CCFE and KIT develop a new water-cooled tungsten monoblock concept for the DEMO divertor target. The work is funded within the EUROfusion DEMO divertor project.

Literally one of the hottest issues in developing the demonstration fusion power plant DEMO is the reactor heat exhaust. Known as the divertor, it is the only area of the reactor wall which comes into contact with the hot plasma edge. The concept developed by CCFE and KIT foresees a DEMO divertor target made up of thousands of water-cooled solid tungsten blocks designed to withstand the high heat and particle fluxes from the plasma.

Temperature drops by 1000 °C

Within a distance of only about 7 mm between the tungsten surface and the cooling water flowing through a copper alloy pipe, the temperature drops from around 1300 to 200 degrees Centigrade. Such a steep temperature gradient poses a challenge for thermal insulation between the tungsten block and the copper alloy pipe. Furthermore, tungsten and copper feature different thermal distortions. As a result, mechanical stress builds up at the tungsten-copper interface. The solution may lie in a suitable intermediate layer placed between the two components.

A spongy interlayer

A CCFE team has been addressing this issue within CCFE’s Technology Programme since 2011. Working with a collaborating team at KIT producing tungsten components, the group came up with the idea that the intermediate layer must be spongy. That way it is able to act as a thermal barrier and also accommodate the differences in deformation of the tungsten and copper components. With the help of an automated design process based on computer models, they proposed an intermediate layer made from copper threads forming a felt-like structure. Together with the group at KIT, mock-ups were produced and tested in CCFE’s laboratory.

The teams will develop their concept further within the 5-year EUROfusion divertor project. More design studies will be carried out and divertor target mock-ups will be produced and tested in the high heat flux test facilities operated by EUROfusion laboratories.