[en] The simplified thermal-hydraulic calculations used in order to asses the feasibility and effectiveness of a Core Rescue System (SSN) for PWRs have been here collected. SSN systems essentially consists of an augmentated (with reference to relief lines) primary system automatic depressurization line, of low pressure borated water accumulator lasting for many hours and of a direct contact condenser for the dissipation of heat to the environment. Although some final verifications have to be completed, it can now be considered well proven that these systems are capable of making a PWR at least ten times safer than most designs. This is due to their characteristics of semplicity, direct detection and action, passive operation. This conclusion relies on analysis and calculations performed at DISP, at ENEA/TERM/MEP and elsewhere. Simple engineering methods as those shown in this report have been and may be useful guide in the use of more accurate, and therefore less flexible, calculatrions