The exclusion zone is an area 30 km around the failed power plant. The zone falls into two parts: an outer part and an inner part which is on the 10 km line from the power plant. To enter the zone you need a permit, and you need further permits to proceed to the inner zone and I assume that you also need a permit to enter Pripyat. If you join an organised tour, they arrange all the permits for you. All you need to do is to bring your passport. Here we have reached the checkpoint for the outer zone.