Metal Embrittlement

DOE-HDBK-6004-99

pressure, a spark energy of 0.02 millijoule can ignite a stoichiometric mixture (29.5% tritium by

volume). The potential for hydrogen fires can be minimized through leak prevention, elimination of

ignition sources, reduction of available oxygen, and/or increased ventilation.

Hydrogen Detonations

The mode of burning in which the flame travels at supersonic speeds is called detonation. Heated to

a high temperature, a mixture of hydrogen and air can spontaneously ignite and detonate. This

temperature is the spontaneous ignition temperature which is a function of composition, pressure and

container size. At one atmosphere of pressure, this temperature is about 540C. Favorable

conditions for detonation are stoichiometric mixture (29.5% hydrogen by volume), high energy

ignition sources and confining surroundings. Unconfined hydrogen-air mixtures do not detonate

unless the ignition source delivers considerable energy in the form of a shock wave.

For a potential hydrogen fire or detonation to be a design basis accident, the safety analysis should

evaluate the frequencies of the required conditions occurring at the same time:

1. Hydrogen isotopes in sufficient concentration,

2. Oxygen in sufficient concentration, and

3. High temperature or ignition source.

To preclude a tritium fire or detonation as a design basis accident, the safety analysis must

demonstrate a low event frequency, typically <10-6/year.

Design features that promote a low event frequency include:

1. Leak tight primary confinement to prevent out leakage of tritium to the secondary confinement,

2. Inert gas in the space between primary confinement and secondary confinement barrier walls, to

prevent oxygen contacting tritium,

3. Monitors to detect tritium out leakage or oxygen inleakage,

4. Minimize ignition sources or high temperatures near the primary or secondary confinement

barriers, and

5. Utilize NFPA rated enclosures (NFPA 70) for electrical equipment in a location potential for

contact with flammable mixtures exist.

Metal Embrittlement

Almost all metals will absorb hydrogen gas in a thin surface layer from which hydrogen will diffuse

deeper into the metal. Additionally, some of the hydrogen isotope tritium will decay to helium-3.

With time and continued exposure, both the diffused hydrogen isotopes and the tritium decay product

helium will embrittle the metal.

Embrittlement alters the material properties of some metals significantly, by reducing ductility which

leads to failure by crack growth at ambient temperature. In addition, some metals containing helium