The 3 MW SL-1 was a U.S. Army experimental nuclear power reactor at the National Reactor Testing Station in eastern Idaho , derived from the Borax Boiling water reactor ( BWR ) design , it first achieved operational criticality/connection to the grid in 1958 . For reasons unknown , in 1961 a technician removed a control rod about 22 inches farther than the prescribed 4 inches . This resulted in a steam explosion which killed the three crew members and caused a meltdown . The event was eventually rated at 4 on the seven-level INES scale
SL-1
SL-1
Nuclear power