North Korea is estimated to have enough weaponized plutonium for four to eight bombs, according to American nuclear scientist Siegfried Hecker.
It wasn't immediately clear to outside experts whether the device exploded Tuesday was small enough to fit on a missile, and whether it was fueled by plutonium or highly enriched uranium. A successful test would take North Korean scientists a step closer to building a nuclear warhead that can reach U.S. shores —seen as the ultimate goal of North Korea's nuclear program.
In 2006, and 2009, North Korea is believed to have tested devices made of plutonium. But in 2010, Pyongyang revealed a program to enrich uranium, which would give the country a second source of bomb-making materials — a worrying development for the U.S. and its allies.
"This latest test and any further nuclear testing could provide North Korean scientists with additional information for nuclear warhead designs small enough to fit on top of its ballistic missiles," Daryl Kimball and Greg Thielmann wrote on the private Arms Control Association's blog. "However, it is likely that additional testing would be needed for North Korea to field either a plutonium or enriched uranium weapon."
Uranium would be a worry because plutonium facilities are large and produce detectable radiation, making it easier for outsiders to find and monitor. However, uranium centrifuges can be hidden from satellites, drones and nuclear inspectors in caves, tunnels and other hard-to-reach places. Highly enriched uranium also is easier than plutonium to engineer into a weapon.
Monitoring stations in South Korea detected an earthquake in the North with a magnitude of 4.9 and the South's Defense Ministry said that corresponds to an estimated explosive yield of 6-7 kilotons.
The yields of the North's 2006 and 2009 tests were estimated at 1 kiloton and 2 to 6 kilotons, respectively, spokesman Kim Min-seok said. By comparison, U.S. nuclear bombs that flattened Nagasaki and Hiroshima during World War II were estimated at 13 kilotons and 22 kilotons, respectively, Kim said.
The test is a product of North Korea's military-first, or songun, policy, and shows Kim Jong Un is running the country much as his father did, said Daniel Pinkston of the International Crisis Group think tank.
The decision to push ahead with a test will be a challenge to the U.N. Security Council, which recently punished Pyongyang for launching the December long-range rocket. In condemning that launch and imposing more sanctions on Pyongyang, the council had demanded a stop to future launches and ordered North Korea to respect a ban on nuclear activity — or face "significant action" by the U.N.
U.N. chief Ban Ki-moon condemned the test in a statement. Japanese officials said they expected the Security Council to meet later to take up the nuclear test.
China expressed firm opposition to the test but called for a calm response by all sides.
The other part of a credible North Korean nuclear deterrent is its missile program. While it has capable short and medium-range missiles, it has struggled in tests of technology for long-range missiles needed to carry bombs to the United States, although it did launch the satellite in December.
North Korea isn't close to having a nuclear bomb it can use on the United States or its allies. Instead, Hecker said in a posting on Stanford University's website, "it wants to hold U.S. interests at risk of a nuclear attack to deter us from regime change and to create international leverage and diplomatic maneuvering room."
Associated Press writers Kim Kwang Hyon in Pyongyang, North Korea; Foster Klug, Hyung-jin Kim, Youkyung Lee and Sam Kim in Seoul, South Korea; and Yuri Kageyama and Malcolm Foster in Tokyo contributed to this report.
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