ABSTRACT In October 2023, deposits retrieved from the X6 penetration of the primary containment vessel in Unit 2 of the Fukushima Daiichi Nuclear Power Station were analyzed and found to contain Ni and S-rich particles with platinum-group elements (Ru, Rh, and Pd) Based on microstructural and compositional analyses combined with existing knowledge of the accident analysis, the origin of Ni and S was inferred to be from structural alloys and injected seawater, respectively, and suggesting high-temperature interactions between core materials and seawater-derived species. To investigate the formation mechanism of the particles, laboratory experiments were performed on the reactions between Ni and MgSO4, the main sulfate species derived from seawater through concentration process. Pelletized mixtures of Ni and MgSO4 powders, with and without Fe addition, were heated at 900°C–1100°C under controlled Ar – H2–H2O atmospheres. Post-test metallographic analysis revealed the formation of Ni, Ni – Mg – O oxides, and Ni3S2. In Fe-containing systems, Fe – Ni sulfides formed, suppressing Ni3S2 production. These results support a mechanism in which seawater-derived MgSO4 reacts with segregated Ni during core degradation, yielding Ni3S2, as observed in Unit 2 deposits. The findings provide new insights into in-vessel chemical processes and offer indirect evidence that seawater reached and interacted with the fuel debris.

How does knowledge production during crises challenge or maintain contemporary institutions? This article investigates expert knowledge on nuclear accidents in France in the aftermath of the Fukushima Daiichi accident in March 2011. It examines the French and European 'lessons from Fukushima' learning exercise, using document analysis, interviews, and ethnographic observation of institutional experts' working practices. The article shows how nuclear safety experts produce limited commensurability. Specifically, through a comparative exercise, they render certain features of the Fukushima accident commensurable by relating them to a diversity of pre-existing evaluation scales and metrics of nuclear safety, while preventing different hypotheses, methods, and data from being brought to bear on nuclear safety. These operations rely on and consolidate experts' epistemic leeway, their discretionary ability to choose from several incommensurable epistemic resources. The article enhances our understanding of the politics of (in)commensurability for expert communities in the context of the pluralization of expert systems. It opens up questions about knowledge on crises, seeing these crises as episodes in which experts redefine the acceptable states of the world.

Even a decade after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on 11 March 2011, fluctuations in atmospheric 137Cs were still observed, and explanations for the fluctuations and their carriers remained elusive. In this study, small fluctuations within 0.0002 Bq∙m−3 were still detected in aerosol samples obtained from January to April, and slightly higher levels of atmospheric 137Cs were observed from May to September in a heavily contaminated area of Fukushima prefecture. Specifically, it is demonstrated that the 137Cs carriers in the aerosol samples were a combination of carbon-containing particles and aluminum-containing particles (Al particles dominated, with the percentage being 68%) in early May, whereas the main 137Cs carriers were carbonaceous particles, with the average percentage being 88% in September and at the end of May, using fluorescent upright microscope and scanning electron microscope equipped with an energy-dispersive X-ray spectrometer quantitatively. Additionally, small particles (less than 2 μm) and medium particles (2–8 μm) of carbonaceous particles had a higher level in the aerosol samples of May and September. Specifically, bacteria (1–1.8 μm) and spores (1.8–10 μm) had a linear relationship with the distribution of atmospheric 137Cs in the aerosol samples of September. In addition, temperature and precipitation were the main impact factors affecting the distribution of 137Cs and their carriers. This observation further suggests that there is still a need for long-term monitoring of atmospheric 137Cs.

Evaluation of sources and seasonal variations of dissolved 137Cs in stream water in forested catchments.

Proficiency test for gamma-emitting radionuclides in seawater: A case study from South Korea.

Spatiotemporal distribution of radionuclides in Korean Coastal Waters pre- and post-Fukushima wastewater discharge.

Evaluating the accumulation potential of organically bound tritium in edible seaweed under dark and light conditions through a deuterium tracer experiment.

Isotopic ratios of plutonium in radioactive particles released by the accident of Fukushima Daiichi Nuclear Power Plant.

KURAMA vs. Safecast: Radiation data comparison in Fukushima following whole-area decontamination.

Spatiotemporal variations and environmental behavior of activity concentrations of natural and artificial radionuclides in soils of Fukuoka, Japan (1980-2019).

Level, accumulation, transfer of radiocesium and plutonium in soil-mushroom systems and their health implications: A review.

Physics-informed optimization for emergency radiation assessment with temporal correction under meteorological uncertainty.

Leaked tritium reveals the source of 137Cs from the Fukushima Daiichi Nuclear Power Plant to the ocean.

Abstract. Nuclear energy plays an important role in global energy supply and mitigates greenhouse gas emissions. Potential environmental and human health risks are associated with the generated radioactive isotopes in the wastewater, especially the accidental release during natural disasters. However, the long-term transport and fate of these radionuclides remain uncertain. Here we employ a state-of-the-art ocean tracer model (MITgcm) to simulate the transport and fate of tritium, carbon-14, and seven other typical radionuclides in the twenty-first-century ocean. We use the discharge of radioactive wastewater from the Fukushima Daiichi Nuclear Power Station, both during the March 2011 earthquake and tsunami and from the subsequent release of stored wastewater, as a case study. The model indicates that the Kuroshio and North Pacific Current will spread the radionuclides over the whole North Pacific basin after three years. The enduring transport of long-term discharge in the Pacific will expand to other ocean basins by 2050. Accumulation of particle-reactive radionuclides in the sediments will mostly be centered in the northwest Pacific till 2050. This study demonstrates the effectiveness of our modeling tool, which can be broadly applied to assess the transport and fate of other types of radionuclides and other nuclear discharges worldwide.

ABSTRACT Due to the temperatures of the reactor pressure vessels at the Fukushima Daiichi Nuclear Power Station maintaining a near-constant level of approximately 373 K for several months post-accident, the internal structural surfaces may have been subjected to both steam and water exposure. Such a condition could potentially induce redistributions of the water-insoluble chemisorbed cesium (Cs) estimated by severe accident (SA) analysis codes. Consequently, this study serves as a foundational investigation to assess the potential for such prolonged Cs redistribution, examining the water-leaching dynamics of Cs chemisorbed on stainless steel 304 (SS304) at room temperature. The Cs-chemisorbed SS304 specimens were prepared at 1273 K and subsequently subjected to water-leaching tests at 303 K for durations extending up to 1200 hours. The results derived from these water-leaching tests revealed that the chemisorbed Cs continued to dissolve even after 1200 hours had elapsed. Moreover, the water-leaching characteristics of the water-insoluble Cs were effectively modeled using the modified Noyes-Whitney equation. These findings underscore the necessity to address the redistributions of water-insoluble Cs estimated by SA analysis codes.

The use of passive cooling systems as a reactor safety measure has become a key approach to preventing future incidents similar to the Fukushima Daiichi NPP accident. These systems operate based on natural circulation, a process driven by temperature differences and elevation between the heat source and heat sink. Key design factors, such as the inclination angle of the rectangular loop, significantly influence this circulation. This study aims to investigate the effects of different inclination angles of the rectangular loop and variations in the initial water temperature in the Water Heating Tank (WHT) on the flow rate and heat removal capability. The research was conducted experimentally using a natural circulation rectangular loop facility, FASSIP-04 Ver.0, which has an inner diameter of 26.64 mm, a rectangular loop height of 2280 mm, and a width of 780 mm. The experimental variations were achieved by adjusting the water temperature inside the WHT to 50°C, 70°C, and 90°C. Meanwhile, the inclination angle of the loop was set to 30°, 60°, and 90° mass flow rate and heat removal capability was influenced by both the loop inclination angle and the water temperature in the WHT. The highest values were observed at a 90° inclination angle and a set temperature of 90°C, with a mass flow rate of 0.0241 kg/s, and heat removal rates of qH = 0.791 kW, qC = 0.489 kW. The resulting buoyancy force was stronger under these conditions, leading to greater heat removal through natural circulation compared to free convection, thereby increasing both mass flow rate and heat removal efficiency.

Knowledge of radioactivity distribution is important for the decommissioning of postaccident plants. We derived methods to obtain the 137Cs contamination density of the inner surface of a pipe using a pinhole-type gamma camera, surface dose rate measurements using a Teletector, a GF10, and a lead-shielded ionization chamber. Using these methods, we obtained the 137Cs contamination density distribution of a Unit 1 standby gas treatment system outdoor pipe at the Fukushima Daiichi nuclear power station. Considering the characteristics of each method, these results were consistent. We obtained useful data for the decommissioning program.

In March 2011, the Fukushima Daiichi Nuclear Power Station (FDNPS) suffered reactor core overheating and fuel melting following the Great East Japan Earthquake and tsunami, producing complex microparticles from vaporized and rapidly solidified nuclear and structural materials. The chemical states and local structures of key elements in these particles, particularly uranium and plutonium, remain poorly constrained. Here, we present the synchrotron-based micro-focused X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) study of microparticles recovered from inside Unit 2 of FDNPS. The particles contain uranium, zirconium, and trace plutonium uniformly incorporated into chemically homogeneous oxide matrices. Two types were identified: uranium-rich particles with cubic UO2 and mixed U-Zr oxides with tetragonal ZrO2, the latter persisting at room temperature, indicating rapid cooling from a high-temperature metastable phase above 1650 °C. Both uranium and plutonium are mainly in the +4 state, with localized valence increases in zirconium-rich regions, suggesting redox-driven charge compensation during crystallization. These results provide direct evidence of melt evolution, actinide mixing, and oxidation-state preservation during severe reactor accidents, informing models of core degradation and strategies for safe decommissioning at FDNPS.

Plastics as vectors of radiocesium in river environments of Fukushima, Japan.

Indoor and outdoor exposure to atmospheric radionuclides in areas affected by the Fukushima nuclear accident.