Watching NHK stream & from various news sources here's what I can tell right now:
Overnight the water levels inside Fukushima 1 Reactor 2 started to drop, so they started pumping seawater in. Several times the course of this, the valve they were using to inject water closed. The water levels then dropped low enough for an internal sensor to alarm. This sensor was set at about 3/4 meter below the top of the fuel rods, a level at which it's very likely that at the very least melting of the top of the core structure and possibly the top of the fuel rods has occurred. Water levels had been brought back up to cover the rods half-way, but then @~6:15am there was a third explosion inside the Reactor 2 structure, believed to have occurred near the torus (lower suppression pool below the reactor). This explosion did not damage the outer (secondary) containment. Afterwards the radiation levels outside the power plant spiked to about 8217 mSv, while the pressure inside the primary containment dropped from 3atm to 1atm. They believe the spike in radiation might have been caused by a release of radioactive steam from the primary cooling system.
XI Wiki



