Engineers are starting the power cord effort with Reactor No. 2 because its outer building has not blown off, thus making it hard to spray in water the way they can with Nos. 1, 3 and 4, according to NHK, Japan’s national broadcaster, which cited power company officials.

The plan was to lay a 1.5-kilometer power cable between Reactor Nos. 1 and 3 to get to No. 2. If they can hook it up, it will theoretically be able to power all six reactors. The main hazard was the exposure of workers.

Unable to contain the catastrophe on its own, Tokyo Electric has received help from Japanese police and fire departments and the country’s Self-Defense Forces. Assistance has started to flood in as well, with nuclear experts arriving from the United States and international agencies. France and South Korea are also providing support.

Overnight in Japan, crews from the Tokyo Fire Department doused water on Reactor No. 3, which was doused earlier Friday by teams from the Self-Defense Forces and the United States military. Workers planned to continue the spraying on Saturday.

In a further sign of spreading alarm on Friday that uranium in the plant could begin to melt, Japan planned to import about 150 tons of boron from South Korea and France to mix with water to be sprayed onto damaged reactors, French and South Korean officials said Friday. Boron absorbs neutrons during a nuclear reaction and can be used in an effort to stop a meltdown if the zirconium cladding on uranium fuel rods is compromised.

Tokyo Electric Power said this week that there was a possibility of “recriticality,” in which fission would resume if fuel rods melted and the uranium pellets slumped into a jumble on the floor of a storage pool or reactor core. Spraying pure water on the uranium under these conditions can actually accelerate fission, said Robert Albrecht, a longtime nuclear engineer.