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Temporal absences in observation records lead to release losses during the source term inversions of atmospheric radionuclide emissions. Consequently, objectively-estimated source terms for the Fukushima accident contain fewer release details and present large discrepancies when compared with the expert-judged one. This paper describes an objective method that can adaptively recover the missing releases caused by the temporal absences of observations. The proposed method assumes that the accident releases of radionuclides are piecewise-constant and comprise both peaks and constant releases. The missing releases are adaptively recovered as either peaks or constant releases by minimizing the total variation of the estimated source term. The proposed method is applied to the Fukushima accident and evaluated against regional airborne and deposited 137Cs observations. The results demonstrate that this method effectively recovers the missing releases, producing a source term that matches the timing of both on-site gamma dose rate peaks and accident events. The retrieved source term improves the simulation of air concentrations and reproduces most of the deposition patterns. This is the first time that an objective method has independently reproduced the details in the expert-judged one for the Fukushima accident. [Display omitted] • Recovery of missing releases caused by temporal absences of observations. • Preservation of both sharp peaks and constant releases. • First independent objective source term matching the expert-judged one. • Agreement with on-site gamma dose rate peak timing and accident events. • Improved spatiotemporal simulation of airborne 137Cs for the Fukushima accident. [ABSTRACT FROM AUTHOR]

The Fukushima Daiichi Nuclear Plant (FDNPP) accident resulted in huge environmental and socioeconomic impacts to Japan. To document the actual environmental and socioeconomic effects of the FDNPP accident, we describe here atmospheric and marine contamination due to radionuclides released from the FDNPP accident using papers published during past five years, in which temporal and spatial variations of FDNPP-derived radionuclides in air, deposition and seawater and their mapping are recorded by local, regional and global monitoring activities. High radioactivity-contaminated area in land were formed by the dispersion of the radioactive cloud and precipitation, depending on land topography and local meteorological conditions, whereas extremely high concentrations of 131 I and radiocesium in seawater occurred due to direct release of radioactivity-contaminated stagnant water in addition to atmospheric deposition. For both of atmosphere and ocean, numerical model simulations, including local, regional and global-scale modeling, were extensively employed to evaluate source terms of the FDNPP-derived radionuclides from the monitoring data. These models also provided predictions of the dispersion and high deposition areas of the FDNPP-derived radionuclides. However, there are significant differences between the observed and simulated values. Then, the monitoring data would give a good opportunity to improve numerical modeling. [ABSTRACT FROM AUTHOR]

In March 2011, severe nuclear accident happened at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) after the gigantic earthquake and following huge tsunami wave. A lot of investigations to assess environmental and radiological impacts of released radionuclides have been conducted by domestic and international organizations. Environmental radioactivity research related to the FDNPP accident has spread widely over different scientific fields due to specific features of the accident, and specifically its impact on the marine environment. The present paper summarizes major lessons learned from the environmental investigations of the FDNPP accident. Environmental radioactivity studies have typical interdisciplinary character; especially physics and chemistry are fundamental as a base of process studies in the environment. In this sight, we review chemical aspects regarding FDNPP-derived radiocesium transfer within and between compartments (atmosphere, ocean and land). We also discuss future trends in investigations of behavior of anthropogenic radionuclides in the environment, important not only for a better understanding of impacts of the FDNPP accident on the environment, but also for improving our general knowledge of the total environment in the Anthropocene era and its protection for the future. • 10 years after the FDNPP accident, the environment has been contaminated mainly by Cs-137. • Understanding of Cs-transfer within and between compartments has considerably improved. • Chemical processes are a key factor of Cs-transfer between aquatic and solid phases. • Migration of Cs-137 in terrestrial and marine ecosystems will dominate in future studies. [ABSTRACT FROM AUTHOR]

A transfer coefficient matrix (TCM) was created in a previous study using a Lagrangian dispersion model to provide plume predictions under different emission scenarios. The TCM estimates the contribution of each emission period to all sampling locations and can be used to estimate source terms by adjusting emission rates to match the model prediction with the measurements. In this paper, the TCM is used to formulate a cost functional that measures the differences between the model predictions and the actual air concentration measurements. The cost functional also includes a background term which adds the differences between a first guess and the updated emission estimates. Uncertainties of the measurements, as well as those for the first guess of source terms are both considered in the cost functional. In addition, a penalty term is added to create a smooth temporal change in the release rate. The method is first tested with pseudo observations generated using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model at the same location and time as the actual observations. The inverse estimation system is able to accurately recover the release rates and performs better than a direct solution using singular value decomposition (SVD). It is found that computing ln ( c ) differences between model and observations is better than using the original concentration c differences in the cost functional. The inverse estimation results are not sensitive to artificially introduced observational errors or different first guesses. To further test the method, daily average cesium-137 air concentration measurements around the globe from the Fukushima nuclear accident are used to estimate the release of the radionuclide. Compared with the latest estimates by Katata et al. (2014), the recovered release rates successfully capture the main temporal variations. When using subsets of the measured data, the inverse estimation method still manages to identify most of the major events in the temporal profile of the release. [ABSTRACT FROM AUTHOR]

The utilization of numerical atmospheric dispersion prediction (NDP) models for assisting the emergency response to emission of radionuclides has been recommended by a working group of the Meteorological Society of Japan. This paper verifies the feasibility of the recommendation through NDP model intercomparison with limited emission source information for the case of the Fukushima Dai-ichi Nuclear Power Plant accident caused by the Great East Japan Earthquake in 2011. According to the recommendation of the working group, the NDP models are run under the assumption of a constant rate of emission during the whole forecast period. This is the worst-case scenario when limited source information is available. Generally, no information is provided on the temporal variability and strength of the emissions, while the source location is known. Surface air radionuclide forecasts are utilized for providing warnings of the risk of inhaling radioactive substances suspended in the low-level atmosphere, whereas column-integrated radionuclide forecasts are utilized for estimating the potential maximum wet deposition of radioactive materials on the ground due to precipitation. The NDP model short-range forecasts were validated with observational data for three locations, at the times when the most serious contamination events occurred at each of the three monitoring stations. The NDP models successfully predicted the risk of surface air contamination and/or ground surface contamination caused by wet deposition in these cases. Particularly, the NDP model forecasts allow us to disseminate warnings at effective lead times before exposure to radiation. The different NDP models gradually deviate their forecasts as the lead time progresses. The deviations may indicate the magnitude of forecast errors. Thus, the use of multi-model forecasts is of greater benefit than the single model forecasts, because forecast error information is suggested. Image 1 • We propose the use of prediction models for environmental emergencies. • We performed model prediction experiments for atmospheric radionuclide dispersion. • The prediction models were validated with the Fukushima nuclear accident data. • Model prediction allows us to disseminate warnings at effective lead times. [ABSTRACT FROM AUTHOR]