Advanced manufacture and characterisation of thick tungsten coatings for nuclear fusion applications
<p>Nuclear fusion is a promising technology for the production of sustainable, clean energy, but its successful implementation is dependent on a number of practical aspects. One important factor is the performance of plasma-facing components (PFCs), such as the divertor or first wall (FW) of a...
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| Format: | Thesis |
| Language: | English |
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2023
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| _version_ | 1826317669853298688 |
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| author | Evans, B |
| author2 | Armstrong, D |
| author_facet | Armstrong, D Evans, B |
| author_sort | Evans, B |
| collection | OXFORD |
| description | <p>Nuclear fusion is a promising technology for the production of sustainable, clean energy, but its successful implementation is dependent on a number of practical aspects. One important factor is the performance of plasma-facing components (PFCs), such as the divertor or first wall (FW) of a tokamak. W is currently the leading candidate to withstand the extreme environment, but establishing a replicable means of manufacture of W components that is faster than the currently-employed diffusion bonding method is essential to achieve commercial viability. It is predicted that 300,000 divertor tiles will be initially required for the International Thermonuclear Experimental Reactor (ITER) alone.</p>
<p>Bonding W to structural materials such as steel and Cu is challenging; the large difference in coefficient of thermal expansion (CTE) results in coating delamination during thermal excursions. Several manufacturing technologies are being actively investigated to improve the production and performance of W coatings.</p>
<p>This thesis focuses on two techniques: vacuum plasma spraying (VPS) and the field-assisted sintering technique (FAST). Optimisation of the VPS process using Taguchi analysis resulted in the production of thick (4 mm) W coatings on steel substrates, exceeding previous best efforts. Finite element modelling (FEM) was used to investigate the residual strains and stresses due to the CTE difference following FAST manufacture, and the effect of substrate texturing on crack propagation.</p>
<p>Thermal stability in fusion-relevant conditions is a key performance indicator of candidate PFCs. Three investigations are reported in this thesis: thermal ageing (up to a week and up to 1000 °C), to observe and analyse any diffusional effects that occur at reactor temperatures; steady-state thermal cycling in HIVE (300–800 °C) and UPP (150–550 °C), to observe crack development and test long-term coating survival; and transient thermal cycling in UPP (up to 800 °C base temperature with an additional 300–800 °C due to transient pulsing), to observe localised high heat flux effects that are common to ‘off-normal’ tokamak events such as edge-localised modes (ELMs). The results obtained led to the following conclusions: oxides and carbides formed through diffusion, which embrittled the coatings; flat steel substrates led to coating delamination, whereas samples with W substrates were only damaged locally by transient heating pulses; texturing of steel substrates improved thermal performance (better conductivity and no coating delamination) of samples during thermal cycling; and thicker coatings (on steel substrates) exhibited fewer, but larger, cracks than thinner coatings.</p>
<p>Following these investigative tests, thermally-cycled samples had an additional 1 mm layer of W applied using the FAST, to assess the viability of this technique to repair damaged coatings. Results were encouraging: pre-existing vertical cracking in the original coating of steel substrate samples caused crack initiation and propagation in the new repair coatings, whereas in W substrate samples no damage was observed; repair coatings were thinner than original coatings and higher concentrations of C had diffused in. Subsequently, samples with repaired coatings were retested in HIVE, whereupon some steel substrate samples either severely cracked or delaminated, whilst W substrate samples remained undamaged.</p>
<p>Overall, alternative production routes for W coatings yielded some promising results that warrant further investigation and development, most notably the potential for in-situ repair of W plasma-facing components (PFCs).</p> |
| first_indexed | 2025-03-11T16:57:35Z |
| format | Thesis |
| id | oxford-uuid:f1db0643-e53e-4caa-a397-80f9012381f1 |
| institution | University of Oxford |
| language | English |
| last_indexed | 2025-03-11T16:57:35Z |
| publishDate | 2023 |
| record_format | dspace |
| spelling | oxford-uuid:f1db0643-e53e-4caa-a397-80f9012381f12025-02-21T10:44:05ZAdvanced manufacture and characterisation of thick tungsten coatings for nuclear fusion applicationsThesishttp://purl.org/coar/resource_type/c_db06uuid:f1db0643-e53e-4caa-a397-80f9012381f1Nuclear fusionFusion reactor wallsMetallurgyMaterialsEnglishHyrax Deposit2023Evans, BArmstrong, DGrant, P<p>Nuclear fusion is a promising technology for the production of sustainable, clean energy, but its successful implementation is dependent on a number of practical aspects. One important factor is the performance of plasma-facing components (PFCs), such as the divertor or first wall (FW) of a tokamak. W is currently the leading candidate to withstand the extreme environment, but establishing a replicable means of manufacture of W components that is faster than the currently-employed diffusion bonding method is essential to achieve commercial viability. It is predicted that 300,000 divertor tiles will be initially required for the International Thermonuclear Experimental Reactor (ITER) alone.</p> <p>Bonding W to structural materials such as steel and Cu is challenging; the large difference in coefficient of thermal expansion (CTE) results in coating delamination during thermal excursions. Several manufacturing technologies are being actively investigated to improve the production and performance of W coatings.</p> <p>This thesis focuses on two techniques: vacuum plasma spraying (VPS) and the field-assisted sintering technique (FAST). Optimisation of the VPS process using Taguchi analysis resulted in the production of thick (4 mm) W coatings on steel substrates, exceeding previous best efforts. Finite element modelling (FEM) was used to investigate the residual strains and stresses due to the CTE difference following FAST manufacture, and the effect of substrate texturing on crack propagation.</p> <p>Thermal stability in fusion-relevant conditions is a key performance indicator of candidate PFCs. Three investigations are reported in this thesis: thermal ageing (up to a week and up to 1000 °C), to observe and analyse any diffusional effects that occur at reactor temperatures; steady-state thermal cycling in HIVE (300–800 °C) and UPP (150–550 °C), to observe crack development and test long-term coating survival; and transient thermal cycling in UPP (up to 800 °C base temperature with an additional 300–800 °C due to transient pulsing), to observe localised high heat flux effects that are common to ‘off-normal’ tokamak events such as edge-localised modes (ELMs). The results obtained led to the following conclusions: oxides and carbides formed through diffusion, which embrittled the coatings; flat steel substrates led to coating delamination, whereas samples with W substrates were only damaged locally by transient heating pulses; texturing of steel substrates improved thermal performance (better conductivity and no coating delamination) of samples during thermal cycling; and thicker coatings (on steel substrates) exhibited fewer, but larger, cracks than thinner coatings.</p> <p>Following these investigative tests, thermally-cycled samples had an additional 1 mm layer of W applied using the FAST, to assess the viability of this technique to repair damaged coatings. Results were encouraging: pre-existing vertical cracking in the original coating of steel substrate samples caused crack initiation and propagation in the new repair coatings, whereas in W substrate samples no damage was observed; repair coatings were thinner than original coatings and higher concentrations of C had diffused in. Subsequently, samples with repaired coatings were retested in HIVE, whereupon some steel substrate samples either severely cracked or delaminated, whilst W substrate samples remained undamaged.</p> <p>Overall, alternative production routes for W coatings yielded some promising results that warrant further investigation and development, most notably the potential for in-situ repair of W plasma-facing components (PFCs).</p> |
| spellingShingle | Nuclear fusion Fusion reactor walls Metallurgy Materials Evans, B Advanced manufacture and characterisation of thick tungsten coatings for nuclear fusion applications |
| title | Advanced manufacture and characterisation of thick tungsten coatings for nuclear fusion applications |
| title_full | Advanced manufacture and characterisation of thick tungsten coatings for nuclear fusion applications |
| title_fullStr | Advanced manufacture and characterisation of thick tungsten coatings for nuclear fusion applications |
| title_full_unstemmed | Advanced manufacture and characterisation of thick tungsten coatings for nuclear fusion applications |
| title_short | Advanced manufacture and characterisation of thick tungsten coatings for nuclear fusion applications |
| title_sort | advanced manufacture and characterisation of thick tungsten coatings for nuclear fusion applications |
| topic | Nuclear fusion Fusion reactor walls Metallurgy Materials |
| work_keys_str_mv | AT evansb advancedmanufactureandcharacterisationofthicktungstencoatingsfornuclearfusionapplications |