What causes a nuclear meltdown?

Answer 1

In a pressurized water reactor, the failure of the main cooling system can cause a meltdown of the core. The primary failure that is of concern is a loss of coolant accident. In this incident, a major leak in the Plumbing (due to a pipe breaking or a weld failing or the like) allows coolant to escape. The escaping coolant cannot be replaced quickly or effectively enough to keep the core from overheating. In addition to the loss of coolant, a loss of pressure occurs as well.

With the loss of coolant in a pressurized water reactor comes the loss of the pressure that keeps the water from turning to steam during normal operation. The temperature of the water in an operating reactor is a couple of hundred degrees above boiling, and only pressure prevents it flashing to steam. With water in the core now turning to steam because of the loss of pressure, the temperatures inside the fuel elements will rise dramatically. Emergency cooling systems will be activated in an attempt to pump water directly into the reactor vessel (which holds the core), but the steam will inhibit this. Steam is a poor "carrier" of heat, and it cannot cool the core sufficiently to keep it from overheating. The fuel, which is welded inside tubes or plates, ends up melting its way through the metal encasing it. With the cladding melted, fuel and fission products (which are extremely radioactive) spill out into the vessel and can escape the primary system altogether. This is a major meltdown.

In the instance of the Japanese reactors, the reactors shut down immediately after the earthquake. But the following tsunami knocked out power to the plant and took down the emergency generators as well. The reactors (numbers one, two and three) were left very hot having been run near full power before being shut down. Though fission had stopped, the decay products from fission were still generating a very large amount of heat (called decay heat). And this heat is easily sufficient to melt the core if cooling is not continued. As no power was available to run cooling system pumps, the reactors overheated (due to decay heat) and partial meltdowns occurred.