The radioactive fuel rods at the No. 2 reactor of the Fukushima No. 1 power station were fully exposed at one point Monday, Tokyo Electric Power Co. said, raising the possibility that it suffered a partial core meltdown.

The utility operating the Fukushima plant later said the level of coolant water in the reactor’s container was raised 2 meters above the base of the rods — which are about 4 meters long.

However, it was not clear if Tepco was able to pump enough coolant into the containment vessel to cool it off. Nevertheless, Chief Cabinet Secretary Yukio Edano told a news conference Monday evening that the situation stabilized after cooling resumed.

Fears of the worst-case scenario — a total core meltdown — are increasing because the No. 2 reactor’s self-cooling system failed and sea water was being pumped in from outside.

Tepco said the water levels fell because the pump temporarily ran out of fuel and workers failed to notice it quickly enough.

It was not immediately clear how long the reactor’s core lay fully exposed or to what extent it heated up in that time.

Also Monday, another hydrogen explosion occurred at the Fukushima No. 1 nuclear power station, this time blowing off the housing of the No. 3 reactor into the sky and injuring 11 people.

Edano gave assurances that the No. 3 reactor’s containment vessel survived the explosion, just like the one at the No. 1 reactor, which blew up its housing on Saturday.

The latest explosion came as the government was busy conducting a massive rescue and body retrieval operation all along the Tohoku region coast, and the threat of massive power outages in the Kanto region.

Like Saturday’s explosion, Monday’s 11:01 a.m. blast at the No. 3 reactor spewed smoke from the building and left only sections of the internal frame visible.

The No. 3 reactor’s fuel rods heated up, reacted with water and caused the release of hydrogen gas, which accumulated and mixed with oxygen to trigger the blast.

Tepco officials considered removing all wall panels to reduce the pressure but felt such a move would be too dangerous.

Tepco has been pumping seawater into the three reactors in a desperate bid to cool them down. But the utility had to temporarily suspend the operation after its seawater storage tank ran dry, apparently resulting in the fuel rods heating up.

The seawater injection stopped at around 1 a.m. due to the water tank shortage but resumed for the No. 3 reactor at 3:20 a.m., according to the nuclear safety agency.

The halt of coolant water injections apparently caused pressure in the reactor container to rise. It also caused radiation at the plant to climb as well, the agency said.

Tepco at one point planned to release radioactive steam from the No. 3 reactor container to depressurize it and ordered workers to vacate the site. But as the pressure later receded, workers resumed operations at the site, the agency said.

The government had warned that this kind of blast was likely to occur.

Because of the explosion, the government told about 500 residents within a 20-km radius of the plant who were in the process of evacuating to get back indoors for the time being.

“According to the data we have gathered, the reactor container remains sound,” Edano told a news conference, adding that radiation levels measured at several locations have not shown a massive leak.

For now, the agency has ruled out widening the evacuation zone.

On Monday, radiation on the plant’s premises rose over the benchmark limit of 500 microsieverts per hour at two locations, hitting 751 microsieverts at the first location at 2:20 a.m. and 650 at the second at 2:40 a.m., according to the report.

Information from Kyodo added

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