Ponente
Descripción
Tungsten is the prime candidate material to manufacture plasma-facing components for tokamak-type fusion reactors, due to its high melting point (Tm~3420 ◦C), mechanical strength and low sputtering yield. However, tungsten exposure to neutron fluxes from the deuterium (D)-tritium (T) plasma reaction generates additional lattice defects in the structure, such as dislocation loops and networks. As a consequence, there is a gradual shift of the ductile-to-brittle transition temperature in tungsten from 200-300 ◦C after manufacture to ~800 ◦C, and also a simultaneous degradation of the thermal conductivity of the material. The additional lattice defects can also affect the helium (He) intake and diffusivity into the material to form nano-bubbles over time, and also the retention of radio-active T atoms from plasma exposure in the tokamak core. Helium arrives into plasma-facing W via two routes, firstly via exposure to a continuous flux of He ions/gas generated in the D-T plasma, and secondly as a product of neutron-induced transmutation reactions within the bulk material, e.g. via the W(n,α)Hf reaction.
Refractory metal alloying in solid solution remains a potential route to delay the expected environmental degradation of tungsten-base fusion components in service, while maintaining many of the more desirable properties of the material, such as its high melting point and strength. Despite the high initial radioactivity of elements such as molybdenum (Mo) and tantalum (Ta), they have been explored in the last decade as potential low alloying strategies in tungsten. Ta activity is significantly reduced by the 100-year target for reduced activation materials, and its presence can delay the generation over time of neutron-induced transmutant elements (e.g. Re, Os). Mo offers comparable thermo-mechanical properties and sputtering yield, whereas long-decay activity could be avoided by isotope enrichment.
In this contribution, I will present an overview of our experimental campaign and results in recent years, in unalloyed W and some of its potential binary alloys using (i) energetic ion beams (i.e. protons, heavy ions) as a surrogate to neutron damage, benchmarked with neutron data from literature where available; (ii) low-energy (eV) He plasmas and higher energy (keV) He ion beams (iii) dual beam irradiations. We will emphasise the mechanisms of structural and surface damage evolution and the potential impact on material’s performance. We will also discuss the limitations/potential of those irradiation approaches to mechanistically understand the material’s behaviour in service environments and to design targeted neutron irradiation campaigns in the future.
References:
[1] I. Ipatova, R.W. Harrison, P.T. Wady, S.M. Shubeita, D. Terentyev, S.E. Donnelly, E. Jimenez-Melero, Structural defect accumulation in tungsten and tungsten-5wt.% tantalum under incremental proton damage, J. Nucl. Mater. 501 (2018) 329-335.
[2] I. Ipatova, R.W. Harrison, S.E. Donnelly, M.J.D. Rushton, S.C. Middleburgh, E. Jimenez-Melero, Void evolution in tungsten and tungsten-5wt.% tantalum under in-situ proton irradiation at 800 and 1000 ◦C, J. Nucl. Mater. 526 (2019) 151730.
[3] I. Ipatova, G. Greaves, S. Pacheco-Gutierrez, S.C. Middleburgh, M.J.D. Rushton, E. Jimenez-Melero, In-situ TEM investigation of nano-scale helium bubble evolution in tantalum-doped tungsten at 800◦C, J. Nucl. Mater. 550 (2021) 152910.
[4] E. Yildirim, P.M. Mummery, T.W. Morgan c, E. Jimenez-Melero, Delayed surface degradation in W-Ta alloys at 400 °C under high-fluence 40 eV He plasma exposure, Fusion Engin. Design 197 (2023) 114061.
[5] E. Yildirim, P.M. Mummery, G. Greaves, C.P. Race, E. Jimenez-Melero, In-situ TEM characterization and atomistic simulation of cavity generation and interaction in tungsten at 800 ◦C under dual W2+/He+ irradiation, Nucl. Mater. Ener. 39 (2024) 101672.
