The Istanbul TRIGA Mark-II reactor is situated on a hill outside the populated city area. Its geographical situation is 41-degrees-06'32 " north latitude and 29-degrees-01'44 " east longitude. In the reactor safety report and in other publications, the safety aspects of the reactor and the I-131 isodose distribution have already been investigated. For this article, the downwind distance of the maximum concentration was investigated for different meteorological conditions of Istanbul. The particle size distribution effect will also be discussed, and the importance of atmospheric parameters will be pointed out.