The 69 MWe prototype
fast breeder reactor Fermi 1 unit was under construction and development at the site from 1956 to 1963. Initial criticality was achieved on August 23, 1963. On October 5, 1966 Fermi 1
suffered a partial fuel meltdown. Two of the 92 fuel assemblies were partially damaged. According to the
Nuclear Regulatory Commission (NRC), there was no abnormal radioactivity released into the environment. Fermi 1 was a
liquid metal (sodium) cooled fast breeder reactor design. It was capable of producing 200 megawatts thermal (MWt) power or 69 MW electrical power with 26% enriched metallic uranium fuel. The
enriched uranium section of the reactor (core) was a in diameter cylinder by 30 inches high and contained 92 fuel assemblies. The core was surrounded by 548 additional assemblies containing depleted uranium. These assemblies were about square by about tall. Only the core section contained the enriched uranium while
depleted uranium was placed above and below within the assemblies. The core also contained 2 control rods and 8 safety rods. The plant was designed for 430 MWt and 125 MWe using a newer uranium oxide fuel, but the plant was closed before the fuel was ever ordered. A 168
MWe oil-fired boiler was added in 1966 to utilize the turbine-generator during periods when the reactor was not producing power. The main cause of the partial meltdown was a temperature increase caused by a blockage in one of the lower support plate orifices that allowed the flow of liquid sodium into the reactor. The blockage caused an insufficient amount of coolant to enter the fuel assembly; this was not noticed by the operators until the core temperature alarms sounded. Several fuel rod subassemblies reached high temperatures of around (with an expected range near 580 °F, 304 °C), causing them to melt. Following an extended shutdown that involved fuel replacement, repairs to vessel, and cleanup, Fermi 1 restarted in July 1970 and reached full power. Due to lack of funds and aging equipment, it was finally shut down permanently on November 27, 1972, and was officially
decommissioned December 31, 1975, under the definition of the
Atomic Energy Commission (AEC). Later, the Nuclear Regulatory Commission (NRC) replaced the AEC and under their new definitions, Fermi was re-designated as being in
SAFSTOR due to some remaining radioactivity at the site. On May 16, 1996, decommissioning was restarted. However, by November 2011 with very little activity remaining, a decision was made to halt further work. It is currently in
SAFSTOR. ==Fermi 2==