These methodologies have been applied to evaluate the reliability of passive systems. A historical review shows that a few methodologies such as REPAS, RMPS, and APSRA have been developed in the past. However, these codes are not well validated for their applicability to passive systems due to the lack of sufficient plant/experimental data, which introduces large uncertainties and errors while evaluating the performance of passive systems. Currently, best estimate codes are used to predict the performance of passive systems. It requires a thorough understanding of the physics of the phenomena, which drive the functional behavior of the system including its failure. Passive system reliability evaluation is a difficult task. These create the issues of “reliability of passive systems”. Other issues are lack of accepted definitions of failure modes of these systems and, difficulty in modeling certain physical behavior of these systems. However, there are serious debates or concerns with regard to their performance or functional capability which must be established adequately due to several technical issues for example: lack of plant operational experience scarcity of adequate experimental data from integral test facilities or from separate effect tests. Apart from simplicity in design, the passive safety systems are considered to be more reliable than the active safety systems. These features make them more safe than the Gen II LWRs even in extreme events. The passive safety system works on the principle of natural laws and does not need an external source of energy or human intervention to actuate. These are called Gen III and Gen III + reactors. Subsequent to accidents at TMI-2, advanced light-water reactors were designed with several passive systems to enhance the defence-in-depth.