During the last decade the European Union funded numerous projects with the aim to pave the way for the development of Gen IV nuclear reactor technologies. The recently started EU project PATRICIA, to which the University of Pisa participates, provides room for further investigations of the involved phenomena in LMFRs (e.g. thermal stratification inside the pool, LBE heat transfer capabilities, transition from forced to natural circulation) by means of both experimental and numerical analyses. In the frame of the PATRICIA project, the ENEA Brasimone Research Centre foresees to carry-out an experimental campaign involving the CIRCE facility. With respect to the previous configurations, the facility will be updated substituting the steam generator, which will exhibit an helicoidal steam generator (THETIS): the experimental campaign will involve the analysis of steady-state and postulated accidental scenarios. The University of Pisa provides numerical support both in the pre-test and post-test phases to assist the design of the experiment. This work reports about the preliminary results obtained by numerical simulations of the CIRCE-THETIS facility. The analyses were performed both using CFD codes (ANSYS Fluent) and STH codes (RELAP5-3D). The analyses mainly focused on temperature and velocity distributions inside the CIRCE pool during the postulated nominal steady-state conditions. In addition, a transient analysis was performed investigating the behaviour of the addressed facility in case of a PLOFA scenario. The limits and capabilities of both the approaches were observed and are discussed in the present work trying to provide guidelines for a correct application of the adopted codes. The obtained results provide interesting suggestions for the experimentalists and represent a valuable support in better setting the experimental conditions and measurements tools layout.
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