Culham engineers are investigating whether parts of a fusion machine could regenerate themselves – a concept that could extend a fusion reactor’s lifetime and cut maintenance costs.
One of the issues for the ‘divertor’ area of the tokamak reactor – the exhaust system through which spent heat and particles are removed from the machine – is the extreme heat from the plasma fuel it comes into contact with. In high power and long pulse tokamaks, the divertor target (on which heat from the fusion reaction is deposited) is typically made of many thousands of individual blocks of tungsten. But in a large power reactor, as a result of interaction with fusion neutrons and energetic plasma particles, the tungsten will become irradiated and degrade. This means a typical tungsten divertor is expected to only have a lifetime of two to three years in a power plant before it would need replacing.
The Liquid Metal Divertor project, funded by EUROfusion, began in 2018, and is a cross-European collaboration with the project being led by DIFFER in the Netherlands and other contributions coming from research units such as ENEA in Italy. UK engineers working on the project believe that using liquid metal as a plasma-facing material means it can take the brunt of the extreme heat and protect the divertor target structure.
The concept would see liquid tin metal held in a porous-like tungsten structure. Although the tin is vaporised by high energy ions arriving from the plasma, it can be replenished from the supply underneath – meaning that the surface is to some extent ‘self-healing’. This feature is thought to limit erosion of tungsten, and so avoid the need for thick tungsten armour which is life-limiting for the divertor.
This could be critical to the viability of the DEMO European fusion power plant.
The liquid metal divertor concept – which Culham’s David Horsley and Shail Desai have both been developing – has now finished its pre-conceptual design stage. In order to manufacture it, tungsten would be fashioned into a compact 3-D structure to hold liquid tin metal.
Mechanical Engineer Shail, who has worked closely on the project, said: “The most obvious application for this is for the mitigation or handling of plasma disruptions and instabilities where the heat-loading on the divertor tiles is expected to be very high – this could be critical to the viability of the EU’s ‘DEMO’ prototype fusion power plant.”
“With a tungsten divertor, you will find parts of the material are eroded away every time there is an instability or disruption at the edge of the plasma – something which significantly reduces the lifetime of the divertor tiles. The Liquid Metal Divertor would extend the lifespan of the divertor significantly.”
He added that with a liquid metal concept, a spike in heat flux on the divertor will see some of the liquid evaporate and form a vapour cloud – protecting the surface from further extreme heat flux. The majority of this vapour is thought to re-deposit onto the divertor surface, effectively recycling the material.
David Horsley, who leads Culham’s work on the Liquid Metal Divertor project, said: “There have been many studies with liquid metals in divertors, but one of the key differences with this project is that we are integrating our design into the baseline geometry defined by the DEMO layout, making it a genuine contender for a power plant.
“Our team has put forward an innovative and strong concept in this Europe-wide project, and one we plan to develop further in the future.”