power plants the Lucens reactor , Switzerland , in 1969. the Three Mile Island accident in Pennsylvania , U.S.A., in 1979. the Chernobyl disaster at Chernobyl Nuclear Power Plant , Ukraine , USSR , in 1986. Other core meltdowns have occurred at: NRX (military), Ontario , Canada, in 1952 BORAX-I (experimental), Idaho, U.S.A., in