Tokyo residents shouldn't worry, Dr. Lim Sang-moo, director of nuclear medicine at the Korea Cancer Center Hospital, said in Seoul.
Parents might want to be more cautious if they have a choice about what water to drink — "nobody wants to drink radioactive water," he said. But "it's not a medical problem but a psychosocial problem: The stress that people get from the radioactivity is more dangerous than the radioactivity itself."
Experts also say iodine-131 dissipates quickly in the air, with half of it disappearing every eight days.
The unsettling new development affecting Japan's largest city, home to some 13 million in the city center and 39 million residents in the greater Tokyo area, came as nuclear officials struggled to stabilize the hobbled reactor 140 miles (220 kilometers) to the north.
The quake and tsunami that struck off the east coast March 11 knocked out the plant's crucial cooling systems.
Explosions and fires have erupted in four of the plant's six reactors, leaking radioactive steam into the air. Progress in cooling down the overheated facility has been intermittent, disrupted by rises in radiation, elevated pressure in reactors and overheated storage pools.
The plant operator had restored circuitry to bring power to all six units and turned on lights at Unit 3 late Tuesday for the first time since the disaster — a significant step toward restarting the cooling system.
It had hoped to restore power to cooling pumps at the unit within days, but experts warned the work included the risk of sparking fires as electricity is restored through equipment potentially damaged in the tsunami.
In a new setback, black smoke billowed from Unit 3, prompting another evacuation of workers from the plant Wednesday afternoon, Tokyo Electric Power Co. officials said, but they said there had been no corresponding spike in radiation at the plant.