SOMA, Japan — A third explosion in four days rocked the earthquake-damaged Fukushima Dai-ichi nuclear plant in northeast Japan early Tuesday, the country’s nuclear safety agency said.

The blast at Dai-ichi Unit 2 followed two hydrogen explosions at the plant — the latest on Monday — as authorities struggled to prevent the catastrophic release of radiation in the area devastated by a tsunami.

The troubles at the Dai-ichi complex began when Friday’s massive quake and tsunami in Japan’s northeast knocked out power, crippling cooling systems needed to keep nuclear fuel from melting down.

The latest explosion was heard at 6:10 a.m. local time on Tuesday, a spokesman for the Nuclear Safety Agency said at a news conference. The plant’s owner, Tokyo Electric Power Co., said the explosion occurred near the suppression pool in the reactor’s containment vessel. The pool was later found to have a defect.

Chief government spokesman Yukio Edano told news agencies that the suppression pool appeared to be damaged after the blast. He said, however, that measuring devices in the area did not indicate an increase in radiation as a result of the damage.

The suppression pool is used to turn steam back into water to cool the reactor and also plays a role in removing radioactive particles from the steam.

If confirmed, it would be the first direct damage to a reactor at the Fukushima Dai-ichi complex since the massive earthquake and tsunami on Friday.

Radiation levels in the air surrounding the plant have risen four-fold after a fresh explosion at the site on Tuesday, the plant operator said.

The radiation reading at 08:31am local time (2331 GMT) climbed to 8,217 microsieverts an hour from 1,941 about 40 minutes earlier, Tokyo Electric Power Co said.

The company said some employees of the power plant were temporarily evacuated following Tuesday morning’s blast.

The accidents — injuring 15 workers and military personnel and exposing up to 190 people to elevated radiation — have compounded the immense challenges faced by the Tokyo government as it struggles to help hundreds of thousands of people affected by twin disasters that flattened entire communities and may have left more than 10,000 dead.

The latest explosion came as Japanese engineers pumped seawater into Unit 2 of the Fukushima Dai-ichi plant after coolant water levels there dropped, exposing uranium fuel rods.

The water drop left the rods no longer completely covered in coolant, thus increasing the risk of a radiation leak and the potential for a meltdown at the Unit 2 reactor, Tokyo Electric Power Co. said.

3 reactors ‘likely’ melting
Workers managed to raise water levels after the second drop Monday night, but they began falling for a third time, according to Naoki Kumagai, an official with Japan’s Nuclear and Industrial Agency.

It now seems that the nuclear fuel rods inside all three functioning reactors at the Fukushima Dai-ichi complex are melting, a senior government official said.

“Although we cannot directly check it, it’s highly likely happening,” Edano said.

Some experts would consider melting fuel rods a partial meltdown. Others, though, reserve that term for times when nuclear fuel melts through a reactor’s innermost chamber but not through the outer containment shell.

Officials held out the possibility that, too, may be happening. “It’s impossible to say whether there has or has not been damage” to the vessels, Kumagai said.

If a complete reactor meltdown — where the uranium core melts through the outer containment shell — were to occur, a wave of radiation would be released, resulting in major, widespread health problems.

Also unknown was the status of any nuclear waste that might be stored at the site, and whether the pools housing used fuel were still being cooled to prevent a radiation release.

The cabinet secretary’s comments followed a hydrogen explosion at Unit 3 on Monday that injured 11 workers and was heard 25 miles away. A similar explosion happened at Unit 1 on Saturday.

Japanese Prime Minister Naoto Kan later said the government was setting up a joint response headquarters with TEPCO to better manage the crisis.

Of all these troubles, the drop in water levels at Unit 2 had officials the most worried.

“Units 1 and 3 are at least somewhat stabilized for the time being,” said Nuclear and Industrial Agency official Ryohei Shiomi. “Unit 2 now requires all our effort and attention.”

The blast occurred as authorities tried to cool the reactor with seawater.