Fabrication routes for advanced first wall design alternatives

Abstract

In future nuclear fusion reactors, plasma facing components have to sustain specific neutron damage. While the majority of irradiation data provides a relatively clear picture of the displacement damage, the effect of helium transmutation is not yet explored in detail. Nevertheless, available results from simulation experiments indicate that 9%-chromium steels will reach their operating limit as soon as the growing helium bubbles extent a critical size. At that point, the material would most probably fail due to grain boundary embrittlement. In this contribution, we present a strategy for the mitigation of the before-mentioned problem using the following facts. (1) The neutron dose and related transmutation rate decreases quickly inside the first wall of the breeding blankets, that is, only a plasma-near area is extremely loaded. (2) Nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium, which would suppress the formation of large helium bubbles for a much longer period. (3) Compared to conventional steels, ODS steels also provide improved irradiation tensile ductility and creep strength. Therefore, a design, based on the fabrication of the plasma facing and highly neutron and heat loaded parts of blankets by an ODS steel, while using EUROFER97 for everything else, would extend the operating time and enable a higher heat flux. Consequently, we (i) developed and produced 14%Cr ferritic ODS steel plates and (ii) optimized and demonstrated a scalable industrial production route. (iii) We fabricated a mock-up with five cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. (iv) Finally, we performed high heat flux tests in the Helium Loop Karlsruhe, applying a few hundred short and a few 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e. 550 • C) was finally exceeded by 100 K. (v) Thereafter, microstructure and defect analyses did not reveal critical defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could

Technological Processes for Steel Applications in Nuclear Fusion

“…Since such imperfections might have to be tolerated in an industrial production, we continued the mockup fabrication and used this as a robustness test. More details are given in [37]. The bonding surface of the EUROFER plate was also milled over (by about 0.1 mm) to remove the oxide layer and to ensure a proper diffusion weld process.…”

“…In summary, the mockup fabrication was performed using solely industrial-based or commercially established processes, which could also be applied to the production of the real components-the breeding blankets. More information on the industrial scale production of the used ODS steel can be found in [37]. The fabrication included two weak points that would allow for assessing the robustness of the design in terms of (1) small imperfections in machining and handling a 2 m 2 size surface of a massive component in the several tons range within an industrial environment and (2) possible crack initiation domains due to fabrication, material, operational, or processing shortcomings.…”

Macroscopic elastic stress and strain produced by irradiation

“…On the other hand, the candidate materials typically selected for the structural components of a fusion power plant all have isotropic cubic crystal structure. It is either the bodycentred cubic structure characterising iron-chromium alloys, ferritic steels and tungsten, or the face-centred cubic structure of copper, copper alloys and austenitic steels [2,[14][15][16][17]. Exposure of these materials to irradiation gives rise to swelling or, in other words, to the predominantly isotropic volume expansion resulting from the accumulation of defects in the material [18][19][20][21].…”

Macroscopic elastic stress and strain produced by irradiation

“…On the other hand, the candidate materials typically selected for the structural components of a fusion power plant all have isotropic cubic crystal structure. It is either the body-centred cubic (bcc) structure characterising iron-chromium alloys, ferritic steels and tungsten, or the face-centred cubic (fcc) structure of copper, copper alloys and austenitic steels [2,[14][15][16][17]. Exposure of these materials to irradiation gives rise to swelling or, in other words, to the predominantly isotropic volume expansion resulting from the accumulation of defects in the material [18][19][20][21].…”