Steam discharging through spargers and blowdown pipes into the Pressure Suppression Pool (PSP) is employed in Boiling Water Reactor (BWR) to prevent overpressure of the reactor vessel and containment. The capability of suppression can be reduced during the operation when the thermal stratification is developed. Direct modeling of steam injection into a water pool with long-term transient is computationally expensive due to the large-scale difference in space and time. To enable such prediction, Effective Heat source and Effective Momentum source (EHS/EMS) models are proposed. In previous work, we demonstrated the implantation of EHS/EMS models in the Computational Fluid Dynamics (CFD) tool and its application to plant simulation. In this work, we use the developed model to further investigate the thermal stratification and mixing in the PSP of a Nordic BWR. The event to be analyzed is initiated by spurious activation of one valve in the safety injection system. The focus of the simulations is to investigate the possibility of stratification development and understand the effects of the activation of different systems on pool behavior. Pool transient is simulated by CFD code (ANSYS Fluent) with EHS/EMS models and the injection conditions of the steam are derived from the simulation results performed by the system-level codes (GOTHIC).