Can they build a nuclear weapon?
Japan is the only non-Nuclear Weapon State with the current capacity to enrich uranium and to reprocess spent nuclear fuel for plutonium production. In addition, Japan has about 10 tons of plutonium stockpile that can be used for weapons production. Japan is predicted to be capable of developing and possessing nuclear weapons in one to three years.
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Sources: http://www.nti.org/learn/countries/japan/nuclear/
Japan
South Korea has no enrichment and reprocessing capability that provide an autonomous path toward a production of fissile materials for nuclear weapons. South Korea will have a difficulty with developing nuclear weapons without acquiring these technologies.
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South Korea
North Korea has conducted five underground nuclear weapon tests as of December 2016. North Korea is estimated to have 10 nuclear weapons.
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