"This is a slow-moving nightmare," said Dr Thomas Neff, a research affiliate at the Center for International Studies, which is part of the Massachusetts Institute of Technology. "This could be a five or a six -- it's premature to say since this event is not over yet."
Experts said that international politics is starting to become evident in the international pressure being put on the Japanese. France's nuclear safety authority ASN said Tuesday it should be classed as a level-six incident.
"We are now in a situation that is different from yesterday's," said ASN President Andre-Claude Lacoste.
An American nonprofit, the Institute for Science and International Safety, went even further.
"This event is now closer to a level six, and it may unfortunately reach a level seven," ISIS said in a statement on its website. ISIS founder David Albright is a physicist who has published global catalogues of nuclear material.
Most, though, say the accident is still well away from the massive explosion that hit Chernobyl.
"People literally probably did not know this was happening, either the instruments were wrong that were measuring the water level, were damaged in the earthquake, or they literally had people scrambling dealing with so many other problems at the plant they had no one's eyes on the level of the water in the spent fuel pools," said Seth Grae, chief executive of nuclear consultancy LightBridge Corp.
"My guess is that if they had, they could have dealt with this before it ever became a big problem ... we're literally talking about a hose."