There is an excellent up-to-date and evolving article on Wikipedia concerning the Fukushima I nuclear accident. Here's a quick synopsis of what instigated the crisis as I understand them.

Only 3 of the 6 cores were online at the time of the quake.

Those 3 cores automatically shut down when the quake hit.

Even though the cores are shut down they still require active cooling due to residual heat generation.

Even the spent fuel must be actively cooled.

Since the plant is no longer generating its own power, it relies on external power from the electrical grid.

The electrical grid was knocked out due to the size of the quake.

The plant has backup generators for such a contingency, and they came online as expected.

About one hour later the tsunami hit which topped the sea wall and flooded the low lying generator building.

This left only batteries to power everything, which would only last about 8 hours.

Spare batteries and portable generators were/are being brought in. However, hooking them up has been problematic as the hookups and routing are in the flooded generator building.

Hope this helps clear up a few things.

Brian

• Fukushima I nuclear accidents - Wikipedia, the free encyclopedia