The accident occurred when the fourth reactor suffered a huge power increase. This led to the core of the reactor exploding. The explosion was so powerful that it blew the 1000 ton steel lid off the reactor. Due to this explosion, large amounts of radioactive materials and fuel were released. This caused the neutron moderator, made of graphite, to start to burn. The fire caused more radioactive fallout to be released, which was carried by the smoke of the fire into the environment.