Nuclear safety in the United States inadvertently reversed pipe restraints at the Diablo Canyon Power Plant reactors, compromising seismic protection systems, which further undermined confidence in nuclear safety. All of these well-publicised events, undermined public support for the U.S. nuclear industry in the 1970s and the 1980s. In 2002, the USA had what former NRC Commissioner Victor Gilinsky termed "its closest brush with disaster" since Three Mile Island's 1979 meltdown; a workman at the Davis-Besse reactor found a large rust hole in the top of the reactor pressure vessel. Recent concerns have been expressed about safety issues affecting a large part of the nuclear fleet of
Nuclear reactor physics to sustain a chain reaction increases as the U content increases, reaching infinity at 94% U (6% U). Concentrations lower than 6% U cannot go fast critical, though they are usable in a nuclear reactor with a neutron moderator. A nuclear weapon primary stage using uranium uses HEU enriched to ~90% U, though the secondary stage often uses lower enrichments. Nuclear reactors with water moderator require at least some enrichment of U. Nuclear reactors with heavy water moderation can operate with natural uranium, eliminating altogether the need for enrichment and preventing the fuel from being useful for nuclear weapons; the