Nuclear Engineering is a branch of engineering dealing with a system under nuclear reaction. Followings are the main subjects of interest to a nuclear engineer.
Reactor physics deal with the behavior of neutron, gamma-ray inside of a nuclear -reactor. Fission of fissile material such as uranium-235 or plutonium-239 produce 2 or 3 fast neutrons and several gamma rays. The fast neutron either loses its energy by scattering, absorbed by a nuclide. Scattered neutron will either escape the system or undergoes another cycle of collsion. Absorbed neutron will either transmute the nuclide to another nuclide or make a fission. The average number of neutron invoking a fission among the neutron produced from a fission should be exactly one to produce energy stably. Main result of reactor physics is the criticality and the power distribution.
Thermal hydraulics dels with the heat removal from a nuclear reactor.