Materials selection for facing nuclear fusion plasma
19 December 2018
As of today, there is still no net electricity contribution from nuclear fusion in our power grids. Here, materials scientist Benjamin Spilker, on behalf of materials search engine Matmatch, explores the world of nuclear fusion and the material characteristics that make research possible.
Plasma physicists were surprised by the behaviour of the plasma more than once in the decades of fusion plasma research. A highly prominent and important discovery has been made with the high-confinement mode (H-mode), in which the plasma suddenly changes its characteristics to a steep pressure gradient at the edge and an overall better confinement.
Even though we have come quite far, our understanding of the underlying mechanisms of the H-mode transition is still incomplete. Thorough modelling efforts are ongoing to reliably predict the operational scenarios and conditions in future fusion devices.
Despite all of the measures that can be taken to control the fusion plasma, the surplus energy and the product of the fusion reactions in the form of helium particles and other impurities need to be removed from the plasma in order to allow for a continuous operation and the generation of electricity.
So-called plasma-facing materials are deployed to protect the vacuum vessel from the hot plasma particles and to efficiently remove the incident power fluxes. Nuclear fusion is sought after as a clean and sustainable energy source. Because of this, all in-vessel components are carefully selected in order to avoid the generation of any long-term radioactive waste that would require geological disposal.
The anticipated deuterium-tritium (DT) fusion reaction generates a continuous flux of highly energetic neutrons. These neutrons carry the potential to knock atoms from their lattice positions, causing material defects, but also to transmute atoms (transform the atom into a different chemical element) if a neutron is captured.
This neutron irradiation environment requires that the materials, which are in place to shield the rest of the machine from these neutrons, do not degrade to the point where they could overheat and fail under operational conditions.
Another important aspect for the environmental impact of fusion is neutron induced activation. When stable atomic nuclei capture a neutron, they can enter an excited state and become radioactive. These newly formed radioactive isotopes decay with a certain half-life time, which should remain within specified limits.
All in-vessel components should exhibit radiation levels below the ‘recycling-level’ (a contact dose rate of ≤10 mSv/h allows for recycling with remote handling) after a maximum of 100 years from their dismounting. The limit of 100 years may seem long but it appears acceptable considering that materials could be fully recycled for the centuries to come. Moreover, it is definitely short compared to the tens of thousands of years that nuclear waste from fission needs to be safely stored and sealed from the environment.
However, low-activation is not the only requirement. Enormous steady state heat fluxes arise in the most loaded areas of the vessel with up to 20 MW/m². For comparison, the radiation flux on the surface of the Sun is about 63 MW/m².
Tungsten appears to be the perfect candidate material to face the fusion plasma directly, but it has two major drawbacks. First, its high atomic number means that even small quantities of tungsten entering the fusion plasma have the ability to cool the plasma down dramatically via radiation. This can lead to a termination of the plasma.
Elements with a low atomic number like beryllium and carbon are significantly less critical when entering the plasma. Second, pure tungsten is naturally highly brittle. It is difficult to machine and the thermally induced mechanical stresses can easily lead to cracking of the material.
Carbon appears almost equal to tungsten in terms of its thermal capabilities, albeit without the drawback of the high atomic number. This is the reason why graphite and carbon fibre composites have been the dominant plasma-facing materials for the first decades of fusion research.
Unfortunately, a major drawback of carbon has been discovered in the course of said research. Even when subjected to only small doses of neutron irradiation of 0.2 dpa (displacements per atom), which could be accumulated in a couple of days in a running fusion reactor, the thermal conductivity of carbon decreases to less than 20% of its original value. As a consequence, the material quickly overheats because the incident heat flux cannot be transferred to the cooling channels efficiently enough.
From these considerations and many others, it has been concluded that all-metal plasma-facing materials, such as beryllium, are the best way to go forwards.
For design engineers working on these projects, Matmatch’s materials comparison platform provides the ideal means of researching and sourcing materials suitable for handling nuclear fusion. By choosing the right materials, engineers and scientists can make a brighter future for fusion power.
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