TMI : A reactor core fusion
Cross-section of the reactor vessel showing the fuel degraded by partial melting of the reactor core. 45% of the fuel melted, including cladding and structure materials to form what is called a "corium". The photography taken by a video camera, which inserted down the vessel 4 years after the accident, shows the fusion of elements of uranium fuel with damages were more important that it had been originally assumed. The reactor core was damaged beyond repair, but there was no major radioactive release into the environment.
IRSN/IAP