ALLEGRO Gas cooled Fast Reactor Rod Bundle investigations with CFD and PIV method

Abstract

The development of a new fuel bundle usually relies heavily on research methods based on computer codes and numerical simulations. The reliability of these computer codes should be compared with measurement results for validation purposes.The main objective of this paper is to present a new measuring system developed at Budapest University of Technology and Economics (BME) Institute of Nuclear Techniques (NTI). The system is designed to investigate the emerging flow structures in GFR relevant 7 pin rod bundle geometry. The created rod bundle is a simplified and reduced version of the ALLEGRO GFR fuel assembly (P/D = 1.208). Our test facility named PIROUETTE (PIv ROd bUndlE Test faciliTy at bmE) is capable of capturing 2D velocity flow fields inside the rod bundle. The fuel rods made of special polymer (FEP), which are practically transparent in water, therefore provide with the opportunity to investigate the effect of different spacer grids even inside the rod bundle. With high resolution 3D printing method different mixing vane spacer grid types (e.g. spacer grid without vane, and spacers with SPLIT and TWISTED type mixing vanes) were manufactured and tested. An uncertainty estimation related to the PIV methodology was also performed.CFD models have been developed for the flow domain, which includes the test section of the PIROUETTE facility and the rod bundle inside it. Separate CFD models were made to describe the different mixing vanes. Mesh independence study was created to provide suitable discretisation of the models. Steady-state RANS calculations were performed with different turbulence models. The measurement results were compared with the results of the CFD modelling. Transient calculations were created to achieve more accurate results. All the calculations were carried out using ANSYS CFX 19.2 code.