Here's a bit of nuke engineering 101, You can also use uranium which the DPRK has in quantity: http://www.huffingtonpost.com/entry/north-korea-nuclear-weapons_us_57d8fe8fe4b0fbd4b7bc6834. 

Using high speed centrifuges like Iran, you can separate the fissile U235 to fuel your nuke from the U238 (which you can turn into fissile plutonium in a nuclear reactor).