Thermal Finite Element Analysis of Ceramic/Metal Joining for Fusion Using X-ray Tomography Data

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Abstract

A key challenge facing the nuclear fusion community is how to design a reactor that will operate in environmental conditions not easily reproducible in the laboratory for materials testing. Finite element analysis (FEA), commonly used to predict components’ performance, typically uses idealised geometries. An emerging technique shown to have improved accuracy is image based finite element modelling (IBFEM). This involves converting a three dimensional image (such as from X ray tomography) into an FEA mesh. A main advantage of IBFEM is that models include micro structural and non idealised manufacturing features. The aim of this work was to investigate the thermal performance of a CFC Cu divertor monoblock, a carbon fibre composite (CFC) tile joined through its centre to a CuCrZr pipe with a Cu interlayer. As a plasma facing component located where thermal flux in the reactor is at its highest, one of its primary functions is to extract heat by active cooling. Therefore, characterisation of its thermal performance is vital.Investigation of the thermal performance of CFC Cu joining methods by laser flash analysis and X ray tomography showed a strong correlation between micro structures at the material interface and a reduction in thermal conductivity. Therefore, this problem leant itself well to be investigated further by IBFEM. However, because these high resolution models require such large numbers of elements, commercial FEA software could not be used. This served as motivation to develop parallel software capable of performing the necessary transient thermal simulations. The resultant code was shown to scale well with increasing problem sizes and a simulation with 137 million elements was successfully completed using 4096 cores. In comparison with a low resolution IBFEM and traditional FEA simulations it was demonstrated to provide additional accuracy.IBFEM was used to simulate a divertor monoblock mock up, where it was found that a region of delamination existed on the CFC Cu interface. Predictions showed that if this was aligned unfavourably it would increase thermal gradients across the component thus reducing lifespan. As this was a feature introduced in manufacturing it would not have been accounted for without IBFEM.The technique developed in this work has broad engineering applications. It could be used similarly to accurately model components in conditions unfeasible to produce in the laboratory, to assist in research and development of component manufacturing or to verify commercial components against manufacturers’ claims.

Layman's Abstract

By harnessing the same process as that which powers the sun, fusion power promises to deliver an effectively limitless supply of energy without producing carbon emissions or long term nuclear waste. To achieve this, plasma is produced that is ten times hotter than the sun’s core and is held in place by superconducting magnets. The edge of this plasma still reaches temperatures of up to 3000 °C and the challenge for engineers and materials scientists is to develop a vessel capable of containing this process.Engineers often use computer models to predict how their design will perform under certain conditions. However, these models tend to be idealised and not include micro scale features, such as defects introduced by the manufacturing process, which will cause unexpected behaviour of the component. An emerging technique shown to have improved accuracy converts three dimensional images of manufactured components into computer models. These images can be collected by various methods, such as CT or MRI scanners similar to those found in hospitals. It was desired to use this image based modelling to make design recommendations for components planned for a fusion reactor.The difficulty with this method is that the models produced have very high resolutions requiring large amounts of computing power, currently available commercial software cannot be used to perform the simulations. This served as motivation to develop specialised software to be run on supercomputers. This enabled successful running of the models by dividing the calculations into manageable chunks to be solved using thousands of computer processors simultaneously. In applying this technique, small voids were found in the component being studied. If gone unnoticed these could have caused the component to fail, but this technique allowed recommendations to be made to reduce this risk.Although developed for use with components for fusion, this technique has a broad application to most engineering fields. As well as being used for research and development, it is envisaged that a streamlined or automated deployment of the technique could be included in a manufacturing line to assist with quality assurance control.

Keyword(s)

3D imaging; CFC; CT; Cu; FEA; FEM; HPC; ITER; X-ray tomography; carbon fibre composites; copper; direct casting; direct casting; divertor; finite element analysis; fusion; high performance computing; image-based; joining; laser flash; open source; parallel computation; supercomputer; thermal analysis; thermal conductivity; tokamak

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