Nuclear reactor accidents in the United States ordered during the late 1960s raised new safety questions and created fears of a severe reactor accident that would send large quantities of radiation into the environment. In the early 1970s, a highly contentious debate over the performance of emergency core cooling systems in nuclear plants, designed to prevent a core meltdown that could lead to the "China syndrome", received coverage in the popular media and technical journals. In 1976, four nuclear engineers —three from GE and one from the Nuclear Regulatory Commission— resigned, stating that nuclear power was not as safe as their superiors were claiming. They testified to
Corium (nuclear reactor) nuclear fuel (the uranium oxide fuel has a melting point of 2865 ˚C, thus the physical nature of the damaged fuel has not been fully determined but it is assumed to have become molten) to melt and form corium. The reactor core isolation cooling system (RCIC) was successfully activated for Unit 3; however, the Unit 3 RCIC subsequently failed, and at about 09:00 on March 13, the nuclear fuel had melted into corium. Unit 2 retained RCIC functions slightly longer and corium is not believed to have started to pool on the reactor floor until around 18:00 on March 14.