Optimising Plasma Sprayed Tungsten Coatings

Tungsten is the key plasma facing material for use in any future nuclear fusion device due to its high melting point, good sputter resistance and low activity. However its refractory nature leads to inherent difficulties in its processing and many traditional production routes are not available. Without tungsten plasma facing materials, there exist no viable concepts for nuclear fusion as a sustainable power supply. As such, it is one of the key areas to develop if fusion is to succeed. Much work has been put into the development of monoblock type structures, where bulk tungsten is directly joined to pipe work carrying coolant, but the behaviour is currently unacceptable due to low fracture toughness and cracking under repeated cycling. An alternative is to use tungsten coatings on a steel of copper substrate. Vacuum plasma spraying is one of the most attractive methods of producing tungsten coatings for this application, but thermal mis-match between the tungsten and substrates such as steel or copper lead to the development of complex residual stresses, which degrade the performance of the coating. Previous work has shown these stresses can causes premature failure of the coatings under thermal cycling.

This project will use recently upgraded vacuum plasma spraying equipment to produce both pure and alloyed tungsten coatings on novel substrates. These substrates have been shown to have promise in reliving some of the residual stress through controlled cracking, but no full characterisation of these has been carried out. These will be characterised using state of the art microscopy and micro-mechanical testing facilities in the department of materials and finite element analysis used to understand the evolution of the stress state. Microscopy will focus on understanding the effects of processing variables on the microstructure and their eventual effect on thermal and mechanical properties. Micro mechanical testing will focus on understanding the local modulus and fracture toughness of both as sprayed and aged coatings. Micro-cantilevers will be manufactured, for the first time, in these materials using Focused Ion Beam machining (FIB). This is a recently developed method at Oxford which allows rapid testing of mechanical behaviour on small volumes of material. By testing with high temperature nanoindentation the mechanical properties (elastic modulus, failure stress and fracture toughness) will be measured not just at room temperature but also at operational temperatures. Finite element analysis will be used to model the behaviour of the coatings using the experimental data to benchmark the model. Additionally for the first time, thermal cycling will be carried out using the HIVE facility at CCFE, UK and JUDITH and FZK Julich. These tests will simulate the thermal cycles experienced in a real reactor. This data will then be fed back into the processing route for improved plasma facing coating design with longer cycles to failure.

This project is funded by the EPSRC CDT in Science and Technology of Fusion Energy. This project falls within the EPSRC Energy research area.

Identifying a solution to the energy problem is crucial to the UK economy and quality of life. In the near term a range of renewable options must be developed, eg wind and solar, but it is unlikely that these will provide the base-load supply required. Nuclear is an option for a carbon-free base-load and, in particular, fusion energy is safe and relatively clean. If it can be achieved, fusion would bring the largest economic benefits to those countries that lead the way to build the first fusion power plants, but ultimately most people in the world will benefit from fusion in some way.
ITER, the largest international science project on Earth, will operate from 2020 to answer the final physics questions and most technology questions required to construct the first demonstration magnetic fusion energy (MFE) power plant, DEMO. We will train the ITER generation of UK fusion scientists who will have the expertise to win time on this key facility against international competition. This is crucial to build experience that will feed into the design of DEMO, ensuring the UK remains at the forefront. EU design studies for DEMO are already under way, with manufacture of prototype components likely to follow soon. There are a number of beneficiaries from this training: (1) it will benefit Culham Centre for Fusion Energy (CCFE), providing well-trained new staff to replace those retiring, keeping the UK at the forefront of fusion energy research, competitive for ITER time and leading elements of DEMO design/prototype development; (2) it will provide expertise for the growing UK industry involvement in fusion, helping to win contracts for ITER and DEMO prototype components; (3) it will ensure the UK has a cadre of fusion experts to advise Government on future directions. We expect to train 60 students in MFE, approximately balanced across plasmas, materials (relevant for IFE also, see below) and related fusion technologies.
For inertial fusion energy (IFE), NIF in the US is the most advanced device in the world, and some expected it would achieve fusion conditions, i.e. ignition. In its 2012 ignition experiments, this did not happen, but the reason why is still uncertain. The immediate need is to understand this, which requires experts to win time on international facilities (including NIF), understand why ignition did not occur and so develop a roadmap to IFE based on the new knowledge. This will benefit the UK Government by providing experts to advise on an appropriate strategy, able to compare the relative merits of IFE and MFE because of our training across both areas. If IFE proves viable, then it will need to integrate fusion technologies in a similar way to ITER and DEMO, bringing benefits to industry. We expect to train 15 students in high energy density physics (HEDP), spanning IFE and lab astrophysics; the MFE materials students' expertise is also relevant for IFE reactor design.
Expertise in HEDP is required by AWE for its science-based approach to underpinning the UK's nuclear deterrent, and is a key element of the UK's strategy to comply with the Test Ban Treaty. The new Orion laser facility at AWE can replicate the conditions in a nuclear warhead, enabling advanced computer codes to be tested. Our students will have the expertise to work with Orion, which requires skilled scientists as it establishes its programme. Also the materials and computational scientists amongst the ~60 MFE students will be of value to AWE.
We will train students in the cooler exhaust plasma of a tokamak. Similar plasma conditions are used in manufacturing industries (coatings, computer chips, etc) so we will develop a skill base that will benefit a number of such companies. Materials research for fusion is also relevant for fission. The popularity of fusion amongst students is a good way to bring outstanding students into the field, providing expertise that benefits the growing nuclear industries and supporting the Government's nuclear policy.