The independent code assessment constitutes an important part in the qualification of an analytical tool for performing licensing or NPP support activities. The consistent application of adopted modeling procedures together with qualified experimental database plays a key role in the code assessment process. It should be mentioned that a consistent code assessment supported by a qualified experimental database is an important step for developing a solid ground for the uncertainty evaluation in the frame of Best Estimate Plus Uncertainty (BEPU) approach. The assessment of RELAP5-3D code capabilities for reproducing various accident and transient in PWR-like experimental Integral Test Facilities (ITF) has been performed at the Nuclear Research Group of San Piero a Grado (GRNSPG) of University of Pisa. The present paper summarizes the first series of this activity, aimed for RELAP5-3D assessment at Large Break LOCA and Small Break LOCA conditions. In this framework, 2 LBLOCA and 2 SBLOCA experimental tests performed at LOBI, LOFT and SPES ITF have been simulated by RELAP5-3D. First, an experimental database has been developed for mentioned facilities and experimental tests of interest. This process has been carried out following IAEA guidelines in Safety Series Report 23 and 52 and other known best practices. The elaboration of great amount of available information and adopting quality assurance procedures resulted in the standardized-form qualified Reference Data Sets (RDS) for each ITF. The second part of the paper discusses development of RELAP5-3D models for LOBI, LOFT and SPES facilities and simulation of the 4 tests, namely LOBI A1-06 and LOFT L2-5 (2A LOCA), and LOBI A1-83 and SPES SB-04 (10% CL LOCA and 6% CL LOCA respectively). All models were developed following the same nodalization techniques adopted at GRNSPG-UNIPI. All calculations were qualified both at steady-state and transient conditions applying the same set of qualitative and quantitative criteria. The judgment of RELAP5-3D code capabilities to reproduce the relevant thermal-hydraulic phenomena has been reported.

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