There's a belief that N Korea got their technology from Pakistan, so their nukes may have been tested by proxy, and don't work very well.
Khan from Pakistan was an engineer in Europe and stole the weapons technology from his employer and managed to buy the critical inverters before they were locked down, otherwise Pakistan still wouldn't have the bomb.
Thank you Eisenhower for your "Atoms for Peace" program. We built Iran and Pakistan's first reactors. They sure behaved themselves didn't they?