Plutonium - hdbk10810045

Spontaneous Heating and Pyrophoricity

DOE-HDBK-1081-94

PYROPHORIC METALS

Plutonium

Plutonium is one of the most widely used pyrophoric materials in the DOE Complex. Some

of the most serious fires occurring within the Complex are caused by the ignition of finely

divided plutonium particles. Several plutonium compounds are pyrophoric. The radioactive

decay of plutonium creates additional concerns such as dispersal of particles in a fire,

pressurization of storage containers, and the production of hydrogen gas during

decomposition of absorbed water.

Properties

Metal, Oxides, and Oxidation

Large pieces of plutonium metal react slowly with the oxygen in air at room

temperature to form plutonium oxides. The rate of oxidation is dependent on

a number of factors. These include (a) temperature, (b) surface area of the

reacting metal, (c) oxygen concentration, (d) concentration of moisture and

other vapors in the air, (e) the type and extent of alloying, and (f) the presence

of a protective oxide layer on the metal surface. The rate of oxidation

increases with increases in the first four factors and decreases with the last.

Alloying can either increase or decrease the oxidation rate, depending on the

alloying metal. Of all these factors, moisture has a large effect on the

oxidation rate and is especially significant in evaluating conditions for storing

plutonium metal and oxide.

Several plutonium oxides can be formed from oxidation of metal or

decomposition of plutonium compounds. Oxide phases corresponding to

sesquioxide (Pu2O3) and dioxide (PuO2) compositions have been identified

and are well characterized. Pu2O3 is pyrophoric in air and rapidly forms

plutonium dioxide while releasing heat. The dioxide is unreactive in air, but

reportedly heats slowly with water vapor at elevated temperatures.

Hydride

Plutonium hydride (PuHx , 2 < x < 3) forms during corrosion of plutonium

metal by hydrogen from water, organic materials, and other sources. Hydride

is rapidly oxidized by dry air at room temperature to produce PuO2 and H2 and

reacts with nitrogen at 250 C to form plutonium nitride (PuN). The quality of

hydride produced depends on the rate of hydrogen formation and on the

magnitude of the hydrogen-containing source. The reactivity of plutonium

hydride in air depends on factors such as particle size, presence/absence of

protective oxide layer, and the hydrogen:plutonium ratio, x. Finely divided

hydride is pyrophoric in air at room temperature. Thus, the only safe practice

is to handle and store hydride in a dry, oxygen-free atmosphere.

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Pyrophoricity