The long-term storage of uranium ingots can form a pyrophoric surface caused by reaction with air and moisture. This is due to the formation of powdered hydride and hydrogen... In many reported incidents, flashing of the fuel or explosions occurred when the containers were opened. The flashing was believed to be the spontaneous ignition of uranium or uranium hydride which became suspended due to the mechanical disturbance of opening the container.
These hazards are more severe with plutonium. In addition, plutonium expands up to 70% in volume as it oxidizes and thus may burst its 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.Chemist [picks up phone]: Hello?
Safety officer: Do you have any plutonium in a can?
Chemist: Uh, yes?
Safety officer: Well, you might want to let it out!
Also, this interesting section:
"The spontaneous ignition of uranium can usually be avoided by storage under dry (without moisture) oil. Plutonium stored under oil has been reported to spontaneously ignite. Plutonium should be stored as pure metal (Pu) or in its PuO2 form in a dry, inert or slightly oxidizing atmosphere...
...For plutonium metal, complete exclusion of oxygen and./or rapid heat removal are the effective ways to extinguish fires. Sand can also be used to smother plutonium fires. Magnesium oxide sand is probably the most effective material for extinguishing a plutonium fire. Magnesium oxide cools the burning material, acting as a heat sink, and also blocks off oxygen. Because of reactivity and criticality concerns, water may not be the appropriate extinguishing agent."Just in case, you know, you had some plutonium lying around...
1. Cournoyer, M.E.; Costigan, S.A.; Schake, B.S. "Why is weapons grade plutonium more hazardous to work with than highly enriched uranium?" J. Chem. Health. Saf. 2015, 22 (4), 2-9. DOI: 10.1016/j.jchas.2014.10.004