A sixth reactor at the Fukushima nuclear power plants has lost its ability to cool the reactor core since the massive earthquake and ensuing tsunami struck Japan Friday, said Tokyo Electric Power Co.(TEPOC), operator and owner of the plants, early on Sunday.

The No. 3 reactor at Fukushima No. 1 nuclear power plant lost the function after No. 1 and No. 2 reactors at the No. 1 plant and No. 1, No. 2 and No. 4 at the No. 2 plant had suffered the same trouble.

At an emergency press conference early Sunday, Chief Cabinet Secretary Yukio Edano said that the reactor is releasing radioactive steam after the malfunction occurred.

An explosion occurred at the Fukushima No. 1 nuclear power plant Saturday, destroying the roof and the walls of the building of the No. 1 reactor's outer container.

Four people were injured at the power plant, but radiation levels dropped quickly after surging for a while following the blast.

Edano said that the TEPOC has begun new cooling operations to fill the reactor with sea water and pour in boric acid to prevent an occurrence of criticality.

He said that there had been no major changes in the results of monitoring radioactivity near the No. 1 reactor.

The top government spokesman warned that there are still 114 people staying within a 10-km radius of Fukushima No.1 nuclear power plants and 180,000 in the 20-km evacuation zone.

All inhabitants have been evacuated from a three-km radius of the No. 2 plant, and authorities have begun evacuating more than 30,000 from a 10-km zone around the plant, he added.

On Saturday authorities expanded the evacuation zone from a 10- km radius for the Fukushima nuclear plants to a 20-km radius.

Separately, the Fire Disaster Management Agency said Sunday that 15 people have been exposed to radioactivity within a 10-km radius of Fukushima No. 1 nuclear power plant.