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Employing Additive Manufacturing for Fusion High Heat Flux Structures

Hancock, David (2018) Employing Additive Manufacturing for Fusion High Heat Flux Structures. PhD thesis, University of Sheffield.

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Abstract

The commercial realisation of nuclear fusion power will require advanced engineering solutions including high heat flux components with higher performance, greater reliability, and longer lifetimes. Additive manufacturing (AM) provides opportunities to produce components with previously unachievable geometries in new and hard-to-manufacture materials. This project introduces the state of the art of fusion high heat flux components and AM and then focuses on applying laser powder bed fusion to high temperature divertor designs. Much of the work was carried out in parallel to the EU FP7 AMAZE project (Additive Manufacturing Aiming towards Zero waste and Efficient production of high-tech metal products). A review of material selection for divertor applications is carried out with an emphasis on the cooled substructure. A parallel, strengths-based approach is undertaken concluding in a series of SWOT (strengths, weaknesses, opportunities, threats) analyses rather than a traditional linear down-selection. Material properties including strength, ductility, thermal expansion, and thermal conductivity are graphically presented as well as derived figures of merit for thermal stress and thermal mismatch with tungsten armour. Radiation damage and compatibility with operational and manufacturing environments are considered and historical summaries of availability and cost are given. By emphasising high temperature operation and acknowledging the inevitability of some nuclear activation beyond the usual 100 year limit, refractory metals and their alloys present themselves as promising candidates, particularly those based on vanadium, tantalum, and molybdenum. A shortage of data for these materials is highlighted, particularly under fusion neutron irradiation, as well as the need for greater understanding of corrosion under relevant conditions. Two novel divertor cooling schemes are then presented and evaluated via concept-level tile-type geometries. The first is a design with multiple small pipes fed from the rear of the component via an in-built manifold and the second employs an enclosed pin-fin array drawing inspiration from the electronics industry. Both highlight features made feasible only by employing AM and use tantalum as the structural material to demonstrate the effect of high-temperature operation on performance. 1D analytical calculations and simple finite element modelling with 150◦C and 600◦C coolant and up to 10 MWm−2 heat flux loading demonstrate improved heat transfer coefficients and more uniform temperature distributions. Performance improvement over conventional designs is likely to be marginal without significant further design optimisation, but the up to 80% reduction in material use compared with conventional concepts, higher thermal efficiency, and opportunity to reduce or relocate pipe joints are highlighted as more significant advantages. Work to develop laser powder bed fusion of tungsten, molybdenum, and tantalum is then presented. First, a summary of context and recent related work is given. A through-lifecycle approach to component development is detailed with the aim of giving an insight into critical issues related to supply chain, process development, material testing, and component build trials. Basic characterisation of size, morphology, and flowability of a selection of powders is used to demonstrate the high variability of current supply. This is followed by determination of first-order build parameters and energy density required for consolidation. Persistent cracking is found, particularly in tungsten and molybdenum, and causes including oxidation and residual stress are posited with suggestions for possible approaches to mitigating these. The results of material testing of small samples are given, including dilatometry, laser flash, and small punch. Small sample numbers and high variability prevent definitive conclusions from being drawn, but trends towards increased brittleness and decreased thermal conductivity are shown and there are indications that the extreme thermal conditions during processing produce β and ω phases of tantalum. Finally a description of a new facility is given, HIVE (Heating by Induction to Verify Extremes), as well as the results of comparative high heat flux testing of two simple copper components - one produced by electron beam melting (EBM) and the other conventionally manufactured. HIVE can apply a constant 10 MWm−2 to a 30 mm x 30 mm test-piece in vacuum which can be cooled using a 200◦C cooling water supply. Thermocouples, thermography, and water calorimetry provide instrumentation. This facility acts as a strategic and previously unavailable intermediate concept validation step between analytical modelling and plasma-surface interaction testing or in-situ qualification. The results presented suggest that convective heat transfer is enhanced by the rough surface of the AM copper part, but that the component’s lower thermal conductivity through the AM copper and across the brazed joint compared to the conventional results in a higher bulk temperature for the same input power indicating a lower overall heat flux handing capability. The project concludes with a summary of key findings and suggestions for future work.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Materials Science and Engineering (Sheffield)
Identification Number/EthosID: uk.bl.ethos.770132
Depositing User: David Hancock
Date Deposited: 11 Mar 2019 15:24
Last Modified: 25 Sep 2019 20:07
URI: http://etheses.whiterose.ac.uk/id/eprint/23009

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