Thermal-hydraulic phenomena for water cooled nuclear reactors

Nuclear Reactor Safety (NRS), Deterministic Safety Assessment (DSA) and Accident Analysis (AA) constitute the general framework for the topic of the present paper. The class of Water-Cooled Nuclear Reactors (WCNRs) is concerned. This includes most of the nuclear reactors in operation, under construction or in advanced design stage. The required licensing process for those reactors, are further necessary elements to establish the context of the performed activity. Best Estimate (BE) system thermal-hydraulic codes are adopted to demonstrate the safety of WCNR based on AA, namely focusing on the class of Design Basis Accidents (DBAs). On the one hand, the validation of BE codes is a necessary step to prove their applicability to calculate accident scenarios. On the other hand the knowledge of accident scenarios is a requirement for the design and the operation of WCNR. The validation of BE codes and the knowledge of accident scenarios needs the identification and the characterization of Thermal-hydraulic Phenomena (T-HP). A list of 116 T-HP is derived in the present paper, based on the documents issued in the last three decades by the Committee on the Safety of Nuclear Installations of Nuclear Energy Agency of The Organization for Economic Cooperation and Development (OECD/NEA/CSNI) and by the International Atomic Energy Agency (IAEA). The T-HP list includes the consideration of Separate Effect Tests (SET) and Integral Effect Tests (IET) relevant in Reactor Coolant System (RCS) and Containment of WCNRs. A dozen WCNR types are considered and include Pressurized Water Reactors (PWRs), Boiling Water Reactors (BWRs), Russian design reactor types (e.g. VVER-440, VVER-1000 and RBMK), pressure tube heavy water reactor designs in Canada (CANDU) and in India (PHWR) and so-called ‘advanced’ reactors (in the text of this paper, they are sometimes assigned as “New Reactors”), which are also equipped with passive systems (for instance, AP-1000 and APR-1400 designed in US and Korea, respectively). Each T-HP can be characterized by one or more parameters or variables which are part of numerical models and constitute calculational results from system codes. A cross link process can be established between T-HP, parameters and DBA scenarios. The basis for the process and selected cross-link examples are provided and discussed. A variety of applications for the T-HP list is envisaged in nuclear thermal-hydraulics. Insights are given in the paper in relation to the use of phenomena: a) to address the scaling issue; b) to distinguish between constitutive equations part of the balance equations and ‘special models’ in BE system codes; c) to prioritize research in nuclear reactor thermal-hydraulics.