Properties of Metals
When exposed to tritium gas, hydriding metals absorb large volumes of tritium to form tritide
phases, which are new chemical compounds, such as UT3. The mechanical integrity of the
original metallic mass is often severely degraded as the inclusions of a brittle, salt-like hydride
form within the mass. Because of this property and their large permeability to hydrogen,
hydriding metals are not to be used for constructing pipelines and vessels of containment for
tritium gas. They have great utility, however, in the controlled solidification and storage of
tritium gas, as well as in its pumping, transfer, and compression.
Uranium, palladium, and alloys of zirconium, lanthanum, vanadium, and titanium are presently
used or are proposed for pumping and controlled delivery of tritium gas. Several of these alloys
are in use in the commercial sector for hydrogen pumping, storage, and release applications.
Gaseous overpressure above a hydride (tritide) phase varies markedly with temperature; control
of temperature is thus the only requirement for swings between pumping and compressing the
In practice, pumping speeds or gaseous delivery rates (the kinetic approach to equilibrium) are
functions of temperature (diffusion within the material), hydride particle size, and surface areas
and conditions. Poisoning of a uranium or zirconium surface occurs when oxygen or nitrogen
is admitted and chemically combines to form surface barriers to hydrogen permeation. In
practice, these impurities may be diffused into the metal bulk at elevated temperature, thereby
reopening active sites and recovering much of the lost kinetics. Other metals and alloys (for
example, LaNi3) are less subject to poisoning, although alloy decomposition can occur.
Helium-3, generated as microscopic bubbles within the lattice of tritides, is not released except
by fracture and deformation of metal grains. This release usually occurs at high temperature or
after long periods of time. When a tritide is heated to release tritium, helium-3 is also released
to some extent. The cooled metal, however, does not resorb the helium-3. The practice of
regenerating a tritide storage bed to remove helium-3 immediately prior to use for pure tritium
delivery is therefore common.
If helium-3 (or another inert impurity) accompanies tritium gas that is absorbed onto a tritide
former, helium blanketing may occur. The absorption rate slows as the concentration of helium
in the metal crevices leading toward active sites becomes high. Normal gaseous diffusion is
often not sufficient to overcome this effect. Forced diffusion by recirculating the gas supply can
be used to overcome blanketing.
Because they generally have high surface areas, graphite samples adsorb large amounts of
hydrogen gas (4 x 1018 molecules/g for a graphite pellet used in gas-cooled reactors). Methane,
protium, and (possibly) water are generated from beta irradiation of the graphite surface.