How is the radius of a nuclear disaster affected area calculated? For example, take the Chernobyl disaster of 1986. The government evacuated a $30\,{\rm km}$ radius. I don't understand why $30\,{\rm km}$, I mean it's radiation, right? It can travel at speed of light...
I have also heard TV crews went there for shooting. I know they stay for a short period of time, but won't they be affected anyway?
 A: I don't know on what assessment was the evacuation area based on. Nevertheless the accident isn't hazardous the way you imply; the melting reactor released radioactive particles and gases that eventually deposited to the ground depending on the meteorological conditions (winds, precipitation). The main and long term danger was because of that material and the vicinity of the site isn't dangerous because the power plant is near (and would somehow radiate to the 30km radius), but because there the concentrations were/are highest. The largest particles fall the fastest and reach the ground very near. Also the release was on for several days, so the wind direction had probably changed during the release and transported the material to multiple directions from the site. Smaller lighter particles and gases were able to travel further and increase radiation levels in a widespread area.
A: Because the radiation is being emitted by a point source (atoms), the ammount of radiation is being reduced by distance Squared. This is the same reason why we get less heat further away form the sun, then nearer to it. 
The other reason is that radiation usually gets quickly absorbed by the medium it passes trough (depending on frequencies of light or the energy of alfa and bety decays). The main problem with radiation is you being near it or actually consuming stuff which is radioactive because it affects you from the inside of the body and continues to radiate when you leave the affected area. The further away you are, the better you're off.
