The new generation of evolutionary nuclear power plants, e.g., the Westinghouse AP600 and the General Electric simplified boiling water reactor, relies on a full reactor coolant system (RCS) depressurization to allow gravity injection from an in-containment tank and thereby assure long-term core cooling. Studies performed to support the licensing process and design of both evolutionary and innovative reactors have shown that cold water injection may, under particular plant conditions, induce a large plant depressurization. Preliminary studies have been performed to support the design of a passive injection and depressurization system (PIDS) based on the idea of depressurizing the RCS by mixing cold water with the RCS hot water and inducing steam condensation in the primary system. The analyses, performed with the RELAP5/ MOD3 computer code, show the response of a typical midsize pressurized water reactor plant [two loops, 600 MW(electric)] equipped with the PIDS. Different RCS injection locations including pressurizer, vessel upper head, and hot leg, and actuation at different residual reactor coolant masses have been investigated. Several factors, including RCS mixing, RCS residual mass at PIDS actuation, PIDS injection flow rate, and steam generator conditions, have been shown to affect the plant depressurization. The PIDS performance has also been verified against the following reference severe accident scenarios: (a) complete station blackout event, and (b) a small-break loss-of-coolant accident and concomitant station blackout event. Preliminary experimental activities to support the PIDS concept have already been performed. Additional experimental activities, including integrated system tests, have been planned to support system development and computer code validation.