The Fukushima I and Fukushima 3 nuclear reactors have both previously experienced explosions, and are in some stage of meltdown.
Today, Kyodo News reports:
Fuel rods at the quake-hit Fukushima ... No. 2 reactor were fully exposed at one point after its cooling functions failed, the plant operator said Monday, indicating the critical situation of the reactor's core beginning to melt due to overheating.
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The utility firm said a hydrogen explosion at the nearby No. 3 reactor that occurred Monday morning may have caused a glitch in the cooling system of the No. 2 reactor.
Similar cooling down efforts have been taken at the plant's No. 1 and No. 3 reactors and explosions occurred at both reactors in the process, blowing away the roofs and walls of the buildings that house the reactors.
It is feared that the No. 2 reactor will follow the same path. To prevent a possible hydrogen explosion at the No. 2 reactor, TEPCO said it will look into opening a hole in the wall of the building that houses the reactor to release hydrogen.
And see this.
There are problems at a total of 6 Japanese nuclear power plants.
Update: There has now been an explosion at reactor number 2. As NHK reports:
The sound of a blast was heard Tuesday morning at the troubled No. 2 reactor of the quake-hit Fukushima No. 1 nuclear power plant, the government said.
The incident occurred at 6:10 a.m. and is feared to have damaged the reactor's pressure-suppression system, the Nuclear and Industrial Safety Agency said, citing a report from the plant's operator Tokyo Electric Power Co.