Video from sky news
These reactor buildings are of the General Electric Mark 1 design. This dates from the late 1960s and it has faced a lot of criticism from anti-nuclear groups. It hasn't been legal for new construction since the 1970s.
A lot went wrong at Three Mile Island, but one of the success stories is that the containment building worked. That building also faced a hydrogen explosion, but the explosion was contained within the structure and the building survived intact.
Here is a diagram of the Mark 1 containment, similar to the ones at Fukushima. You can recognize the square shape of the building. It can accommodate several different sizes of reactor. The containment vessel is within the building. It has the shape of an upside down light bulb. It seems that the containment vessel is still intact, although some of the concrete structures around it have been blown away by the hydrogen explosion.
One of the questions that needs to be asked is if a more modern design would have performed better. The latest containments, if I remember correctly, can survive about 200 atmospheres of pressure. The Japanese kept the pressure in this one under 8 atmospheres.
The image below is one of those pictures which really tells a story. On the left is a before picture of the plant. Note all that equipment close to the ocean! On the right is a picture taken after the tsunami. Note that a lot of stuff isn't there anymore! This plant was massively damaged by the tsunami, and it is no surprise that it is in big trouble. There is a much better interactive version of this picture here. Scroll down their page to find it.
Obviously this was a very extreme event. One of the lessons that needs to be learned here is that the prediction of geological hazards is an imprecise business, and that a safety factor should be applied above and beyond the earth scientist's worse case projections.