In the United States Navy, 121X is the officer designator for a Nuclear Propulsion Training Officer (Nuclear Power School Instructor). Nuclear Power School Instructors work with Nuclear Propulsion Officer Candidates on advanced math equations.
Other officer designators in the Navy include:
122X - Nuclear Engineering Officer (Naval Reactors)
123X - Permanent Military Professor (RL)
130X - Air Warfare - Pilot or NFO
You can become a Navy Nuclear Instructor by being accepted into the NUPOC program.