[en] This report presents the results from the International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO) collaborative project (CP) on Advanced Water Cooled Reactor Case Studies in Support of Passive Safety Systems (AWCR), undertaken under the INPRO Programme Area C. INPRO was launched in 2000 - on the basis of a resolution of the IAEA General Conference (GC(44)/RES/21) - to ensure that nuclear energy is available in the 21st century in a sustainable manner, and it seeks to bring together all interested Member States to consider actions to achieve innovation. An important objective of nuclear energy system assessments is to identify 'gaps' in the various technologies and corresponding research and development (R and D) needs. This programme area fosters collaboration among INPRO Member States on selected innovative nuclear technologies to bridge technology gaps. Public concern about nuclear reactor safety has increased after the Fukushima Daiichi nuclear power plant accident caused by the loss of power to pump water for removing residual heat in the core. As a consequence, there has been an increasing interest in designing safety systems for new and advanced reactors that are passive in nature. Compared to active systems, passive safety features do not require operator intervention, active controls, or an external energy source. Passive systems rely only on physical phenomena such as natural circulation, thermal convection, gravity and self-pressurization. Passive safety features, therefore, are increasingly recognized as an essential component of the next-generation advanced reactors. A high level of safety and improved competitiveness are common goals for designing advanced nuclear power plants. Many of these systems incorporate several passive design concepts aimed at improving safety and reliability. The advantages of passive safety systems include simplicity, and avoidance of human intervention, external power or signals. For these reasons, most advanced type of reactors have adopted passive safety systems such as gravity driven water pool, isolation condenser, accumulator and other passive heat removal systems. Since 1991, the IAEA has made various efforts to improve economics, safety, and reliability of evolutionary or innovative reactors through the implementation of a variety of activities related to passive safety systems. In particular, the coordinated research programme (CRP) launched in 2004, titled Natural Circulation Phenomena, Modelling and Reliability of Passive Systems, focused on the use of passive safety systems in new generation of nuclear power plants. Following this CRP, the IAEA supported the implementation of the collaborative project on Advanced Water Cooled Reactor (AWCR) Case Studies in Support of Passive Safety Systems launched in 2008. The objectives of the current collaborative project were to investigate natural circulation phenomena related to the selected AWCR systems, such as: (1) steady state, stability and startup of single-phase and two-phase natural circulation reactor systems, and (2) theoretical and experimental studies on mixing and stratification in large water pools with immersed heat exchangers. The reactor systems concerned are the advanced heavy water reactor (AHWR) of India, the Central Argentina de Elementos Modulares (CAREM-25) of Argentina, and the advanced power reactor plus (APR+) of the Republic of Korea. During the past three years, India, Argentina, and the Republic of Korea have actively participated in implementing this project and, according to the terms of reference, each country has successfully conducted an assigned case study associated with natural circulation and thermal stratification phenomena. In-depth review and discussion on case study results were made at the final consultants meeting held in December 2011 at the IAEA Headquarters in Vienna, Austria during which the final report was also reviewed. In order to simulate the AHWR main heat transport system, an integral test loop (ITL) was set up at Bhabha Atomic Research Centre (BARC) and the stability state of the ITL was investigated and experiments were conducted in ITL simulating the startup procedure for different powers and pressures. Major findings and conclusions of the AWCR case studies are the following: - All mechanical devices considered are able to suppress the instability in the loop and especially the spool piece with helical coil and bellow. It can be concluded that the helical coil is an effective tool to suppress instability in single-phase natural circulation systems. - The instabilities get suppressed in both single-phase and two-phase natural circulation systems in the presence of nanoparticles in the water. The increase in the loop flow rate in the presence of nanoparticles was found to be proportional to the concentration of nanoparticles. - A wide area in the stability map has been detected at condensation power values close to 0.75 MW in which the stability margins are considerably incremented. This region offers excellent conditions to guarantee the reactor stability for a wide range of operation conditions. - The use of fewer shrouds gives a higher flow rate and improved inventory utilization but multidimensional effects are not accounted for in RELAP5 code simulation. Therefore, further studies on the shroud effect based on integral simulation with isolation condenser system (ICS) and CFD simulation are required. - As per simple analyses using CFX code and the separate effect tests, a horizontal heat exchanger shows better performance regarding thermal stratification in large pools. The reason is that the horizontal heat exchanger provides a greater driving force for natural convection and hence higher circulation rates than a vertical heat exchanger