By Mayumi Negishi and Raju Gopalakrishnan
TOKYO (Reuters) - Workers battled to staunch radiation leaks at a Japanese nuclear plant on Thursday, almost two weeks after it was disabled by an earthquake and tsunami, but some experts saw signs of the crisis being brought under control.
Hundreds of workers have been desperately trying to cool down the six reactors and spent fuel ponds at the Fukushima Daiichi plant, 240 km (150 miles) north of Tokyo, since the March 11 disaster, including pumping in seawater or dropping water from the air.
Two of the reactors are now seen as safe in what is called a cold shutdown, but the other four remain volatile, emitting steam and smoke periodically and raising radiation levels in the vicinity.
But that does not mean the situation is out of control, the experts said.
"The reactors are more stable as time progresses," said Peter Hosemann, a nuclear expert at the University of California, Berkeley.
"By now, the decay heat is greatly reduced and it becomes easier to supply sufficient water for cooling. As far as we know, the containments are holding and the radiation levels have dropped."
But he added: "We might see some more release of radioactive material, mostly due to the water going through the systems."
Officials at Tokyo Electric Power Co, the plant's operator, said the delay in repairs was at least partly due to underestimates of the damage from the disaster.
Upturned roads, broken pipes and debris hindered transport of equipment and replacement parts, and one official said connecting cables to wet equipment in the dark interior of the reactor buildings had been "far more challenging than expected."
After more than a week, workers managed to connect power to the reactors, but since seawater has been used to cool the plant, checks are needed on all systems before electricity can be switched back on.
Once coolers are switched on, reactor temperatures should fall rapidly and the plant could be on its way to being declared safe, the experts said.
"The problem of testing all the systems has caused some delays," said Tony Irwin, a former nuclear plant manager who now lectures at Australia National University.
"Obviously there must have been quite a lot of building damage. It doesn't seem to be too much of a risk because it all seems to be stable. As soon as you can resume cooling, it should be OK."
A Japanese nuclear expert said the main risk was from continued radiation leaks, and the risk of criticality, or a self-sustaining nuclear reaction, was low.
"The possibility is not zero, since nobody can look inside the reactors to see what is happening," said Yoshiaki Oka, a professor at Tokyo's Waseda University.
"The nuclear fuel rods could cause the reactor core to melt, the core could heat up and melt equipment around it. But as long as we make sure heat is exiting from recirculation pumps and the spent nuclear fuel is covered in water, that risk is remote."
Hosemann, of the University of California, Berkeley, said the crisis should die down early next month.
"It's still a bit early to make an exact time prognosis, but my guess is in a couple of weeks the reactors will be cool enough to say the crisis is over," he said.
"It will still be important to supply sufficient cooling to the reactors and the spent fuel pools for a longer period of time. But as long as this is ensured and we don't see any additional large amount of radioactivity released, I am confident the situation is under control."
(Editing by Michael Watson)