The immersion model based on the meteorological dispersion of the cloud can not be applicable to the estimation of exposure dose near the release point. And it is also important to estimate the exposure of personnel in site of the reactor, and results of the estimation would be useful to know the exposure pattern of ⁴¹A which is released in normal operation of the reactor. In this paper the estimation was made of the external dose near the release point, comparing with those in immersion case, and the aspect of external dose from radioactive cloud in whole area of interest was figured out.
The calaulation was carried out under the following assumptions:
(1) The cloud is of the coning shape with a vertex angle 20° which corresponds to category D (stable) in Meade and Pasquill's classification.
(2) The condition of continuity of radioactivity holds and the distribution of the radioactive concentration is Gaussian in the radial direction of the cross section of the cloud.
The results show that the external dose near the release point from the coning cloud is in the same order as the maximum dose in immersion case and never exceeds it. Figures were given showing the effect of variation of release point height and iso-dose distribution near the release point. The error resulted from some simplifications of the original equation was within 10%.