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DOE-HDBK-1129-99
detection species, since it travels rapidly through cracks and can be easily detected at very low
levels.
5.1.1.b Joining
Components of tritium systems are commonly joined by welding. Welds are normally designed so
they can be non-destructively inspected by a suitable method such as radiography or ultrasonic
testing. Also, the weld design should minimize so-called "virtual leaks" on the interior of tritium
containing volumes. Examples of weld practices to minimize virtual leaks include 1) using full
penetration welds where possible, and 2) welding feed-throughs to the interior wall surface (not on
the outside, which would leave a gap on the inside around the feed-through that is difficult to
outgas). Standard weld rod filler materials are chosen, depending on the base alloy. Every effort
should be made to reduce or eliminate welding residual stresses.
There is less experience using other joining methods such as brazing or high temperature
soldering in tritium systems. Dissimilar materials may have to be joined by a transition junction,
using an intermediate material to enable proper welding and accommodate differences in thermal
expansion and other properties.
Tritium gas permeates austenitic stainless steels, and, over time, 3He is created by beta decay of
tritium in solution in the material. Welding stainless steel containing solute helium is difficult
because intergranular cracking can occur. During welding, the solute helium agglomerates at
grain boundaries and forms both intergranular cracks in the heat-affected zone and pores in the
fusion zone. Low-heat-input weld techniques have been shown to mitigate this problem to some
degree; however, weld repair of helium-containing stainless steel is normally difficult to perform
without some cracking.
All-metal mechanical joints are also a sound way to join components in tritium systems. Typically,
copper, silver-plated nickel, or silver-plated stainless steel have been used as gaskets.
Commercial high- and ultrahigh- vacuum fittings are normally compatible with tritium.
5.1.1.c Surface Coatings and Treatments
Aluminum and aluminide coatings have been successfully employed on stainless steel to reduce
permeation into and through the steel. These coatings can be applied on large items using a
proprietary fluidized bed furnace, having a controlled atmosphere (in the so-called "calorization"
process). Gold has also been used as a permeation barrier in some applications and is often
applied over a thin nickel buffer layer ("strike") that has been applied to the bulk metal (e.g.,
stainless steel) after proper surface preparation.
Several companies treat stainless steel surfaces using various proprietary electrochemical
processes to "passivate" the surface. These processes probably enrich the chrome content of the
surface oxide, polish the surface (thereby reducing the effective surface area), and remove carbon
and hydrogen from near the surface. All of these microstructural changes may be desirable for
tritium systems in which the process gas must remain at high purity. Capillary lines that route gas
to mass spectrometers are commonly passivated, to reduce changes of gas composition by
isotope exchange, while the gas flows from the sample location to the mass spectrometer.
Passivated surfaces reduce the rate of isotope exchange in hydrogen isotope mixtures, probably
by reducing the catalytic effect of the surface decomposing hydrogen isotope molecules to atoms,
which enables isotope exchange. Vacuum systems having surfaces treated in this way evacuate
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