My name is Jose and I am going to tell you my fusion adventure in Germany. Fusion reactors need solid pellet injectors for fuelling the plasma. I started learning about pellets in the 18’ summer during an internship in Garching (Germany). I quickly note that the pellet extruders employed were in a very early phase of design and requires improvement. Firstly, because of the high reliability necessary to maintain a continuous flow of pellets in larger operations as it would be expected for DEMO and secondly, because the difficulties to get hydrogen isotopes in a good quality solid ice form.

Pellet injection for DEMO

Then, I got in touch with the KIT to propose them designing a pellet extruder able to satisfy these requirements. It was a big coincidence because they want to do the same. There were no designs coming from Europe in this kind of device and no one able to fulfill DEMO reactor specifications.

The case of my study was a design of a pellet injector for DEMO. This means I had to consider deuterium-tritium mixtures and the effects of these isotopes in the liquid and solid phases. The research was really interesting, I found some surprising details that make me more and more involved in the topic. As an example, I found that tritium gets frozen at larger temperatures than deuterium. I did deep research on these isotopes. I found all the thermodynamic characteristics through extrapolations from experiments on deuterium and hydrogen. I studied the behavior of liquid and solid mixtures in this kind of device. Then, I also had to investigate the rheological model that describes the behaviour of these materials. Curiously at solid states before becoming a crystal it behaves like the toothpaste.

Analysing the extruder

I also have done a technology analysis to determine which kind of device is more interesting for the desired pellet extrusion. The result of these research was that the twin-screw extruder in counter-rotating intermeshing mode is the most adequate system for fusion because it produces very well mixed solids and transports the mass in a linear way forcing it at high pressures.

For those not familiar with this kind of extruders, most important is that the operation of the screws can either melt or form solid materials. In our case, the screws are inside a copper barrel that cools down the material flowing in the gaps of the screws. Thanks to this cooling and the pressure produced by the rotation of the screws in the closed barrel, the material is finally forced by a small tube (nozzle) where it gets the shape of a solid cylindrical rod. Afterward, this rod would be cut in small pellets that would be launched to the DEMO plasma. The work on research was quite complete and in the KIT I found a lot of information. I have read books that give me a good perspective to approach the solutions. This allowed me to jump in a further step, the considerations for design an extruder.

Using a script, I was able to obtain the pressure, temperature, geometries, speed and torque necessaries to produce an ice rod in a continuous extrusion as DEMO would require. The work was not easy, but I was fortunate to be in the correct place to obtain all the necessary information.

The people from the vacuum group helped me and was so kind, one of them even make for me a program able to read the geometries produced by my script and represent the extruder screws in a 3D figure.

I would like to acknowledge KIT, EUROfusion, and FuseNet for providing me the necessary funding to live in Karlsruhe and enjoy the city and the work. Karlsruhe is a big city merged in a student atmosphere where I make a lot of friends. I would like to come back one day. It was definitely an unforgettable experience.

- José-Manuel Trueba-Cutillas