So, this is (surprise!) a bit more complex. Yes, heavier chunks landed nearby. Those were the ones with the surface to mass ratio low enough to make air drag almost negligible and were governed by basic kinematics and advection. (Taken from Partial Differential Equations with Fourier Series and Boundary Value Problems by Nakhlé H. Asmar, 2nd edition) The rest travelled much farther, and it didn't matter which isotope they contained. It depends on moisture, wind strength and direction, initial height those 'dust' particles achieved etc etc etc. Notice that on maps like this one: you get concentrated spots. That's where it was accumulating due to large-scale air currents, rainfall and, basically, weather.But I do remember that the heavy particules that have the longer half life could not travel too far with the wind - which is why the 10km zone is the most dangerous one. And that living organisms absorb those particules so you should not eat fish, berries, mushrooms - even if thy are abundant.
