A compelling collection of criticality accidents, mostly involving Uranium shifting or collecting into a critical configuration. Sometimes Plutonium. Via a commenter at http://www.damninteresting.com/bitten-by-the-nuclear-dragon/ which is a readable account of Slotin’s fatal screwdriver mishap.
H/t to +Måns Rullgård
http://www.orau.org/ptp/Library/accidents/la-13638.pdf