This paper is an outome of a study on the development of the conceptual design of advanced fast reactors that have passive and inherent abilities. Safety performance indicators are evidenced by the ability of the reactor to remain under control and can operate on a new equilibrium shortly after an accident. Simulations are performed when an accident due to unprotected loss of flow (ULOF) and unprotected loss of heat sink (ULOHS) occurs at the reactor. The passive safety system is realised by the reactor vessel auxiliary cooling systems (RVACS), before its performance is evaluated. The simulation results show that the current power reactors can survive the ULOF and ULOHS accident. The optimisation of the reactor and the RVACS results in higher natural circulation level, indicating the reactors satisfy the inherent or passive safety standards. The results from the 'accident' show there is a large safety margin to the maximum temperature within the fuel, cladding, and coolant.