This animation shows the spread of radiation based on readings taken of the concentration levels of cesium 137 over Europe between April 26 and May 6, 1986.
Download the note accompanying the animation - "Modeling the Europe-wide dispersion of cesium 137 in the air following the Chernobyl accident" (pdf) .fr
.
North Poland, Gdynia it was 850km from Chernobyl.