Electricity is expected to be reconnected to four reactors at the stricken Fukushima nuclear power plant in northern Japan on Saturday.
The country's Nuclear and Industrial Safety Agency says it expects to restore electricity at numbers 1, 2, 5 and 6 reactors on Saturday and to numbers 3 and 4 on Sunday.
Further work is needed before any attempt is made to restart the water pumps that, it is hoped, will cool down overheated fuel rods.
But engineers have managed to restart a diesel-powered pump that is now being used to help cool reactor 5. Military trucks have also been spraying water for a second day.
Radiation has been escaping from the plant since the earthquake and tsunami of 11 March led to a series of hydrogen explosions and fires at buildings housing the reactors.
The head of the International Atomic Energy Agency, Yukiya Amano, says the situation there remains serious.
But if the pumps can be made to work, experts say it could mark the beginning of the end of Japan's nuclear nightmare.
Threat level increased
Japan has upgraded the severity of the disaster from four to five, on a scale of one to seven.
France's Nuclear Safety Authority puts the threat level even higher at six, which would make it the worst nuclear incident since thosands of people were killed in Chernobyl, Ukraine, in 1986.
Japan's decision grades the situation at Fukushima as an "accident with wider consequences".
The country's nuclear officials say the decision was prompted by core damage to two of the plant's four reactors.
Meanwhile, the Japanese Government has acknowledged that it could have reacted more quickly to the earthquake and tsunami.
It says contingency plans failed to anticipate the scale of the disaster.
Radiation may have reached US
Radiation particles apparently released by the Fukushima plant have been detected on the west coast of the United States.
Diplomatics sources say a facility in the US that monitors for breaches of the comprehensive nuclear test ban treaty has registered tiny amounts of radioactivity that reportedly pose no health threat.
The International Atomic Energy Agency said earlier that it would start measuring radiation levels in Tokyo straight away, in the hope of reassuring the Japanese public.