There were four: NS savannah (US): 1962 to 1971, too expensive to operate NS Mutsu (Japan): 1972 to 1992, neutron leak Otto Hahn (Germany): 1970 to 1982,

There were four: NS savannah (US): 1962 to 1971, too expensive to operate NS Mutsu (Japan): 1972 to 1992, neutron leak Otto Hahn (Germany): 1970 to 1982, converted to diesel power Sevmorput (Russian): 1988 to 2007, converted to world's first nuclear powered drilling ship