Experts cautious on signs of Japan nuclear progress

LONDON/OSLO (Reuters) - Japan's reconnection of power to its earthquake-damaged reactors is a big step in managing its nuclear crisis, experts said on Monday, but smoke and concerns about food safety showed the dangers are far from over.

The U.S. Nuclear Regulatory Commission said that the crisis appeared to be on the verge of stabilising.

"We believe that the spent fuel pools on units 3 and 4 which had been two components that were a significant safety concern, that the situation there is stabilized," said Bill Borchardt, chief operations at the NRC.

He said that the containment at units 1, 2 and 3, appeared to be functional and that there was water being injected into the reactor vessels at the units. "I would say optimistically that things appear to be on the verge of stabilizing."

Engineers have 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.

Robin Grimes, head of the centre for nuclear engineering at Imperial College in London, said that the smoke did not seem to be accompanied by a spike in radiation readings.

"There is more than a glimmer of light at the end of the tunnel," he said.

PHASE ONE

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."

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 Organisation 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.

France's IRSN radiation protection and nuclear safety institute said there were also worries about a build-up of tonnes of salt from sea water used as an emergency measure to cool reactors and spent fuel rods.

As the water boiled, it left behind salt deposits

"Salt is a concern, but it's a secondary concern. The main concern is cooling the plant," said Jerome Joly, director of Nuclear Defence Expertise at the IRSN. He said that a build-up of salt might block valves or be corrosive.

(Additional reporting by Raju Gopalkrishnan and Elaine Lies; Editing by Jeremy Laurence and Patrick Graham)