Breaking the ice cleverly — a contribution to fusion research

Current forecasts predict a worldwide increase in primary energy and electricity demand. To meet this demand and at the same time meet the enormous challenges of climate change, CO₂-neutral renewable energy sources such as wind and solar power play an important role. But nuclear fusion power plants could also make their contribution to electricity generation to cover the base load in the future.
 

However, there is still a long way to go before the first power plant of this kind will be built. And on this way — in the truest sense of the word in Latin — is the major international research project ITER (International Thermonuclear Experimental Reactor). The world’s largest nuclear fusion reactor to date, which is currently under construction in Cadarache in southern France, is intended to demonstrate for the first time that a net energy gain is technically possible when hydrogen is fused into helium — that is, with a process that takes place similarly in the sun. In contrast to the sun, the two hydrogen isotopes deuterium and tritium are used for this purpose because of their higher efficiency.

This energy gain, fed by the strong nuclear force, is only effective over very short distances in the atomic nuclei. For fusion to take place at all, a large amount of energy must first be expended to overcome the repulsive Coulomb forces of the positively charged atomic nuclei: The hydrogen is heated to extremely high temperatures, between 100 and 200 million degrees Celsius, and must at the same time be held together. Since, at these temperatures, the hydrogen is no longer a gas but a plasma, it can be influenced by means of magnetic fields and thus enclosed in a ring-shaped magnetic field cage (tokamak principle). If this plasma is sufficiently dense and enclosed for long enough at high temperatures (Lawson criterion), the fusion processes are setting in, releasing enormous amounts of energy. On the one hand, this energy is used to further heat Breaking the ice cleverly — a contribution to fusion research the plasma, and on the other hand, it is released to the outside, namely to the blanket, the inner structure of the plasma vessel: A coolant is heated up, and the steam generated via a heat exchanger powers a turbine, thus driving a generator to produce electricity.

One challenge is to keep the plasma in a controlled state. So-called plasma disruptions — suddenly occurring disturbances leading to a loss of the plasma-confining plasma current and accompanied by high energy release — can cause high thermal and mechanical loads on reactor components and lead to damage. That would result in additional maintenance time for the replacement of these components. Therefore, a system called disruption mitigation system (DMS) is being installed for ITER to reduce the effects of such disruptions. Its operating principle is to inject fragments of, for example, frozen hydrogen and neon into the plasma within a short period of time. For this sake, cylindrical pellets of the appropriate material are frozen at temperatures of minus 268 degrees Celsius and then shot onto a shattering unit at speeds of up to 1,800 kilometers per hour, where they fragment under the enormous impact load. The effectiveness of the DMS depends on the optimal size and velocity distribution of the fragments. Therefore, it is important to know how the impact conditions affect the fragmentation properties. To this end, the ITER DMS Task Force has specifically established a program to characterize and study fragmentation experimentally.

As part of this program, Fraunhofer EMI is developing numerical models and computer codes to simulate and analyze the complex fragmentation process as part of an ongoing research project. The above-mentioned experiments serve, among other things, to calibrate and validate the developed models and procedures. The validated models will then be used to significantly expand the experimentally determined database and to make predictions for the fragment size distribution under different boundary conditions. The goals here are to optimize the design of the shattering unit and to derive guidelines for optimized impact conditions in order to obtain the desired fragmentation properties. Or, to put it somewhat casually: It is about answering the question of how to break the ice most cleverly.