In recent years more and more attention was dedicated to the development and detailed analysis of supercritical water cooled reactors. In particular several studies (e.g.: [2,4]) outlined that the SuperCritical water cooled Fast Reactor (SCFR) presents new-features that require a careful analysis of its neutronics safety parameters. Specifically it is considered of the highest importance to assess the void effect accurately, given the fact that a negative void reactivity is a fundamental requirement for the inherent safety of water cooled reactors; since, differently from a sodium cooled reactor, Loss Of Coolant Accidents (LOCAs) are a Design Basis Accident (DBA) for SuperCritical Water cooled Reactors (SCWR). With the aim of accurately estimating the void reactivity coefficient several models were then developed and employed pursuing the objectives of improving the void effect to overcome the computational uncertainties and investigating the possibility of performing coupled neutronics/thermal-hydraulics calculations using MCNP4C and SIMMER-III. The paper will then first illustrate the outcome of the initial void reactivity effect investigations and then the details of the advanced models that were developed to analyze the problem more accurately. Concluding then design modifications are suggested and the results relative to the previously mentioned coupling are discussed.