Failure of multiple backup systems caused near-meltdowns
In the aftermath of the earthquake and tsunami that struck northeastern Japan on March 11, engineers are flooding three nuclear reactors with seawater in an effort to cool their radioactive cores and to prevent all of their nuclear fuel from melting down. Explosions have been recorded at two of the reactors, but do not seem to have breached the crucial inner containment vessels.
The grimmest situation is at the final reactor, where water stopped flowing temporarily on March 14, exposing the fuel rather than cooling it. Much now depends on the containment vessels that shield the highly radioactive reactor cores. Even a full meltdown does not necessarily mean that the reactors will release large amounts of radioactive material — as long as the vessels remain intact.