There are a lot of “smart” devices that support you in your daily life. But “smart” surely also serves fusion. Smart alloys are a possible way of preventing the release of radioactive material in the event of an accident when operating future fusion reactors.

W – THE MATERIAL’S SOLUTION?

Tungsten (W) is currently the prime candidate for the plasma-facing wall of a fusion experiment. It features a high melting point and low tritium retention. Another factor that meets the requirements of a future fusion reactor wall, are its low sputter yields. Sputter yield is the term for the number of sputtered or dispersed target atoms per incoming projectile. A unity sputter yield means that each incoming atom sputters one target atom. Despite these advantages, there are also some drawbacks in using W as plasma-facing material: W is inherently brittle and its ductile-to-brittle-transition temperature even increases along with high temperature as well as neutron irradiation. In order to improve the material’s ductility and enhance its toughness, different approaches must be investigated.

Janina Schmitz at the linear plasma device at Forschungszentrum Juelich. The experiment tests material for future fusion reactors. Picture: Tobias Wegener (CC-BY-NC-SA 3.0)

Janina Schmitz at the linear plasma device at Forschungszentrum Juelich. The experiment tests material for future fusion reactors. Picture: Tobias Wegener (CC-BY-NC-SA 3.0)

THINGS SHOULD NOT GO LOCA

In order to build a safe fusion experiment, we must consider extraordinary events in addition to the operational scenario. These might put additional constraints on the development of plasma-facing components (PFCs). In case of a so-called LOCA (loss-of-coolant-accident), the cooling of the first wall fails and, as a consequence, the wall temperature may rise up to 1200 ºC for several months. With additional air ingress, W oxidises easily and radioactive volatile tungsten oxide (WO3) is formed. This requires elaborate measures to prevent this gas from being released into the environment. In order to finally achieve an intrinsically safe fusion operation, it would be best to entirely avoid the mobilisation of the wall material.

SMART ALLOYS – A POSSIBLE SOLUTION

Smart alloys might provide an answer to this demanding question. Alloys are materials consisting of two or more compounds, for example, metallic compounds. Often they are designed to improve the materials‘ properties. Steel for example is an alloy made up mostly of iron, but containing a few percent of chromium (Cr) and other elements that altogether improve the corrosion resistance of the material.

W-based self-passivating ‘smart alloys’ are intended to suppress the WO3 formation and thus passivate the oxidation properties compared to pure W. The alloy’s smartness consists of its ability to adapt to two kinds of operation scenarios. When the smart alloy comes into contact with oxygen, its elements form a dense protective oxide layer at the surface and thus prevent tungsten mobilisation.

In its normal operational mode as plasma-facing material, preferential sputtering of light alloy elements causes the depletion of these alloying elements at the surface. It forces the alloy to behave like pure W during reactor operation.

At the same time, the self-passivation proper-ties of the smart alloy should remain unaffected by the plasma impact as the alloying elements in the bulk material remain.

For the currently most promising alloys, Cr is used as passivating element, while yttrium (Y) serves as an active element to improve self-passivation. Oxidation tests carried out at Forschungszentrum Jülich (FZJ) demonstrated a significant improvement in oxidation suppression for the WCrY system. And initial plasma tests at the linear plasma device PSI-2 showed no impact on the oxidation behaviour.

ADVANCED W MATERIALS FOR DEMO

Smart alloys are possibly a solution in the event of LOCA. However, for DEMO, the next step after ITER and the first fusion powerplant, the need for long-lasting PFCs with additional stable mechanical properties during plasma operation is just as important. The development of advanced materials designed to meet the demanding requirements of a fusion operation is progressing. Material solutions tailored to ensure low erosion and long lifetime PFCs are moving forwards. Yet their behaviour in regimes beyond regular reactor operation must be examined more closely. At FZJ, we are currently also working on improving the various drawbacks of pure W. We are also looking into the use of tungsten fibre reinforced composites (Wf/W) as a solution to the inherent brittleness. At the same time, we are developing self-passivating smart alloys in order to improve the material during LOCA events.

Ultimately, our efforts must be combined and a composite designed which has suitable properties for both the operational and accidental reactor scenario, and which is geared to any eventuality.

NEUTRON ACTIVATION

info iconNeutron activation is where energetic neutrons induce radioactivity in a material. Free neutrons are captured by atomic nuclei, pushing them into an excited state. These atoms then release decay radiation in order to return to a stable state. The neutrons produced in fusion reactors are not confined to the plasma and can hit the wall material, which will predominantly be made of tungsten in future devices. Continuous activation will degrade the tungsten, which will then need to be replaced and disposed of as low-level radioactive waste.

Clear

authorbox_Janina-SchmitzThe idea of creating a Sun- like power generation source on Earth can only be realised by the joint efforts of fusion researchers from all over the world. With my PhD work within the Fusion Doctoral College framework, I want to contribute to this aspiring project. At FZJ, and elsewhere in Europe, we are developing tungsten-based smart alloys for future fusion power plants like DEMO. My research is focussed on the plasma compatibility of smart alloys.

Janina Schmitz (25) from Germany is currently based at: Forschungszentrum Juelich (FZJ)/Ghent University. (Picture: private)