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
Reactor physics deal with the behavior of neutron, gamma-ray inside of a nuclear reactor. Fission of a 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 interests of reactor physics are the criticality and the power distribution.
Thermal hydraulics
Thermal hydraulics deals with the heat removal from a nuclear reactor. The heat generated at a nuclear fuel are transferred to a coolant. The interest of thermal hydraulics is to keep the material temperature inside of a nuclear reactor below the limiting one during operation as well as possibble accident.