An analysis has been performed for the OECD/PKL experiments on boron dilution during small break loss-of-coolant accidents ( SBLOCA) and loss of residual heat removal system ( RHRS). The objective of the analysis is to evaluate and validate the system code capabilities on boron dilution/ concentration transients. A focus is set on both the correct prediction of the thermal-hydraulic ( T-H) parameters and of the boron dynamics in the pressurized water reactor (PWR) system. The experiments chosen for the analyses are the test E3.1 for loss of RHRS and the tests F1.1 and F1.2 for SBLOCA of the OECD/PKL project. The RELAP5/MOD3.3 ( Patch02) was used for the calculations. For the SBLOCA simulating tests F1.1 and F1.2, calculations have shown that the general trends of the system behavior and T-H phenomena, such as water level in SG, coolant temperature, boron concentration, and coolant flow rate, were well simulated. For the loss of RHRS simulating test E3.1, the equilibrium pressure and core outlet temperature generally agreed with the test results. Therefore, the amount of heat removal from the core thorough the water-filled steam generators (SGs) by reflux condenser heat transfer mode was simulated by the code. However, some discrepancies between the analysis and the test were found for the local parameters, such as the water level in the pressurizer, the amount of deboration, and local temperature fluctuation in the loop seal.