Earthquake-damaged reactors at the Fukushima Daiichi nuclear plant have been reconnected to the power supply.
Authorities have made progress in managing the nuclear crisis at the plant in north-east Japan, triggered by the country's most devastating earthquake and tsunami on March 11.
Power has been connected but not switched on to start up most of the coolers and pumps, which may have been badly damaged. Just one pump has been activated.
The damaged reactors and their spent fuel pools at the nuclear power plant urgently need to be cooled by air-conditioners and water pumped in.
U.S. Nuclear Regulatory Commission chairman Gregory Jaczko said that radiation levels at the plant appeared to be falling, although some nuclear experts were not quite as positive.
"I am not sure if the crisis has passed but it is definitely a step in the right direction," said Peter Hosemann, a professor at the University of California Berkeley's Nuclear Engineering Department.
"It is getting better. However, we don't know if the pipes and connections and pumps still work at this point or what works and what not. But having power makes external water supply easier."
Hundreds of engineers have worked around the clock inside an evacuation zone at the plant to contain the accident.
The most badly damaged reactors are No. 3 and 4, which were both hit by explosions last week.
Japan's nuclear safety agency said pressure was rising in the most threatening reactor, No. 3, which contains highly toxic plutonium, and this may have to be released by "venting" steam, a step taken last week that discharged low levels of radiation into the atmosphere.
Reactors 5 and 6 have not been much of a problem since a diesel pump was activated last week, cooling down both the reactors and their spent fuel ponds.
The other reactors are damaged but more or less stable, although the spent fuel cooling pond at reactor 4 is also a particular worry. The reactor's core was drained only last November and the radioactive spent fuel transferred to the pond.
"There was already spent fuel in there so there was quite a high load of spent fuel in that pond," said said Tony Irwin, a former nuclear plant manager who now lectures at Australian National University. "That has been giving the main radiation effects on site."
He added: "I think it's all going in a good direction, but we are not at a point where we can say we are totally happy."
Engineers have been spraying the coastal complex with thousands of tonnes of sea water so fuel rods will not overheat and emit more radiation.
However some experts have raised the concern that the sea water has now turned into radioactive waste water and questioned whether there has been any measurement of its radiation effect.
"That is the hidden part of this catastrophe," said Najmedin Meshkati, a nuclear and environmental expert at the University of Southern California in Los Angeles.
Japanese authorities have acknowledged that some of the water may be spilling into the ocean, but said they doubted it would have any effect on human health. They agreed it needed to be monitored.
Overall experts have been more optimistic than they were earlier in the crisis.
Professor Murray Jennex at San Diego State University in California said: "Things are not getting worse and that's actually good news right now.
“The longer they go, the cooler the stuff starts to get, and the less likely there is to be a severe problem."