Nonhydriding Metals:
The mechanical integrity of nonhydriding metals within the presence of tritium is excellent since the electron bands hold away the energy of colliding beta particles without disrupting the metal bonding or structure. These metals form the most general class of tritium containment structural materials. They commonly involve 304L, 316L, 321, 21-6-9, and Nitronic stainless steels, as well as aluminum and copper. An Inconel, Ni-Cr alloys, and 400-series stainless steels are generally not selection since of corrosion or hydrogen embrittlement sensitivity. On high pressures of tritium gas, therefore, classical hydrogen embrittlement, as well as helium-3 embrittlement, could occur in accepted materials. For instance, for 304L stainless steel samples exposed to 9 kpsi of tritium at 423 K for 6 months and then aged 1.5 years, fracture toughness decreased through a factor of 6. By this, a factor of two could be attributed to helium-3 alone.
Substantially different fracture modes are observed among aged tritium-loaded and unloaded steel specimens. A Helium-3 is vastly less soluble in metals than is hydrogen (tritium); helium pockets (bubbles) form along with high internal pressures. Hydrogen embrittlement also contributes to this effect.
Permeative escape rates of tritium by nonhydriding metals are generally acceptable at temperatures below 100º C to 300º C and for thicknesses of 0.1 cm or more. For 304 stainless steel 0.3 cm thick along with a 1000-cm2 surface area exposed on one side to tritium gas of 1 atm pressure at 300 K, a permeability is 1.6 x 10-4 Ci/day (t0.9 = 7 hours). The temperature dependence of permeation is frequent astounding.
Cross-contamination among nonhydriding metals and tritium does occur frequent sufficient to be troublesome. The Oxide layers on metals often contain hydrogen and are further covered with a thin adsorbed carbonaceous film when originally grown in room air. Ahead exposure to such a surface, tritium gas might become contaminated over hours or days with hundreds to thousands of parts per million of protium (as HT) and methane (as CT4) as the surface layers are radiolyzed, replaced, and contaminated through the material. Since diffusion of tritium in the bulk material is commonly slow at room temperature and the extent of surface oxide contamination might greatly surpass the bulk contamination of a component. Cross-contamination could be minimized through minimizing material surface areas, selecting an impermeable material along with a thin or nonexistent oxide layer, and managing cleanliness.