Access to sustainable, sufficient and economically viable energy sources, mitigation of climate change, and avoiding harmful environmental and health impacts are vital to growing world population. The new generation fission reactors can offer a remarkable contribution to this end by extending the availability of nuclear fuel resources to thousands of years. Nuclear energy causes negligible greenhouse gas and fine particulate emissions. However, strict control on safety of reactors and fuel cycle facilities as well as safe and timely nuclear waste disposal and improved proliferation resistance are prerequisites of a positive net contribution to the wellbeing of the whole society. In addition to electricity production, new applications: process heat for industrial use and hydrogen generation, are foreseen. Recycling of actinides, use of thorium as an additional fission fuel, and reduction of nuclear waste generation will improve the position of nuclear fission as a long term sustainable energy source. Increased international attention has recently been devoted to reactor concepts that differ essentially from the existing light water reactors. Basic processes of these new concepts, known as Gen IV reactors, are fundamentally different from those used today. Some new features may create new type safety challenges. They also aim to push nuclear reactor technology to completely new regimes of performance parameters, thus raising engineering challenges. In order to allow Finland to benefit from these new technologies and influence their development, it is necessary to join relevant international projects, develop domestic expertise on critical technologies involved, and participate in ongoing international efforts to develop safety requirements for them. The technical potential of Gen IV will be shown in a time span of 15 - 20 years as the first demonstration plants are to be built. The NETNUC project is a multidisciplinary consortium of national and international partners to carry out basic research to generate scientific knowledge needed for Gen IV reactors and to educate a new generation of

The EPR is designed to cope with severe accidents, involving core meltdown. A specific melt spreading area has been designed within the containment. This core catcher will be flooded by water, which transfers the decay heat to the containment heat removal system. To improve cooling, horizontal flow channels made of cast iron are located also below the core catcher. STUK, the radiation and nuclear safety authority in Finland, wanted an independent study of the functionality of the core catcher design. Effect of the presence of insulation material and boric acid in the cooling water was to be studied, as well as the general behavior of the system in different phases of the flooding of the core melt spreading area. To verify the function of the core catcher design, a scaled down test facility was built at Lappeenranta University of Technology. Since there are some physical restrictions of a test facility computational tools were applied especially for the tests where steady state conditions could not be reached without endangering the integrity of the test facility. This paper introduces the Volley test facility, computational simulations and compares them with the test results. Simulated temperatures of those Volley tests, which could be run until steady state conditions, are very close to the measured temperatures. It can be concluded also, that the temperatures are evidently below the cast iron melting point with heat fluxes used in the tests, if there is a small flow inside the cooling channels or even in case when only a few adjacent cooling channels are totally dry.