Uncovering the Key 137Cs Emission Sources Contributing to High-Deposition Zones After the Fukushima Accident

Understanding the transport of ¹³⁷Cs released during the Fukushima accident remains challenging, as existing source terms fail to adequately capture the critical emissions leading to the high-deposition zone. For the problem, this study presents an objective inverse reconstruction method that uses total deposition and atmospheric concentration data. The deposition data is used to extract the a-priori emissions by novel identifying the critical temporal formation process of these depositions in high-deposition areas, with the help of the WRF-Chem model, and deriving the corresponding emissions. This deposition-based prior was then fused with the concentration data within an inversion framework, compensating the spatiotemporal information of incomplete data and ensuring the continuity feature of the emissions.

The reconstructed source term reveals two prominent emission peaks on March 15, 2011, occurring between 10:00-11:00 and 14:00-15:00. These peaks align with in-situ pressure measurements and accident analysis, suggesting that they were caused by pressure drops in the primary containment vessels of Units 3 and 2, respectively. This finding provides an explanation for the observation of spherical ¹³⁷Cs particles, likely formed through the condensation of vaporized or liquefied substances. The reconstructed source term also independently validates the widely adopted reverse estimation results by JAEA.

Simulations based on the reconstructed source term show significantly better agreement with various observational data than simulations using other source terms. The two-peak emission pattern accounts for the high-deposition areas, supporting the accuracy of the reconstruction. Furthermore, the proposed method outperforms traditional direct fusion approaches that combine deposition and atmospheric concentration data, which often fail to provide satisfactory results due to insufficient temporal information in deposition observations.

This new method offers a powerful tool for multi-observation fusion, providing a more accurate extraction of temporal information from total depositions. It represents a significant advancement in source term reconstruction, especially for complex nuclear accidents. The approach has broader implications for understanding the transport of short-lived radionuclides, with potential applications in iodine emission reconstruction, thyroid dose evaluation, and improving future environmental assessments in nuclear accident scenarios.