A calculation model for prediction of dose in case of an accidental atmospheric release from nuclear power plants

This report proposes a calculation model to estimate the gamma exposure rate due to radioactive clouds released from a nuclear power plant under an accident condition. This method is based on a plume segment atmospheric transport model. A plume segment is considered to be a sequence of puffs. The gamma exposure rate from a puff is assumed to be a function of σsub(z), the ratio σsub(y)/σsub(z) (= β) and r. Here, σsub(y), and σsub(z) are standard deviations of a single isotropic puff, in the crosswind, and vertical directions, respectively, and r is the distance from an evaluation point to the center of the puff. The standard exposure rate, namely, the exposure rate from the puff that has standard values for σsub(z) and β, is estimated from the predetermined approximate polynominal equations. These equations describe the relation between the exposure rate of a single puff and r. The exposure rate of a puff that has an arbitrary values for σsub(z) and β is calculated by interpolating the standard exposure rate. The exposure rates at an evaluation point are calculated by summing up the exposure rate from each puff which forms plume segment. The features of the model proposed in this report are as follows; 1) The exposure rate at the evaluation point within a perimeter about 10 km from a nuclear power plant, where the submersion model is not effective, can be calculated. 2) By incorporating a three-dimensional wind field model into this model, we can take account of spatial variations in the wind field. 3) The exposure rates can be calculated every 10 mins to 1 h in response to temporal changes of meteorological conditions and release rates. (author)