[en] The support state model employed in the Progetto Unificato Nucleare (PUN) Probabilistic Safety Study was an effective means for the analysis of intersystem dependencies for modeling and quantifying accident sequences. The model identifies the various plant system interactions and defines the complete spectrum of support system conditions, in such a way as to significantly reduce intersystem dependencies in frontline event trees. The support state model allows for eminent insights into the understanding of plant system dependencies and provides an immediate perception of support system failures that are critical for the safety of the plant. It is a very effective model for an accurate quantification of accident sequence frequencies