Starting in 2006 North Korea has conducted three nuclear tests. All three weapons tested were fueled by plutonium, a by-product of nuclear reactor operations. In the decade before he was finally stopped in February 2004 Pakistani metallurgist and nuclear scientist A.Q, Khan managed to sell uranium-enrichment technology, which he had earlier stolen from the European consortium URENCO in the 1970s, to three countries: Iran, Libya, and North Korea. Iran got thousands of centrifuges used to separate the lighter U235 isotope from the heavier and more common U238 isotope in order to enrich uranium, ostensibly for peaceful uses. Iran is presently subject to severe Western economic sanctions and is presently negotiating with the five permanent members of the UN Security Council and Germany (P5 + 1) over the lifting of sanctions in exchange for Tehran limiting its enrichment activities. Libya gave up its military nuclear ambitions in 2003 following the American-British invasion of Iraq. That leaves just one country: North Korea.
On November 20, 2013 South Korean Defense Minister Kim Kwan-jim told the South Korean National Assembly that North Korea had perfected the technology to build a uranium-fueled bomb. If this is accurate, it will mean that North Korean nuclear development will be harder to monitor because instead of just monitoring a single old nuclear reactor at Yongbyon, Korean, Japanese and American intelligence agencies will have to locate and monitor the locations where the Korean centrifuges are at work. There is a bright side to this: uranium bombs are harder to miniaturize than plutonium bombs. Shrinking the prototype weapons so that they can be delivered by missiles over long distances is crucial to developing a credible deterrent force. But it is possible that Beijing has provided North Korea with Chinese nuclear weapons designs as they provided to Pakistan in the 1980s.
Pyongyang does not really need a credible second-strike nuclear deterrent as its main use for the nuclear weapons is to blackmail Seoul, Tokyo, and Washington into providing economic aid and technology to North Korea. This has been the situation since the George W. Bush administration. As long as North Korea can build a bomb deliverable by MiG-23 fighter bombers it can credibly threaten to bomb the South Korean capital Seoul. It would just need to put one or two bomb-carrying MiGs in the middle of a swarm of MiG-21s and MiG-23s on route to Seoul to provide a credible risk of delivering the bomb.