According to Robert Farley, America’s spotty nuclear safety history suggests that “there is a far greater likelihood that North Korea will accidentally drop a nuclear weapon on itself than on South Korea, Japan, or the United States”:
A 2012 Center for Strategic and International Studies presentation [pdf] highlighted many of the difficulties associated with preventing a nuclear mishap in North Korea. Although the report focused on problems in production and in the fuel cycle, the difficulties associated with developing proper weapon handling techniques are even more challenging. The relatively early stage of North Korea’s nuclear program means that safety procedures remain in their infancy, and thus that the potential for accident is high. At the same time, in part because of the program, North Korea is isolated from the knowledge and expertise of the international community with respect to nuclear weapons handling and safety protocols.
Moreover, the small size of North Korea’s arsenal and the apparent paranoia of Pyongyang’s military and political leadership may necessitate a nearly constant alert status, putting pressure on personnel and increasing the chances of an accident. And indeed, this same paranoia (even, to some degree, with respect to China) may make North Korea particularly unlikely to agree to any transparency in its nuclear program.