Open pool research reactors play a crucial role in industry, medicine, scientific research and training. Ensuring its safety involves the use of widely accepted computer codes, such as RELAP5, that can predict the progression of accidents and evaluate reactor performance during transient events. These codes need a continuous validation process against various accident scenarios to ensure the reliability of the results. Two Loss of Heat Sink events (LOHS) took place previously in the McMaster Nuclear Reactor. One is the Loss of Forced Circulation in the Secondary Side event that happened in the year 2020, and the second is the Pool Temperature Experiment conducted at the McMaster Nuclear Reactor (MNR) in March 2023. These two events became a perfect opportunity to validate the safety analysis tools used by the NOF (Nuclear Operations and Facilities) staff. The focus was on validating the MNR RELAP5 model, particularly on the simulation of a loss of heat sink (LOHS) accident caused by the loss of the secondary pump. This study elucidates the validation results of the RELAP5 code for these two events and also under steady state conditions. A particular finding of this research was that reactor pool cooling transients prior to the start of a loss of heat sink accident (LOHS) can have an impact on the pool heating rate due to the pool thermal stratification. In these cases, the common assumption of an initial homogeneous temperature profile in the pool might not be accurate and could lead to underestimating the core temperature. With the help of CFD simulations it was possible to adjust the RELAP5 model, by providing a stratified temperature profile of the pool to be used as initial condition for the simulations. This led to more accurate estimations of the pool heating rate during the LOHS. Moreover, a sensitivity analysis on the pool nodalization showed that a minimum of two vertical pipes interconnected laterally by cross flow junctions is needed for the accurate analysis of this kind of transients. / Thesis / Master of Applied Science (MASc)