(Corrects paragraph 6 to read “...smoke did not seem to accompanied...” inserts dropped word “not”)
By Kate Kelland and Alister Doyle
LONDON/OSLO (Reuters) - Japan's reconnection of power to its earthquake-damaged reactors is a major step in managing its nuclear crisis, experts said on Monday, but smoke from two reactors and worries about food safety showed the crisis was far from over.
"There is more than a glimmer of light at the end of the tunnel," said Robin Grimes, head of the center for nuclear engineering at Imperial College, London.
The head of the U.N. atomic agency, Yukiya Amano, said the situation "remains very serious." But he added "I have no doubt that this crisis will be effectively overcome."
Engineers rigged power cables to all six reactors at the Fukushima complex and started to pump water at one of them to reverse overheating that has caused the worst atomic crisis since the 1986 Chernobyl disaster in what is now Ukraine.
Smoke was seen from reactors No. 2 and 3 at the crippled plant and the plant operator said it did not know the reason. A brief release of grey smoke postponed efforts to cool reactor 3 -- the only one to use highly radioactive plutonium.
Grimes said that the smoke did not seem to be accompanied by a spike in radiation readings.
"It looks like reactor No. 3 is the one posing most problems...but the situation is still not clear for several of the reactors," said Astrid Liland, of the Norwegian Radiation Protection Authority.
A restoration of power will let workers pump more water to help cool reactors and spent fuel ponds "assuming the equipment can still be operated," said Laurence Williams, professor of nuclear safety at the John Tyndall Institute in Britain.
"Once the reactors can be reflooded we can breathe a sigh of relief that phase one is finished," he said of the crisis caused by a tsunami after an earthquake on March 11 that has left 21,000 people dead or missing.
The World Health Organization said that radiation detected in food -- such as vegetables and milk -- was worse than previously thought. Most toxic has been radioactive iodine and indications of radiocaesium.
"The few measurements of radiation reported in food so far are much lower than around Chernobyl in 1986, but the full picture is still emerging," said Malcolm Crick, Secretary of the U.N. Scientific Committee on the Effects of Atomic Radiation.
Jim Smith, a leading expert on Chernobyl at Britain's Portsmouth University, said that the measurements from food and water were important because they helped confirm that "there has been a very serious release of radioactivity" at Fukushima.
"This doesn't mean that consumption of these products is necessarily an immediate threat, as limits are set so that foodstuffs can be safely consumed over a fairly long period of time," he said.
NOT SO OPTIMISTIC
On Sunday, the situation had looked more promising.
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."
Nuclear experts in the United States and elsewhere were not quite as sure.
"I am not sure if the crisis has passed but it is definitely a step in the right direction," Peter Hosemann, a professor at the University of California Berkeley's Nuclear Engineering Department, said of the reconnection of power.
France's IRSN nuclear safety institute said there were also worries about a build-up of salt from sea water used as an emergency measure to cool reactors and spent fuel rods.
It said in a statement that any build-up of crystallized salt could have an "impact on the cooling of the core, and a risk of blocking valves."
(Additional reporting by Raju Gopalkrishnan and Elaine Lies; Editing by Jeremy Laurence and Patrick Graham)