"We're nearly five days after the fission process was stopped, the levels of radioactive iodine will only be about two-thirds of where they were at the start, some of the other very short-lived, very radioactive material will be gone altogether by now," he said.
"The situation may recede or deteriorate and lead to a massive radiation leak to the atmosphere," said Professor Javier Dies, head of Nuclear Engineering at the Polytechnic University of Barcelona. "As things stand, this cannot be ruled out."
Greenpeace's Beranek said that heavy pollution from cesium could make some areas of Japan near the plant uninhabitable, at least for decades, as happened around Chernobyl. Pflugbeil also said some areas might be off-limits.
Laurence G. Williams, Professor of Nuclear Safety at the John Tyndall Institute for Nuclear Research in Britain, said he did not see a Chernobyl-type blast as likely.
"I can't think of anything at the moment that would drive that explosive force," he said. "It will just be a melting, or a degrading, heating up of the fuel which will just crumble into a heap like what happened at Three Mile Island."
Richard Wakeford, of the Dalton Nuclear Institute at the University of Manchester, said in a statement that words like "apocalypse" and "catastrophe" were "utterly inappropriate" about the situation at Fukushima and could cause unnecessary panic.