In a research activity that SIET has been conducting for years about safety systems for light water reactors (LWRs), attention has been paid to developing two passive injection systems representing an innovative solution in mitigating the consequences of loss of coolant accidents. Both systems allow the completely passive injection of cold water into a pressurised vessel. They are triggered by a low-level signal and work on the base of phenomena like natural circulation and condensation. The simplest system, Sistema Iniezione Passiva 1 (SIP-1), injects water contained in a tank into a circuit at the same pressure as the circuit. The most complex system, injection cyclic system (ICS), injects cold water, by filling cyclically a proper tank with the water stored in an atmospheric pressure pool. Thanks to the ENEA sponsorship, this activity has been conducted in three steps: the definition of the conceptual design of the systems; the application of the Relap5 code to simulate their behaviour; and the proposal of their specific applications to pressurised and boiling LWR. In this paper, both systems are presented in their structural and operating characteristics together with the main results of the code application for their simulation. Some proposals of application of SIP-1 and ICS to pressurised water reactors and boiling water reactors are also shown. The developments and reached goals of the prosecution of the research are also summarised here, together with future needs.
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