Testing an innovative heat pipe concept for future tokamaks

Could replacing failed plasma-facing components in the wall of a fusion reactor ever be as easy as changing a light bulb? That was the ambitious idea behind a collaborative project between CCFE / UKAEA (UK), Sandia National Laboratories (USA) and DIFFER (Netherlands), when the group ran a proof of principle demonstration earlier this year.

The new approach is based on high temperature heat pipes which provide super-efficient heat removal with no moving parts or need for direct cooling. It is hoped that worn out or damaged wall components could be simply unplugged and replaced using a small robot – a big gain in time compared to the present method of extracting large heavy modules from the inside of the reactor which also involves cutting and re-welding of cooling circuits.

Heat is lost by thermal radiation from the heat pipe
to the water cooled walls of Magnum-PSI.

The principle of the heat pipes is akin to a saucepan with a small amount of water in the bottom and a tight-fitting lid, whereby the steam will transfer heat from the base of the pan to the lid, where it condenses and runs back to the bottom. High temperature heat pipes transfer heat in a similar passive way but contain liquid lithium rather than water and use a wick rather than gravity to drive the return flow of the liquid. Although they have been studied for over 50 years, high temperature lithium filled heat pipes are not routinely manufactured. Richard Nygren from Sandia National Laboratories in Albuquerque said: "For our first experiment we were lucky to be able to purchase from Aavid Thermacore a simple tantalum heat pipe filled with lithium – this had been left over from a 1980s programme so wasn't optimised for our application."

It's so hot it's cool

Lithium filled heat pipes operate in the temperature range 900°C to 1700°C meaning they will be able to function on the wall of a reactor at an optimum temperature for neutron damage resistance. They are also so hot they naturally lose heat by emitting visible and infra-red light, just like an incandescent light bulb. UKAEA's Guy Matthews, who co-ordinated the project, said: "You can think of our heat pipe as a thermal transformer which converts an intense localised heat load from the plasma into a much gentler and more uniform source of thermal radiation".

Left to right: Scott Silburn (UKAEA), John Rosenfeld (Aavid/Thermacore),
Guy Matthews (UKAEA), Richard Nygren (Sandia National Laboratory),
Tom Morgan (DIFFER)

Testing facility: Magnum-PSI

In order to carry out the demonstration, the collaborative group were given access by DIFFER to the superconducting steady state linear plasma device Magnum-PSI in Eindhoven. "This was a pretty unusual experiment which was a perfect match to our machine's capabilities" said Tom Morgan who heads DIFFER's plasma-materials interactions research. "Our team were keen to contribute, and nobody quite knew what would happen but what we saw was spectacular."

The heat pipe ran for about two hours with no direct cooling at a peak temperature of around 1000°C but ended when the lithium leaked out as the plasma power was stepped up. The hope is that more optimised materials would achieve a longer demonstration. Surface temperature was measured with two webcams specially adapted for near infra-red imaging by Scott Silburn (UKAEA).  

"Our data shows we were able to operate the heat pipe in a fusion relevant range of power density, and although we had hoped the heat pipe would last a lot longer, the lithium leak did demonstrate that the impact on the plasma device was tiny compared to a water leak from a conventional component, because the heat pipe contains only a few grams of lithium" said Scott.

The various collaborators presented their results at the recent Symposium on Fusion Technology (SOFT) conference in Sicily and are now working towards a design with optimised materials.