The Collaborative International Dictionary of English v.0.48:

Nuclear physics \Nu"cle*ar phys"ics\, n.
   That branch of physics which studies the structure of and
   phenomena observed in atomic nuclei.
   [PJC]

WordNet (r) 3.0 (2006):

nuclear physics
    n 1: the branch of physics that studies the internal structure
         of atomic nuclei [syn: nuclear physics, atomic physics,
         nucleonics]