Nuclear alert

I hope the Japanese authorities are telling the truth about the nuclear reactor building which exploded as a consequence of the earthquake.

We are told that while the outer shell at the Fukushima plant did indeed explode (as seen on YouTube), the inner core, within its steel cradle, remained apparently unimpaired. In which case, why exactly did the outer shell explode? What caused it to do so?

I ask not rhetorically, but out of interest. And also suspicion. They have been as secretive, the Japs, as were the Soviets when Chernobyl blew up and the first we knew of it was from a Norwegian meteorological team.

Incidentally, I’m in favour of nuclear power.