This view from Japan state broadcaster NHK shows a Japanese Self Defense Force helicopter dropping water on reactor 3 at the Fukushima Daiichi nuclear power plant Thursday. The reactor is the smaller steam-emitting building to the right of the tower' the helicopter is at top center dropping the water.

The Tokyo Electric Power Co. has successfully restored power to part of a dangerously overheating nuclear power plant damaged by last week's earthquake and tsunami, a key move in efforts to cool the site.

The power company, also called Tepco, connected a power line to reactor 2, one of six at the Fukushima Daiichi power station about 140 miles northeast of Tokyo. The site has six reactors' keeping them and their associated spent-fuel ponds cool has been a problem ever since the massive magnitude 9.0 earthquake and resulting tsunamis knocked out power and damaged auxiliary cooling pumps.

The International Atomic Energy Agency reported that the new power cable was connected to reactor 2, which suffered a serious explosion Monday that breached its primary containment vessel. Due next for a connection is reactor 3, which suffered an explosion of its own and which is fueled by a more hazardous combination of uranium and plutonium oxide.

Tepco had been trying to connect the power line yesterday, but radiation risks postponed the work.

The situation at the reactors is dire. A fire earlier this week forced most of the plant's staff to evacuate, complicating efforts to stabilize temperatures. Today, highly unusual efforts to keep the fuel from overheating included spraying 30 tons of water from trucks and dropping several loads of water from a helicopter.

The power line means cooling reactors with electrical pumps will be much easier. Japanese state broadcaster NHK reported today that the tsunami damaged the main pumps, so makeshift ones will be used instead.

Tepco already had resorted to cooling reactors 1, 2, and 3 with seawater, another last-ditch move because seawater is highly corrosive. Reactors 4, 5, and 6 had been shut down before the quake because of an inspection, but they, too, are a problem because of hot fuel in their spent-fuel ponds. Reactor 4's pond was the site of a serious fire already.

This illustration, based on GeoEye satellite photo viewed through Google Earth, shows the locations of the six Fukushima Daiichi nuclear reactors.