Development of Kalman Filter Based Source Term Estimation Model (STEM)

Information on source term is necessary for calculation of radiological doses to the population during accidental release conditions. Kalman Filter is a widely used technique for source term estimation. A Kalman Filter (KF) based Source term Estimation Model (STEM) has been developed and used to estimate the source term for Fukushima accident. Concentration measurements carried out by Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) for I-131 and Cs-137 at nine different locations on the same day i.e. (20th March, 2011) are utilized in the model. Here, the release rate is estimated for an assumed meteorological condition (wind speed = 0.5 m/s and stability = A). STEM utilizes linear KF method to estimate source term for known source location and receptor coordinates. Model results indicate that the source term for I-131 works out to be 1.59E + 11 Bq/s and that for Cs-137 is 3.52E + 10 Bq/s. Assuming release rate to be constant, the total activity released in the atmosphere from 12th March, 05:00 JST to 20th March 20:00 JST works out to be 1.18E + 17 Bq for I-131 and 2.62E + 16 Bq for Cs-137. This estimate is in line with the same carried out by other agencies.