Uranium is in the news due to negotiations with Iran. Uranium to be enriched is converted to uranium hexafluoride. If Iran plans to enrich theirs further it is probably stored as uranium hexafluoride, not uranium metal.
Wikipedia has an article on uranium hexafluoride. Uranium hexafluoride - Wikipedia
It is a volatile, white solid, sublimation temp (at atmospheric pressure) 56.5 °C = 134 F.
The word volatile causes confusion. It is a technical word meaning the substance forms vapors and evaporates. (Media sometimes uses volatile to mean explosive. Volatiles need not be explosive although they may be when the vapors are flammable. They may be hazardous when toxic. Volatile is also used to describe explosives or personalities when they are unpredictable.)
Sublimation means the material passes from solid to vapor without melting. By definition, the sublimation temperature is when the vapor pressure of the material reaches one atmosphere or 15 psi. Vapor pressure increases with temperature implying that at room temperature uranium hexafluoride will be emitting vapors perhaps like moth balls.
The phase diagram shows uranium hexafluoride can be liquified at pressures above 22 psi and temperatures over 147 F.
To deal with volatility, uranium hexafluoride is probably stored in cylinders. The low pressure required implies a light wt cylinder similar to a propane cylinder can be used. Maintaining it as a liquid is probably convenient for handing and processing.
U-235 is fissile. When critical mass is reached, a nuclear reaction occurs releasing energy. Google reports the critical mass of U-235 hexafluoride at about 50 kg (110 lb).. That implies UF6 is in cylinders weighing under 100 lb depending on degree of enrichment. The density of 5.09 kg/l implies 10 liter (abt 2.5 gal) max containers when fully enriched.
Its NFPA hazard diamond rates it 4 (highest) for heath hazards due mostly to radioactivity but also toxicity of uranium compounds–causes kidney damage. It is rated 2 for reactivity.
We expect Iran’s uranium to be contained 2 to 3 gal cylinders weighing less than 100 lb. If ruptured the cylinders are expected to let the material evaporate.