Reactors 1 & 3 had hydrogen explosions in the upper structure that held the refueling crane and covered the spent fuel pool. It has been suspected that reactor 2 had a hydrogen explosion in the torus that is used to cool steam being vented from the reactor pressure vessel, cracking it. This the likely source of the very contaminated water in the trenches around the turbine buildings. The valves from the RPV to the torus were closed after the suspected explosion and crack. This water from the torus would have boiled in the secondary containment and condensed after leaking from the secondary containment.

According to the latest IAEA Briefing on Fukushima Nuclear Accident (29 March 2011, 16:30 UTC) at http://www.iaea.org/newscenter/news/tsunamiupdate01.html

The RPV has not been breached in any of the reactors and the non empty reactors (1-3) are being cooled with fresh water.