Dictionary.net

a disaster comparable to a nuclear meltdown; "there is little likelihood of a meltdown comparable to the American banking collapse in March 1933"

severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping

1. The point where nuclear core overheats and melts causing a core failure. Known to be a catastrophic event it causes the loss of radioactive materials containment, which are released into the environment. 2. Non-nuclear events with actual or perceived catastrophic endings, as if it were a nuclear meltdown. A person may experience an emotional meltdown due to actual or perceived overwhelming stress.