Image: R&D engineer and pellet injection scientist Trey Gebhart from the Blanket and Fuel Cycle Group in the ORNL’s Fusion Energy Division with a pellet injection system test stand for fusion research and development. The group is a world leader in the research and development of pellet injections.
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Image credit: ORNL
A team of experts from Oak Ridge National Laboratory is tackling one of the greatest challenges facing the international ITER fusion device: converting cold gas into solid pellets the size of wine corks to keep the ITER’s plasma in check.
The Blanket and Fuel Cycle group in the ORNL’s Fusion Energy Division is researching the design and construction of the ITER Disruption Mitigation System (SPI = Shattered Pellet Injection).
“We have the infrastructure, we have the knowledge,” said Trey Gebhart, R&D engineer and pellet injection scientist who is leading the effort. “We know what it takes to deliver the most reliable design.”
Learning to tame ITER disruption “would certainly be a big deal,” said physicist Dave Rasmussen, who oversees the pellet injection and disruption reduction teams for the US ITER. “ITER’s plasma is the size of a fusion reactor that could inform future fusion power plants. It would be a great achievement to solve this problem along the way. “
ORNL pioneered SPI technology a decade ago, building on the laboratory’s years of experience in pellet injection – systems that shoot deuterium, tritium, or hydrogen fuel pellets into fusion device plasma.
However, SPI systems do not fire plasma. Rather, they switch off the plasma when instabilities occur that threaten the surfaces of the machine walls facing the plasma. This process is known as interference mitigation.
Over the years, ORNL has supplied SPI systems to fusion experiments around the world. Now these experts are helping to adapt this technology to the much larger scale of ITER, by far the largest tokamak in the world, which is currently being assembled in France.
ITER SPI pellets are roughly the size of wine corks and will be over 15 times larger than those used in tokamaks today. The ORNL team is currently testing 28mm diameter pellets and will begin testing 23mm pellets shortly.
“Scaling from small pellets to ITER-sized pellets poses many challenges,” Gebhart explained.
Gebhart has developed a special test stand to master these challenges, from the cold pellet formation to the fiery finale.
Pellets begin as a gas that is cooled in a tube with liquid helium at –269 degrees Celsius until it solidifies. Although it is informally referred to as “freezing,” the gas actually desublimates and goes directly from the gas to the solid.
Just as larger ice cubes freeze more slowly, larger pellets take more time to desublimate. Gebhart’s first attempt took about an hour – twice as long as it takes for ITER. So the team gave these larger pellets a head start by developing a special pre-cooling heat exchanger using liquid nitrogen at -196 degrees Celsius, which cut pellet formation time by half.
However, the desublimation of deuterium, an isotope of hydrogen made up of a proton, an electron, and a neutron, is much easier than hydrogen – the preferred material for ITER.
Renowned ORNL employee Larry Baylor, who heads the Blanket and Fuel Cycle group, explained the problem. When pellet gas cools down in a pipe, molecules initially stick to the inner surface and desublimate. The pellet forms layer by layer from the outside to the inside and eventually fills the entire space in the pipe. But hydrogen, a poor conductor of heat, does not desublimate as well as deuterium.
To solve this problem, Baylor measured and modeled this previously poorly understood “condensation coefficient” for hydrogen.
“When the cylindrical ring of material gets thicker, these molecules no longer adhere as well when they get to the surface,” explained Baylor. “They ricochet because they don’t lose all of their thermal energy because of this heat transfer problem.”
The ORNL team also had to rethink how the pellet is shot into the tokamak. On other tokamaks, a valve based on a magnetic circuit will open to release high pressure gas on the pellets – but magnetic circuits will not work in ITER’s high magnetic fields. That is why Gebhart has improved a special flyer plate valve that works in these areas.
Shooting bullets in a precise path is another challenge. Since the tubes that guide the pellets into the tokamak are unusually narrow with ITER, the flying pellets cannot deviate from an arrow-straight path by more than 1 degree. The larger ITER pellets require more force to send into the tokamak. However, if you apply this force too quickly, the pellet will break.
“You have to concentrate on this narrow goldilocks spot that your pellet can get out intact,” explained Gebhart.
Another technical challenge awaits at the other end of the flight path. Due to the design, the pellets hit a curved section of the pipe wall at 250 meters per second and break into pieces. These fragments spread throughout the plasma and cool it down as the plasma uses energy to strip electrons from the cold atoms.
The ORNL team is now investigating how the pellet speed, pellet material and pipe angle affect the all-important splinter distribution. They discovered clues from some interesting sources, including a study of shrapnel from WWII.
“We use a statistical fragmentation model that was actually derived in the 1940s to understand how grenades fragment when they explode,” explains Gebhart. “Pellet material is very brittle, similar to some metals.”
All of these elements help improve the system’s performance.
“In the past everything was just: ‘Shoot the pellet, you get what you get and it does its job,'” said Gebhart. “Now we’re slowly learning that we can tweak it to get better results.”
UT-Battelle leads ORNL for the DOE’s Office of Science, the largest single funder of basic research in the physical sciences in the United States. The Science Office is working to address some of the most pressing challenges of our time. More information is available at energy.gov/science.
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