ABSTRACT
U), the most likely fuels for weapons applications are Pu and U highly enriched in
U (HEU or Oralloy). Plutonium and HEU were first produced in the 1940s by the United States and the Soviet Union at the inception of their nuclear weapons programs. Since that time, large quantities of both have been produced in many countries — including some without acknowledged weapons programs. Plutonium and HEU are difficult to produce, requiring a large initial investment in facilities and the mastery of dozens of engineering and scientific disciplines. Both fuels require extensive physical protection and safeguarding, in part to prevent criticality accidents and (in the case of Pu) the accidental release of a highly toxic substance.
