Materials for plasma facing components in fusion technology
Nuclear fusion reactors require specific materials and manufacturing methods to balance effectiveness and efficiency. Wood Nuclear Senior Project Manager, Dr Garreth Aspinall, talks about material choices when designing and developing plasma facing components for fusion applications.
ITER is aiming to become the first experimental fusion device that produces more energy than is required to power its plasma. To achieve this, various organisations across the world have been collaborating to build the largest ever tokamak – a magnetic confinement fusion machine – in the south of France. The fusion reaction occurs in the plasma, a gas of charged particles, inside the tokamak vessel at the centre of the ITER machine.
The plasma is contained by blanket modules, which surround the plasma to provide shielding, cooling and, in future designs, the tritium breeding capability required for power-generating fusion. Each blanket module measures approximately 1m by 1.5m and weighs up to 4.6 tonnes. All have a detachable first wall panel (FWP) that directly faces the plasma to remove the heat load and a main block designed for neutron shielding. The blanket, with a surface of 600m², is one of the most critical and technically challenging components in ITER.
Making a shield
In simple terms, ITER aims to replicate nuclear fusion reactions similar to those that take place inside the sun. Therefore the FWPs form the lining of a box designed to contain a fusion plasma with temperatures up to 150 million degrees Celcius.
There are two types of FWPs within the ITER machine – normal heat flux (NHF) and enhanced heat flux (EHF). They are designed and manufactured using different technologies to accommodate their required heat load. ITER needs more than 440 FWPs, 215 of which will be supplied by Fusion for Energy (F4E), which is responsible for the European Union’s contribution to the project.
Each FWP also measures about 1m by 1.5m in the plasma-facing area, with a stainless steel heatsink base in which sits a composite copper alloy, copper chromium zirconium (CuCrZr). The FWPs have to absorb very high heat fluxes and high energy neutrons, and to achieve this, each panel is covered with approximately 700 beryllium tiles of 10mm-thickness.
The ITER Organization provides the design of the panels to domestic agencies, such as F4E. These designs, with their overall geometry and dimensions, are supplied to Wood, which has worked on plasma-facing components for more than 20 years. These are then elaborated on and developed so the panels can be made.
The starting point is to use computer-aided three-dimensional interactive application CAD design to produce complex 3D models, providing a visual representation of components and the basis for generating 2D drawings.
This facilitates the complex machining of stainless steel and CuCrZr to obtain the parts required for the FWP components, which is done by Leading Metal-Mechanical Solutions, Wood’s joint venture partner based in Northern Spain.
Full-scale prototype manufacture was completed in June 2019. The first full-size panel produced by the consortium has 696 beryllium tiles bonded to its plasma facing surface. All manufacturing processes have successfully been applied to the FSP, without any technical failures, and factory acceptance tests (FATs) have been successfully completed, resulting in a prototype for delivery to F4E to go through high heat flux testing.
Testing it out
Wood’s full-scale prototype involves nearly 2,000 component parts in its manufacture, each varying significantly in their size and complexity. There are three separate assembly stages, and each culminate in a hot isostatic pressing process to bond the components together.
This design uses approximately 2,900kg of material to produce a single first wall panel. As processes are refined, it is expected that the numbers of components and raw material required per panel will be reduced, leading to more sustainable first wall panel production.
These material savings are achieved through design efficiencies and the use of an integrated can concept.
This concept enables the complex geometry of the FWPs to be manufactured and assembled with increased efficiency, and reduces the number of parts required within the component, minimising the complexity of the assembly and the amount of welding.
Ultrasonic testing is applied to demonstrate that the hot isostatic pressing process has been completed successfully and the various parts of the component have consolidated as required. This testing comprises a probe scanning the surface of the component and detecting the feedback from the bonds and interfaces within.
Visual and dimensional inspections are carried out at the end of the project as part of the FATs process. These include pressure, flow and hot helium leak tests to prove the cooling circuit’s integrity for use in ITER. Finally, F4E intends to perform a high heat flux qualification test to assess and qualify the component in conditions similar to those expected in the ITER device.
Inside an operational tokamak where fusion is occurring, the plasma will erode the reactor lining. This severely limits the choice of materials for fusion tokamaks. Most metals with low atomic numbers have low melting points, such as lithium or magnesium. Even aluminium, which is heavier than is desirable, would melt under plasma conditions.
Beryllium, however, is ideal because it is one of the lightest engineering materials and has a sufficiently high melting point. Lighter than aluminium and stiffer than steel, it is already used in highly specialised engineering applications, mainly for space and defence projects. The downsides are that it is difficult to machine and produces hazardous particles. Beryllium is a known carcinogen and inhalation of its dust, even in very small quantities, can lead to chronic beryllium disease.
Making the beryllium tiles for the FWP armour is complex, due to the hazardous nature of the material. The tiles are machined to the required geometry and polished to the final surface finish and dimensions. Then they are chemically etched and coated, to aid bonding.
To ensure staff safety, Wood developed swabbing methods to check for surface contamination, introduced air monitors to assess worker exposure and developed analytical techniques to identify very low concentrations of beryllium and beryllium oxide.
Beryllium will not be suitable for larger power generating fusion reactors, such as Spherical Tokamak for Energy Production and Demonstration Power Station, which are expected to succeed ITER. However, Wood is working with UKAEA to identify suitable materials and bonding techniques of advanced alloys of titanium and zirconium, which may be used in future breeder blanket modules and divertor designs.
Wood and Leading Metal-Mechanical Solutions have mastered the technical demands of FWP manufacture but work is ongoing to develop and refine the design, manufacturing sequence and processes, with the aim of delivering a reliable, cost-effective and efficient product.