Storage and Handling - hdbk10810049

Spontaneous Heating and Pyrophoricity

DOE-HDBK-1081-94

PYROPHORIC METALS

reactor purposes, uranium fuel elements are always encased in a metal jacket (usually

zirconium or stainless steel).

Uranium in finely divided form is readily ignitable, and uranium scrap from machining

operations is subject to spontaneous ignition. This reaction can usually be avoided by

storage under dry (without moisture) oil. Grinding dust has been known to ignite

even under water, and fires have occurred spontaneously in drums of coarser scrap

after prolonged exposure to moist air. Because of uranium's thermal conductivity,

larger pieces generally have to be heated entirely to their ignition temperature before

igniting. Moist dust, turnings, and chips react slowly with water to form hydrogen.

Uranium surfaces treated with concentrated nitric acid are subject to explosion or

spontaneous ignition in air.

The pyrophoric characteristics of uranium are similar to those of plutonium except

that uranium forms do not ignite as easily as those of plutonium. Both form

pyrophoric oxides and hydrides. Both react violently with water and are best stored in

their oxide form (UO2, PuO2) in dry, inert atmospheres. Uranium metal (U) releases

hydrogen gas (H2) when allowed to react with water. The hydrogen may then react

with the metal to form uranium hydride (UH3) which may in turn react with oxygen in

the air to form stable uranium oxide (UO2 ) and hydrogen gas (H2 ). This sequence of

events is given by the equations

U + 2H2O

UO2 + 2H2 + Q

U + 3/2H2

UH3 + Q

4UH3 + 7O2

4UO2 + 6H2O + Q .

Note, in all instances, heat ( Q) is liberated, which increases the rates of reaction.

Under a dry, slightly oxidizing atmosphere, however, uranium corrodes quiescently

according to the equation

U + O2

UO2 + Q .

The heat generated from slow corrosion is not sufficient to ignite the uranium.

Storage and Handling

The storage of the many forms of uranium is beyond the scope of this primer. For a

complete discussion of uranium storage, the reader should refer to the draft

Assessment of Uranium Storage Safety Issues at Department of Energy Facilities

(referenced).

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Pyrophoricity