Improving Condensation Modelling in RELAP5: From Code Modification to Uncertainty Analysis of HERO-2 Experimental Data

In recent decades, international interest has grown in the design and implementation of evolutionary reactors based on passive systems. The design of such systems requires reliable and validated numerical tools capable of simulating phenomena driven by very small forces, especially when compared to active systems. For this reason, several international research projects aim to assess the capabilities and limitations of numerical tools in modelling passive systems and their associated physical phenomena. The HERO-2 facility was designed to provide preliminary experimental data for characterizing bayonet tubes and exploring their potential application as Steam Generators (SGs) in advanced nuclear reactor designs, such as Small Modular Reactors (SMRs). Following the agreement between the Italian Ministry of Economic Development and the ENEA, multiple experimental campaigns were conducted, and a RELAP5 (R5) input deck of the facility has been developed. Considering the RELAP5 limits in simulating condensation phenomena encountered in previous studies, the primary objective of this study is to enhance the capabilities of the code in simulating condensation phenomena in horizontal pipes under natural circulation conditions with the implementation of Thome correlation and, in the second instance, to re-evaluate the numerical model of the HERO-2 facility. Moreover, a comprehensive uncertainty analysis (UA) is carried out to identify the key parameters influencing the simulations. The analysis revealed that the simulation results are strongly affected by the filling ratio uncertainties, a given initial condition that, together with the power supplied, determines the most important thermal-hydraulic (T/H) test parameters, such as the saturation pressure, the void fraction, mass flow rate, etc. Overall, the study provides a deeper understanding of the factors governing passive system performance and highlights the importance of accurately characterizing the experimental boundary and initial conditions in the verification and validation activities of a T/H code.