North Korea's Nuclear Bombs
North Korea's nuclear bombs, in their first two underground tests used plutonium from their small stockpiles mined in the 1990's. This type of bomb creates an explosion like the one used three days after the first one in 1945. Two were dropped on Nagasaki. Their more recent nuclear bomb tests have US officials more concerned.
Despite the cloaking efforts to conceal the tests since 2006, the US has been able to detect that NK is producing the highly enriched uranium (HEU) type of bomb. This type of bomb relies on natural uranium and modern enrichment technologies that Iran has been well skilled at, and no doubt, have shared with NK. The first bomb dropped on Nagasaki was a HEU type. Very destructive. The enrichment abilities of NK are worrisome because they are obviously developing rapidly with the help of Iran. NK has always had plutonium, but enrichment is recent.
The recent belligerent threats only serve to heighten concern as to what capabilities they really do have. It is largely a guessing game to some degree,