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.

But in a full meltdown — which could occur within hours if all cooling water was lost and the rods became completely uncovered — melting is all but guaranteed, as thousands of fuel pellets fall to the bottom of the reactor and heat themselves into a molten pool at several thousand degrees Fahrenheit.

While it is considered highly unlikely that a full meltdown would result in a nuclear chain reaction, experts said, such lava-like fuel might breach the reactor’s pressure vessel and then its containment, leading to widespread release of radioactivity.