Water levels temporarily fell at Tokyo Electric Power Co.’s No. 3 reactor at the Fukushima Dai-Ichi plant 135 miles north of Tokyo, raising the possibility of a hydrogen explosion, Chief Cabinet Secretary Yukio Edano said in Tokyo today.

Asia’s largest utility is battling to prevent a meltdown of two reactors at the nuclear power station by flooding them with water and boric acid to eliminate the potential for a catastrophic release of radiation into the atmosphere. The station lost power to keep the reactor core cool after the March 11 earthquake, the largest ever recorded in Japan.

The “likelihood of success should be fairly high,” Dale Klein, a professor of nuclear engineering at the University of Texas at Austin and former chairman of the U.S. Nuclear Regulatory Commission, said of the seawater flooding. “This should have been part of their overall strategy to keep the core covered and cooled.”

Radiation rose yesterday after a hydrogen leak caused a blast that destroyed the walls of the No. 1 reactor. Four workers were injured in the explosion, while no damage was reported to the container holding the reactor’s radioactive core, according to Tokyo Electric.

Radiation levels peaked at 13:52 p.m. local time and have declined since, Tokyo Electric spokesman Naoki Tsunoda said.

Wind Directions

Winds in the area of the Fukushima plant are blowing at less than 18 kilometers (11 miles) per hour generally in an easterly direction, according to a 12 p.m. update from the Japan Meteorological Agency.

The International Atomic Energy Agency, in partnership with the World Meteorological Organization, forecasts that winds will blow any atmospheric radiation northeast over the next three days, according to an IAEA statement.

Tokyo Electric is preparing to vent gas from containment areas at neighboring nuclear power station, Fukushima Dai-Ni, spokesman Akitsuka Kobayashi said yesterday. The station has four reactors.

The IAEA said Japan has informed the Vienna-based agency that casualties have risen among workers at the reactor site. A crane worker was killed at the Fukushima Dai-Ni plant and seven emergency response workers have been injured, including four hurt in yesterday’s explosion, the IAEA said in a statement on its website.

Tokyo Electric will start power outages in parts of the greater Tokyo area from tomorrow, according to a company statement.

Nuclear Meltdown

Inadequate cooling of the reactor core may lead to a meltdown, the most dangerous kind of nuclear power accident because of the threat of radiation releases, according to the Nuclear Regulatory Commission. The 1979 partial meltdown at the Three Mile Island reactor in Pennsylvania failed to breach the containment building, according to the commission.

“Only a small amount of active particles made it outside and were released into the atmosphere, so there were no consequences for the population,” Rafael Arutyunyan, first deputy director of Institute for Safety of Nuclear Energy at the Russian Academy of Sciences, said on Russian television over the weekend in reference to Three Mile Island. “That’s the way we’re heading at the moment” in Fukushima, he said.

‘Station Blackout’

The Fukushima complex lost power after the earthquake when its reactors shut automatically and a backup generator failed, making it difficult to circulate cooling water, Tokyo Electric has said. Without circulation, water within the reactor can boil away, exposing the hot fuel rods and starting a meltdown, Klein said. Three Mile Island operators exposed the core by mistake, the U.S. concluded.

“The difference here is that people understand what’s happening,” Klein said. “They are just having difficulty getting the equipment to work because of the very adverse conditions of both an earthquake and a tsunami.”

The type of accident that involves the loss of both the electrical grid and backup power on site is known as a “station blackout,” said Ken Bergeron, a physicist and former staff member at Sandia National Laboratories, where he worked on nuclear reactor accident simulation.

“It’s considered to be extremely unlikely, but the station blackout has been one of the great concerns for decades,” he told reporters on a conference call. “We are in uncharted territory. We are in the land where probability says we shouldn’t be and we are hoping that all of the barriers to release of radioactivity will not fail.”

Chernobyl Meltdown

There are six reactors at the Dai-Ichi site. The unit being flooded, No. 1, is a General Electric Co. boiling-water reactor model that is capable of generating 439 megawatts of power and began commercial operation in 1971, according to the International Atomic Energy Agency.

If the seawater-flooding attempt fails, engineers may have to pump in sand and cement to entomb the reactor, Peter Bradford, another former member of the Nuclear Regulatory Commission, said on the press conference call.

That ended contamination from the 1986 Chernobyl accident in Ukraine, where the meltdown of a reactor without a containment building killed at least 28 workers, the United Nations Scientific Committee on the Effects of Atomic Radiation said in a 2011 report.

Death Toll

Thousands were evacuated in Japan as workers vented radioactive gas yesterday from the Fukushima plant. The death toll from the quake and the tsunami that followed topped 970, with more than 700 missing, 1,683 injured and 350,000 in emergency shelters. The death toll may reach 10,000, national broadcaster NHK reported, citing local police.

Tokyo Electric took almost two years to restart power generation at the Kashiwazaki Kariwa nuclear plant in the country’s northwest after a 6.8 magnitude temblor on July 16, 2007, caused a fire and radiation leaks at the world’s biggest atomic energy station.

Nuclear energy provides almost 30 percent of Japan’s electricity, with total capacity of about 47,000 megawatts, with plans to increase that to 40 percent by 2017, according to the World Nuclear Association. The nation’s first reactor began operating in 1966 and there are 54 reactors in the country.

--With assistance from Yuriy Humber, Michio Nakayama, Aki Ito and Aaron Sheldrick in Tokyo, Anna Shiryaevskaya in Moscow, Kim Chipman in Washington, Jim Polson in New York and Rachel Layne in Boston. Editors: Amit Prakash, Peter Langan