White smoke rose from the No. 3 reactor at the Fukushima No. 1 nuclear plant and radiation levels rose at one point Wednesday, but the government later played down the possibility of grave damage to the containment vessel.

Correcting an earlier remark, Chief Cabinet Secretary Yukio Edano told reporters in the afternoon that the government now believes the water pool for spent nuclear fuel at the No. 3 reactor probably heated up, causing steam to rise.

The containment vessel is the last line of defense for containing lethal radioactive materials, and significant damage would pose grave safety concerns.

“The possibility of any great damage to the containment vessel is low,” the government’s emergency headquarters said in a statement.

But evaporation of the water in the spent fuel rod pool poses another kind of threat.

If the fuel rods were to melt, high amounts of radiation could be released into the environment. The pool is not in the containment vessel.

Unlike the reactor itself, the fuel pool is not protected by a containment vessel and the roof of the No. 3 building was blown away by an earlier hydrogen explosion.

The temperature of the water in the spent fuel pool at the No. 4 unit also spiked Wednesday. The reactor had caught fire a day earlier after a hydrogen blast created two big holes in the facility’s wall.

Providing more water is urgently needed to prevent the fuel rods from melting. In a race against time to cool the water pool, the government dispatched a Self-Defense Forces C-47 helicopter carrying a bladder to dump water into the pool.

But the plan was canceled for the day because of the abnormally high level of radiation escaping from the plant.

It was later reported that a Metropolitan Police Department water cannon was requested to pump watere into the overheating facilities.

Earlier in the day, the nuclear safety agency said the radiation level briefly reached 10 millisieverts per hour at the plant’s main gate at 10:40 a.m.

Still, that was lower than the 400 millisieverts per hour — a level equivalent to roughly 400 times that at which people can be safely exposed in one year — that was recorded Tuesday and the maximum so far reported at the plant after apparent hydrogen blasts hit the No. 2 and No. 4 reactors.

Radiation levels had dropped to 1.5 millisieverts per hour at the main gate by 4 p.m.

Tokyo Electric Power Co., operator of the nuclear plant, instructed its officials to evacuate the area.

Despite a series of events that further raised fear of radiation leakage, the government said it doesn’t intend to expand the evacuation zone.

At present, residents within a 20-km radius have been ordered to evacuate and people between a 20- to 30-km radius have been instructed to stay indoors.

Seawater continued to be pumped into all the Nos. 1, 2 and 3 reactors, but water levels were still not high enough to cool all of the fuel rods.

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