Investigation of three-dimensional effects on natural circulation nuclear reactors: Chimney recirculation

This paper explores the intricate interplay of three-dimensional effects on the operational dynamics of nuclear reactors employing natural circulation (NC). A review of thermal-hydraulic phenomena (THP), with insights into three-dimensional THP and NC, as well as of their role in deterministic safety assessment has been performed. Moreover, some ongoing research directions relevant to NC reactors have been summarized, showing that they do not address the concerns highlighted in the present work. In this work, attention is directed towards reactors featuring a long chimney, also called riser, in which upward movement of hot fluid happens, together with a downflow with lower temperature in the surrounding downcomer. The study delves into the potential occurrence of buoyancy driven recirculation phenomena within the chimney and the consequential risk of cold fluid ingress into the core region. Such occurrences may precipitate in instabilities including the neutronic-thermal hydraulic feedback. In this work, we consider single phase flow including the presence of subcooled void and two-phase flow in the core region which are typical respectively of NC iPWRs and NC BWRs. Conceptually simplified RELAP5-3D nodalizations have been adopted, in which the chimney region has been divided into a peripheral annular part and a central cylindrical region. Recirculation in the chimney is affected by core power, which also determines NC flow inside the vessel. The performed study is preliminary considering that no experimental data is available, and the system code RELAP5-3D is used instead of a more powerful CFD code, perhaps more suited in case of single-phase conditions.