RCS Depressurization is a phenomenon in which the Pressure tube reactors, especially of the CANDU type, have a low-pressure vessel calandria – under an internal pressure near atmospheric. The calandria vessel is immersed into the water contained inside a concrete structure – the calandria vault. A very small LOCA (in capacity of Pressure and Inventory Control (PIC) system) along of reactor coolant system (RCS)-outside core area and without operator actions to earliest and manual shutdown the reactor, even all the safety systems are available, can go to a severe phase. The method concerns the use of a safety auto-depressurization of the pressurizer (ADS) by means of a new safety valve block located on the line to the degasser-condenser (DC). Then, the DC water and that of the later collected in the storage tank of PIC, by feed-bleed system (with DC as source pressure) goes to RCS via purification lines and, in the following when these sources are exhausted, the RCS water supply need to be from an infinite source.