How is Uranium-235 stored to avoid accidental critical mass? How is Uranium-235 stored to avoid it going critical?
 A: As pointed out in PM 2Ring's comment, storage of enriched U-235 is done with great care. This is of particular concern in facilities which either enrich raw uranium or reprocess reactor fuel or the components of decommissioned atomic and nuclear weapons, or where radioactive processing waste is stored. 
In a process line handling uranium, each machine in the process sequence is either limited in size or subject to strict operating procedures to prevent the accumulation of a critical mass inside it while it is in operation. The same is true for each of the material transfer operations which occur between the processing machines. 
Failure to follow the processing rules can result in a chain reaction occurring inside the processing machinery. A google search on nuclear accidents will furnish you with a list of such incidents. 
A: Fissionable metals are sometimes formed in to < critical shapes that cannot be easily arranged into a critical geometry. One shape resembles a 'dog bone' or 'dumbbell' and limits the proximity and contact area of like pieces. The shape keeps the Uranium density sub-critical no matter how one places the 'bones' in contact... if one had the opportunity and were foolish enough to try.
