 ORNL shifted its focus to a civilian version of the meltdown @-@ proof Molten Salt Reactor ( MSR ) away from the military 's " daft " idea of nuclear @-@ powered aircraft . The Molten @-@ Salt Reactor Experiment ( <unk> ) set a record for continuous operation and was the first to use uranium @-@ 233 as fuel . It also used plutonium @-@ 239 and the standard , naturally @-@ occurring uranium @-@ 235 . The MSR was known as the " chemist 's reactor " because it was proposed mainly by chemists ( ORNL 's Ray Briant and Ed Bettis ( an engineer ) and NEPA 's Vince Calkins ) , and because it used a chemical solution of melted salts containing the actinides ( uranium , thorium , and / or plutonium ) in a carrier salt , most often composed of beryllium ( BeF2 ) and lithium ( LiF ) ( isotopically depleted in Lithium @-@ 6 to prevent excessive neutron capture or tritium production ) – FLiBe . The MSR also afforded the opportunity to change the chemistry of the molten salt while the reactor was operating to remove fission products and add new fuel or change the fuel , all of which is called " online processing " . 
