Development and Performance of the ANDES/COBRA-III Coupled System in Hexagonal-Z Geometry

Abstract

In this paper, the extension of the nodal diffusion code ANDES, based on the ACMFD method, to hexagonal geometry is presented, as well as its coupling with the thermal-hydraulic (TH) code COBRA-IIIc/MIT-2 for such hexagonal problems. In extending the ACMFD method to hexagonal assemblies, triangular-Z nodes are used, taking advantage of the mesh refinement capabilities implicit within that geometry. The existing TH coupling for Cartesian geometry applications has also been extended to hexagonal problems, with the capability to model the core using either assembly wise channels (hexagonal mesh) or a higher refinement with six channels per fuel assembly (triangular mesh). Achieving this level of TH mesh refinement with COBRA-IIIc code provides a better estimation of the in-core 3D flow distribution. Therefore, the work presented here introduces an improvement in the TH VVER core modelling, where in the best case scenario, just twenty axial layers and one channel per fuel assembly were used. As a result, the neutronic and thermal-hydraulics (N-TH) coupled code, ANDES/COBRA-IIIc, extensively verified in Cartesian geometry cores analysis, can also be applied to full threedimensional VVER core problems. Some verification results are provided, corresponding to 2nd exercises (HZP and HP steady states) of the OECD/NEA- VVER-1000 Coolant Transient benchmark and to the HZP and HFP steady states of the V1000CT2-EXT2 NURESIM benchmark.