The nuclear reactor forms the heart of a nuclear power plant. Fundamental to a nuclear reactor are nuclear physics and reactor physics, which deal with the basic aspects of the design of nuclear reactors. This knowledge is essential for understanding reactor behavior during normal operation as well as abnormal conditions. Nuclear engineering is an excellent technology by which tremendous amounts of energy is generated from a small amount of fuel. In addition to power generation, numerous applications are expected in the future. In addition to being used in energy generation, neutrons are expected to be widely used as a medium in nuclear reactions. Here, nuclear energy refers to the energy released in nuclear fission. This occurs because of the absorption of neutrons by fissile material. Neutrons are released by nuclear fission, and since the number of neutrons released is sufficiently greater than 1, a chain reaction of nuclear fission can be established. This allows, in turn, for energy to be extracted from the process. The amount of extracted energy can be adjusted by controlling the number of neutrons. The higher the power density, the greater the economic efficiency of the reactor. Ultimately, this means careful control of neutron distribution. If there is an accident in a reactor system, the power output will run out of control. This situation is almost the same as an increase in the number of neutrons. Thus, the theory of nuclear reactors can be considered to be the study of the behavior of neutrons in a nuclear reactor. The design of nuclear reactors is such that there is a balance between the production of neutrons in fission reactions and the loss of neutrons due to capture or leakage. The study of such a process is known as nuclear reactor theory or nuclear reactor physics [22-24].