What happens if the fukushima plant explodes?

Actually, at the time this question was written, the plant had already undergone three explosions.

According to the New York Times, nuclear fuel was thrown about 1.6 km (1 mile) from the plant, and the ground in the are was made radioactive.

The reactor containment vessels failed sufficiently to leak large amounts of radioactive material into the sea. Later the water coming from the reactors and spent fuel pools was contained in storage tanks.

Windborn radioactive material has been carried to various places near the plant, requiring about 100000 people to be evacuated.

You should understand, however, that the explosions at the Fukushima Daiichi plant were not nuclear in nature, even though they did through radioactive material around. They were caused when an air-hydrogen mix in the reactor buildings ignited and exploded.