Plutonium, uranium 233 or 235; fissionable material.
Plutonium, uranium enriched in the isotope 233 or in the isotope 235, and any other material that the Nuclear Regulatory Commission, pursuant to the provisions of the Atomic Energy Act of 1954, Section 51, determines to be special nuclear material.
U-233, U-235, or Pu-239. These are the only materials which can be used to make a fission device
Special nuclear material is a term used by the Nuclear Regulatory Commission of the United States to classify fissile materials. The NRC divides special nuclear material (SNM) into three main categories, according to the risk and potential for its direct use in a clandestine nuclear weapon or for its use in the production of nuclear material for use in a nuclear weapon.