A partial meltdown of nuclear fuel rods has occurred in two, or perhaps three, nuclear reactors at the Fukushima Daiichi power plant in Japan. What does that mean exactly?

    “A meltdown is when the uranium dioxide fuel melts*. The melting temperature of uranium dioxide is 2,865 degrees Celsius,” said Martin Bertadono, a nuclear engineer at Purdue University.

    Under normal conditions, the uranium dioxide fuel rods* are kept at a temperature below their melting point. Heat emitted* by the rods gets carried away by the water that surrounds it, which is constantly* being pumped through the nuclear core housing the rods. But if the water doesn’t get pumped through quickly enough, it heats up too much and starts boiling and evaporates*.

    “As the water boils, the level of water in the reactor drops. If the level drops below the top of the fuel, it starts heating up. If this goes on long enough, about an hour, the uranium begins to melt,” Bertadono said.

    “If the water is allowed to boil off completely the whole reactor could melt down,” he said.

    By flooding the failed nuclear reactors with seawater, technicians at the Fukushima plant seem to be avoiding a full meltdown, but a partial meltdown has already happened.

    What happens when there’s a partial meltdown?

    “When the fuel melts, the [nuclear] fission* products can be released from the fuel into the reactor pressure vessel, and then into the containment* if the coolant* leaks from the reactor vessel,” said Taiwo Temipote, a nuclear scientist at Argonne National Laboratory.

     (SD-Agencies)