APPENDIX ADOE-HDBK-1017/1-93Properties of MetalsWhen exposed to tritium gas, hydriding metals absorb large volumes of tritium to form tritidephases, which are new chemical compounds, such as UT3. The mechanical integrity of theoriginal metallic mass is often severely degraded as the inclusions of a brittle, salt-like hydrideform 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 fortritium gas. They have great utility, however, in the controlled solidification and storage oftritium gas, as well as in its pumping, transfer, and compression.Uranium, palladium, and alloys of zirconium, lanthanum, vanadium, and titanium are presentlyused or are proposed for pumping and controlled delivery of tritium gas. Several of these alloysare in use in the commercial sector for hydrogen pumping, storage, and release applications.Gaseous overpressure above a hydride (tritide) phase varies markedly with temperature; controlof temperature is thus the only requirement for swings between pumping and compressing thegas.In practice, pumping speeds or gaseous delivery rates (the kinetic approach to equilibrium) arefunctions of temperature (diffusion within the material), hydride particle size, and surface areasand conditions. Poisoning of a uranium or zirconium surface occurs when oxygen or nitrogenis admitted and chemically combines to form surface barriers to hydrogen permeation. Inpractice, these impurities may be diffused into the metal bulk at elevated temperature, therebyreopening active sites and recovering much of the lost kinetics. Other metals and alloys (forexample, 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 exceptby fracture and deformation of metal grains. This release usually occurs at high temperature orafter long periods of time. When a tritide is heated to release tritium, helium-3 is also releasedto some extent. The cooled metal, however, does not resorb the helium-3. The practice ofregenerating a tritide storage bed to remove helium-3 immediately prior to use for pure tritiumdelivery is therefore common.If helium-3 (or another inert impurity) accompanies tritium gas that is absorbed onto a tritideformer, helium blanketing may occur. The absorption rate slows as the concentration of heliumin the metal crevices leading toward active sites becomes high. Normal gaseous diffusion isoften not sufficient to overcome this effect. Forced diffusion by recirculating the gas supply canbe used to overcome blanketing.Because they generally have high surface areas, graphite samples adsorb large amounts ofhydrogen 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. MS-02Page A-4Rev. 0-A
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