© Jiji Press/AFP/Getty ImagesMedical staff checks radiation levels of a resident in Koriyama city, Japan.
Tokyo Electric Power Co. engineers tried to stabilize three nuclear reactors damaged by the biggest earthquake in Japan's history after the plant was struck by a second explosion and as water levels dropped at one reactor, exposing fuel rods and increasing the threat of a meltdown.

The cooling system failed at the Dai-Ichi No. 2 reactor today, said Tokyo Electric, which runs the Fukushima nuclear plant 220 kilometers (135 miles) north of the nation's capital. Fuel rods at the reactor may have melted after becoming fully exposed, Chief Cabinet Secretary Yukio Edano told reporters.

A hydrogen explosion occurred at the No. 3 reactor today, following a similar blast on March 12 at the No. 1 reactor that destroyed the walls of its building. The utility has been flooding the three reactors with water and boric acid to reduce the potential for a large release of radiation into the atmosphere following the March 11 earthquake-generated tsunami that smashed into the plant, disabling electricity supply and backup generators.

"They are managing the situation, they have very qualified personnel there," Gennady Pshakin, a nuclear expert based in Obninsk, Russia, said by telephone. "We will have a week or 10 days of this uncertainty, but the situation should normalize. What we need is for the water supply to be constant."

Radiation Concerns

U.S Navy ships and planes involved in earthquake rescue efforts were moved after radiation was detected on three helicopters.

"Low level radioactivity" was detected on 17 air crew members when they returned to the USS Ronald Reagan aircraft carrier operating about 100 miles northeast from the plant, Navy spokesman Jeff Davis said in an e-mail.

Japanese officials yesterday evacuated more than 200,000 people and handed out iodine, used to protect the thyroid from radioactivity, as they extended an exclusion zone around the plant to 20 kilometers.

Winds in the area of the Fukushima plant are blowing at less than 10 kilometers (6 miles) per hour generally in a northeasterly and northerly direction, according to a 9 a.m. update from the Japan Meteorological Agency today.

Core Intact

The vessel containing the No. 3 reactor's radioactive core is intact after today's blast, Edano said. The likelihood of a large radiation leak is very small, even as radiation levels at the No. 3 reactor are rising, said Edano, the government's spokesman.

Tepco, as Asia's biggest utility is known, said at least four employees and two contractors were injured in the blast. The company's shares slumped 24 percent.

There are six boiling-water reactors at the Fukushima Dai- Ichi plant, three of which were shut for maintenance before the earthquake.

Unit No. 1 is a General Electric Co. (GE) model that can generate 439 megawatts of power and began commercial operation in 1971, according to the International Atomic Energy Agency. The No. 2 reactor was built by GE Toshiba and the No. 3 by Toshiba Corp.

Tokyo Electric said the sea water used to cool the two reactors is being retained at the plant. "We will look into what we will do about the water," spokesman Shogo Fukuda said by telephone today.

Flooding the reactors with sea water renders them useless for future power production.

The disaster at Fukushima isn't the first quake-related accident for Tokyo Electric.

A 6.8-magnitude temblor on July 16, 2007, caused a fire and radiation leaks that shut down the Kashiwazaki Kariwa nuclear plant, the world's biggest. It took almost two years to restart.