The atmospheric source term for the Fukushima Daiichi nuclear power plant accident in March 2011 has been estimated by a Gaussian puff based atmospheric dispersion model. The scanty meteorological data available at irregular time intervals are utilized to demonstrate the utility of such data along with a simplified modeling approach to derive useful information. The source terms for I-131 and Cs-137 have been estimated as a function of time from the observed values of activity concentration in the air and deposited activity on the ground. The model results suggest that during 12th March 2011–16th March 2011, 9.29 x 10¹⁶ Bq of I-131 and 6.15 x 10¹⁵ Bq of Cs-137 might have got released to the environment.