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Formation of oxide solid solution is the key to the fate of nuclear fuel debris. – A new insight into the chemistry of fuel debris for safe storage, treatment and future disposal – 

2022.06.13

We demonstrated that the formation of an oxide solid solution makes drastic changes to the chemical properties of the fuel debris matrix, and determines its durability in water. This finding was achieved through our collaborative research between Tohoku University, the Nuclear Science and Engineering Center at JAEA, and Kyoto University.

The aim of our research project is to reveal the chemical properties of fuel debris which is essential for safe handling of this highly radioactive and pyrogenic substance during the course of decommissioning of TEPCO’s Fukushima Daiichi Nuclear Power Station. To reach this goal, we synthesized a series of simulated fuel debris samples from UO2 (nuclear fuel material), Zr (the base metal of fuel cladding alloy), and stainless steel (structural material in a nuclear reactor). By using this simulated fuel debris, we conducted experiments for predicting phase composition and evaluating the chemical durability of fuel debris.

The synthesized simulated debris samples contained a characteristic phase that has a crystal structure corresponding to UO2, but contained foreign elements, such as Zr added as a starting material and Fe from the stainless steel. This is the formation of the oxide solid solution. We conducted leaching experiments using the simulated debris samples in pure water and in seawater. The experimental results showed that leaching of actinide elements, which have high radiotoxicity, was reduced with the formation of an oxide solid solution inside the sample.

Based on the experimental results, we concluded that the formation of a solid solution of UO2 with Zr and Fe improves the long-term durability of the fuel debris matrix in water. The results of our study will serve as an important finding for planning and designing the mid-term storage and future disposal of the 1F fuel debris. We will further study the chemical properties of fuel debris, such as the dependence on the pyrochemical conditions during fuel debris generation, and make efforts to support the 1F decommissioning from a scientific viewpoint.

The results of this study have been published in Journal of Nuclear Materials. This study was supported by the JAEA Nuclear Energy S&T and Human Resource Development Project through concentrated wisdom [grant number JPJA18P18071886].

JAEA HP Press release (Japanese only)

Mesospheric ionization during substorm growth phase – Latest observations and simulations reveal the possible link between space and atmosphere – 

2022.06.10

A research group composed by Japan Atomic Energy Agency (JAEA), the Graduate University for Advanced Studies (SOKENDAI), National Institute of Polar Research, the University of Tokyo, Osaka University, and Nagoya University discovered the mesospheric ionization during substorm growth phase based on the combination of latest observations and simulations.

Many studies have been conducted about the impact of energetic charged particles on the atmosphere during geomagnetically active times, while quiet time effects are poorly understood. We identified two energetic electron precipitation (EEP) events during the growth phase of moderate substorms and estimated the mesospheric ionization rate for an EEP event for which the most comprehensive dataset from ground-based and space-born instruments was available. The mesospheric ionization signature reached below 70 km altitude and continued for ~15 min until the substorm onset, as observed by the PANSY radar and imaging riometer at Syowa Station in the Antarctic region. We also used energetic electron flux observed by the Arase and POES 15 satellites as the input for the air-shower simulation code PHITS to quantitatively estimate the mesospheric ionization rate. The calculated ionization level due to the precipitating electrons is consistent with the observed value of cosmic noise absorption. The possible spatial extent of EEP is estimated to be ~8 h MLT in longitude and ~1.5 degree in latitude from a global magnetohydrodynamic simulation REPPU and the precipitating electron observations by the POES satellite, respectively. Such a significant duration and spatial extent of EEP events suggest a non-negligible contribution of the growth phase EEP to the mesospheric ionization. Combining the cutting-edge observations and simulations, we shed new light on the space weather impact of the EEP events during geomagnetically quiet times, which is important to understand the possible link between the space environment and climate.

The results of this study have been published in Journal of Space Weather Space Climate (https://doi.org/10.1051/swsc/2022012) on June 6, 2022.

JAEA HP Press release (Japanese only)

NMB4.0, an Integrated Nuclear Fuel Cycle Simulator, is Now Available – A platform capable of calculating the entire nuclear power cycle has been developed and opened to contribute to strategic planning for the development of advanced energy systems – 

2022.04.20

Nuclear Transmutation System Development Group led by Kenji Nishihara in Nuclear Science and Engineering Center, have developed a method for evaluating future nuclear energy utilization scenarios in collaboration with Masahiko Nakase, assistant professor, Tomohiro Okamura, graduate student, and Kenji Takeshita, professor of the Laboratory for Zero-Carbon Energy (ZC Lab.), Institute for the Creation of Science and Technology, Tokyo Institute of Technology. The simulator “NMB4.0” was developed and released free of charge on March 15.

A quantitative evaluation of the advantages and disadvantages of each power source is essential for developing an energy strategy that can respond to various international situations with a view to decarbonization in Japan. In particular, for the use of nuclear energy, it is necessary to estimate in advance the amount of uranium resources, plant size, nuclear fuel cycle size, and waste generation required in order to determine which reactor type and nuclear fuel cycle should be adopted. However, all existing simulators in Japan are not open to the public, making it difficult to have a cross-sectional discussion for comprehensive strategic planning.

JAEA and ZC Lab. have jointly developed this simulator believing that it is essential to have a simulator that can serve as a foundation for vigorous discussion of future energy and nuclear energy strategies. “NMB4.0” is equipped with a fast calculation algorithm and can be flexibly configured with a wide range of nuclear fuel cycle processes such as fuel fabrication, reprocessing, and disposal, as well as various reactor types and their combinations.

Moreover, “NMB4.0” has been released free of charge to the public, and developers and users are widely invited. In the future, with the aim of becoming a standard simulator both domestically and internationally, we will work to build a research platform to realize evaluation research across the entire energy field, including other power sources, while further enhancing evaluation functions such as economic efficiency and environmental impact.

JAEA HP Press release (Japanese only)

Detailed radioactive plume dispersion and dose calculations considering buildings in local areas – Development of a local-scale high-resolution atmospheric dispersion and dose assessment system (LHADDAS) – 

2022.03.07

Nuclear Science and Engineering Center developed a local-scale high-resolution atmospheric dispersion and dose assessment system (LHADDAS) for safety and consequence assessment of nuclear facilities and emergency response to nuclear accidents or deliberate releases of radioactive materials in built-up urban areas. LHADDAS calculates inhomogeneous distributions of air concentrations, surface deposition of radionuclides released into the atmosphere and consequent air dose rates with regards to the shielding effects of surrounding buildings in a local-scale of several kilometers from the emission source.

Conventional atmospheric dispersion simulation systems, such as SPEEDI and WSPEEDI developed by JAEA, were designed to provide atmospheric-dispersion predictions to assess the environmental impacts and radiological doses to the public in the case of nuclear accidents. Therefore, these atmospheric dispersion simulations are targeted at a regional scale of approximately 100 km × 100 km with a grid resolution of several hundred meters. However, they can neither reproduce inhomogeneous distributions of air concentrations, surface deposition of radionuclides, caused by complex wind flow patterns influenced by buildings, nor estimate consequent air dose rates considering the shielding effects of surrounding buildings.

We combined atmospheric dispersion and air dose calculation to develop a new simulation code. Firstly, we utilized the local-scale high-resolution atmospheric dispersion model using large-eddy simulation (LOHDIM-LES). It can perform detailed simulations of complex turbulent flows and plume dispersion with regards to individual buildings that are resolved by a fine grid resolution. Secondly, the dose calculation code powered by lattice dose-response functions (SIBYL), which can quickly estimate air dose rates by a three-dimensional radiation transport scheme, was also used. Furthermore, the real-time dispersion simulation model based on the lattice Boltzmann method (CityLBM) was recently developed. By integrating these calculation codes, LHADDAS has been developed.

LHADDAS can provide a more realistic analysis method than wind tunnel experiments for plume dispersion under the actual meteorological conditions in the safety assessment of nuclear facilities. It is also useful for detailed pre/post-analyses of radiological dose exposures for emergency workers in nuclear accidents, the general public and first responders, in case of terrorist attacks, in urban/central districts. It can also be used for real-time consequence assessment in emergency response to terrorist attacks in urban/central districts.

JAEA HP Press release (Japanese only)

What is going to happen to the radiocesium contamination of forest trees in Fukushima in the future? – Newly developed computational model elucidated the contamination mechanism and enabled the future prediction – 

2022.01.06

Nuclear Science and Engineering Center developed a computational model (SOLVEG-R) that predicts land-surface dynamics of radioactive materials in detail. By using the model, we clarified contamination mechanism of wood of forest trees affected by the radiocesium (137Cs) fallout of the TEPCO’s Fukushima Daiichi Nuclear Power Plant (hereinafter referred to as ‘FDNPP’) accident and enabled predictions of the concentration of radiocesium in the wood in the future.

In the forests affected by the FDNPP accident, trees became contaminated with radiocesium and commercial use of wood has been regulated. To enable eventual reactivation of the commercial use, understanding the future evolution of concentration of radiocesium in wood is necessary. In the present study, we propose a model, SOLVEG-R, that calculates dynamic behavior of radiocesium in forests on the basis of water cycle and tree growth in forests influenced by actual meteorology and soil condition. The model was applied to two contaminated forests of cedar plantations and natural oak stands in Fukushima, and the results elucidated the pollution mechanism of wood at these forests. It was shown that the transfer of radiocesium to the trees occurred mostly through surface uptake of radiocesium trapped by needles (cedar) and bark (oak) during the accident and the subsequent contamination of the wood proceeded through enduring internal recycling of the radiocesium absorbed on these tree surfaces. It was suggested that the concentration of radiocesium in the wood begun to decrease from the year 2020 (cedar) or 2017 (oak), as a result of removal of radiocesium from the tree surfaces by precipitation wash-off and leaf-fall. As affected by dilution effect from the tree growth, the decrease of the concentration proceeded at a rate of 3% per year, which was rapider than the decrease through radioactive decay of radiocesium (2% per year).

SOLVEG-R is able to calculate radiocesium dynamics in forests without any calibration of the modeled processes by observational data and therefore it is applicable to various forests in Fukushima. Besides, the model accounts for dynamics of radiocesium in forest-floor organic layer and tree’s growth. The model can be therefore used to predict radiocesium concentration in trees after forest decontamination activities or forest managements such as removal of forest-floor organic layer and new planting of tree seeds after deforestation.

JAEA HP Press release (Japanese only)