Since they did not see an increase in onsite or offsite rad levels following the explosion, that supports the hypothesis that containment is intact.Which speaks to the strength of those reactors, to have weathered a 9.0 earthquake, a 10 meter tsunami, AND numerous 6.0 or worse aftershocks, and still be intact. The problems stemmed from a loss of coolant, which was caused by a loss of power to the coolant pumps. The newer generation of the reactors does not rely on electric power to run the coolant through, and around the reactor, so those will be extremely reliable.
It's my understanding that the Fukushima reactors, being the Generation II design, are some of the oldest active reactors in Japan, and were due to be 'retired' in the next few years, when the Gen. III, which doesn't have the coolant pump issue, come online.
My guess is that the event exceeded the design basis earthquake but they were still able to get those units shutdown safely (i.e., scam and initial cooldown). The trips on seismic event worked. The emergency diesels started and provided initial site emergency power in the station blackout event that followed the earthquake. It was the common-mode failure of the tsunami taking out the diesels that started the whole accident sequence. That will probably be the result for the rest of the industry, looks at post-seismic phenomena and take steps to mitigate the consequences of those.