Multi-scale analysis of gas cooled reactors through CFD and system codes

The aim of this paper is to define the space scales related to the thermalhydraulic phenomena in the case of a Gas Cooled Reactor (GCR), and to propose a coherent set of numerical descriptions adapted to each scale. These descriptions will have to allow, by coupling simulations on different scales, to perform parametric studies for the design, or very precise safety studies in specific zones of the reactor. After having identified the various scales, the various types of couplings will be listed. This document is limited primarily to the description of the vessel (core and plenums), even if other elements of the primary circuit can benefit from a multi-scales description (exchangers, decay heat removal system, for example). Lastly, a section is devoted to the first step of coupling with the CFD code Trio U and the system code CATHARE. The paper has the following structure: 1. Introduction; 2. The different scales; 2.1. The system scale (tens of meters); 2.2. The vessel scale (metres); 2.2.1. The plenums and the downcomer; 2.2.2. The core; 2.3. The subassembly scale (centimeters); 2.3.1. The subassembly; 2.3.2. The gaps between subassemblies; 2.4. The fuel element scale (centimeters); 2.5. The fuel ball scale (micrometers); 3. Coupling methods; 3.1. Two possible approaches of coupling; 3.1.1. Domain Decomposition Method without overlapping; 3.1.2. Domain Decomposition Method with overlapping; 4. First realizations; 4.1. The subassembly scale: Blocking of Helium channels in a HTGR; 4.1.1. Description of the study; 4.1.2. Main results; 4.1.3. Conclusion; 4.2. The plenum scale: Thermal fluctuations in the lower plenum of a HTGR; 4.2.1. Description of the study; 4.2.2. Example of results; 4.2.3. Conclusion; 4.3. Scale of the core; 4.3.1. Application to a pebble bed reactor; 4.4. Preliminary work of coupling between the system code CATHARE and the CFD code Trio U; 5. Conclusion. To summarize, this document underlines the various scales, relating to the thermalhydraulic phenomena, in the Gas Cooled Reactors. The different pointed scales in the reactor are: the system, the vessel, the subassembly, the fuel element and the fuel ball. For each scale, we propose adapted numerical modelling using a 'CFD' approach, or a 'porous media' approach, or a 'system' approach. The coupling methods between the previous different approaches are treated. A last section is devoted to the first realizations, performed with the CFD code Trio U and the system code CATHARE. These applications concern different scales: we have seen CFD simulations to treat the scale of the subassembly or the lower plenum in a HTGR. Other simulations cover the scale of the core of a pebble bed reactor, using a porous media approach. First work about the coupling a system code and a CFD code is also presented. For the future works, many guidelines could be investigated, in particular about the coupling of the various descriptions, for instance: - to develop the multi-scales coupling, for example to be able to simulate the core at two scales, one scale for the modelling of the core with a porous media approach, and on the other hand a high description at the subassembly scale; - to develop the multi-physical coupling between neutronic code and thermalhydraulic code in order to perform complex and realistic calculations of the core; - to improve the coupling between system codes and CFD codes in order to propose powerful numerical tools for the study of the gas cooled reactors