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
Nuclear power of the fission reactor power-level, principally the nuclear engineering reliance on a negative Void coefficient of reactivity, a form of Passive nuclear safety. The chain reaction can also be manually stopped completely by inserting control rods into the reactor core. While again depending on the reactor design, a mixture of both passive and active Nuclear reactor safety systems, the most well known being the Essential service coolant system, operate to remove decay heat. Alongside this, a number of further Defense in depth (nuclear engineering) principles are relied upon, such as physical containment barriers. Some serious nuclear and radiation accidents have