For some years now, studies have proved the feasibility and the interest of providing simulation solutions on the functional and detailed I&C specifications of nuclear units. This is a first step to detect design errors upstream of I&C engineering process. Indeed, the rupture in the design process between detailed specifications and final programmed or wired I&C systems (different tools and different people/staffs are involved for these activities) increases the risk of errors detected late, during on-site requalification tests. Generally, platform tests are performed to verify the final implementation of I&C, but they are often limited to logical functions and performed in open-loop. For several years, EDF Research and Development has been working jointly with EDF engineering units to add I&C closed-loop verifications in design process, especially by studying Model-In-the-Loop (MIL) and Hardware-In-the-Loop (HIL) simulations. In HIL simulation, an I&C platform is connected to a numeric model that simulates the plant dynamic behavior. The advantage to connect control system to a process model is that exchanged data, namely process and I&C data are more realistic and have a functional meaning. It presents a real interest to have physical feedbacks (for example water level or pressure in a circuit) for the verification of analog controls. Furthermore, the use of real programmed I&C systems allow to perform more realistic tests by taking into account the hardware characteristics of the platform (filtering time, delays, etc.). This approach ultimately increases confidence level in engineering studies before on-site tests and earn time and money by detecting problems during platform tests. The paper details the approach adopted by EDF R & D for the implementation of HIL simulation (in terms of tools, testing platforms), and will present its application on a concrete case study of an analog regulation modification, on 1300MW French Nuclear Power Plants.