Research reactors are essential to the implementation of a nation’s nuclear program, and can be used for research and training, material testing, neutron activation analysis, production of radioisotopes for medicine or industry, and other purposes. More than 220 research reactors with different power, fuel type, and neutron energy are currently operating in 53 countries [11]. The structure and power of all research reactors are quite simple with low power, temperature, and pressure when compared to nuclear power plants. Under Reduced Enrichment Research and Test Reactor (RERTR) program [12], almost all operational research or test reactors were converted from highly enriched uranium (HEU) to low enriched uranium (LEU) fuel but they still kept the purpose in utilizations and applications. Three main factors related to the existence of the research reactor are management, operation, and utilization. From a management point of view, the reactor must be in good condition and operating staff or managers can know clearly or deeply about the reactor in practice and parameters as well. In terms of safety operation, the reactor must meet or exceed the design requirements for safety in physics, thermal hydraulics, and adequate operation. The reactor also has a design to meet for safe operation even in abnormal, transients, and accident conditions. Depending on the characteristics of the reactor, the utilization can be exploited as much as possible. The purposes of application of the reactor must be explicitly defined before building and operating.
In general, reactor physics can be divided into two problems: statics and dynamics, along with reactor kinetics and burn-up. In statics calculations, the time variable in transport or diffusion neutron equations is ignored. Multiplication factor or reactivity and neutron flux distributions or power distribution are the most important characteristics derived from static neutronics calculations. For thermal hydraulics, the safety parameters need to be evaluated including fuel, fuel cladding, and coolant temperatures, other safety parameters (ONBR or DNBR) under maximum nominal power, and inlet coolant temperature condition. Reactor kinetics describe the behavior of a reactor based on the insertion or withdrawal of reactivity in reactor core at time step intervals. Three-dimensional (3-D) reactor kinetics is crucial and must be considered for any reactor in normal and transient/accident conditions. During the simulation’s subsequent time steps, the power distribution of the hottest fuel assembly (FA) in radial and axial directions within the reactor core can be determined by using 3-D reactor kinetics computer code. Fuel burn-up is also a very important process that directly influences the properties and safety of a reactor. Changing fuel compositions such as the production of actinide isotopes and fission products, and reducing the reactor’s excess reactivity or core lifetime are the two most significant factors affecting reactor characteristics. The burn-up process of a reactor occurred according to operation time in day, month, or even yearly timescale. The burn-up distribution of FAs, excess reactivity, and other parameters are important for core and fuel management in addition to enhancing safe operation and effective utilization as well.