As part of the collaboration between Universidad Politecnica de Madrid (UPM), Indizen Technologies and the Spanish Nuclear Safety Council (CSN), the Integrated Safety Assessment (ISA) methodology, developed by the Spanish Nuclear Safety Council (CSN), has been applied to a thermo-hydraulical analysis of Total Loss of Feed Water sequences in a Westinghouse 3-loop PWR. The ISA methodology allows among others obtaining the Damage Domain, i.e., the region of the space of uncertain parametere where certain damage limit is exceeded. Apart of typical uncertain parameters for the physical model, ISA considers operator actuations times (like RCP trip, AFW recovery and begining of Feed & Bleed…) and /or stochastic uncertain phennomena. Given a particular safety limit, several data of every sequence are necessary in order to obtain the damage exceedance frequency of that limit. These data are obtained from the results of the simulations of transients performed with MAAP code. The results show the capability and necessity of the ISA methodology, or similar, in order to obtain accurate results that take into account time uncertainties.Copyright © 2013 by ASME