By Fredrik Dahl VIENNA (Reuters) - About 140 cases of missing or unauthorized use of nuclear and radioactive material were reported to the U.N. atomic agency in 2013, highlighting the challenges facing world leaders at a nuclear security summit next week. Any loss or theft of highly enriched uranium, plutonium or different types of radioactive sources is potentially serious as al Qaeda-style militants could try to use them to make a crude nuclear device or a so-called "dirty bomb", experts say. Denis Flory, deputy director general of the International Atomic Energy Agency (IAEA), said most of the reported incidents concerned small quantities of radioactive material. In a "dirty bomb", conventional explosives are used to disperse radiation from a radioactive source, which can be found in hospitals, factories or other places that may not be very well protected. Source: Reuters