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TOKYO, March 21 (Reuters) - Japanese authorities have taken a major step in managing a nuclear crisis by connecting all six earthquake-damaged reactors to power supply, but it's too soon to say the crisis has reached a turning point, experts said on Monday.
Power has been connected but not switched on to crank up most coolers and pumps, which may have been badly damaged in the quake and tsunami that on March 11 triggered the world's worst nuclear accident since Chernobyl.
Only one pump has been activated.
The damaged reactors and their spent fuel pools at the Fukushima Daiichi nuclear power plant, 240 km (150 miles) north of Tokyo, urgently need cooling from air-conditioners and from water pumped in.
U.S. Energy Secretary Steven Chu, asked by CNN whether the worst of Japan's 10-day nuclear crisis was over, said: "Well, we believe so, but I don't want to make a blanket statement."
U.S. Nuclear Regulatory Commission Chairman Gregory Jaczko added that radiation levels at the plant appeared to be falling.
But nuclear experts in the United States and elsewhere 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."
At Fukushima, 300 engineers have worked around the clock inside an evacuation zone to contain the worst nuclear accident since Chernobyl, Ukraine, in 1986.
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.
Japanese authorities say they have established power lines to all the reactors. 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.
"Reactors 5 and 6, they are now in what's called cold shutdown, and the spent fuel cooling ponds are at normal temperatures," said Tony Irwin, a former nuclear plant manager who now lectures at Australian National University.
"They are in the sort of situation now we would like to see 1, 2, 3 and 4 in."
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 Irwin. "And that has been giving the main radiation effects on site."
He said of the efforts at the plant: "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.
Najmedin Meshkati, a nuclear and environmental expert at the University of Southern California Los Angeles, said the measures were necessary but raised a fresh, and serious, concern.
"Where does the sea water drain?" he asked. "This is now radioactive waste water. Has there been any measurement of its radiation effect?
"I am interested to know how this water is being disposed, if it is being disposed or just allowed to drain to sea. That is the hidden part of this catastrophe."
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, however, experts were more optimistic than they were earlier in the crisis.
"My read is, that they're at least holding their own," said Murray Jennex, professor at San Diego State University, California.
"Things are not getting worse. 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." (Additional reporting by Elaine Lies; Editing by Mark Bendeich and Jeremy Laurence)