Tokyo Electric Power Co. acknowledged for the first time possible damage to core pressure containers at the crippled Fukushima nuclear plant--the last line of defense in preventing radioactive materials from spewing out.

TEPCO officials told reporters Monday morning that despite the continuous pumping in of water to cool down the No. 1, No. 2 and No. 3 reactor cores, water levels were not rising as expected, meaning the pressure containers may not be completely sealed off.

The water, which is believed to be mixing with radioactive materials from the fuel rods within, is likely leaking from the pressure containers, they said.

The tsunami that hit the Fukushima No. 1 nuclear power plant after the March 11 Great East Japan Earthquake knocked out the emergency generators at the three reactors. The systems that circulate water within the pressure containers to cool the fuel rods also stopped working.

Workers have been pumping in water through pipes to the pressure container to submerge the fuel rods and directly cool the decay heat that continues to be emitted even after the reactors were stopped.

But the water level meters for the three reactors have not risen as expected.

TEPCO officials said a possible reason the water levels were not rising sufficiently were breaches in the lower part of the pressure containers. They said they did not know what caused the possible damage.

A pressure container holds nuclear fuel pellets placed in metal rods that have been bundled together. A containment vessel, located within the building housing the reactor, surrounds the pressure container.

The pressure containers at the Fukushima plant are made of steel 16 centimeters thick. The lower part of the containers have openings for measuring and other equipment. Water may be leaking from around those parts, the officials said.

TEPCO has cited the possibility that fuel rods may have been damaged due to overheating after being exposed above the waters surface in the core.

According to experts, if the fuel rods are damaged and crumble, the melted fuel could fall and accumulate into a lump at the bottom of the pressure container. In that scenario, it would be more difficult to cool because of the fuels larger surface area. The experts also said damaged fuel rods could have reached high temperatures that melted the walls of the pressure container.

However, TEPCO officials said major structural damage was unlikely and the soundness of the pressure container has probably been maintained because the pressure within the container remains at higher levels than the outside atmosphere.

TEPCO officials are trying to restart the cooling systems that circulate water so that they can stop discharging the water to the outside. But they have run into problems in obtaining a stable electric power source.

For now, they said the pumping will have to continue to cool the fuel rods, which raises concerns about highly contaminated water leaking steadily through breaches in the pressure containers.

At the same time, the Nuclear Safety Commission of Japan said Monday that extremely high levels of radiation have not been recorded outside the reactor buildings, making it possible to continue pumping in water and releasing steam to cool the core, even if water continues to leak.

The NSC also commented on the water accumulated at the basement of the turbine building of the No. 2 reactor that had radioactivity levels about 100,000 times above the normal level.

The commission said the water likely flowed into the turbine building from the core containment vessel after coming in contact with fuel rods that had temporarily melted.