Geographical distribution of plutonium derived from the atomic bomb in the eastern area of Nagasaki

Assessment of soil-soil solution distribution coefficients of global fallout 237Np and 239Pu in Japanese upland soils

Neptunium-237 and ²³⁹Pu are important radionuclides in the safety assessment related to geological disposal of radioactive waste because of the possibility of long-term exposure to humans. Mobilities of these radionuclides in the environment are of particular importance for their radiation dose evaluation; therefore, in this study, we have made the assessment of the soil-soil solution distribution coefficient (K_d, L/kg) using global fallout ²³⁷Np and ²³⁹Pu in Japanese upland soils. The K_d values were determined by extracting these radionuclides from 24 soil samples using a laboratory batch method. The desorption K_d values of ²³⁷Np ranged from 3.3 × 10² to 1.0 × 10⁴ L/kg, and their geometric mean (GM) and arithmetic mean (AM) were 1.7 × 10³ L/kg and 2.6 × 10³ L/kg, respectively. The desorption K_d values of ²³⁹Pu were found to vary from 9.4 × 10³ to 7.1 × 10⁴ L/kg, and their GM and AM were 3.3 × 10⁴ L/kg and 4.0 × 10⁴ L/kg, respectively. In Japanese upland soils, the K_d value of ²³⁹Pu was one order of magnitude higher than that of ²³⁷Np.

Soil-soil solution distribution coefficients of global fallout 239Pu and 237Np in Japanese paddy soils

²³⁹Pu and ²³⁷Np are long-lived radionuclides that emit alpha rays, and once released into the environment, they are present there for a long period of time. Therefore,²³⁹Pu and ²³⁷Np are important radionuclides in the safety assessment related to geological disposal of radioactive waste because of the possibility of long-term exposure to humans. Mobilities of these radionuclides in the environment are of particular interest; therefore, in this study, we have made the first-time determination of the soil-soil solution distribution coefficient (K_d, L/kg) using global fallout ²³⁹Pu and ²³⁷Np in soil. The K_d values were determined by extracting these radionuclides from 23 soil samples using a laboratory batch method. The desorption K_d values of ²³⁹Pu were found to vary from 3.2 × 10³ to 1.4 × 10⁵ L/kg, and their geometric mean (GM) and arithmetic mean (AM) were 2.3 × 10⁴ L/kg and 3.2 × 10⁴ L/kg, respectively. The desorption K_d values of ²³⁷Np ranged from 8.9 × 10² to 2.1 × 10⁴ L/kg, and their GM and AM were 4.1 × 10³ L/kg and 6.4 × 10³ L/kg, respectively. A comparison between the obtained K_d values of ²³⁹Pu and ²³⁷Np indicated that the former were about 6-fold higher than the latter in Japanese paddy field soils. Pearson's correlation analysis suggested that the main factors contributing to the sorption and desorption of Pu isotopes in Japanese paddy soils were related to the amounts of Fe and Mn oxides, while the sorption and desorption of ²³⁷Np in them would be controlled by an ion-exchange reaction, and/or complex-formation with organic matter, especially in Fluvisols.

Spatiotemporal distribution and fluctuation of radiocesium in Tokyo Bay in the five years following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident

A monitoring survey was conducted from August 2011 to July 2016 of the spatiotemporal distribution in the 400 km² area of the northern part of Tokyo Bay and in rivers flowing into it of radiocesium released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. The average inventory in the river mouth (10 km²) was 131 kBq⋅m^-2 and 0.73 kBq⋅m^-2 in the central bay (330 km²) as the decay corrected value on March 16, 2011. Most of the radiocesium that flowed into Tokyo Bay originated in the northeastern section of the Tokyo metropolitan area, where the highest precipitation zone of ¹³⁷Cs in soil was almost the same level as that in Fukushima City, then flowed into and was deposited in the Old-Edogawa River estuary, deep in Tokyo Bay. The highest precipitation of radiocesium measured in the high contaminated zone was 460 kBq⋅m^-2. The inventory in sediment off the estuary of Old-Edogawa was 20.1 kBq⋅m^-2 in August 2011 immediately after the accident, but it increased to 104 kBq⋅m^-2 in July 2016. However, the radiocesium diffused minimally in sediments in the central area of Tokyo Bay in the five years following the FDNPP accident. The flux of radiocesium off the estuary decreased slightly immediately after the accident and conformed almost exactly to the values predicted based on its radioactive decay. Contrarily, the inventory of radiocesium in the sediment has increased. It was estimated that of the 8.33 TBq precipitated from the atmosphere in the catchment regions of the rivers Edogawa and Old-Edogawa, 1.31 TBq migrated through rivers and was deposited in the sediments of the Old-Edogawa estuary by July 2016. Currently, 0.25 TBq⋅yr^-1 of radiocesium continues to flow into the deep parts of Tokyo Bay.

Plutonium concentration and isotopic ratio in soil samples from central-eastern Japan collected around the 1970s

Obtaining Pu background data in the environment is essential for contamination source identification and assessment of environmental impact of Pu released from the Fukushima Daiichi nuclear power plant (FDNPP) accident. However, no baseline information on Pu isotopes in Fukushima Prefecture has been reported. Here we analyzed 80 surface soil samples collected from the central-eastern Japan during 1969–1977 for ^239+240Pu activity concentration and ²⁴⁰Pu/²³⁹Pu atom ratio to establish the baseline before the FDNPP accident. We found that ^239+240Pu activity concentrations ranged from 0.004 –1.46 mBq g⁻¹, and ²⁴⁰Pu/²³⁹Pu atom ratios varied narrowly from 0.148 to 0.229 with a mean of 0.186 ± 0.015. We also reconstructed the surface deposition density of ²⁴¹Pu using the ²⁴¹Pu/²³⁹Pu atom ratio in the Japanese fallout reference material. The obtained results indicated that, for the FDNPP-accident released ²⁴¹Pu, a similar radiation impact can be estimated as was seen for the global fallout deposited ²⁴¹Pu in the last decades.

Depositional records of plutonium and (137)Cs released from Nagasaki atomic bomb in sediment of Nishiyama reservoir at Nagasaki

In a sediment core of Nishiyama reservoir at Nagasaki city, depth profiles of (240)Pu/(239)Pu isotopic ratio, (239+240)Pu and (137)Cs activities were determined. Sediments containing plutonium and (137)Cs, which were deposited immediately after a detonation of Nagasaki atomic bomb, were identified in the core. Observed below the sediments were macroscopic charcoals, providing evidence for initial deposit of the fallout of the Nagasaki atomic bomb. This is the first entire depositional records of plutonium and (137)Cs released from the Nagasaki atomic bomb together with those from atmospheric nuclear tests.