The core is placed in the pressure vessel. The pressure vessel is a thick steel vessel that operates at a pressure of about 7 MPa (~1000 psi), and is designed to withstand the high pressures that may occur during an accident, but not a full melt down.
The explosions that we see, are not the containment structures. The "core" casing-catcher (main containment) of the plant, is intact. The plant is stable. Some radiation was released when the pressure vessel was vented. Most radioactive isotopes from the activated nitrogen and noble gases have since (decayed). There is damage to the first containment (Fuel Damage-Melt Down)
Both the main containment structure and the secondary containment structure are housed in the reactor building. The reactor building is an outer shell that is supposed to keep the weather out, but nothing in.
Leaking coolant does not mean that the main containment structure has failed.
The Zircaloy fuel rod barrier did its job, and the core was shut down within seconds of the Earthquake. But because the cooling cannot be restored, the core will eventually melt (after several days), and will "likely" be contained in the containment structure, as it has been cooling down for several days now.
Yes... I said "Likely Contained"