Question:

Please answer Fast!!! I'm in class and need help I can't find this answer anywhere

Answer:

It didn't. The last nuclear explosion in Japan was in 1945. The vented steam and hydrogen caused by the overheated fuel rods exploded outside the containment vessel, damaging the exterior of the buildings.

Here is an informative article describing the situation:
http://bravenewclimate.com/2011/03/13/fu…

And here is where you find current, factual status information:
http://www.iaea.org/newscenter/news/tsun…

And a slide presentation that describes the effects:
http://www.slideshare.net/iaea/fukushima…

And here is a chart that helps make sense of the numbers:
http://www.xkcd.com/radiation/