Characterization and validation of thermal and boron mixing phenomena in the downcomer and lower plenum of an integrated primary system nuclear reactor

The main objective of this work is to present the preliminary results obtained in the recent experimental campaign performed at the DIMNP (Dipartimento di Ingegneria Meccanica, Nucleare e della Produzione) of University of Pisa with a stainless steel mock-up, simulating the downcomer and lower plenum of a small scale PWR (Pressurized Water Reactor). In particular, the facility, characterized by a scaling factor of 1/5 in respect to a reference reactor having steam generators integrated inside the vessel, was designed in order to provide relevant data to study the mixing phenomena occurring during a DVI (Direct Vessel Injection) line break accident, SB-LOCA (Small Break Loss Of Coolant Accident), and to validate commercial Computational Fluid Dynamic (CFD) codes. In the first part of the report a detailed description of the experimental facility is provided including the hydraulic circuit and the data acquisition system. In this context, the correct test procedure has been supported and validated by numerical simulations performed using the RELAP5 code. The second part ofthe work is mainly focused on the numerical simulations, performed using the FLUENT CFD code, of the mixing phenomena occurring during water injection into the vessel from the two lines representing the two DVIs. The final and most important part of the report was devoted to the description of the first perforrned experimental activity and to the criticai analysis of the obtained test results. The four experimental tests analysed in this part given useful in formation concerning the thermal mixing in the lower plenum of the facility test section.