This work concerns a specific passive system design for advanced pressurized water reactor (PWR). The studied system first relies on passive safety condensers, which are increasingly being used in the design of new generation nuclear power plants (NPP). These condensers are typically immersed in large water tanks that function as a cold source or heat sink. They have to be situated in a sufficient elevation enabling a two-phase natural circulation mode, with both a condensing phase of the steam extracted from the steam generator, and a gravity draining return for the condensates to the steam generator. Such power extraction can be used for a relatively long period of time depending on the pool size. The present research is based on the use of a portion of the energy stored in this boiling water volume as a hot source for a thermodynamic cycle via an immersed heat exchanger. The power generated by this cycle will be used as an autonomous supply for various critical components, in addition to existing systems. This technology used here which can convert electricity from low-grade heat is similar with existing technologies already used for the valorization of renewable (biomass, solar, geothermal) or industrial waste heat at low and medium temperature (<150°C) into electricity. An efficient technology is the organic Rankine cycle (ORC), which has been used at laboratory and industrial scales for about several decades. However, two major issues hamper the use of ORC in the context of PWR: the nature of the hot source (water at 100°C) and the requirement for system reliability and robustness. Aside from these two challenges, there are the usual constraints associated with this type of cycle: maximizing energy performance, using an environmentally friendly fluid, and minimizing space requirements. An experimental test bench with a boiling water pool and an ORC with immersed evaporator was built to address this problem. The design of the immersed evaporator is explained, as well as the correlations used. The system reliability is then studied through the investigation of off-nominal situations (degraded heat transfer at ORC evaporator, ''high'' temperature of ORC condenser) for a first approach of reliability assessment. This study gives first elements for the demonstration of the adaptability of a partial admission axial micro-turbine to the variation of cold source temperature and to the entry of two-phase fluid in Novec649TM. All the experimental results will be used to validate the theoretical model of the ORC (condenser - evaporator - turbine) in order to design the ORC at scale 1.