A Source Term Estimation Method for a Nuclear Accident, using Atmospheric Dispersion Models

The rapidly evolving series of events experienced during the Fukushima Dai-ichi nuclear power plant accident highlighted the need for a capability for rapidly defining the evacuation zones needed to protect people from the radiation exposure. To address this need, the University of Tokyo recently began a 3-year research program, sponsored by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT). In addition to the organizations from the Japan, this research program is being conducted in collaboration with a coalition of international partners from the United States (US) and European Union (EU) who are leading researchers in the fields of atmospheric dispersion and estimation of source term parameters for airborne contaminants. The amount of radioactive substances released during the accident, i.e. the source term, is critical to determine a reasonable evacuation zone. In the absence of reliable information on the plant condition and direct measurements of a release, the source term can be determined through inverse modelling from observed data of radiation dose, concentration in the air and deposition amount on the ground. Such data are acquired from a variety of sources that may include observations from stationary monitoring posts, aircraft, and vehicles. In the present study, the source term is calculated using a Transfer Coefficient Matrix that is obtained from a combination of atmospheric dispersion model calculations (e.g WRF/CHEM, RAMS/HYPACT, Gaussian plume model etc.). This Source Term Estimation (STE) system is validated against wind tunnel experimental concentration data of Tokai-Daini nuclear station and using gamma radiation data from a research reactor in Mol, Belgium. In this study we found that our STE approach, based on Gaussian plume model, can estimate the release rate of I-131 from the Fukushima Dai-ichi Nuclear Power Plant by the same order of magnitude (e.g. PetaBq/hr), which was obtained by WSPEEDI, WRF/CHEM.