Advanced Multi-Step Brazing (AMSB) for fabrication of new type of W/stainless steel first-wall component with ODS-Cu intermediate layer

The novel method “Advanced Multi-Step Brazing (AMSB)” has been developed to fabricate a new type of “divertor” and “first-wall” heat removal component in a fusion reactor. This study is focused on the latter component, in which a tungsten (W) sheet is jointed through AMSB to a stainless steel (SUS) substrate via an oxide dispersion strengthened copper (ODS-Cu) intermediate layer. The principle of AMSB is a repetitive application of the advanced brazing technique (ABT). The initial purpose of the ABT was to braze W to ODS-Cu (GlidCop®) with the Ni-11 %P filler material. Later, we confirmed that the ABT is able to produce a very tough GlidCop® and SUS (GlidCop®/SUS) joint. One of the major advantages of GlidCop®/SUS joints is physically strong tolerance against the repetitive brazing heat-cycle. Thus, a repetitive application of the ABT does not cause any negative effects against post-brazed GlidCop®/SUS joints, and hence AMSB can be applied for fabricating a single heat-removal component with multiple joint interfaces. A small-scale sample of the new type of first-wall component was fabricated through two-step brazing. At first, the GlidCop® plate was jointed to the SUS by the ABT, and then a thin W-sheet with the thickness of 0.254 mm was jointed to GlidCop®/SUS by the ABT. If a large area of the first-wall surface in the fusion reactor can be covered with such a thin W-sheet, the amount of hydrogen isotopes trapped on the first-wall surface could be significantly reduced, compared to other fabrication methods such as a vacuum plasma spray W (VPS-W). The VPS-W is theoretically less dense than a W-sheet and often contains pore structures, which could act as effective trapping sites for hydrogen isotopes. A small-scale sample of the new type of first-wall component with a W-sheet (W/GlidCop®/SUS) was successfully fabricated to overcome the above disadvantages. In addition, the W surface of the component showed low retention characteristics of hydrogen isotopes compared with other W surfaces, e.g., atmospheric plasma-sprayed W (APS-W).