How did the explosion happen in Japan?

Answer 1

There were several explosions; but the two most notable were:

-The nuclear reactor explosion in Sendai, and

-The oil refinery explosion in Ichihara.

The nuclear reactor explosion:

It occurred when the engines powering the cooling system failed in the earthquake. Without constant cooling, the radioactive material inside the reactor heated up, causing immense pressure. The tank burst because it couldn't support that pressure. To date (March 12, 2011), the Japanese government has ordered an evacuation of a 20 mile radius around the plant and is also supplying victims with iodine tablets (which blocks some absorption of radioactivity).

The oil refinery explosion:

So far, the trigger of the blaze is unknown, but officials say it is likely that a gas line was burst in the initial earthquake. More information will be available when an investigation can be made into the cause.