soil samples from Fukushima exclusion zone ranged from 5.01 × 10^-8 - 6.16 × 10^-7 with a mean value of 2.51 × 10^-7.

Isotopic ratios of uranium and caesium in spherical radioactive caesium-bearing microparticles derived from the Fukushima Dai-ichi Nuclear Power Plant

Spherical radioactive caesium (Cs)-bearing microparticles (CsMPs) were emitted during the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March, 2011. The emission source (timing) and formation process of these particles remain unclear. In this study, the isotopic ratios of uranium (²³⁵U and ²³⁸U) and caesium (¹³³Cs, ¹³⁴Cs, ¹³⁵Cs, and ¹³⁷Cs) isotopes in the five spherical CsMPs (ca. 2 μm in size) sampled at 50 km west of the FDNPP were determined using secondary ion mass spectrometry and laser ablation-ICPMS, respectively. Results showed that the ²³⁵U/²³⁸U ratios of CsMPs were homogeneous (1.93 ± 0.03, N = 4) and close to those estimated for the fuel cores in units 2 and 3, and that the Cs isotopic ratios of CsMP were identical to those of units 2 and 3. These results indicated that U and Cs in the spherical CsMPs originated exclusively from the fuel melt in the reactors. Based on a thorough review of literatures related to the detailed atmospheric releases of radionuclides, the flow of plumes from the FDNPP reactor units during the accident and the U and Cs isotopic ratio results in this study, we hereby suggest that the spherical CsMPs originate only from the fuel in unit 2 on the night of 14 March to the morning of 15 March. The variation range of the analysed ²³⁵U/²³⁸U isotopic ratios for the four spherical particles was extremely narrow. Thus, U may have been homogenised in the source through the formation of fuel melt, which ultimately evaporating and taken into CsMPs in the reactor and was released from the unit 2.

236U and radiocesium in river bank soil and river sediment in Fukushima Prefecture, after the Fukushima Daiichi Nuclear Power Plant accident

Almost 8 years after the Japanese Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, data for ²³⁶U and ²³⁶U/²³⁸U have mainly remained limited to only a few heavily contaminated samples. In the present study, activities of ²³⁶U, ¹³⁴Cs, and ¹³⁷Cs, along with ²³⁴U, ²³⁵U, ²³⁸U, in 15 river bank soil and 10 river sediment samples, were measured by ICP-MS/MS and γ spectrometry. The ¹³⁴Cs activities and ¹³⁴Cs/¹³⁷Cs activity ratios (decay-corrected to March 11, 2011) in these 15 river bank soil samples were from 74.8 to 3.88 × 10⁵ Bq kg^-1 and from 0.944 to 1.02, respectively; and in these 10 river sediment samples were from 87.1 to 1.86 × 10⁵ Bq kg^-1 and from 0.904 to 0.990, respectively. The ²³⁶U activities and ²³⁶U/²³⁸U atom ratios in these soil samples were in the respective ranges of (0.139-17.6) × 10^-5 Bq kg^-1 and (0.259-3.83) × 10^-8; and in these sediment samples were in the respective ranges of (0.884-27.0) × 10^-5 Bq kg^-1 and (1.12-5.04) × 10^-8. For one river sediment core sample, ¹³⁴Cs and ²³⁶U activities decreased with the depth indicating ¹³⁴Cs and ²³⁶U accumulated in the river sediment with time. Unlike ¹³⁴Cs, no clear evidence of FDNPP accident-derived ²³⁶U has been found in this study, although further monitoring is encouraged to establish the background database on ²³⁶U/²³⁸U for its potential application as a tracer in environmental studies.

Isotopic compositions of 236U, 239Pu, and 240Pu in soil contaminated by the Fukushima Daiichi Nuclear Power Plant accident

Six years after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, data for ²³⁶U and ²³⁶U/²³⁸U have remained limited to a few heavily contaminated samples. In the present study, activities of ²³⁶U, ²³⁹Pu, and ²⁴⁰Pu, along with other U isotopes in 46 soil samples both heavily and lightly contaminated by this accident were measured by inductively coupled plasma–mass spectrometry (ICP-MS) and triple-quadrupole ICP-MS. The ²³⁶U activities and ²³⁶U/²³⁸U atom ratios in these soil samples were in the range of (0.469–24.4) × 10⁻⁵ Bq kg⁻¹ and ((0.099–1.35) × 10⁻⁷), respectively. Higher ²⁴⁰Pu/²³⁹Pu atom ratios (0.245–0.312) and ²³⁸Pu/^239+240Pu activity ratios (0.859–1.62) indicated Pu contamination originated from this accident and global fallout in some samples. For those soil samples along with black substances collected along roads in Fukushima Prefecture, high linear correlations were presented between ²³⁶U activities and ^239+240Pu activities (Pearson’s r = 0.755, p < 0.01), and between ²³⁶U activities and ²³⁸Pu activities (Pearson’s r = 0.844, p < 0.01). The analysis of these soil samples confirmed the release of ²³⁶U, although in trace amounts, during the FDNPP accident.

Airborne Plutonium and non-natural Uranium from the Fukushima DNPP found at 120 km distance a few days after reactor hydrogen explosions

Plutonium (Pu) and non-natural uranium (U) originating from the Fukushima Daiichi Nuclear Power Plant (FDNPP) were identified in the atmosphere at 120 km distance from the FDNPP analyzing the ratio of number of atoms, following written as n(isotope)/n(isotope), of Pu and U. The n((240)Pu)/n((239)Pu), n((241)Pu)/n((239)Pu), n((234)U)/n((238)U), n((235)U)/n((238)U) and n((236)U)/n((238)U) in aerosol samples collected before and after the FDNPP incident were analyzed by accelerator mass spectrometry (AMS) and inductively coupled plasma mass spectrometry (ICPMS). The activity concentrations of (137)Cs and (134)Cs in the same samples were also analyzed by gamma spectrometry before the destructive analysis. Comparing the time series of analytical data on Pu and U obtained in this study with previously reported data on Pu, U, and radioactive Cs, we concluded that Pu and non-natural U from the FDNPP were transported in the atmosphere directly over a 120 km distance by aerosol and wind within a few days after the reactor hydrogen explosions. Effective dose of Pu were calculated using the data of Pu: (130 ± 21) nBq/m(3), obtained in this study. We found that the airborne Pu contributes only negligibly to the total dose at the time of the incident. However the analytical results show that the amount of Pu and non-natural U certainly increased in the environment after the incident.

Isotopic compositions of (236)U and Pu isotopes in "black substances" collected from roadsides in Fukushima prefecture: fallout from the Fukushima Dai-ichi nuclear power plant accident

Black-colored road dusts were collected in high-radiation areas in Fukushima Prefecture. Measurement of (236)U and Pu isotopes and (134,137)Cs in samples was performed to confirm whether refractory elements, such as U and Pu, from the fuel core were discharged and to ascertain the extent of fractionation between volatile and refractory elements. The concentrations of (134,137)Cs in all samples were exceptionally high, ranging from 0.43 to 17.7 MBq/kg, respectively. (239+240)Pu was detected at low levels, ranging from 0.15 to 1.14 Bq/kg, and with high (238)Pu/(239+240)Pu activity ratios of 1.64-2.64. (236)U was successfully determined in the range of (0.28 to 6.74) × 10(-4) Bq/kg. The observed activity ratios for (236)U/(239+240)Pu were in reasonable agreement with those calculated for the fuel core inventories, indicating that trace amounts of U from the fuel cores were released together with Pu isotopes but without large fractionation. The quantities of U and (239+240)Pu emitted to the atmosphere were estimated as 3.9 × 10(6) Bq (150 g) and 2.3 × 10(9) Bq (580 mg), respectively. With regard to U, this is the first report to give a quantitative estimation of the amount discharged. Appreciable fractionation between volatile and refractory radionuclides associated with the dispersal/deposition processes with distance from the Fukushima Dai-ichi Nuclear Power Plant was found.