Loss of Coolant Accident: parametric analysis of the initial period of the depressurization of a BWR vessel with the use of the RELAP4/MOD5 code" (in Italian)

The document (in Italian) deals with the analysis by computer code RELAP4 of experimental data measured in the PIPER facility. PIPER was installed at the Scalbatraio Laboratory managed by IIN (Istituto di Impianti Nucleari) of University of Pisa. PIPER consisted of a pressure vessel equipped with electrical heaters and rupture nozzle, other than suitable instrumentation (typically temperature and pressure transducers). Namely original instrumentation was developed to measure the jet thrust, the jet impingement loads and the load on internal components following the break opening and the propagation of the depressurization wave upstream from the rupture into the vessel. Pioneering RELAP applications to the analysis of experimental data demonstrated the validation level of the code. Namely, in this report, experimental data of blowdown are compared with code calculations results. The bases were put for better understanding of phenomena like Two-Phase Critical Flow (TPCF), depressurization rate, effect of initial subcooling and of relative position between initial collapsed level and elevation of exist nozzle axis. Key reported data (experiment and corresponding calculation results) are transient pressure and fluid temperatures at different positions in the vessel and in the exit nozzle.