Development of mesh generation method in a fast reactor fuel assembly

Kikuchi, Norihiro ; Imai, Yasutomo*; Yoshikawa, Ryuji ; Tanaka, Masaaki ; Ohshima, Hiroyuki

In the Japan Atomic Energy Agency, a detailed thermal-hydraulic analysis code named SPIRAL based on the finite element method (FEM) is being developed to evaluate the detailed thermal-hydraulic properties of fuel assemblies (FAs) in sodium-cooled fast reactors (FBRs). Because the quality of the computational grid (elements) used in the calculations has a significant impact on the prediction accuracy, the allocation of high-quality elements in the wire-spacer-type FA pin bundle region is an important issue for numerical analysis. Although a commercial mesh generation program (mesher) with CAD data of FA's geometric shape can be considered as one measure, it is an extremely complicated task to perform element division of complex FA region. Therefore, to efficiently allocate high-quality elements, we developed a mesher that automatically performs element division in the FA region using the FA's geometric shape (design information) and meshing parameters as input conditions. This report describes the details of the mesher's various meshing models and their usage. To regularly allocate the computational grid for the complex FA region, the mesher first divides the region into multiple blocks using a multi-block method, then generates boundary-fitted curvilinear coordinate grids for each block region, and finally integrates them into a single FA mesh system. In addition, a combination of hexahedral elements and prism-shaped elements is arranged to maintain element continuity between adjacent block regions. Element division for both the normal FAs surrounded by a hexagonal cross-section tube and the irregular FAs, inside which a duct is installed to promote the discharge of molten fuel, is possible. The development of this mesher has made it possible to accurately and efficiently perform element division of complex FA region on various conditions.