http://i.imgur.com/CckjP.jpg (X)
The reactor is the red cylinder in the middle, the containment is the pear-shaped cavity it sits in, closed on top with the yellow hood and connected to that torus in the lowest-level which in case of an accident is supposed to condense the steam back into water, thereby removing heat from the containment.
The part that's exploded is the flimsy metal "hut" on top of the concrete building.
(X) from another HN comment I can no longer find, sorry, no attribution for who posted it initially
NRC: "Reactor Concepts Manual | Boiling Water Reactor (BWR) Systems"
http://www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf
Here's one with labels:
Magdi Ragheb, "Containment Structures"
https://netfiles.uiuc.edu/mragheb/www/NPRE%20457%20CSE%20462...
There are six, and units 4,5,6 were already in a cold state before the tsunami (for maintenance); they're not involved.
You could be thinking of Fukushima Daini, which has four reactors.