Study on the RELAP5 break models making reference to the LOBI BL-30 experiment

Integral Test Facilities (ITF) experiments are one of the main tools for the validation of the models used inside the best-estimate thermalhydraulic system codes. The LOBI-MOD2 is a single plus a triple loop (simulated by one loop) ITF electrically heated to simulate a 1300 MWe Pressurized Water Reactor (PWR) with scaling factor equal to 1/712 for the core power, volume and mass flow. The Two-Phase Critical Flow (TPCF) historically constituted an important issue in development and related predictions of system thermalhydraulic codes as RELAP5. In the present paper, following studies performed in the past and considering the experimental database of the LOBI experiment BL-30, a novel evaluation of the TPCF models is performed. The evaluation aims at identifying differences in the performance among RELAP5 TPCF models using a modified nodalisation of the LOBI that is obtained by adding a 'properly scaled containment'. In this way, not only pressure in the primary system (measured during the experiment), but also pressure in the containment is determined that depends upon the TPCF. The measured signals of primary system pressure temperature, levels, etc. are taken as reference to show the capabilities of TPCF while the predicted pressure in containment is used only to show the impact that different TPCF models have upon the prediction. The key parameter involved for the prediction of the containment pressure is the energy release through the break. It is shown that current TPCF models produce 'very' different energy releases into the containment, with relatively 'minor' effects upon the prediction of primary system performance. No large discrepancies were found in the application of the two TPCF models.