• gen-IV
• CFD
• flow blockage
• thermal hydraulic
• heavy liquid metal
• nuclear reactors

The present master thesis is devoted to the Computational Fluid Dynamic (CFD) analysis of Heavy Liquid Metal (HLM) cooled Fuel Bundles to be adopted in Gen-IV nuclear reactors. The thesis has been carried out in collaboration with the ENEA Brasimone research center (Italy).
The large scale pool CIRCE facility, placed at the Brasimone research centre, isupgraded with the ICE (Integral Circulation Experiment) test section.The ICE fuel pin bundle (FPS), consists of wrapped 37 electrical pins arranged in a hexagonal/triangular fashion, fixed by three spacer grids. The FPS is instrumented with thermocouples to measure wall and central channel temperatures in different subchannels.
To analyze the behavior around the pins, depending on their position within the section, a CFD computational study has been carried out on fluid flow and heat transferin the ICE bundle. Several geometrical models have been built to approximate the real 37-pin ICE bundle: starting from the periodic subchannel, 7-pin, 19-pin, 37-pin; all of these models are described by a detailed computational domain representative of an angle of 60deg with lateral rotational periodic boundary conditions, with a resolution of y+=1 at the wall in order to solves explicitly the viscous sub-layer.The SST-omega and RSM-omega turbulence models have been adopted and the grid independence was assessed. A very good agreement has been obtained in the comparison with experimental results and known heat transfer correlations, at various mass flow rates, from nominal (w≈1m/s, Re≈10^5) to reduced PLOFA condition (w≈0.15m/s, Re≈1.5•10^4). In addition, the detailed analysis of the local effects provided good indications to the bundle designers.
A preliminary CFD analysis is also presented on the fluid flow and heat transfer in the case of flow blockage in a HLM cooled FA, both for the open square named ELSY (European Lead-cooled SYstem) and for the wrapped hexagonal closed ALFRED (Advanced Lead Fast Reactor European Demonstrator).
The computational domain of a single FA has been simulated with an active region preceded by a short non-active entry region to have fully developed flow at the beginning of the active region. A constant power distribution with no reactivity temperature feedback has been considered as conservative engineering approach.
The study evidenced two main effects: a local effect in the wake/recirculation region downstream the blockage and a global effect due to the lower mass flow rate in the blocked subchannels, which respectively lead to a temperature peak behind the blockage and at the end of the active region. A comparison between the two FA, on the base of the equal area fraction blocked, indicate that in the open FA the velocity far upstream the occlusion remains unchanged, and therefore the mass flow rate across the FA is not altered, while the closed FA offers a better detectability of the blockage.