Extension of the sub-channel code ANTEO+ to the mixed convection regime

Liquid Metal cooled Fast Reactors, and notably Heavy Liquid Metal cooled systems, are promising options for achieving, in a relatively short term, the various advantages brought about by Small Modular Reactors (SMRs). To increase safety, natural circulation in the primary circuit to cool the core, even at rated power, is also considered, possibly bringing the core thermal exchange dynamics inside the mixed convection regime. To address, at the outset of the design process, thermal-hydraulics aspects, suitable tools must be employed, like the sub-channel code ANTEO+. ANTEO+ was, however, specifically built for the forced convection regime, trying to maximize modeling efficiency inside that regime; to make it applicable also for these SMRs concepts, its validity domain has been extended to the mixed convection regime. In this work, such extension process is outlined, presenting the new code structure, including the solved set of equations and models in the new anticipated application domain. A thorough validation for sub-channel and clad outer temperatures is also presented, confirming the ability of ANTEO+ in reproducing experimental data, homogeneously, in its new anticipated validity domain, with a relatively high degree of accuracy when compared to a reference tool like COBRA-IV-I-MIT. The need for Heavy Liquid Metal-cooled experiments dedicated to validation for sub-channel tools has also been highlighted so to greatly enhance confidence in the final code accuracy assessment, ultimately easing its use in the design phase.