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Fusion Energy: Materials In Direct Contact With An Artificial Sun

Fusion Energy: Materials In Direct Contact With An Artificial Sun

As of today, there is still no net electricity contribution from nuclear fusion in our power grids. What are the challenges that separate our current status from a time, where a significant fraction of the grid base load is provided by fusion power plants? What are the provisions that the nuclear fusion community has chosen to account for the bad reputation of anything connected to the word ‘nuclear’? In the following, these questions will be addressed.

In a previous article, it was mentioned that a plasma at 150 million K is a frantic beast to tame. A large variety of highly sophisticated diagnostic tools, for example, active probing by laser and particle beams [1], and attenuators, for example in the form of special magnetic coils, are used to control the shape and position of the fusion plasma on the sub-millisecond time scale.

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 [2]. 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 [3].

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.

Fusion Energy: Materials In Direct Contact With An Artificial Sun
Figure 1: Schematic view of the many different processes relevant to the interaction of a fusion plasma with the surface of a wall component [4]. A comprehensive understanding of all the processes and the synergies between them is vital to develop durable plasma facing materials. H: hydrogen, D: deuterium, T: tritium, PFC: plasma facing component, ɣ: gamma ray.

Elemental considerations in the nuclear environment

Nuclear fusion is sought after as a clean and sustainable energy source. Thus, 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 [4]. 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.

Elements that can be used for the fabrication of structural materials fulfilling the low-activation requirement are Be, C, Cr, Fe, Si, Ta, Ti, V, and W. With a finely selected mixture of these elements, reduced activation ferritic martensitic (RAFM) steels such as EUROFER97 and F82H were developed, which can be used at temperatures of up to 823 K [5]. Small quantities of more strongly activating materials for e.g. alloying or doping can be acceptable within calculated limits.

Fusion Energy: Materials In Direct Contact With An Artificial Sun
Figure 2: Estimated dose rate as a function of the time after the exposure to a typical fusion neutron spectrum with 20 MWy/m² for selected pure metals [6]. Metals in green are activated within acceptable limits, can be recycled relatively quickly, and no geological disposal is needed after 100 years. Metals in red are activated too strongly by fusion neutrons and can only be used/tolerated in trace quantities.

What about the heat?

In the previous section, you have seen that only a very few elements can be applied in the fusion reactor vacuum vessel in order to minimise any negative environmental impact. 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 [7]. For comparison, the radiation flux on the surface of the Sun is about 63 MW/m² [8], which makes the title quite accurate, doesn’t it?

To preserve the structural integrity of the material subjected to these loads and to remove these power fluxes efficiently with active cooling, two requirements have to be met, namely a high melting point and a high thermal conductivity.

Fortunately, tungsten fulfills the low-activation requirement, has the highest melting point of all metals in pure form, with 3695 K, and also has a relatively high thermal conductivity of approximately 167 W/(m·K), which is about eight times higher than the value of conventional steel. As you can see in figure 3, the other low-activation elements also score fairly high in terms of the melting point, especially carbon.

Fusion Energy: Materials In Direct Contact With An Artificial Sun
Figure 3: Melting point for the different elements in the periodic table [9]. Note that carbon does not melt but sublimes directly from the solid to the gas state at 3915 K [10]. Tungsten has the highest melting point of all metals in pure form with 3695 K. Elements without a colour code either do not solidify at standard pressure (He), are short-lived (e.g. a half-life of 22 minutes for the most stable Fr isotope [11]), or are only artificially producible.

Plasma-facing materials past, present and future

From the considerations above, tungsten appears to be the perfect candidate material to face the fusion plasma directly. But, as always in life, nobody is perfect. Tungsten 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 [12].

As a consequence, the material quickly overheats because the incident heat flux cannot be transferred to the cooling channels efficiently enough. In contrast, beryllium and tungsten lose only a small fraction of their thermal conductivity under comparable neutron irradiation conditions.

From these considerations and many others, it has been concluded that all-metal plasma-facing materials are the best way to go forwards. ITER, the world’s largest fusion experiment, currently under construction, will employ beryllium for the largest fraction of the vacuum vessel wall and tungsten for the divertor region, where the strongest heat loads are expected.

Although it can be tolerated in the plasma up to quite large amounts, beryllium is not suitable as plasma-facing material in a fusion power plant because of its high susceptibility to erosion. Candidate materials to replace beryllium covering the major fraction of the plasma-facing surface area are, for example, the above mentioned RAFM steels.

Once in operation, a tungsten-rich surface naturally forms on these steels through a process called “preferential sputtering”. In principle, the least erosion resistant atoms are sputtered away first, gradually enriching the surface with the most erosion-resistant ingredient of the steel, namely tungsten. If the characteristics of such a tungsten-rich surface are still insufficient to meet the operational requirements, the steel could also be covered with a tungsten coating.

In the far future, reactions other than the DT reaction could be targeted, which proceed without neutron irradiation but require significantly higher temperatures. For these aneutronic fusion reactors, carbon-based composites could make a great comeback as plasma-facing material.

The insights gained from the DT fusion research can establish the foundation for various approaches to energy/electricity generation via fusion. Moreover, fusion spinoff technologies like the Millimeter-Wave Thermal Analyzer [13] and compact cyclotrons for cancer therapy [14] are already making a real-world impact.

"I deem materials science to be the indispensable link between phenomenal ideas and their real world application and would like to share this perception in all facets with interested minds".


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