The term nuclear materials have been defined under Title 18, Part 1 of the US Code.
According to the US Code, the term "nuclear material" means:
"Any material containing plutonium; uranium not in the form of ore or ore residue that contains the mixture of isotopes as occurring in nature; enriched uranium, defined as uranium that contains the isotope 233 or 235 or both in such amount that the abundance ratio of the sum of those isotopes to the isotope 238 is greater than the ratio of the isotope 235 to the isotope 238 occurring in nature; or uranium 233".
Any person who illegally uses or disperses any nuclear material or nuclear byproduct material, thereby causing death or bodily injury to any person or considerable damage to property and/or environment, shall face a fine and imprisonment for any term of years or for life.
It can be mentioned here that the term "nuclear byproduct material" means any material which contains a radioactive isotope created by a process of irradiation in the functioning of a nuclear reactor or accelerator.
(More: http://www4.law.cornell.edu/uscode/html/uscode18/usc_sup_01_18_10_I_20_39.html)
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