Severe accidents (SA) in LWR occur when reactor vessel water decreases and there is no available water for core cooling. During SA the reactor core could, after partial/total melt down, pour into the lower plenum. The study of the interaction of melt fuel with water is the objective of MFCI (Melt Fuel Coolant Interaction). MFCI is an important issue in reactor safety . A SA can lead to energetic (steam explosion) or non-energetic (melt quenching) interactions in-vessel or ex-vessel. FARO facility at JRC-Ispra simulated MFCI experiments under real conditions. Pre-test/post-test analytical activities using COMETA code were carried out. The code was assessed based on FARO tests. Limitations and uncertainties in the application to a full plant needed to be identified and quantified. The PhD research objective was achieved expanding the knowledge in MFCI. It was complemented with application of COMETA to conditions not experimented before, developing and improving COMETA sources and verifying code consistency, analysing and unifying the COMETA simulations. An analytical study was carried out to illustrate the MFCI inside a NPP SA sequence.