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This thesis studies the cooling of rod bundles within the Advanced Gas-cooled Reactor (AGR) fuel route at non-design conditions using a variety of methods. The aims of the thesis are (i) to contribute to the general understanding of the detailed flow, heat transfer, and turbulence phenomena in AGR rod bundles. (ii) to develop a 3-D porous model software package for the thermal-hydraulics analysis of the fuel route. Of primary concern to this project are scenarios where the fuel bundle is distorted as a result of being dropped or damaged during refuelling operations. The model developed herein will complement the current 1-D thermal codes in use at EDF Energy. This is particularly for the cases where the latter would be excessively pessimistic or inaccurate due to their inability to capture the 3-D characteristics of the flow. The open-source, co-located, and segregated Computational Fluid Dynamics (CFD) solver Code Saturne developed by EDF has been used. For the first aim, three studies have been carried out, that is, (a) Large Eddy Simulation (LES) study of natural circulation in a short 0.25 m enclosed bundle (b) Large Eddy Simulation study of natural circulation in a 1 m tall enclosed bundle and (c) Reynolds Averaged Navier Stokes (RANS) study of forced convection in a damaged bundle. In a short bundle, (a) the flow is largely laminar and constrained to the thin boundary layers around the fuel rods and containment wall. Away from the walls, in the core, the flow is stagnant. The vertical temperature distribution is heavily stratified. The natural circulation flow in the 1 m domain is heavily influenced by a vertically developing boundary layer on the containment surface, which is initially laminar but transitions to turbulence at about a quarter of the height from the top. The Nusselt number on the containment wall can be correlated using a well established expression over a vertical plate in a free space. Laminar boundary layers observed in both the long and short domains compare very well with similarity solutions, though for those over the fuel rods, the curvature needs to be considered. Forced convection in a damaged WheatSheaf bundle shows the flow to swirl around the rods as it is diverted to regions of less resistance through the rod gaps. Hot spots on the fuel at any axial location are found on the leeward side of the cross-flow. To fulfil the requirements of the second aim a thermal-hydraulics code for the fuel route, named FREEDOM has been developed. FREEDOM aims to predict AGR fuel component temperatures under potential fault conditions while the fuel is being handled or stored within the AGR fuel route. FREEDOM has two modes, one for intact and another for damaged fuel. The main focus of this thesis is on damaged fuel. The model comprises of two domains, the fluid domain computed using Code Saturne and the solid domain computed using Syrthes. In the fluid domain, the porous media representation is used to simplify the mesh generation and lessen computation cost. Thermal conduction and radiation are associated with the solid domain. The two domains are coupled together through the exchange of temperatures and heat transfer coefficients. In addition, oxidation due to the fuel and carbon deposit have been modelled considering both diffusion and reaction dynamics controlled conditions. The code is validated by performing experimental and code-to-code comparisons for a variety of flow conditions and idealised geometries. Forced convection comparisons were in good agreement and natural convection comparisons ranged from good to acceptable. The validated FREEDOM has then been successfully used to support a safety case argument by taking into account the three-dimensional effects for the flow, radiation, and solid conduction.

The nuclear industry has a long history of usage of reliable and well-validated software for design and safety analysis of nuclear power generation systems. However, the majority of codes employed are based on the assumption that the flow is essentially one-dimensional, and they often provide excessively conservative predictions when three-dimensional phenomena have a significant influence. The aims of this study are twofold: - Furthering the understanding of three-dimensional flow patterns that may occur within the AGR fuel stringer during refuelling operations. - Developing a numerical tool for refuelling cooling analysis capable of supporting the existing methodology in scenarios where neglecting three-dimensional phenomena would affect significantly the accuracy of the calculations. The software used in the calculations comprised of Code_Saturne, a finite volume CFD solver, and Syrthes, a finite element solver for thermal conduction and radiation. Both solvers are open source and are developed by EDF. To pursue the first objective, a LES of natural convection in an enclosed fuel pin bundle was carried out using the WALE model. The computational domain represented a 60 degrees sector of a 250mm long pin bundle. The roughness of the pins was neglected. The results of the calculation showed a pattern in which fluid motion takes place in thin boundary layers adjoining the active walls, separated by a stagnant core region. It has been found that the flow between the various pins resembles natural convection in a rectangular enclosure and that the local Nusselt number correlation takes the same form as that of the cavity flow. The second line of work resulted in the development of POSTR, a numerical three-dimensional tool designed for routine usage in refuelling cooling calculations. The tool is based on the approximation of the fuel pin bundle as a porous medium to reduce the computational cost. A two-scale approach has been followed, with a coarse grid used for the calculation of the modelled quantities and a finer mesh used for the solution of the governing equations. A solver for conduction and thermal radiation in the solid components of the stringer is coupled with the CFD model for the gas, and a simple turbulence model is adopted. A demonstration of the capabilities of the code has been performed. The predictions of the code have also been validated against experimental data experiment for scenarios encompassing forced, natural, and mixed convection conditions. The solid temperatures predicted by POSTR are found to be in good agreement with those calculated by detailed CFD calculations and by legacy software formerly used in the industry, as well as with the measurements taken in a large scale experiment. Recommendation for the usage of the tool are provided and directions for future work are suggested. Two main original contributions of this study can be highlighted: - To the knowledge of the author, the LES presented in this thesis is the first numerical investigation of turbulent natural convection in an enclosed bundle of concentrically arranged pins. The observations described in the thesis could provide useful guidance for further investigation on the topic. - The hybrid approach between CFD and sub-channel analysis introduced in this thesis, based on the use in the calculation of two meshes at two different scales has been adopted in subsequent research projects on coarse-grid CFD, carried out at the Heat, Flow and Turbulence Research Group of the University of Sheffield.