Storage and Handling - hdbk10810047

Spontaneous Heating and Pyrophoricity

DOE-HDBK-1081-94

PYROPHORIC METALS

skulls tend to ignite readily and achieve a high initial temperature which lasts

until melting occurs and the surface area is reduced.

The oxide layer that forms during burning limits the oxidation rate of

plutonium. The burning process is similar to that of a charcoal briquette. The

ignition temperature of plutonium metal depends on the factors that increase

the oxidation rate. Finely divided plutonium metal, such as metal powder of

fine machine turnings, ignites near 150 C. This temperature is easily reached

if a coexisting pyrophoric material such as a hydride spontaneously ignites at

room temperature. Bulk or massive plutonium characterized as having a

specific surface area less than 10 cm2 /g requires temperatures in excess of

400 C to ignite. Many plutonium fires have occurred because samples

containing finely divided metal have spontaneously ignited. Fires have not

occurred with well-characterized metal existing in large pieces that have

higher ignition temperatures. Thus, massive plutonium is not considered

pyrophoric or capable of self-ignition.

An investigation of two instances in which kilogram-sized plutonium pieces

were observed to "spontaneously ignite" in air at room temperature showed

that they had been exposed to unlimited sources of hydrogen for extended

periods, and that the samples were thermally insulated when ignition occurred.

The amount of hydride present on these massive pieces apparently generated

sufficient heat to cause ignition. These observations emphasize the need for

well-characterized materials.

Storage and Handling

Plutonium should be stored as pure metal (Pu) or in its dioxide (PuO2 ) form in a dry,

inert or slightly oxidizing atmosphere. The formation of oxide from metal is

accompanied by a large volume expansion (up to 70%) which may bulge or breach

the primary container. Case studies show that mechanical wedging resulting from this

expansion can even breach a second metal container, resulting in localized

contamination release and possible exposure of personnel. Oxidation of the metal and

rupture of the container by mechanical wedging are prevented if the storage container

is hermetically sealed. Plutonium radioactively decays producing alpha particles and

helium molecules. Over long-term storage, helium buildup can contribute to the

pressurization of containers.

Plutonium sesquioxide and hydride should be converted to plutonium dioxide before

storage. Primary and secondary containers should be hermetically sealed and contain

no plastics or other materials that decompose as a result of radiation exposure.

For a more complete discussion of plutonium storage issues, refer to DOE/DP-123T,

Assessment of Plutonium Storage Issues at Department of Energy Facilities,

January 1994.

Rev. 0

Page 35

Pyrophoricity