Those are two broad areas. I would imagine that one will take courses in reactor theory and fluid mechanics in which one would learn both subjects.
A fission reaction releases 2 or 3 neutrons in addition to the two fission product radionuclides and gamma radiation. Some fission products also release neutrons, and that population is called 'delayed neutrons' which allow for control of the reactor during withdrawal of control rods. Fission neutrons are born with energies in the low MeV range, and they must be slowed or 'moderated' to less the 0.1 eV, or 7-8 orders of magnitude in energy to increase the probability of causing additional fissions. Transport theory and its approximation, diffusion theory, addresses how a population of neutrons behave in a reactor environment.
Fluid mechanics describes how fluids behave when flowing through structures such as pipes, or in the case of a reactor, through the core. The fuel assemblies impose a resistance, or drag, on the coolant. The pressure, in conjunction with temperature, is important as it influences the heat transfer mechanism, forced convection, and it is important in pressurized water reactors (PWRs) to limit nucleate boiling toward the top of the core (the coolant condition must remain below the point of departure from nuclear boiling, DNB), whereas in boiling water reactors (BWRs) must remain below a critical power at which boiling transition, or dryout, occurs.