Nuclear reactor accidents in the United States reactors and research facilities. This led to the introduction of the Price-Anderson Act in 1957, which was "an implicit admission that nuclear power provided risks that producers were unwilling to assume without federal backing". Nuclear reactor accidents continued into the 1960s with a small test reactor exploding at the Stationary Low-Power Reactor Number One in Idaho Falls in January 1961 resulting in three deaths which were the first fatalities in the history of U.S. nuclear reactor operations. There was also a partial meltdown at the Enrico Fermi Nuclear Generating Station in Michigan in 1966. The large size of nuclear reactors
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