Nuclear reactor design and analysis is done with knowledge and data from funda­mental fields such as nuclear reactor physics, nuclear reactor kinetics and plant control, nuclear thermal-hydraulic engineering, nuclear mechanical engineering, and nuclear safety engineering. There are various calculation models from simple to detailed, and their selection depends on the purpose. Simple models have the advantage of giving a better understanding of calculation results and physical phenomena, and easy improvement of calculation codes. With today’s high com­putational performance, it is common to use a calculation code based on detailed models from the viewpoint of generic programming. Since the accuracy and application range of calculation results depend on models and data, it is necessary to imagine the physical phenomena and comprehend the validity of the calculation results based on the fundamental knowledge concerning the calculation target.

The nuclear reactor calculation is classified broadly into the reactor core calcula­tion and the nuclear plant characteristics calculation. The former is done to clarify nuclear, thermal, or their composite properties. The latter is done to clarify dynamic and control properties, startup and stability, and safety by modeling pipes and valves of the coolant system, coolant pump, their control system, steam turbine and con­denser, etc. connected with the reactor pressure vessel as well as the reactor core.