On 28th March 1979, at Three Mile Island nuclear power plant in USA, part of the core melted in the # 2 reactor. The TMI-2 reactor was destroyed. The cause of the accident was the little malfunction in the secondary cooling circuit which allowed temperature in the primary coolant to rise. This caused the reactor to shut down automatically. This situation developed because the level controls turned off the coolant to the reactor when they detected presence of cooling water near the top of the tank. The water at the top was not because of the tank got completely filled, it was because the water was too little in the tank that it got boiled and swelled up to the top of the tank.