Schematic of the reactor TMI-2
The Three Mile Island plant unit involved in the accident was unit 2 (TMI-2). The scheme shows the main components of this pressurized water reactor. The accident originated in a failure of the main pumps water supply in the non-nuclear part of the reactor combined with a false signal of a reserve valve (discharge valve of the pressurizer) declared closed while it remained open. The overheating of the reactor caused the melting of the heart, but the containment of the reactor building was not affected..
Source NRC