Pool type reactor is a kind of MTR research reactors (RRs). The reactor core is immersed in a tank containing a huge amount of coolant which is normally used for cooling, moderation, and shielding purposes. If a break occurs at the bottom of this tank or at a large piping connected to it, pool discharge takes place and core uncover will be predicted. Loss of Coolant Accident (LOCA) is one of the most important postulated accidents considered in the design basis of nuclear reactors. From the lessons learned after Fukushima accident, the assessment of low probability severe accidents such as the RR tank break with using best estimate codes must be considered. During the simulation of LBLOCA in an MTR reactor using RELAP5/Mod3.3, some confusion in the results is observed. Therefore, a simple system consisting of a tank filled with water and a discharge valve is assumed to investigate the best nodalization for LBLOCA simulation using RELAP5 Code. Multiple nodalizations for the system are presented. It is noticed that the nodalization used in the analysis and some other user's options in the input file affects the simulation results. Also, a simple mathematical system of equations expressing the case is modeled using FORTRAN program to verify the best nodalization.