Hydrogen and deuterium plasma interactions with brazed first wall elements

Four different high temperature brazes (Zr, 90wt%Ni.10%Ti, 90Cu.10Ti and 70Ag.27Cu.3Ti, nominal composition prior to brazing) were used to join isotropic fine grain graphite and TZM, a high temperature alloy of molybdenum. The general response of the brazes to a high flux deuterium plasma bombardement was examined using samples whose exposed surface intersected the braze line. Only in the case of Zr, which is known for its hydride forming properties, near-surface layers chipped off in the braze region directly exposed to the plasma. However, in graphite-shielded Zr-braze regions no disintegration of the interface was observed. The other brazes showed no visible attack at all. The interaction of a hydrogen plasma and a braze surface was studied in more detail by bombarding specimens in the PISCES-B facility. In this case the graphite was removed completely to enable an exaggerated plasma attack onto the bare braze. Even under these testing conditions the brazes CuTi and NiTi showed no particular sensitivity towards hydrogen. To prove the thermal stability of the brazed joints each quality was heat treated for 1 hr and 10 hrs, respectively, 50o below the softening temperature of the braze in 1 bar of Ar or 96vol%Ar/4%H2. After the heat treatment no changes or damages were observed in the brazes AgCuTi and CuTi, whereas interstices are found in the widened NiTi-interlayer after 10 hrs in both, Ar and Ar/H2. Zr suffers different microstructural changes in particular after treatment in Ar/H2 atmosphere. (Authors) (also appeared in Fusion Technology 1990, p. 411-415)