The difference between a partial meltdown and a full meltdown at a nuclear plant is enormous, both in the degree of damage and in the potential release of radiation, experts in nuclear power said.

A partial meltdown, like those suspected at two reactors in northeastern Japan over the weekend, may not necessarily mean that any of the uranium fuel in the core has melted, experts said. The fuel rods may be only damaged, a portion of them having been left uncovered by cooling water long enough to crack, allowing the release of some radioactive elements in the fuel.