Following is a draft version, which is subject to be updated or revised. Your comments and feedbacks are welcome!
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As of December 2013, eight countries have declared that they have nuclear weapons. According to Article IX-3 of the Nuclear Non-Proliferation Treaty (NPT), “a nuclear-weapon State is one which has manufactured and exploded a nuclear weapon or other nuclear explosive device prior to 1 January 1967.” China, France, Russia, the United Kingdom, and the United States meet this requirement, and have acceded to the NPT as nuclear-weapon States (NWS) which are permitted to possess nuclear weapons under the treaty.
The three other countries that have tested nuclear weapons—after January 1, 1967—and declared having nuclear weapons are India, Pakistan and North Korea. India and Pakistan have never been parties to the NPT. North Korea declared its suspension from the NPT in 1993 and its withdrawal in 2003. Israel, a non-NPT state, has maintained a so-called “ambiguous policy” by neither confirming nor denying having nuclear weapons, although it is widely considered that it has a nuclear weapons capability (no evidence has yet been found that Israel ever conducted a nuclear test). In this report these 4 states that have publicly declared or are believed to own nuclear weapons are referred to as “nuclear-armed states.”
None of the nuclear-weapon/armed states, except France which declares the ceiling of the number of nuclear weapons, has declassified the exact number of nuclear weapons in its arsenal. The status of nuclear forces shown in table 1-1 below is based on the estimates produced by the Stockholm International Peace Research Institute (SIPRI). According to the data, in spite of the reduction of 2,000 nuclear weapons from the previous year, approximately 17,000 nuclear weapons still exist on the earth, and the US and Russian nuclear stockpiles together constitute more than 90 percent of them.
SIPRI estimates that China, India and Pakistan have added about 10 warheads each in the course of the past year. China’s Foreign Ministry spokesman Hong Leiat refuted SIPRI’s analysis at a daily news briefing: “China has never deployed nuclear weapons in other countries, and China does not participate in any form of the nuclear arms race and has always kept its nuclear capabilities at the minimum level required for national security.” Regarding the China’s nuclear arsenal, while SIPRI estimates that China has 250 nuclear warheads, one scholar estimates that it deploys 800-900 warheads, and another considers that it possesses approximately 3,000 nuclear weapons. Such wide-ranging estimates are derived from China being the least transparent about nuclear weapons among the five NWS since, contrary to the other NWS, China has never released any information on the number of nuclear weapons and delivery vehicles it possesses.
(Drafted by Hirofumi Tosaki, CPDNP)
 On this point, Bruno Tertrais explains the reasons as following: “Stockpiles include weapons which are not entirely functional (when exactly does an atomic device become a ‘nuclear weapon’?), or which are used for non-destructive testing. As a result, giving an exact number can be difficult, misleading, and/or be accurate just for a given day.” Bruno Tertrais, “Comments on Hiroshima Report of March 2013,” Hiroshima Report Blog: Nuclear Disarmament, Nonproliferation and Nuclear Security, October 29, 2013, http://hiroshima-report.blogspot.jp/2013/10/op-ed-bruno-tertrais-comments-on.html.
 Zhou Wa, “China defends use of nuclear warheads,” China Daily, 4 June 2013, http://www.asianewsnet.net/China-defends-use-of-nuclear-warheads-47524.html.
 Viktor Yesin, “China’s Nuclear Capabilities,” Aleksey Arbatov, Vladimir Dvorkin and Sergey Oznobishchev, eds., Prospects of China’s Participation in Nuclear Arms Limitation (Moscow: Institute of World Economic and International Relations, Russian Academy of Sciences, 2012), chapter 3.
 Phillip A. Karper, “Strategic Implications of China’s Underground Great Wall,” Georgetown University Asian Arms Control Project, 26 September 2011.