Properties of Metals
Many compatibility concerns can be raised for tritium/material interactions.
The mechanical integrity of the material
The escape rate of tritium into and through the material
Contamination of tritium by the material and vice versa
Gettering capabilities of a substance for tritium
Mechanical integrity is a function of how well the material dissipates the energy of colliding beta
particles and how well it excludes tritium from its bulk. Cross-contamination occurs when
materials contain hydrogen or carbon in their bulk or at their surface or when the materials absorb
a significant amount of tritium.
Gettering capabilities are largely a function of alloy overpressure. The process of gettering is the
removal of gases by sorption; either adsorption, absorption, or chemisorption. In absorption the
atoms of the gas dissolve between the atoms of the alloy. In adsorption and chemisorption, the
molecules of the gas adhere to the surface of the alloy. The difference between adsorption and
chemisorption is the type and strength of bonds that hold the molecules to the surface.
Because of its radioactive, chemically-reducing, and diffusive properties, tritium gas interacts with
almost all materials. Tritium gas permeates and degrades many useful polymeric materials (for
example, organics such as pump oils, plastics, and O-rings). This action causes a loss of
mechanical properties within months or years.
Tritium gas diffuses through glass, especially at elevated temperatures. The beta rays activate
the reduction of Si-O-Si bonds to Si-OT and Si-T bonds, and mechanical properties may be lost
over a period of years.
Some metals, such as uranium, are directly hydrided by tritium gas. These metals form a
chemical compound and their mechanical properties are altered within minutes or hours.
However, some metals, such as stainless steels, are permeated by tritium, but do not lose their
mechanical properties unless the tritium pressure is hundreds of atmospheres for several years.