The recent development of small modular reactors needs to be followed by safety analysis using the newest available tools. This work focuses on one type of reactor, SEALER, which is a small lead cooled reactor intended for remote communities in Canada. Simulations of a loss of flow transients are performed using a CFD-based model that was specifically developed for this project. The CFD geometry includes the entire primary circuit with some simplifications. The fuel channel, steam generator and pumps use a simple geometry with momentum source and heat source/sink. Free surface level is modelled with the multiphase volume of fluid (VOF) method. The CFD part of the model is coupled to a custom code for heat transfer in the fuel rods and point kinetics for neutronics. Transient results show that core temperatures do not increase significantly and stay well below coolant boiling and fuel melting points. The CFD-based model presented here is compared against a lumped-parameter model using the same transient. It is shown that the evolution of the mass flow and temperature is significantly different and more detailed with the CFD-based model. Finally, the influence of the moment of inertia of the pump flywheel on the transient is explored.