The greatest hazard in a nuclear reactor accident does not come from the direct effects of the explosion or the fire but the subsequent release of large amounts of radioactive materials. If radioactive material in the form of smoke or dust escapes into the atmosphere it will be carried around the globe by winds. In the immediate area it will descend to the ground as lethal fallout.
The most disastrous nuclear reaction accident happened at Chernobyl, near Kiev, on April 26th 1986. The fallout spread north and west into Europe and produced hazardous contamination as far away as Italy. A 30km radius around the reactor was so badly contaminated it has been permanently evacuated. The reactor was completely destroyed by the explosion and fire and the remains were entombed in concrete. Besides the 31 reactor operators and firefighters who lost their lives an expected 3000 delayed casualties are expected from cancer induced by the radiation in the subsequent 20 years.
The design of the reactor lead to problems as water circulated through the moderator - graphite. Here the water was designed to inhibit the reaction but this causes an instability as the water can overheat. In this case the power increased to such values the fuel rods shattered, the energy released boiled the water forming a large volume of steam which blew the lid off the reactor. The explosion also causes a fire in the graphite and released large quantities of radioactive particles into the atmosphere.