Before the definitive shutdown of Phénix fast reactor, a pool-type sodium-cooled prototype, occurred in the year 2009, the French Commissariat a l’Energie Atomique et aux Energies Alternatives (CEA) decided to carry out a final set of experimental tests, in order to gather data and additional knowledge on relevant Sodium Fast Reactors (SFR) operation and safety aspects. One of these experiments was the dissymmetrical configuration test, which has been selected as a benchmark transient on H2020 SESAME project. ENEA and Sapienza University of Rome are participating, in collaboration, in the benchmark using RELAP5-3D© code, developed by Idaho National Laboratory (INL). The analysis of the thermal hydraulic characteristics and the plant behavior focuses on the accurate core cooling prediction in natural circulation, by assessing the flow and the temperatures in the various reactor components. Aiming to investigate the asymmetric thermal hydraulic behavior inside the pool, the nodalization scheme has been developed using a detailed parallel pipe with cross-junctions for the hot and cold pool and the core has been simulated assembly per assembly, including blanket and reflector, in the first seven rings, ready for a proficient coupling with a three-dimensional neutron kinetic analysis code. The scope of the present study is to verify if a detailed thermal–hydraulics pool model could add useful data to show the differences which occur in each loop and into the pools, caused by the azimuthal asymmetry of the boundary conditions. Blind calculations results are here presented and discussed, highlighting the details added with the multidimensional nodalization scheme, compared to a mono-dimensional model. The paper will be a first step on the comparison of a Phénix mono-dimensional and three-dimensional thermal-hydraulic nodalization and on the RELAP5-3D© code assessment against the dissymmetrical experimental results.

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