Carbides and Nitride

PYROPHORIC METALS

DOE-HDBK-1081-94

Spontaneous Heating and Pyrophoricity

Carbides and Nitride

Plutonium carbides, oxycarbides, and nitride are reactive and potentially

pyrophoric materials that could pose handling problems if exposed to air or

oxygen-containing atmospheres. These compounds react readily with moisture

to form gaseous products such as methane, acetylene, and ammonia. Because

plutonium compounds of this type have been prepared at several sites and may

have been "temporarily" stored under special conditions (hermetically sealed

within an inert atmosphere) without first oxidizing them, caution should be

exercised in opening cans that might contain such materials.

Reactions Involving Water

Water vapor accelerates the oxidation of plutonium by oxygen and reacts

directly with the metal. Oxidation is about ten times higher in humid air than in

dry air at room temperature. For this reason, plutonium metal has routinely

been handled in a very dry atmosphere such as one with a -40 C dew point.

Inerting of glove boxes and enclosures for handling plutonium with nitrogen or

argon is effective in reducing metal oxidation only if it also excludes water

vapor. Rapid oxidation does not occur if oxygen is present at a level of 5% in

nitrogen or argon. However, if 1.3% moisture (50% relative humidity)

accompanies the oxygen, then rapid metal oxidation can be anticipated.

Plutonium dioxide can adsorb up to 8% of its weight as water on the surface.

The quantity absorbed is a direct function of the surface area of the oxide.

The principal hazard associated with absorbed water is pressurization of a

sealed oxide container through any of several separate processes including

evaporation of water, radiolysis to form oxygen and hydrogen, or direct

reaction with the oxide to form a higher oxide and hydrogen gas.

Pressurization of oxide containers can be prevented by use of sealed containers

fitted with durable, high-efficiency metal filters. Although gases can escape

without release of plutonium-containing particles, air (possibly moist) is able to

enter the container.

Pyrophoricity

When heated to its ignition temperature, plutonium reacts at an accelerated

oxidation rate, which sustains continued oxidation. The burning temperature

depends on the rate of heat dissipation to the surroundings and the rate of heat

generation, which is dependent on the surface area of oxidizing metal.

Temperatures of plutonium fires usually exceed the melting temperature of

plutonium metal (640 C) which causes the material to consolidate into a

molten configuration. As such, finely divided metal, turnings, and casting

Pyrophoricity

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