Results and Discussion

Simplified Meteorological Condition

Before carrying out simulations with the calculated meteorological variables from MM5, this study conducted a series of preliminary simulations with a set of simplified meteorological conditions, that is, a uniform and constant wind field of 1 m s−1, horizontally uniform but vertically varying vertical eddy diffusivities based on the calculated values of MM5, and prescribed intensities of uniform precipitation. The LPRM was the same as the main simulations. The purpose of these simulations is to discuss dependency of deposition and air concentration on deposition parameters.

The results are shown in Figs. 14.2 and 14.3. The dependency of dry deposition on the deposition velocity is quite simple. The dry deposition rate from the plume monotonously decreases with travel time, which is the elapsed time from release, mainly from horizontal and vertical diffusion causing air concentration decreases (Fig. 14.2a). The dry deposition rate for the 0.3 mm h−1 case is about 11.4 % of that for the 3 mm h−1 case at the travel time of 6 h. This ratio is almost same with the ratio of deposition velocities, but slightly modified by the change in the air concentration. The concentration for the 3 mm h−1 case is about 88 % of that of the 0.3 mm h−1 case (Fig. 14.2b). These results imply that the dry deposition process does not significantly reduce the amount of radioactivity in air within a period of several hours.

The dependency of wet deposition on the scavenging coefficient is somewhat complicated (Fig. 14.3a). A larger scavenging coefficient does not always cause greater deposition. Among the four cases, the largest scavenging coefficient, which corresponds to the rain intensity of 6.6 mm h−1, resulted in the largest wet deposition

Fig. 14.2 Variation of the calculated values of (a) dry deposition rate and (b) air concentration with travel time. Dry deposition velocities, Vd, are 0.3, 1.0, and 3.0 mm s−1

only in the first 0.6 h from the release. The wet deposition of this case becomes the smallest after a travel time of 1.7 h. On the other hand, the wet deposition in the case with the smallest scavenging coefficient (rain intensity of 0.37 mm h−1) is the largest among the four cases after the travel time of 3.9 h. This complicated result is caused by the substantial depletion of the plume by scavenging (Fig. 14.3b). In the cases with large scavenging coefficient, the air concentration rapidly decreases with travel time.