• boro
• PKL
• PWR
• Relap5
• termoidraulica

The interest on scenarios dealing with the boron dilution events in PWR-NPP has been increased among the scientific community in the last decades so that research programs have been performed to investigate the boron issue. The nuclear industry pays a special care to the same issue due to the implications on the safety and licensing aspects for the presently operated and new designed Nuclear Power Plants.
The boron issue is entirely addressed to five main associated aspects: the formation of diluted boron plugs in specific zones of the primary system, the transport of these slugs, the mixing of the boron diluted plugs, the deboration and boration processes (loss or gain of boron from primary system, respectively) and the reactivity feedback due to entrance of the borated diluted plug in the core.
These aspects may be investigated at system level (dilution and boron transport) and at local level (boron mixing) with Thermal Hydraulic System Codes and CFD Codes respectively, in parallel or subsequently to dedicated experimental test campaigns for the investigation on the process mechanism and for the constitution of an experimental data base of test results to be used for code validation.
The assessment of various accident scenarios which might give rise to boron dilution and transport shows that in particular Small Break LOCAs and Natural Circulation scenarios should be investigated to this respect. In fact, during the SBLOCA a diluted borated water slug is formed in the loop seal after a certain period characterized by the natural circulation flow rate at core inlet almost equal to zero or non existent; then the sudden restart of the circulation following primary coolant mass inventory recovery eventually after ECCS actuation is able to transport the boron diluted slug to core inlet.
The present research activity is devoted to the analysis of the boron issue through the boron dilution and transport processes based on the analysis of the results of the experimental test campaign performed at the PKL Integral test facility (under OECD/CSNI projects) addressed to scenarios of Small Break LOCA and Natural Circulation. The study of the experimental tests, the simulation of the experiments using the Relap5 System Code, the code assessment against these experiments and the characterization of the boron dilution and transport phenomena have been carried out. The boron distribution in the primary system has been characterized, the results of the simulation compared with analogous results of Cathare code and a scaling calculation to NPP have been performed.
In order to fulfil the requirement of the OECD/SETH and OECD-PKL projects the data and the results of the boron dilution PKL test campaign are proprietary and cannot be disclosed. Since, in the present thesis, an extensive analysis of the results of the tests and the performance of the system codes to predict the relevant phenomena has been performed and the experimental data are compared with the calculations results, the respect of the above mentioned requirement imposes the restriction on the publication of the chapters 2 and 3 of the thesis and the appendixes.