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 reactor physics purifying, and isotopically separating new uranium ore. In practice, both the difficulty of handling the highly radioactive fission products and other political concerns make fuel reprocessing a contentious subject. One such concern is the fact that spent uranium nuclear fuel contains significant quantities of Pu, a prime ingredient in nuclear weapons (see breeder reactor). Short-lived reactor poisons in fission products strongly affect how nuclear reactors can operate. Unstable fission product nuclei transmute into many different elements ("secondary fission products") as they undergo a decay chain to a stable isotope. The most important such element is xenon, because the isotope Xe,