The Fukushima accident led to concerns about enhancement of safety for the new design nuclear plants and also for the existing fleet of nuclear power plants (NPPs) by introducing passive safety systems. An important objective is to increase the grace time for a plant operator to establish an alternate heat sink in the case of a station blackout (SBO) accident. Efforts made by RATEN (https://www.raten.ro/?lang=en) in the frame of H2020 PIACE projects were to implement a passive safety system in the CANadian Deuterium Uranium 6 (CANDU 6) project. In this project, RATEN was in charge of the engineering design and computational modeling aspects, required to integrate a passive safety system in the existing CANDU 6 project. In order to design a passive safety system, a 3-day SBO accident was credited to occur at the Cernavoda Unit 2 NPP, a CANDU 6–type reactor. An isolation condenser (IC) system capable of transporting the total energy produced in the reactor core due to decay heat was designed and modeled. The engineering design solutions were made by RATEN CITON (http://www.citon.ro/english_index.html), and the thermal-hydraulic analysis was performed by RATEN ICN (https://nuclear.ro/en/) using the RELAP5 computer code (https://relap53d.inl.gov/SitePages/Home.aspx) to confirm natural circulation both in the secondary and the primary circuits during the SBO accident and heat transfer capability of the IC with and without noncondensable gases. The passive safety system design consists of four (4 × 33%) closed loop independent circuits, one for each steam generator. Each loop has an IC design to transport 0.66% of nominal thermal power of the reactor. In order to avoid a rapid transient during reactor cooldown, the system is endowed with four noncondensable gas tanks (one for each IC), connected to the outlet header of each IC, provided for reducing the IC heat flux simultaneously with reactor core residual heat decrease. The design concept was adapted to the CANDU 6 reactor power and the specific layout of the Cernavoda site, starting from the Advanced Lead-cooled Fast Reactor European Demonstrator (ALFRED) patent (the demonstrator of lead fast reactor technology) passive system, to increase the plant operator grace time from 23 h (current situation) to more than 72 h.