Fusion materials

Materials World magazine
,
27 Sep 2019

Works are in place to advance the knowledge and ensure technology transfer of commercially viable nuclear fusion power plants.

UK demand for electricity varies between 30–40GW, with peaks in the winter months, not including the requirement for electric vehicles. Nuclear fission contributes approximately 20% of the energy mix but the majority of electricity is supplied by gas or coal-fired plants, at 40% and 10% respectivelyTherefore, there is a case for baseline electricity generation of at least 10–20GW from sustainable sources.

Electricity from a nuclear fusion power plant represents an attractive scenario, pursued in earnest worldwide since the 1960s. The most technologically mature approach is based on the tokamak, a doughnut-shaped vacuum vessel, in which a plasma at 100-200 million degrees Celsius is confined by magnetic fields.

The plasma typically contains deuterium (²H) and tritium (³H), which react to form helium (⁴He) and a high-energy 14 mega electron volts neutron. Heat caused by these neutrons in the walls of the vessel can then be used for future electricity generation. The inputs to the reaction are deuterium, a naturally occurring and relatively abundant hydrogen isotope, and lithium, which is used to create the radioactive tritium.

One of the key challenges for a commercially feasible and realistic fusion power station is plant availability. In general, plant availability is limited by the lifetime of materials and components inside the reactor, as they are exposed to extremely harsh environments, including neutron and ion irradiation, high temperatures, hydrogen gas and cyclic loading.

In 2017, the Department for Business, Energy and Industrial Strategy (BEIS) gave a grant to the UK Atomic Energy Authority, in order to establish Fusion Technology Facilities (FTF). Two of the programmes under FTF are the Materials Technology Laboratory (MTL) and the Joining and Advanced Manufacturing (JAM) programme, both based at the UKAEA laboratory in Culham, near Oxford.

Their remit is to establish the research and technology pipeline and supply chain required to enable commercially viable power plants with fusion energy. This includes transferring fusion know-how and requirements to industry and academia, but equally partnering with other industry sectors, such as aerospace or automotive, to exploit synergies.

Small-scale testing

MTL is establishing itself in support of the technological development of fusion energy. The laboratory aims to develop novel materials for use in fusion, for example vanadium alloys and nanostructured steels, to establish the materials supply chain, enable fusion-relevant environmental testing, standardise small specimen testing, and perform fusion design criteria validation testing.

The engineering and analysis test centre includes a range of high-resolution universal testing machines with hydraulic and electro-mechanical load frames, operating in tension or compression up to 50kN. They have argon or moderate vacuum environments ranging from cryogenic temperatures up to 1,000°C, and are flexible to allow for building engineering performance on fusion materials and modifying equipment to meet experimental needs. These systems work on small-scale samples up to a few centimetres with non-contact examination technologies such as digital image correlation. Instrumented indenter testing is used to evaluate micro to macro properties, which may lead to advance in-situ testing techniques.

Specific effects such as high magnetic fields or liquid metal corrosion will be added to provide the UK with specialist capabilities in this field. This is all supported with analytical equipment such as scanning electron microscopes to support mechanistic understanding of effects. Other areas of research include rapid evaluation of the mechanical performance of additive manufactured products and vanadium alloys research with existing collaborative work on V44 and ODS V44. Some of the leading work is on developing novel alloys with international collaborations such as Southwestern Institute of Physics in China.

The irradiation damage and complex environments of fusion mean only small volumes of materials that have undergone this damage type will be available before fusion reactors are operational. This is a big challenge and means the mechanical performance of proposed materials can only be determined using very small-scale samples. This is a research challenge MTL is addressing by evaluating and going through the standardisation process towards qualifying the results of small-scale samples, to predict full-scale components. One technique MTL is helping to standardise is the small punch technique, which only uses 8mm-diameter, 0.5mm-thick samples to determine mechanical performance.

A 1mm or 2.5mm diameter ball or hemispherical ‘punch’ is driven into the sample at a constant displacement rate, which results in a load-displacement curve. While analogous to a stress-strain curve, a direct comparison cannot be made because the stress state is not uniaxial. Correlations between this curve and conventional uniaxial mechanical properties exist, typically related to yield stress and ultimate tensile strength. When a constant load is applied to the specimen, the creep properties can be studied and similar uniaxial correlations made. The qualification of these new qualified small-scale testing techniques for fusion will also enable them to be used for other industries.

Advanced manufacturing

Once materials have been selected for a particular application, the next step is the manufacturing and inspection of components. This is the remit of the JAM programme. Fusion components must withstand extreme environments and often include joints between dissimilar or exotic materials. These joints need good thermal conductivity as well as structural integrity to handle the high heat loads subjected to them and must withstand cyclic loading at high temperatures due to pulsed operation. JAM specialises in the development of the critical material joining and manufacturing technologies required to deliver fusion.

The key objectives of JAM are:

  • An active research programme on joining and manufacturing components for fusion
  • Hosting and developing testing equipment
  • Providing expert knowledge in fusion as well as intellectual property and expertise in joining, advanced manufacturing and testing which will be accessible to UK industry to help them access ITER contacts and other future fusion work, and
  • Working to transfer technology and expertise between sectors, particularly into and out of fusion.

While it has an internal research strand, a significant aspect of JAM is enabling universities, research organisations and industry to be part of the fusion manufacturing technology supply chain. Projects are underway through PhD studentships, UKAEA-funded research and collaborative funding bids. This encourages technology transfers both in and out of fusion, such as with the aerospace and nuclear fission industry.

Recent work has highlighted potential applications of additive manufacturing for fusion applications. Building on previous experience, methods are being exploited to create novel cooling architectures, use novel materials, and incorporate functionally graded joints in plasma-facing components, as well as reduce manufacturing costs. Work is ongoing at multiple universities with projects and PhD students, as well as SMEs and larger industrial organisations.

Other key themes for JAM are the development of fusion compatible non-destructive testing (NDT) techniques and condition monitoring sensors, and manufacturing for maintenance. The development of NDT and in-situ monitoring techniques compatible with conditions of the manufacture and operation of fusion components are active research areas. These involve assessment and improvements to well-established techniques, as well as increasing the technology readiness of new methods and concepts.

The unusual materials needed for fusion means these techniques need to be investigated in conjunction with an understanding of the material properties. The team has been using neutron imaging techniques at ISIS, the neutron and muon source in Harwell, UK, to analyse critical components and manufacturing processes. An advantage of neutron imaging over X-ray is that neutrons are significantly more penetrating through tungsten, one of the often-used fusion materials.

Attenuation of the particle beam is correlated with the materials it passes through, so by capturing transmission patterns at set sample angles, the internal features of the component, including flaws in material or construction, are revealed. These can be included in a digital model of the component for virtual testing. This approach has allowed us to inspect, for example, the integrity of brazed tungsten to copper interfaces of as-manufactured components and components exposed to high heat fluxes of 10-20MW/m2 with a resolution down to 50-100mm. Standard test routes for these components use ultrasound – about a 1mm detection limit – or destructive metallographic analysis.

The team also operates the UKAEA’s compact high heat flux test facility (HIVE), which delivers fusion-relevant heat fluxes up to 18 MW/m2 to cooled or uncooled samples within a vacuum environment by applying pulses of radiofrequency electromagnetic radiation. Temperature sensors, infrared cameras and digital image correlation are the main instruments available for in-situ analysis.

HIVE’s flexible component architecture facilitates efficient feasibility testing of novel prototypes, materials, or technologies while digital twinning techniques allow efficient planning and rapid interpretation of experimental data – together accelerating technology development.

As well as testing joints and prototypes, HIVE has the capability characterising new sensors and measurement techniques such as those needed for embedded condition monitoring. The facility can create the conditions of temperature or incident heat flux to the sample in order to simulate such extreme environments. The data then allows scientists and engineers to evaluate properties such as the heat transfer, validate modelling or simple to inspect how well their component or material has survived.

Maximising fusion potential

Fusion has significant materials challenges to overcome in order to reach the point where power generated by fusion will be a standard part of the UK’s energy mix. Some of these issues are unique to the field, however, there is significant overlap with other sectors. The FTF facilities and research programmes are pursuing collaborative projects that bring together diverse expertise and allow technology transfer.

The testing methods described advance the understanding of materials under fusion-relevant conditions, which can then feed into power plant designs. They allow new materials to be investigated and current ones to be driven through qualification processes, demonstrating that they can play their part in the long future of safe and reliable energy generation.


This article is published under UK Department for Business, Energy and Industrial Strategy © Crown Owned Copyright 2019/UKAEA

 

*Mike Gorley is Materials Technology Programme Manager, Heather Lewtas is Joining and Advanced Manufacturing Programme Manager and Frank Schoofs is Engineering Innovation Section Leader at UK Atomic Energy Authority.