Belgian Nuclear Research Centre SCK.CEN explains the benefits of working with OCAS.
“SCK.CEN (Belgian Nuclear Research Centre ) has a long standing tradition of working together with OCAS on material related topics,” explains Dmitry Terentyev, Head of Expertise Group on Structural Materials and Programme Manager of Fusion R&D at SCK.CEN. “While looking to improve the material characteristics of a component that needs to withstand operating temperatures between approx. 2000° and 150-200°C, we launched a collaboration with OCAS.”
“While tungsten (W) is being main candidate to create an armour for plasma facing components in ITER – fusion reactor under construction in France, it has a principal drawback – brittleness. Commercially produced W already has a rather high ductile to brittle transition (about 300°C). Under prolonged operation in fusion environment it will raise up further, causing a risk for structural integrity of the component. Application of composites and high entropy alloys based on refractory metals (tungsten and chromium) are potential options to alleviate the initial brittleness as well as improve resistance to the harsh irradiation damage caused by post products of fusion reaction. The rich metallurgical expertise of OCAS and the long standing nuclear material science knowledge of SCK.CEN were put in synergy to launch a joint project.”
The scientific paper “Development of chromium and chromium-tungsten alloy for the plasma facing components: Application of vacuum arc melting techniques”, has been accepted for publication in Journal of Nuclear Materials; Volume 536, 1 August 2020, 152204. This work was a collaboration between the Belgian Nuclear Research Centre, SCK CEN, (Mol, Belgium), University of Liège, (Liège, Belgium), Max-Planck-Institute für Plasmaphysik, (Garching, Germany) and OCAS NV (Zelzate, Belgium). Authors: D. Terentyev, T.Khvan, J.-H.You, N.Van Steenberge.
Design of plasma-facing components (PFC) for DEMO divertor unravels new challenges to be met by the in-vessel materials. Embrittlement induced by 14 MeV neutrons in the baseline first wall material – tungsten (W) endangers structural integrity of PFCs. Chromium (Cr) and/or Cr–W alloy is currently considered as a candidate material in the design of mid heat flux PFCs as structural body of the monoblock. Cr has the superior mechanical properties in the low temperature range where the commercial tungsten products are brittle. However, the fabrication of Cr requires high level purity control and is therefore challenging for mass production.
In this work, vacuum arc melting (VAM) equipment is employed for the fabrication of chromium (Cr) and Cr-10at.%W alloy targeted for the PFC applications. VAM techniques represents new promising alternative route with a high upscale potential. VAM fabrication improves Cr quality by avoiding the introduction of interstitial impurities, while the produced ingots can be further mechanically treated as well as solution alloyed by W to design a dedicated microstructure thus enhancing important mechanical properties, e.g. yield strength and fracture toughness.
The produced heats of pure Cr and Cr–10 W are investigated by means of chemical and microstructural analysis as well as by mechanical testing. The obtained results are compared with those obtained for pure Cr and W products fabricated by Plansee (Austria). The VAM-produced pure Cr (without any thermo-mechanical optimization) shows the transition to ductility deformation mode just above the room temperature proving the principal advantage of this fabrication route. Solid solution with 10% of W significantly improves the proof stress while sustaining good ductility at elevated temperatures. The ductile to brittle transition in Cr–10%W is observed around 300 °C, which likely can be reduced further by thermo-mechanical treatment. The bending strength of the tested pure Cr grades is considerably lower than that of pure tungsten, when compared at the technologically relevant temperature i.e. 300-500 °C. Whereas the bending strength of Cr–10%W constituted about 80% of the strength of pure tungsten. Hence, the developed VAM Cr and Cr–W alloys require a next step assessment with respect to neutron irradiation testing and improvement by thermo-mechanical treatment.
“The collaboration with our external partner in the field of synthesis of high melting point elements/alloys is a great initiative, opening up a broad range of fascinating exploratory domains!”
“A good knowledge of the application requirements is of key importance for the development of a suitable material. Even though at OCAS we only have a minimal understanding of what happens in a nuclear reactor, our long-standing partnership with SCK.CEN ensures the needed exchange of information with the specialists of SCK.CEN to co-develop good lab materials for advanced, international R&D programmes.”