主题：日本福岛核电站传来爆炸声 -- 葡萄

Air pressure inside reactor 2 rose suddenly when the air flow gauge was accidentally turned off. That blocked the flow of water into the reactor, leading to the water level dropping and the rods being exposed at about 2300 local time (1400 GMT).

Exposure for too long a period of time can damage the fuel rods and raise the risk of overheating and possible meltdown.

Chief Cabinet Secretary Yukio Edano said there were signs that the fuel rods were melting in all three reactors at Fukushima Daiichi.

The fuel rod exposure at Fukushima Daiichi number 2 reactor is potentially the most serious event so far at the plant.

A local government official confirmed the fuel rods were at one point largely, if not totally exposed; but we do not know for how long.

Without coolant around the rods, temperatures can rise to hundreds of degrees Celsius, almost certainly resulting in some melting.

This opens the possibility of a serious meltdown - where molten, highly radioactive reactor core falls through the floor of the containment vessel and into the ground underneath.

However, engineers appear to have restored some water flow into the reactor vessel and if they are successful, temperatures will begin to fall again rapidly.