atomic weight calutron cascade chemical element chemical reaction cyclotron depleted uranium deuterium electric field enriched uranium evaporation finland fissile fluorine formate gas centrifuge gaseous diffusion girdler sulfide process heavy water helikon vortex separation process hydrogen hyperfine structure infrared multiphoton dissociation isotope kinetic isotope effect laser lithium magnetic field manhattan project mass spectrometry molecular laser isotope separation national superconducting cyclotron laboratory natural uranium nuclear proliferation nuclear weapon oak ridge national laboratory riken reaction rate refractory rjukan south africa tritium united states uranium uranium hexafluoride uranium pentafluoride vortex tube zippe-type centrifuge