Construction of system for evaluation of environmental emergency dose

On emergency case at nuclear power station (NPS), the prediction of environmental activity concentration and dose rate is very important for decision of countermeasure of radiation protection. These predictions are able to be figured, in Japan, according to the guide which shows simplified calculation models and SPEEDI (System for Prediction of Environmental Emergency Dose Information) network that belongs to Science and Technology Agency. On the other hand, electric power company has its own system because we need to grasp the situation of the diffusion of activity in site before startup of SPEEDI. TEPCO has been using the system like this since 1988. But the calculation is based on old series of two dimensional numerical simulation models, so we are reconstructing system to meet the necessity of details and new functions. We constructed the system of three dimensional numerical dispersion simulation model (prototype) around the NPS that is worked by EWS on the assumption that the discharged radioactive gaseous wastes are noble gaseous, radioactive iodine and particles. The range for evaluation are 10km*10km*0.5km and 50km*50km*1km. In the NPS, meteorological data and discharged source data are gathered by on-line network at 10-minutes interval and we will be able to evaluate the environmental emergency dose and predict it by using the GPV (Grid Point Value) data that is reported by Meteorological Agency. In case of emergency disaster at NPS, when the radioactive gaseous waste is discharged we need to evaluate the radiation exposure rate. To do the prompt evaluation on the basis of discharged circumstances after startup of discharge and observed meteorological conditions we adopt PIC (Particle In Cell) method for dispersion model analysis, Mass-Conservation model for preparation of air flows, cell dose model for external exposure and evaluation equation of ICRP Pub 30 for internal exposure. The calculation models and codes are confirmed by comparing them to the analysis solution of concentration and dose rate in air at the condition of flat geographical feature and unchanged weather, and the experimental concentration at field tracer diffusion experiment. The meteorology numerical prediction model that is used in this system is Local Circulation Assessment and Prediction System, LOCALS in short. This model is a hydrostatics model including turbulent closure model. As the real time prediction, we use numerical forecasting data (GPV data) at 20km interval in horizontal direction produced by Japan Meteorological Agency as initial and boundary conditions of the model. The series of three dimensional numerical simulation models make more detailed evaluation result that the condition of geographical features is taken into consideration. And, for example, under the pressure of necessity of primary containment vessel vent as the severe accident we will be able to decide the right time when the activity should be discharged. We, TEPCO will use the system that we present at this program as a prototype, and we will introduce this system that evaluates discharge and weather conditions online to the each NPS. (J.P.N.)