Japan's Daiichi 1 reactor north of the capital Tokyo began leaking radiation after the 8.9-magnitude earthquake triggered a tsunami, and swiftly prompted fears of a nuclear meltdown.
But experts said pictures of mist above the plant suggested only small amounts of radiation had been expelled as part of measures to ensure its stability, far from the radioactive clouds that Chernobyl spewed out when it exploded in 1986.
Yaroslov Shtrombakh, a Russian nuclear expert, said a Chernobyl-style meltdown was unlikely.
"It's not a fast reaction like at Chernobyl," he said. "I think that everything will be contained within the grounds, and there will be no big catastrophe."
The Chernobyl nuclear reactor exploded and caught fire, sending a cloud of radiation over much of Europe.