Release of plutonium isotopes into the environment from the Fukushima Daiichi Nuclear Power Plant accident: what is known and what needs to be known

Plutonium in the coastal seawater around Chinese nuclear power plants: Sources, distribution, and environmental implications

Coastal surface seawater samples within 30 km around ten Chinese nuclear power plants (NPPs) were systematically investigated. The 239+240Pu activity concentration in the samples varied from 0.226 mBq/m3 to 3.098 mBq/m3, meanwhile the 240Pu/239Pu atom ratios ranged from 0.151 to 0.353. Besides, the Pacific Proving Ground (PPG) close-in fallout and the global fallout were recognized as two primary sources of Pu in these samples. The 239+240Pu activity concentration as well as the PPG contribution showed similar trends as the Kuroshio intrusion path and the coastal currents in the China Seas, illustrating long-range transport and consuming of PPG derived Pu in the coastal China Seas. Moreover, accumulation of PPG sourced Pu in the Beibu Gulf were observed and was attributed to the continuous invasion of the high isotopic Pu that remobilized from the South China Sea (SCS).

Fukushima Daiichi Nuclear Power Plant Accident: Understanding Formation Mechanism of Radioactive Particles through Sr and Pu Quantities

The Fukushima Daiichi Nuclear Power Plant accident released considerable radionuclides into the environment. Radioactive particles, composed mainly of SiO2, emerged as distinctive features, revealing insights into the accident's dynamics. While studies extensively focused on high-volatile radionuclides like Cs, investigations into low-volatile nuclides such as 90Sr and Pu remain limited. Understanding their abundance in radioactive particles is crucial for deciphering the accident's details, including reactor temperatures and injection processes. Here, we aimed to determine 90Sr and Pu amounts in radioactive particles and provide essential data for understanding the formation processes and conditions within the reactor during the accident. We employed radiochemical analysis on nine radioactive particles and determined the amounts of 90Sr and Pu in these particles. 90Sr and Pu quantification in radioactive particles showed that the 90Sr/137Cs radioactivity ratio (corrected to March 11, 2011) aligned with core temperature expectations. However, the 239+240Pu/137Cs activity ratio indicated nonvolatile Pu introduction, possibly through fuel fragments. Analyzing 90Sr and Pu enhances our understanding of the Fukushima Daiichi accident. Deviations in 239+240Pu/137Cs activity ratios underscore nonvolatile processes, emphasizing the accident's complexity. Future research should expand this data set for a more comprehensive understanding of the accident's nuances.

Level, distribution and sources of Np, Pu and Am isotopes in Peter the Great Bay of Japan sea

Transuranium elements such as Np, Pu and Am, are considered to be the most important radioactive elements in view of their biological toxicity and environmental impact. Concentrations of 237Np, Pu isotopes and 241Am in two sediment cores collected from Peter the Great Bay of Japan Sea were determined using radiochemical separation combined with inductively coupled plasma mass spectrometry (ICP-MS) measurement. The 239,240Pu and 241Am concentrations in all sediment samples range from 0.01 Bq/kg to 2.02 Bq/kg and from 0.01 Bq/kg to 1.11 Bq/kg, respectively, which are comparable to reported values in the investigated area. The average atomic ratios of 240Pu/239Pu (0.20 ± 0.02 and 0.21 ± 0.01) and 241Am/239+240Pu activity ratios (3.32 ± 2.76 and 0.45 ± 0.17) in the two sediment cores indicated that the sources of Pu and Am in this area are global fallout and the Pacific Proving Grounds through the movement of prevailing ocean currents, and no measurable release of Np, Pu and Am from the local K-431 nuclear submarine incident was observed. The extremely low 237Np/239Pu atomic ratios ((2.0-2.5) × 10-4) in this area are mainly attributed to the discrepancy of their different chemical behaviors in the ocean due to the relatively higher solubility of 237Np compared to particle active plutonium isotopes. It was estimated using two end members model that 23% ± 6% of transuranium radionuclides originated from the Pacific Proving Grounds tests, and the rest (ca. 77%) from global fallout.

Spatial distribution and modelling of 239+240Pu in the sediments and seawater columns of the South China Sea and Indian Ocean

In order to investigate the 239+240Pu potential influence in the ocean, and develop a new method for rapidly monitoring radioactive pollution, the 239+240Pu spatial distribution in the South China Sea (SCS) and the Indian Ocean (IND) sediments is analyzed by SF-ICP-MS (ELEMENT 2). The inventory-weighted mean activities of 239+240Pu were 0.413 ± 0.333 mBq/g, 0.128 ± 0.044 mBq/g, and 0.483 ± 0.606 mBq/g in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 239+240Pu activity spatial distribution in the SCS sediments was influenced by the current, the vertical distribution of Pu in seawater, and the transport of particulate matter. The 239+240Pu activity spatial distribution in the IND sediments could be impacted by Antarctic Intermediate Water. The average of 240Pu/239Pu atomic ratios were 0.258 ± 0.034, 0.219 ± 0.031, and 0.212 ± 0.028 in the sediments of the SCS, eastern IND, and Arabian Sea, respectively. The 240Pu/239Pu atomic ratios in the SCS and IND indicate that Pu from the Pacific Proving Ground (PPG) is transported to the IND via the SCS internal current and transverse ocean currents within Indonesia. In addition, a seawater advection-dispersion equation (S-ADE) model is established based on the actual physical processes of radionuclides in the seawater column and well fitting results were obtained (R2 = 0.49 to 0.99). The 239+240Pu data and the geographic information from the sample site were used to correct the Pu distribution in the seawater. The calculated 239+240Pu mean concentrations in the surface seawater were 2.465 mBq/m3 and 2.205 mBq/m3 for the SCS and the eastern IND seawater, respectively, and the result is consistent with the previous measurements. Then, the 239+240Pu stored in the study area of SCS and eastern IND was estimated to be approximately 1.0-1.4% of the global ocean based on the model. This study provides a useful model for guiding and designing future monitoring of pollution by anthropogenic Pu and other isotopes.

Spatial distribution, source identification, and transportation paths of plutonium in the Beibu Gulf, South China Sea

To investigate the spatial distribution and source of plutonium isotopes in the Beibu Gulf, surface sediments were collected and analyzed using sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). The activities of 239+240Pu in surface sediments ranged from 0.012 to 0.451 mBq/g (mean: 0.171 ± 0.138 mBq/g, n = 36), indicating a decreasing trend in a counterclockwise direction from the southern bay mouth. The counterclockwise decreasing trend in the south of the bay mouth is similar to the current in the Beibu Gulf. The 240Pu/239Pu atom ratios in surface sediments ranged from 0.156 to 0.283 (mean: 0.236 ± 0.031, n = 36), slightly higher than that of the global fallout value of 0.18. This suggests that the Pu in the Beibu Gulf was a combination of global fallout and Pacific Proving Ground (PPG). The average contribution of the plutonium (Pu) derived from the PPG in the sediment was estimated to be 52 % ± 24 %.

A novel strategy for Pu determination in water samples by automated separation in combination with direct ICP-MS/MS measurement

Methods for Pu determination in water samples has been longtime studied but they generally involved tedious manual operations. In this context, we proposed a novel strategy for accurate determination of ultra-trace Pu in water samples by the combination of fully automated separation with direct ICP-MS/MS measurement. A recently commercialized extraction resin TK200 was used for single-column separation due to its distinctive nature. Acidified waters up to 1 L were directly loaded to the resin at high flow rate (15 mL min^-1) with omitting the frequently used co-precipitation process. Small volumes of dilute HNO₃ were used for column washing, and Pu was efficiently eluted within only 2 mL 0.5 mol L^-1 HCl-0.1 mol L^-1 HF with a stable recovery (65%). This separation procedure was fully automated under the control of user program, meanwhile the final eluent was compatible for direct ICP-MS/MS measurement without extra sample treatment. In that way, both the labor intensity and reagent consumption were minimized compared with existing methods. With the high decontamination (10⁴ to 10⁵) of U in the chemical separation and the further elimination of uranium hydrides under oxygen reaction model during ICP-MS/MS measurement, the overall interference yields of UH⁺/U⁺ and UH₂⁺/U⁺ were down to 10^-15. The limits of detection (LODs) of this method reached 0.32 μBq L^-1 for ²³⁹Pu and 2.00 μBq L^-1 for ²⁴⁰Pu, which were much lower than those stipulated in the general guidelines for drinking water standards, suggesting this method was promising in routine or emergency radiation monitoring. Furthermore, the established method was successfully applied in a pilot study to determine global fallout derived Pu in surface glacier samples with extremely low concentrations of ^239+240Pu, which suggested the method would also be feasible in glacial chronology studies in the future.

Radioactivity research in mosses from typical Karst Regions in Leye Tiankeng, Southern China

Radionuclides in environmental ecosystems have ecotoxicity and health impact on human and environment, so radioactive contamination has always been one of the global concerns. This study mainly focused on the radioactivity of mosses collected from the Leye Tiankeng Group in Guangxi. The activities of ^239+240Pu measured by SF-ICP-MS and ¹³⁷Cs measured by HPGe in moss and soil samples are as follows: 0-2.29 Bq/kg in mosses and 1.5-11.9 Bq/kg in soils for ¹³⁷Cs, and 0.025-0.25Bq/kg in mosses and 0.07-0.51Bq/kg in soils for ^239+240Pu. The range of ²⁴⁰Pu/²³⁹Pu atom ratios (0.201 in mosses and 0.184 in soils) and ^239+240Pu/¹³⁷Cs activity ratios (0.128 in mosses and 0.044 in soils) indicated that the ¹³⁷Cs and ^239+240Pu in study area were mainly contributed by global fallout. ¹³⁷Cs and ^239+240Pu showed similar distribution in soils. However, their behaviors in mosses were quite different due to the differences in the growth environment of mosses. The transfer factors of ¹³⁷Cs and ^239+240Pu from soil to moss varied in different growth stages and specific environments. A weak positive correlation among ¹³⁷Cs, ^239+240Pu in mosses and soil-derived radionuclides suggested that resettlement was predominant here. The negative correlation between ⁷Be, ²¹⁰Pb and soil-derived radionuclides indicated that ⁷Be and ²¹⁰Pb came from atmospheric components, while the weak correlation between them suggested that their specific sources were different. The Cu and Ni were moderately enriched in mosses here due to the use of agricultural fertilizers, At the same time, Zn was at a high level in the Lilang area, where transportation was more developed.

Reconstructing the chronology of the natural and anthropogenic uranium isotopic signals in a marin sediment core from beppu bay, Japan

The long-lived U isotopes, ²³³U and ²³⁶U, have been used increasingly in recent years as marine circulation tracers and for identifying sources of uranium contamination in the environment. The sedimentation histories of these two U isotopes in combination with natural ²³⁸U were reconstructed for an anoxic sediment core collected from Beppu Bay, Japan, in the western North Pacific Ocean showing good time resolution (less than 2.6 y/sample). The ²³³U/²³⁶U atom ratio showed a prominent peak of (3.20 ± 0.30) × 10^-2 around 1957 which can be attributed to the input from atmospheric nuclear weapons tests including thermonuclear tests conducting in the Equatorial Pacific. The integrated ²³³U/²³⁶U ratio of (1.64 ± 0.08) × 10^-2 for the sediment was found to be in relatively good agreement with the representative ratio published for global fallout (∼1.4 × 10^-2). A prominent increase in the authigenic ratio of ²³³U/²³⁸U_a,s in the leached fraction (1.39 ± 0.11 × 10^-11) and the bulk digestion (1.36 ± 0.10 × 10^-11) was also observed around 1957. This reflects the input supply of ²³³U to the seawater which is known to have a relatively constant ²³⁸U content. The authigenic ²³⁶U/²³⁸U_a,s ratio (0.18 ± 0.02 × 10^-9) obtained for 1921 increased from the early 1950's to a maximum of (6.59 ± 0.60) × 10^-9 around 1962. The variation in this ratio represents well the introduction history of U into the surface environment without site-specific U contamination and the time profile is also consistent with the ¹³⁷Cs signature. This work thus provides a benchmark for the long-term use of the isotopic U composition as an input parameter for seawater circulation tracers and as a chronological marker for anoxic sediments and sedimentary rocks. Especially the ²³³U/²³⁶U ratio may serve as a key-marker for the new geological age Anthropocene.

Temporal evolution of plutonium concentrations and isotopic ratios in the Ukedo - Takase Rivers draining the Difficult-To-Return zone in Fukushima, Japan (2013-2020)

In 2011, the Fukushima Dai-Ichi Nuclear Power Plant (FDNPP) accident released significant quantities of radionuclides into the environment. Japanese authorities decided to progressively reopen the Difficult-To-Return Zone after the decontamination of priority reconstruction zones. These areas include parts of the initially highly contaminated municipalities located to the north of the FDNPP, including Namie Town, an area drained by the Ukedo and Takase Rivers. Eleven years after the accident, research focused on the spatial distribution of plutonium (Pu) and radiocesium (Cs) isotopes at contrasted individual locations. To complement previous results, the current research was conducted on flood sediment deposits collected at the same locations after major flooding events during eleven fieldwork campaigns organised between 2013 and 2020 at the outlet of the Ukedo and Takase Rivers (n = 22). The results highlighted a global decrease of the Pu and ¹³⁷Cs contents in sediment with time during the abandonment phase in the region, from 2013 (238.20 fg g^-1) to 2020 (4.28 fg g^-1). Furthermore, based on the analysis of the ²⁴⁰Pu/²³⁹Pu isotopic ratios, the plutonium transiting these rivers (range: 0.166 - 0.220) essentially originated from the global fallout (0.180 ± 0.014 (Kelley et al., 1999)). Sediment showed contrasted properties in the two investigated rivers, which is likely mainly the result of the occurrence of Ogaki Dam on upper sections of the Ukedo River as it strongly impacts the material supply from this river to the Pacific Ocean. A statistical analysis highlighted the strong correlation between Pu activity concentrations and ¹³⁷Cs activities in both rivers, confirming that both radionuclides are transported with a similar pathway. Despite it was detected early after the accident (2011-2013), the current research demonstrates that plutonium originating from FDNPP is no longer detected in these rivers draining the Difficult-To-Return Zone at the onset of the reopening of the area to its former inhabitants.

Short review of plutonium applications for the sediment transport studies

The transport of sediments in surface waters is a natural process involving the relocation of the sediments themselves and the nutrients, pollutants and radionuclides associated with them. Plutonium isotopes have proven to be a useful tool for studying this process over several decades. In this article, we review the characteristics of the behaviour of plutonium in the water column and the main directions of its use to study the transport of sediments at different scales. The characteristic isotopic fingerprints of the sources and their known input functions, as well as the good reactivity of the particles, favour the widespread use of plutonium for the study of the chronology of sediments and deposits and for studies of the fate and migration pathways of sediments at different scales. While other radionuclides are losing relevance due to the short half-life the ²³⁹Pu and ²⁴⁰Pu will remain useable for a long time.

Distribution and migration of 239,240Pu in soil profiles in North China

The source, vertical distribution and migration behavior of plutonium in five soil profiles (from forest, grassland and desert areas) in northern China were investigated. The average ²⁴⁰Pu/²³⁹Pu atomic ratio of 0.184 ± 0.022 observed in these samples is in good agreement with the reported value of global fallout, suggesting that the global fallout is the major source of plutonium in northern China. The ^239,240Pu inventories in five soil profiles ranges from 43.3 Bq/m² to 175 Bq/m², lying in the reported range for global fallout in the similar latitude band. The effective convection velocity (0.04-0.16 cm/y) and effective dispersion coefficient (0.13-0.41cm²/y) of plutonium in different soil profiles derived using the CDE model varies significantly, attributed to multi-factors including location, topography, climate and soil types. The results showed that the migration of plutonium in grassland soil is significantly slow compared to other type of soil, especially desert soil.

An overview of plutonium isotopes in soils, China: Distribution, spatial patterns, and sources

Plutonium (Pu) is an anthropogenic radionuclide which has drawn significant attentions due to its radiotoxicity, and the sources of plutonium linked with nuclear accidents and contaminations. The ²⁴⁰Pu/²³⁹Pu atom ratio is source dependent and can be used as a fingerprint to determine the sources of radioactive contaminant. However, the distribution and sources of plutonium in soils of China have not yet been systematically studied at a national scale up to date. The distribution, spatial patterns, and sources of plutonium in soils of China were discussed in this work. The concentrations of ^239,240Pu are in the range of 0.002-4.824 mBq/g with a large variation, and the ^239,240Pu concentrations in surface soils increase with the increasing latitude, which affects by multi-factors such as organic matter and particle size, etc. The inventories of ^239,240Pu are in the range of 7.31-554 Bq/m². The weighted average of ²⁴⁰Pu/²³⁹Pu atom ratios (0.180 ± 0.004) in all surface samples is good agreement with the ratio of global fallout (0.180 ± 0.014) of the nuclear weapons tests, this indicate that the major source of plutonium in China is global fallout. However, among some sites, distinctly lower ²⁴⁰Pu/²³⁹Pu atom ratio compared to the global fallout values were observed in the northwest China, indicating a significant contribution from other source besides the global fallout. Furthermore, the spatial clustering patterns of hot spots (high values) and cold spots (low values) for plutonium showing the clear associations with nuclear tests, especially the Chinese Lop Nor nuclear weapons tests (CNTs) and the Semipalatinsk nuclear weapons tests (STS). Radioactive material including plutonium from the STS or CNTs was transported by the prevailing westerlies to the northwest China. This review about the fingerprints and distribution of plutonium in soils of China will help researchers to establish a reference database for future radiation risk assessment and environmental radioactive management.

Distributions of fallout 137Cs, 239+240Pu and 241Am in a soil core from South Central China

The source and vertical distribution of ¹³⁷Cs,^239+240Pu and ²⁴¹Am activity concentrations in a soil core from Hunan Province, China were investigated. The maximum ¹³⁷Cs and ^239+240Pu activity concentrations were 15.45 ± 0.76 mBq/g and 0.819 ± 0.066 mBq/g, respectively. While the maximum ²⁴¹Am activity concentration in samples obtained from the core was 0.341 ± 0.019 mBq/g. The ²⁴⁰Pu/²³⁹Pu atom ratio and the ¹³⁷Cs/^239+240Pu activity ratio were 0.183 ± 0.011 and 19.5 ± 1.8, respectively, and both were consistent with the characteristic value of global fallout. The integrated ²⁴¹Am/^239+240Pu activity ratio for global fallout was also re-estimated. The measured ²⁴¹Am/^239+240Pu activity ratio (average 0.43 ± 0.07) in the samples was very close to the estimated value (0.45), which suggested their ²⁴¹Am also came from the global fallout. Regarding the vertical distribution of ¹³⁷Cs, ^239+240Pu and ²⁴¹Am in these red soil samples, all these radionuclides had higher concentrations in upper layers of several centimeters of soil while they had slightly lower concentrations in lower soil layers down to 30 cm. Vertical distributions of ¹³⁷Cs/^239+240Pu and ²⁴¹Am/^239+240Pu activity ratios indicated the migration velocity was Am ≈ Pu > Cs. The intrinsic chemical properties of the radionuclides as well as soil type and properties (acidic, nutrient-deficient and low in organic matter and cation exchange capacity) might be reasons for the differences in their migration behaviors.

Distributions and impacts of plutonium in the environment originating from the Fukushima Daiichi Nuclear Power Plant accident: An overview of a decade of studies

This paper reviews the current knowledge on plutonium (Pu) isotopic composition (the atom or activity ratios) and activity concentrations of ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu resulting from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011. In this critical review, we document the characteristic values of Pu atom or activity ratios (fingerprints) and present their spatial distributions around the FDNPP site. Based on multiple Pu fingerprints (²³⁸Pu/^239+240Pu activity ratio, ²⁴⁰Pu/²³⁹Pu atom ratio, and ²⁴¹Pu/²³⁹Pu atom ratio), we clarify that Pu contamination from the FDNPP accident occurred in a restricted terrestrial area, while Pu in the Northwest Pacific Ocean is still predominately sourced from the Pacific Proving Grounds (PPG) and global fallout. Using a simple two end-member mixing model, we calculate average contributions of Pu from the FDNPP accident of 13 ± 20% (n = 180) in soil samples, 55 ± 32% (n = 38) in leaf litter samples, and 67 ± 26% (n = 129) in air dust/black substances. In the marine environment, the PPG source average contributions are 45 ± 15% (n = 76) in seawater and 42 ± 12% (n = 48) in sediments. The spatial distributions of Pu atom or activity ratios based on existing studies suggest that: 1) in the terrestrial region investigated 80 km northwest of the FDNPP site, the Pu contamination is mainly observed in an area within a 50 km distance, and 2) in the terrestrial region investigated 60 km southwest of the FDNPP site, the Pu contamination is mainly observed in an area within a 30 km distance. Studies of Cs-bearing radioactive particles indicate that Pu occurs as Pu oxide, and the fuel fragments containing Pu that were released from the reactors to the surrounding environment are associated with micron-scale Cs-bearing radioactive particles. We note that the fractionation between Pu and other radionuclides occurred after release. These new findings about the Pu fingerprints around the FDNPP site will help researchers to establish a reference background database for future environmental risk assessment and geochemical study there.

Analysis of particles containing alpha-emitters in stagnant water at torus room of Fukushima Dai-ichi Nuclear Power Station’s Unit 2 reactor

Particles containing alpha (α) nuclides were identified from sediment in stagnant water in the torus room of the Fukushima Dai-ichi Nuclear Power Station(FDiNPS)’s Unit 2 reactor. We analyzed uranium (U), which is the main component of nuclear fuel, using scanning electron microscopy (SEM). Other α-nuclides (plutonium [Pu], americium [Am], and curium [Cm]) were detected by alpha track detection and the morphology of particles with α-nuclides were analyzed by SEM-energy dispersive X-Ray (EDX) analysis. Several uranium-bearing particles ranging from sub-µm to several µm in size were identified by SEM observation. These particles contained zirconium (Zr) and other elements which constituted fuel cladding and structural materials. The 235U/238U isotope ratio in the solid fractions that included U particles was consistent with what was found for the nuclear fuel in the Unit 2 reactor. This indicated that the U of similar fuel composition had made finer. The α-nuclide-containing particles identified by alpha track analysis were several tens to several hundred µm in size. The EDX spectra showed that these particles mainly comprised iron (Fe). Since the amount of α-nuclide material was very small, Pu, Am, and Cm were adsorbed on the Fe particles. This study clarifies that the major morphologies of U and other α-nuclides in the sediment of stagnant water in the torus room of FDiNPS’s Unit 2 reactor differed.

Distribution of plutonium isotopes in soils between two nuclear test sites: Semipalatinsk and Lop Nor

Plutonium (Pu) has attracted attention as an environmental tracer due to its radiotoxicity and the possibility of sources linked with nuclear accidents in recent years. Plutonium isotopes (^239,240Pu) were detected at trace levels in soils collected from the Xinjiang region located between the Semipalatinsk nuclear test site and China's Lop Nor nuclear test site. Little is known regarding the spatial variation of ^239,240Pu in soils from this region. This study reports the use of Sector Field Inductively Coupled Plasma Mass Spectrometry (SF-ICP-MS) methods to distinguish between Pu isotopes derived from global fallout and nuclear weapon tests. We found that the ^239,240Pu activity concentrations ranged from 0.035 to 1.338 mBq/g; the ²⁴⁰Pu/²³⁹Pu atomic ratios were 0.157-0.223 with a weighted average of 0.180 ± 0.002, corresponding with the expected average global fallout ratio of 0.180 ± 0.014. This indicated that global fallout is the major source of Pu in the study region. The ^239,240Pu inventories in these soils ranged from 23.67 to 222.7 Bq/m², corresponding with those from other areas in China and other countries within the latitude range. Our Pu isotope data was supplemented with other published Pu data for soils collected in the vicinity of the Semipalatinsk nuclear test site and Lop Nor nuclear test site. Results indicate that ^239,240Pu inventories and ²⁴⁰Pu/²³⁹Pu atomic ratios in soils exhibit large variations with distance from the Semipalatinsk nuclear test site. High deposition and accumulation of Pu, and low ²⁴⁰Pu/²³⁹Pu ratios were observed in close-in fallout and downwind regions of the Semipalatinsk nuclear test site and China's Lop Nor nuclear test site.

Isotopic signatures of plutonium and uranium at Bikar atoll, northern Marshall Islands

We present plutonium (Pu) and uranium (U) isotopic fingerprints (or signatures) in environmental samples collected at Bikar Atoll. Bikar is the second -most northern atoll of the Republic of the Marshall Islands, and therefore an important reference point to evaluate the extension of the regional fallout from the Pacific Proving Grounds (PPG) in Bikini and Enewetak Atolls. Previous studies have shown that regional fallout from atmospheric nuclear weapon testing (NWT) in Bikini and Enewetak has resulted in elevated levels of fallout radionuclides in this atoll. In order to optimally interpret the isotopic fingerprints, we compare our results with data obtained in eleven certified reference materials, representing different contamination sources. As well as ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Pu, ²³⁸U and ²³⁵U, this study also encompasses less commonly reported radionuclides such as ²⁴²Pu, ²⁴⁴Pu and ²³⁶U. We show the importance of combining numerous fingerprints for improved assessment of the source of a nuclear contamination. In samples from Bikar, Pu and U isotope ratios were found to vary within narrow ranges. Pu and U fingerprints suggest that regional fallout from the Castle Bravo test in March 1954 was the main source of the contamination. This was further confirmed by two different age dating approaches that estimated 1954 as the year of the contamination. We demonstrate that use of an exponential function to approximate the yield of heavy radionuclides in thermonuclear explosions with increasing mass is a valid approach for estimating the age of a contamination. We show that, if sufficient radionuclide activity concentration measurement results with low uncertainties are available, this method is robust.

Level, distribution and sources of plutonium in the northeast and north China

Concentrations of ²³⁹Pu and ²⁴⁰Pu in 163 surface soil samples and five soil cores collected from the northeast and north China were analyzed using the radiochemical separation combined with inductively coupled plasma mass spectrometry measurement. The average ²⁴⁰Pu/²³⁹Pu atomic ratios (0.185 ± 0.018) for all surface soil samples indicated that the global fallout is the major source of plutonium in the studied region. The ^239,240Pu concentrations of the surface soil ranged from 0.002 mBq/g to 4.82 mBq/g, lying in the range of the reported results in the areas with similar latitude, except for a few samples. The distribution of ^239,240Pu in this region is controlled by the deposition of plutonium in the atmosphere and its preservation in the soil, which were affects by multi-factors such as topography, climate, utilization of the land and vegetation coverage. The analytical results could be used as the baseline data for the assessment of the impact of nuclear activities in the past and the future.

237Np and 241Am as Fingerprints in the Major River Basins of Southern China and North South China Sea: A Land-Sea Perspective

During the global nuclear weapon tests, large amounts of radioactive elements are released into the environment. Long-period actinide elements such as Np, Pu, and Am exhibit different behaviors in various environments, and their isotope characteristic fingerprints are of great significance for studying the distribution, migration, and prediction of radioactive pollutants. To investigate the distribution of ²⁴¹Am and ²³⁷Np in southern China, activities of ²⁴¹Am (0.008 ± 0.012-0.932 ± 0.066 Bq/kg) and ²³⁷Np (0.037 ± 0.003-1.458 ± 0.063 mBq/kg), ²³⁷Np/²³⁹Pu atom ratios (0.055 ± 0.003-0.864 ± 0.054), and ²⁴¹Am/^239+240Pu activity ratios (0.033 ± 0.075-15.870 ± 0.477) in 95 surface sediment samples collected from the northern South China Sea and major river basins were analyzed for the first time. Due to the different scavenging mechanisms of Am and Pu, ²⁴¹Am is preferentially concentrated by sinking the particulate, while plutonium is scavenged in the coastal area, resulting in a higher ²⁴¹Am/^239+240Pu activity ratio in estuary and coastal areas. The distribution of ²³⁷Np shows obvious spatial inhomogeneity as the high migration rate. The relevant fingerprint characteristic has changed greatly and needs to be updated urgently. As a result of the convergence of land and sea, ²⁴¹Am, ^239,240Pu, and ²³⁷Np are dominated by terrestrial sediments and deposited in the coastal area of southern China, which should be paid more attention to. This work can establish China's current neptunium radioactivity database, and the difference in Np, Pu, and Am scavenging processes may be a powerful tool for evaluating the impact of the Pearl River Estuary salt tide.

Temporal variability of Pu signatures in a 210Pb-dated Sphagnum peat profile from the Northern Ural, Russian Federation

The peat archives are one of the stratigraphic records revealing clearly physical, chemical and biological signals of human influence on the Earth System since the 1950s, at least. The presented study was aimed mainly to identify the level and origin of anthropogenic radionuclides such as ^{238, 239, 240}Pu in a ²¹⁰Pb-dated peat profile derived from the Northern Ural, Russian Federation. As stated, the vertical variability of ²⁴⁰Pu/²³⁹Pu isotopic compositions reflects the nuclear weapons testing history with the maximum in the 1960s and small regional impact most likely of high-yielded tests in the 1950s as well as Chinese detonations in the 1970s. Peat accumulations rates were similar to those obtained in adjacent areas, whereas ²¹⁰Pb flux slightly exceeded the reference level established for adequate latitude belt.

Determination of ultra-trace level plutonium isotopes in soil samples by triple-quadrupole inductively coupled plasma-mass spectrometry with mass-shift mode combined with UTEVA chromatographic separation

Although triple-quadrupole inductively coupled plasma-mass spectrometry (ICP-MS/MS) has become an attractive technique for the measurement of long-lived radionuclides, the abundance sensitivity, isobaric and polyatomic ions interferences seriously restrict the application. The spectral peak tailing and uranium hydrides (UH⁺, UH₂⁺) from ²³⁸U have a serious influence on the accurate measurement of ²³⁹Pu and ²⁴⁰Pu, especially for the ultra-trace level plutonium isotopes in the higher uranium sample. A new method was developed using ICP-MS/MS measurement in mass-shift mode with collision-reaction gas combined with a chemical separation procedure. As O₂ readily converted Pu⁺ ion to PuO₂⁺, while disassociated the interfering diatomic ions of interfering elements (U, Pb, Hg, Tl, etc.), the interferences from these elements were completely eliminated if plutonium was detected as PuO₂⁺ at the m/z more than 270. By the mass filter in MS/MS mode combined with O₂ as reaction gas the lower peak tailing of ²³⁸U⁺ (<5 × 10^-12) was significantly suppressed. By this way, the ²³⁸UO₂H⁺/²³⁸UO₂⁺ atomic ratio was reduced to 4.82 × 10^-9, which is significantly lower than that of other collision-reaction gas modes. Interferences from Pb, Hg and Tl polyatomic ions were also completely eliminated. Thus, accurate measurement of ultra-trace level ²³⁹Pu in high uranium sample solutions with the ²³⁹Pu/²³⁸U concentration ratio of 10^-10 was achieved by the mass-shift mode with 0.15 mL/min O₂/He + 12.0 mL/min He as collision-reaction gas, and high elimination efficiency of uranium interferences up to 10¹⁴ can be obtained by combination with the chemical separation using a single UTEVA resin column. The developed method can be applied to accurately determine the fg level ²³⁹Pu in high uranium samples, such as large-size deep seawater, deep soil and sediment, uranium debris of nuclear fuel.

The level, distribution and source of artificial radionuclides in surface soil from Inner Mongolia, China

Mid- and long-half-life artificial radioisotopes in the earth's surface environment are of great concern to the environmental radiation risk assessment. As nuclear fuel and fission products, ²³⁹Pu, ²⁴⁰Pu, ²⁴¹Am, ⁹⁰Sr and ¹³⁷Cs in soils from Inner Mongolia of China were analyzed with a modified systematic separation procedure combined with ICP-MS and LSC measurements, to study the level, distribution and source of artificial radionuclides in the region. The radioactivity and inventory of ¹³⁷Cs (0.26-28.3 Bq/kg, 0.5-5.4 kBq/m²), ^239+240Pu (0.05-1.26 Bq/kg, 20-229 Bq/m²), ²⁴¹Am (0.036-0.35 Bq/kg, 11-81 Bq/m²) and ⁹⁰Sr (1.2-7.6 Bq/kg, 0.39-1.7 kBq/m²) all lie in the range of the global fallout. Vertical distributions of these radionuclides were examined for two soil core samples SC14025 and SC14038, and great differences were observed between these two sample locations. For SC14025 where little human disturbance to soil occurred, both ¹³⁷Cs and ^239+240Pu have a subsurface activity maximum followed by an exponential decay. Fittings base on CDE model gives a small downward migration velocity of about 0.097 cm/y for both Pu and ¹³⁷Cs. Source identification for SC14025 and SC14038 soil cores with ²⁴⁰Pu/²³⁹Pu (average of 0.180 ± 0.017 and 0.164 ± 0.035, respectively), ¹³⁷Cs/^239+240Pu (average of 25.3 ± 0.6 and 25.6 ± 3.0, respectively) and ²⁴¹Am/^239+240Pu (average of 0.56 ± 0.08 and 0.60 ± 0.09, respectively) ratios consistently indicated that anthropogenic radionuclides in Xilingol region are mostly from the global fallout of atmospheric nuclear weapons tests in the last century. According to the geographical distribution of the radioactivity level, the high radioactivity level in the east of Inner Mongolia probably results from enhanced deposition by the blocking of the Great Khingan Range.

Differentiating Fukushima and Nagasaki plutonium from global fallout using 241Pu/239Pu atom ratios: Pu vs. Cs uptake and dose to biota

Plutonium (Pu) has been released in Japan by two very different types of nuclear events - the 2011 Fukushima accident and the 1945 detonation of a Pu-core weapon at Nagasaki. Here we report on the use of Accelerator Mass Spectrometry (AMS) methods to distinguish the FDNPP-accident and Nagasaki-detonation Pu from worldwide fallout in soils and biota. The FDNPP-Pu was distinct in local environmental samples through the use of highly sensitive ²⁴¹Pu/²³⁹Pu atom ratios. In contrast, other typically-used Pu measures (²⁴⁰Pu/²³⁹Pu atom ratios, activity concentrations) did not distinguish the FDNPP Pu from background in most 2016 environmental samples. Results indicate the accident contributed new Pu of ~0.4%-2% in the 0-5 cm soils, ~0.3%-3% in earthworms, and ~1%-10% in wild boar near the FDNPP. The uptake of Pu in the boar appears to be relatively uninfluenced by the glassy particle forms of fallout near the FDNPP, whereas the ^134,137Cs uptake appears to be highly influenced. Near Nagasaki, the lasting legacy of Pu is greater with high percentages of Pu sourced from the 1945 detonation (~93% soils, ~88% earthworm, ~96% boar). The Pu at Nagasaki contrasts with that from the FDNPP in having proportionately higher ²³⁹Pu and was distinguished by both ²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu atom ratios. However, compared with the contamination near the Chernobyl accident site, the Pu amounts at all study sites in Japan are orders of magnitude lower. The dose rates from Pu to organisms in the FDNPP and Nagasaki areas, as well as to human consumers of wild boar meat, have been only slightly elevated above background. Our data demonstrate the greater sensitivity of ²⁴¹Pu/²³⁹Pu atom ratios in tracing Pu from nuclear releases and suggest that the Nagasaki-detonation Pu will be distinguishable in the environment for much longer than the FDNPP-accident Pu.

Transuranic nuclides Pu, Am and Cm isotopes, and 90Sr in seafloor sediments off the Fukushima Daiichi Nuclear Power Plant during the period from 2012 to 2019

The ²³⁸Pu, ^239+240Pu, ²⁴¹Am, ²⁴²Cm, ^243+244Cm and ⁹⁰Sr concentrations in seafloor surface sediments collected at three sampling stations off the Fukushima Daiichi Nuclear Power Plant (FDNPP) site during the period from 2012 to 2019 were determined to elucidate the impacts of the FDNPP accident onto their concentrations in coastal sediments and to discuss the sources of the measured radionuclides. The ^239+240Pu, ²⁴¹Pu and ²⁴¹Am concentrations and ²⁴⁰Pu/²³⁹Pu atom ratios in a sediment core were also determined to allow comparison of their inventories between this study and previously reported values and to identify the Pu sources. The ²³⁸Pu, ^239+240Pu, ²⁴¹Am and ⁹⁰Sr concentrations showed no remarkable temporal variations; no significant increases in concentrations after the FDNPP accident were observed; these concentrations were comfortably within the previously reported concentration range; and no detectable ²⁴²Cm and ^243+244Cm amounts were observed in surface sediments. The observed ²³⁸Pu/^239+240Pu activity ratios were approximately two orders of magnitudes lower than those for the damaged FDNPP reactor core inventories and the observed values in terrestrial samples after the accident. The ^239+240Pu, ²⁴¹Pu and ²⁴¹Am inventories in the sediment core were 389 ± 5, 503 ± 33 and 214 ± 3 Bq m^-2, respectively. The ^239+240Pu inventory was about an order of magnitude greater than the expected cumulative deposition density of global fallout from atmospheric nuclear weapons testing due to an enhanced scavenging effect. The ²⁴⁰Pu/²³⁹Pu atom ratios in the sediment core ranged from 0.239 to 0.246 with a mean value of 0.242 ± 0.002; these ratios were clearly greater than the mean global fallout ratio of 0.18. The results for ²³⁸Pu/^239+240Pu activity ratios and ²⁴⁰Pu/²³⁹Pu atom ratios reflected a mixture of global fallout and Pacific Proving Grounds (PPG) close-in fallout Pu rather than Fukushima accident-derived Pu. The sediment column inventory for ^239+240Pu originating from the PPG close-in fallout was calculated as 166 Bq m^-2, which corresponded to 43% of the total inventory. A significant amount of the PPG-derived Pu has been transported by ocean currents and then preferentially scavenged in the coastal waters of Japan.

Particulate plutonium released from the Fukushima Daiichi meltdowns

Traces of Pu have been detected in material released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March of 2011; however, to date the physical and chemical form of the Pu have remained unknown. Here we report the discovery of particulate Pu associated with cesium-rich microparticles (CsMPs) that formed in and were released from the reactors during the FDNPP meltdowns. The Cs-pollucite-based CsMP contained discrete U(IV)O₂ nanoparticles, <~10 nm, one of which is enriched in Pu adjacent to fragments of Zr-cladding. The isotope ratios, ²³⁵U/²³⁸U, ²⁴⁰Pu/²³⁹Pu, and ²⁴²Pu/²³⁹Pu, of the CsMPs were determined to be ~0.0193, ~0.347, and ~0.065, respectively, which are consistent with the calculated isotopic ratios of irradiated-fuel fragments. Thus, considering the regional distribution of CsMPs, the long-distance dispersion of Pu from FNDPP is attributed to the transport by CsMPs that have incorporated nanoscale fuel fragments prior to their dispersion up to 230 km away from the Fukushima Daiichi reactor site.

First study of 237Np in Chinese soils: Source, distribution and mobility in comparison with plutonium isotopes

In this study, the distribution and migration of ²³⁷Np and ^239+240Pu in soils in the vicinity (<5 km) of Qinshan and Tianwan Nuclear Power Plants in China were studied, which is the first specific study of global fallout ²³⁷Np in Chinese soils. The ²³⁷Np and ^239+240Pu concentrations in surface soils showed large spatial inhomogeneity. A remarkable ^239+240Pu concentration (4.783 mBq/g) was observed in a surface soil near Qinshan NPP and stands for the ever reported highest value in the Chinese soils. The inventories of ^239+240Pu in two Qinshan and Tianwan soil cores were estimated to be 128.8 Bq/m² and 121.0 Bq/m², respectively; while the ²³⁷Np inventories were 0.039 Bq/m² and 0.035 Bq/m² at these sites, respectively. The ²⁴⁰Pu/²³⁹Pu atomic ratios in these soils indicated that the global fallout is the main source of Pu in these regions. However, the non-isotopic ²³⁷Np/²³⁹Pu atomic ratio in environmental soil is not a sensitive indicator for source identification. Furthermore, we conducted pilot study on the migration behaviors of ²³⁷Np and ^239+240Pu in soil core at Qinshan site with the Convection-Dispersion Equation (CDE) model. The obtained apparent dispersion coefficients of ²³⁷Np (2.82 ± 2.06 cm²/y) was 5 times higher than that of ^239+240Pu (0.57 ± 0.16 cm²/y), proving that ²³⁷Np has stronger migration ability than Pu isotopes in the Qinshan soil. Finally, we predicted that with the increase of migration time, both ²³⁷Np and ^239+240Pu concentration in the soil will gradually become more evenly distributed among different soil layers due to the dominant dispersion effects.

Radionuclides in the environment around the uranium mines in Guangxi, China

Uranium and plutonium are both poisonous radioactive elements, which are very harmful to human health and environment. Therefore, it is of great significance to study the distribution of ²³⁸U concentration and ^239+240Pu activity in the uranium mine surrounding soils. We have collected some surface soil sediments within 2 km of two uranium mines and a solid waste management center in Guangxi Province. The ²³⁸U concentration in these study areas is in the range of 1.44-83.91 mg/g, and the ²³⁸U concentration in the A uranium mine surrounding surface soils is higher than that in the B uranium mine and the solid waste management center. While the B uranium mine and the solid waster management center don't pollute the surrounding soils because the ²³⁸U concentrations in their surrounding soils are similar to the average ²³⁸U concentration in the soil. The ^239+240Pu activities in soil samples collected around the two uranium mines and the solid waste management center are close ranged from 0.06 mBq/g to 0.51 mBq/g. Moreover, the ²⁴⁰Pu/²³⁹Pu atom ratios in our study samples are ranged from 0.15 to 0.23, which indicate the Pu may come from the global fallout. In addition, we study heavy metals in our collected samples, only heavy metal Tl has weak positive correlations with ²³⁸U concentrations and ^239+240Pu activities. And there is a weak positive correlation between ²³⁸U concentrations and ^239+240Pu activities.

Plutonium and other radionuclides persist across marine-to-terrestrial ecotopes in the Montebello Islands sixty years after nuclear tests

Since the 1956 completion of nuclear testing at the Montebello Islands, Western Australia, this remote uninhabited island group has been relatively undisturbed (no major remediations) and currently functions as high-value marine and terrestrial habitat within the Montebello/Barrow Islands Marine Conservation Reserves. The former weapons testing sites, therefore, provide a unique opportunity for assessing the fate and behaviour of Anthropocene radionuclides subjected to natural processes across a range of shallow-marine to island-terrestrial ecological units (ecotopes). We collected soil, sediment and biota samples and analysed their radionuclide content using gamma and alpha spectrometry, photostimulated luminescence autoradiography and accelerator mass spectrometry. We found the activity levels of the fission and neutron-activation products have decreased by ~hundred-fold near the ground zero locations. However, Pu concentrations remain elevated, some of which are high relative to most other Australian and international sites (up to 25,050 Bq kg-1 of 239+240+241Pu). Across ecotopes, Pu ranked from highest to lowest in the following order: island soils > dunes > foredunes > marine sediments > and beach intertidal zone. Low values of Pu and other radionuclides were detected in all local wildlife tested including endangered species. Activity concentrations ranked (highest to lowest) terrestrial arthropods > terrestrial mammal and reptile bones > algae > oyster flesh > whole crab > sea turtle bone > stingray and teleost fish livers > sea cucumber flesh > sea turtle skin > teleost fish muscle. The three detonations (one from within a ship and two from 30 m towers) resulted in differing contaminant forms, with the ship detonation producing the highest activity concentrations and finer more inhalable particulate forms. The three sites are distinct in their 240/239Pu and 241/239Pu atom ratios, including the Pu transported by natural process or within migratory living organisms.

Level, distribution and sources of plutonium in the coastal areas of China

Plutonium is an important radioactive element in view of its biological toxicity and environmental impact. The two most important isotopes of plutonium, ²³⁹Pu and ²⁴⁰Pu, in the surface soil from the coastal area of China were determined using radiochemical separation combined with inductively coupled plasma mass spectrometry (ICP-MS) measurement. The average of the measured ²⁴⁰Pu/²³⁹Pu atomic ratios in all these samples is 0.186 ± 0.021, which is in good agreement with the ratio of global fallout of the nuclear weapons tests. This indicates that the major source of plutonium in this region is global fallout, and there is no measurable release of plutonium from any nuclear power plant along the coast of China. The ^239,240Pu concentrations in all 71 soil samples range from 0.002 mBq/g to 0.670 mBq/g, which fall in the range of the reported values in the background area not directly influenced by nuclear activities. The anthropogenic disturbance of sampling sites, vegetation coverage of land, precipitation rate and organic substance content and erosion of soil have significant influence on the level of plutonium in the surface soil.

Analysis of cable force on steady-state motion of remotely operated vehicle for reaction pool underwater welding

Remotely operated vehicle is a reliable and efficient tool in routing inspection of reactor pools of nuclear power plants. While, there is still no study on steady-state motion of remotely operated vehicle which is crucial for reaction pool underwater welding reported. In practice, the cable force has been considered as a critical factor affecting the vehicle’s operation. To evaluate and predict the disturbing effect caused by the cable mounted on the tailing of a vehicle, a numerical simulation can be employed. In this work, we set up a theoretical model of a cable partially in reaction pool water and air to validate the remotely operated vehicle design and reduce the prototype developing time. We programmed the model governed by an ordinary differential equation set, which was considered as an initial value problem following a dimensionless treatment to be solved. The influence by factors (i.e. velocity, water depth, entry water angle, water density and cable length on tension, normal shear force, and binormal moment) was quantified by a numerical method. The test of a cable force was carried out to verify an authenticity of the three-dimensional mechanical model and a numerical method. The results demonstrated that the presented method could be used to evaluate the effect of real environment factors on a remotely operated vehicle steady-state motion and held a potential to improve the remotely operated vehicle design and control strategy.

Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power Plant site within two months after the severe nuclear accident

The marine environment is complex, and it is desirable to have measurements for seawater samples collected at the early stage after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident to determine the impact of Fukushima-derived radionuclides on this environment. Here Pu isotopes in seawater collected 33-163 km from the FDNPP site at the very early stage after the accident were determined (May 2011, within two months after the accident). The distribution and temporal variation of ²³⁹Pu and ²⁴⁰Pu were studied. The results indicated that both ^239+240Pu activity concentrations (from 0.81 ± 0.16 to 11.18 ± 1.28 mBq/m³) and ²⁴⁰Pu/²³⁹Pu atom ratios (from 0.216 ± 0.032 to 0.308 ± 0.036) in these seawater samples were within the corresponding background ranges before the accident, and this suggested that Fukushima-derived Pu isotopes, if any, were in too limited amount to be distinguished from the background level in the seawater. The analysis of Pu isotopic composition indicated that the major sources of Pu in the seawater after the accident were still global fallout and the Pacific Proving Ground close-in fallout. The contribution analysis showed that the contributions of the Pacific Proving Ground close-in fallout in the water column of the study area ranged from 26% to 77% with the average being 48%.

Insights into potential consequences of fusion hypothetical accident, lessons learnt from the former fission accidents

From previous catastrophic fission nuclear accidents, such as the Chernobyl and Fukushima accidents, researchers learnt the lessons that external hazard beyond design basis or human errors could result in severe accidents and multi-failure of the confinements although they were considered as very-low-probability events and not requested to be paid much attention to according to the current nuclear safety regulations. Fusion energy is always regarded as a safe and clean energy. However, massive quantity of radioactivity still exists in the fusion reactor and is possible to be released into the environment. The environmental pollution and potential public consequences due to severe accidents of fusion reactor remain largely unexplored. In this contribution, we intended to investigate the hypothetical accident to envelop the worst but probable consequences of fusion reactor, and compare with historic Chernobyl and Fukushima accidents under assumed environmental conditions. It was demonstrated that, the radiation consequences of a hypothetical fusion accident would be much less severe than fission accidents, e.g. an INES 7 accident could not appear in a fusion reactor, as in the Chernobyl and Fukushima nuclear accidents. However, it would still be disastrous and the publics close to site might be exposed to "potentially lethal" radiation dose.

The transfer of fallout plutonium from paddy soil to rice: A field study in Japan

Reported transfer factor (TF) values of Pu from paddy soil to rice are rather scarce, despite the radiotoxicity of Pu and the irreplaceable role of rice in Asian peoples' diets. Here, we conducted a field study to investigate the transfer of global fallout Pu from paddy soil to rice grain (hulled rice) in Japan. The ²⁴⁰Pu/²³⁹Pu atomic ratios in two rice grain samples out of 16 samples were determined and the ratios corresponded well with the global fallout value. The soil-to-rice TF_Pu in 12 Japanese prefectures ranged from 4.5 × 10^-6 to 1.2 × 10^-4 with a geometric mean of 3.3 × 10^-5. The TFs of rice obtained in this study were compatible to the TFs for the broad heading "cereals" compiled in the IAEA Technical Report Series No. 472. Weak correlations were found between the TF and the investigated soil characteristics such as soil pH and loss on ignition. Regarding the TFs for cerium (Ce) and thorium (Th) which are commonly considered as Pu analogues, we observed no significant correlations between the log(TF_Pu) and log(TF_Ce) or log(TF_Pu) and log(TF_Th). On the other hand, interestingly, a significantly positive correlation (r = 0.795, p < 0.001) was observed between log(TF_Pu) and log(TF_U). In view of the observed similarity of TF values for U and Pu from soil to rice, we thought that using the easy-to-measure TF_U to estimate TF_Pu from soil to rice might be suggested although the mechanism was unclear.

Spatial pattern of plutonium and radiocaesium contamination released during the Fukushima Daiichi nuclear power plant disaster

Plutonium and radiocaesium are hazardous contaminants released by the Fukushima Daiichi nuclear power plant (FDNPP) disaster and their distribution in the environment requires careful characterisation using isotopic information. Comprehensive spatial survey of ¹³⁴Cs and ¹³⁷Cs has been conducted on a regular basis since the accident, but the dataset for ¹³⁵Cs/¹³⁷Cs atom ratios and trace isotopic analysis of Pu remains limited because of analytical challenges. We have developed a combined chemical procedure to separate Pu and Cs for isotopic analysis of environmental samples from contaminated catchments. Ultra-trace analyses reveal a FDNPP Pu signature in environmental samples, some from further afield than previously reported. For two samples, we attribute the dominant source of Pu to Reactor Unit 3. We review the mechanisms responsible for an emergent spatial pattern in ^134,135Cs/¹³⁷Cs in areas northwest (high ¹³⁴Cs/¹³⁷Cs, low ¹³⁵Cs/¹³⁷Cs) and southwest (low ¹³⁴Cs/¹³⁷Cs, high ¹³⁵Cs/¹³⁷Cs) of FDNPP. Several samples exhibit consistent ^134,135Cs/¹³⁷Cs values that are significantly different from those deposited on plant specimens collected in previous works. A complex spatial pattern of Pu and Cs isotopic signature is apparent. To confidently attribute the sources of mixed fallout material, future studies must focus on analysis of individual FDNPP-derived particles.

Sources and scavenging of plutonium in the East China Sea

The ²⁴⁰Pu/²³⁹Pu atom ratio and ^239+240Pu activity of seawater in the East China Sea (ECS) was measured in order to examine the Pu sources and elaborate Pu scavenging process. High ²⁴⁰Pu/²³⁹Pu atom ratios (0.187-0.243, average = 0.221 ± 0.017) in the surface water and water column were observed during 2011, implying of non-global fallout Pu sources. The distribution of ²⁴⁰Pu/²³⁹Pu atom ratio in the ECS was in agreement with the introduction pathway of the Kuroshio, showing a decreasing trend away from the outer shelf. An even higher ²⁴⁰Pu/²³⁹Pu atom ratios (0.243-0.263, average = 0.253 ± 0.007) were observed in the Kuroshio, indicating the non-global fallout Pu signal from the Pacific Proving Grounds (PPG). Using a two end-member mixing model, the Pu source from the PPG contribution was calculated to be 36 ± 9% in the ECS seawater. The ^239+240Pu activities of surface seawater were in the range of 2.00-2.95 mBq m^-3 in the ECS. The spatial distribution of ^239+240Pu activity in the surface seawater showed an increasing trend from the outer shelf to the nearshore. Moreover, ^239+240Pu inventory of water column at the station DH23 in the ECS was calculated to be ~0.29 Bq m^-2, which was 1-3 orders of magnitude lower than the estimates of sediment cores in the ECS shelf (9-407 Bq m^-2). Such differences were determined by the high degree Pu scavenging efficiency in the ECS and high Pu input carried by terrestrial sediments from the Yangtze River. Finally, both ²⁴⁰Pu/²³⁹Pu atom ratios and ^239+240Pu activities were identical before and after the Fukushima nuclear accident (FNA), suggesting that the impact of the FNA on the ECS was negligible.

Plutonium isotopic signatures in soils and their variation (2011-2014) in sediment transiting a coastal river in the Fukushima Prefecture, Japan

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in a significant release of radionuclides that were deposited on soils in Northeastern Japan. Plutonium was detected at trace levels in soils and sediments collected around the FDNPP. However, little is known regarding the spatial-temporal variation of plutonium in sediment transiting rivers in the region. In this study, plutonium isotopic compositions were first measured in soils (n = 5) in order to investigate the initial plutonium deposition. Then, plutonium isotopic compositions were measured on flood sediment deposits (n = 12) collected after major typhoon events in 2011, 2013 and 2014. After a thorough radiochemical purification, isotopic ratios (²⁴⁰Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu and ²⁴²Pu/²³⁹Pu) were measured with a Multi-Collector Inductively Coupled Mass Spectrometer (MC ICP-MS), providing discrimination between plutonium derived from global fallout, from atmospheric nuclear weapon tests, and plutonium derived from the FDNPP accident. Results demonstrate that soils with the most Fukushima-derived plutonium were in the main radiocaesium plume and that there was a variable mixture of plutonium sources in the flood sediment samples. Plutonium concentrations and isotopic ratios generally decreased between 2011 and 2014, reflecting the progressive erosion and transport of contaminated sediment in this coastal river during flood events. Exceptions to this general trend were attributed to the occurrence of decontamination works or the remobilisation of contaminated material during typhoons. The different plutonium concentrations and isotopic ratios obtained on three aliquots of a single sample suggest that the Fukushima-derived plutonium was likely borne by discrete plutonium-containing particles. In the future, these particles should be isolated and further characterized in order to better understand the fate of this long-lived radionuclide in the environment.