The (Il?)legality of Nuclear Weapons Tests Under International Law—Filling the Possible Legal Gap by Ensuring the Comprehensive Test Ban Treaty’s Entry into Force

Daniel Rietiker
March 16, 2017


Two nuclear weapons test conducted by the Democratic People’s Republic of Korea (DPRK) in 2016 brought the topic of nuclear testing under international law to the fore again in the year that the Comprehensive Test Ban Treaty (CTBT), the most relevant international instrument in this field, was supposed to be celebrating the twentieth anniversary of its adoption. Recent missile tests by the DPRK in February and March of this year have also highlighted concerns over the state’s nuclear program and increased anxiety in the region.[1] Further tensions can be expected from the already complicated relations with Russia. Indeed, in an interview with Arms Control Today at the beginning of 2016, former U.S. Defense Secretary William Perry declared the following:

First of all, let me say that I don’t have any inside information from anybody in the Russian government . . . . It’s just that I know they are developing new nuclear weapons. They’ve said so themselves. I’m confident that that will lead the weapons designers to request new tests. That will put [Russian President Vladimir] Putin up to a decision. . . . One of the reasons the political costs will not be as high as they should be is because we have not yet ratified the CTBT. That makes a decision to test easier for him.[2]

These recent events and statements raise the question of the legality of potential nuclear tests under international law, which will be examined in the following Insight.

The Legal Norms Prohibiting Nuclear Weapons Tests

Nuclear testing has fortunately become rare in recent years. Only the DPRK has tested nuclear weapons in this century, namely on Oct. 9, 2006; May 25, 2009; Feb. 12, 2013; Jan. 6, 2016; and Sept. 9, 2016.[3] The legal norms restricting states’ freedom to conduct nuclear tests are mainly established by the treaties discussed below.

Early Treaties

The Partial Test Ban Treaty (PTBT or the Moscow Treaty), a product of the Cold War, was negotiated and concluded in the aftermath of the Berlin crises of 1961 and the Cuban missile crisis of 1962.[4] It was adopted on August 5, 1963, as a tripartite agreement between the United States, the United Kingdom, and the United Soviet Socialist Republic (USSR), and entered into force on October 10, 1963.[5] 126 states have ratified this treaty to date, including India, Israel, and Pakistan.[6] The DPRK, in contrast, is not a party.

The PTBT only prohibits a “nuclear weapon test explosion, or any other nuclear explosion in the atmosphere, in outer space, under water and in any other environment if such explosion causes radioactive debris to be present outside the territorial limits of the State under whose jurisdiction or control such explosion is conducted.”[7] Consequently, states parties would not be prevented from conducting underground tests unless they cause transboundary fallout.

The PTBT’s most important weakness lies in its lack of implementation provisions, making it more difficult for an independent statement to be made about a possible breach.[8] Also, a party can denounce this treaty on the basis of a withdrawal clause:

Each Party shall in exercising its national sovereignty have the right to withdraw from the Treaty if it decides that extraordinary events, related to the subject matter of this Treaty, have jeopardized the supreme interests of its country.[9]

Even though this clause sets certain limits, its language is vague and the applicable criteria broad, conceding a wide margin of appreciation to the states parties.[10]

The United States and the USSR agreed on further mutual restraint, prohibiting underground tests exceeding 150 kilotons.[11] Being of limited scope, it also contains a similar withdrawal clause to the PTBT.

Additionally, states have ratified instruments prohibiting nuclear testing in the context of denuclearizing certain zones and regions.[12] Russia, for instance, is bound by the 1959 Antarctic Treaty[13] and Protocol No. 3 to the 1985 South Pacific Nuclear Free Zone (Rarotonga Treaty) by which Russia, like the other four states recognized under the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) as possessing nuclear weapons,[14] have agreed not to test nuclear weapons within the zone.[15]

1996 CTBT

Status of the Treaty

The UN General Assembly adopted the CTBT on September 10, 1996.[16] It contains a comprehensive prohibition of nuclear weapons tests, even though it does not proscribe nuclear weapons testing by way of computer simulations or conducting so-called sub-critical tests (i.e. those not resulting in nuclear explosions).[17]

Unfortunately, the CTBT has not yet entered into force in spite of the current 164 ratifications.[18] Under Article XIV, all states with nuclear power and/or research reactors, as listed in its Annex 2, have to ratify the treaty before it enters into force. The vast majority of those states have done so, including the United Kingdom, France, and Russia, but the CTBT still must be ratified by China, the DPRK, Egypt, India, Iran, Israel, Pakistan, and the United States.[19]

Legal Effects of the CTBT—The Example of Russia

The question arises whether states such as Russia, seventeen years after its ratification in 2000, are still bound by the CTBT. A legal analysis of this question starts from Article 18 of the 1969 Vienna Convention on the Law of Treaties (VCLT). The customary nature of this provision is well recognized.[20] According to Article 18(a), “a State is obliged to refrain from acts which would defeat the object and purpose of a treaty when it has signed the treaty or has exchanged instruments constituting the treaty subject to ratification, acceptance or approval, until it shall have made its intention clear not to become a party to the treaty.” Article 18(b) imposes the same duty on a state that “has expressed its consent to be bound by the treaty, pending the entry into force of the treaty and provided that such entry into force is not unduly delayed.”

What do those provisions mean in concrete terms for Russia? Two questions have to be addressed: First, what kind of act would defeat the “object and purpose” of the CTBT and, second, what does “unduly delayed” mean?

Acts that run counter to CTBT Article I(1) (Basic Obligations), would certainly defeat the object and purpose of the treaty. Under that provision, a “State Party undertakes not to carry out any nuclear weapon test explosion or any other nuclear explosion, and to prohibit and prevent any such nuclear explosion at any place under its jurisdiction or control.” In other words, a new nuclear test (by Russia for instance) would not per se breach the CTBT, but it might violate Article 18 VCLT.[21]

The question of whether the CTBT’s entry into force has been “unduly delayed” is more difficult to answer. In the absence of a precise temporal indication in Article 18(b) VCLT and due to the fact that proposals to introduce such indications had been rejected during the preparatory work for the VCLT,[22] the term “unduly delayed” has to be construed with flexibility, taking into account the particular circumstances. It is noteworthy to mention that both states that have ratified the CTBT as well as some states that have signed the CTBT have confirmed at the biannual conferences on facilitating the entry into force of the CTBT their commitment not to act counter to the object and purpose of the treaty pending its entry into force.[23]

It is fair to conclude that the CTBT’s entry into force, after so many years, has been “unduly delayed.” Such a conclusion may be deduced from the particular nature of arms control treaties. In fact, they are often considered “interdependent treaties” based on a strong reciprocity between states parties.[24] Once this critical balance is defeated, the continuation of treaty implementation by other states might no longer be expected for reasons of justice and equality.[25] In other words, states such as Russia, France, and the United Kingdom, when ratifying the CTBT, might have had legitimate expectations that the United States, China, and others would follow on the same path.

As indicated above, Article 18(a) VCLT contains the same obligation not to defeat the object and purpose of the treaty for states, such as the United States, that have signed but not ratified the CTBT, for the period between signature and actual ratification of the treaty. There is no condition of “unduly delayed” here, but the states can demonstrate their intention not to become parties. While the United States was instrumental in negotiating the CTBT,[26] it has not ratified it yet, although the Obama administration constantly stressed its support for the treaty.[27] The treaty is, however, not likely to receive strong support from the Trump administration.

Other Sources of International Law

International law, including arms control law, is not restricted to treaties. Article 38 of the Statute of the International Court of Justice enumerates the sources of international law, listing “international conventions,” “international custom,” “general principles of law,” as well as “judicial decisions and the teachings of the most highly qualified publicists.” Regarding nuclear weapons tests, it has been suggested that customary international law might come into play, at least regarding atmospheric, underwater, outer space, and celestial bodies testing, thanks mainly to the relevance of environmental protection in modern international law.[28] George Bunn, the first general counsel of the U.S. Arms Control and Disarmament Agency, advocated in an article published in 1999 that a norm against nuclear testing already existed, at least a “politically” binding one, even without the CTBT’s entry into force.[29] He based his conclusion on the practice of states (including national moratoria), the reasons given in support of their behavior, and international condemnation of nuclear testing.[30]

In the absence of a legally binding court decisions establishing such a customary rule, Russia or other states willing to test nuclear weapons might nevertheless rather easily plead the contrary. This is even more so in the case of underground testing, even though state practice seems to have remained largely consistent for many years with only the DPRK having carried out such tests, as mentioned above.

What else, from a strictly legal point of view, could prevent Russia or other states from resuming nuclear testing? The UN Security Council (UNSC) has repeatedly condemned the DPRK’s nuclear tests and qualified them as a threat to peace and international security.[31] On the other hand, it did not, as such, qualify them as a breach of international law. In comparison, in resolutions concerning the use of chemical weapons in Syria, the UNSC used clear and explicit language, for example, in Resolution 2118 in 2013 condemning “in the strongest terms any use of chemical weapons in the Syrian Arab Republic, in particular the attack on 21 August 2013, in violation of international law” (emphasis added).

Finally, in a resolution adopted on September 23, 2016, thus only some days after the twentieth anniversary of the CTBT, the UNSC reiterated the importance of its entry into force.[32] Without underestimating the political significance of this resolution, it is suggested here that the UNSC did not declare nuclear weapons tests per se illegal under international law.


The legality of nuclear tests is not certain under international law, and the relevant sources of international law are incomplete and fragmentary. The question of legality depends on the concrete circumstances of the test, in particular the environment—underground, atmospheric, outer space, under water, and transboundary. Certain treaties establish a clear prohibition, but they are limited and contain withdrawal clauses. Moreover, in spite of the high number of ratifications and the fact that the CTBT’s preparatory commission established its structure many years ago and has started its verification activities, the CTBT is still not in force and needs the ratification of several key states. In this situation, it is also uncertain whether Article 18 VCLT is still pertinent. Finally, it cannot be asserted convincingly that a general prohibition of nuclear weapons testing derives from other sources of international law.

The moment to celebrate the twentieth anniversary of the adoption of the CTBT would have been in 2016. It is in the hands of a few states, especially the United States, to save the treaty, since it is generally considered that its ratification would trigger a virtuous circle pushing China and then other states to ratify as well.[33]

About the Author: PhD, lecturer at the University of Lausanne, Member of the Adjunct Faculty, Suffolk University Law School, Visiting Fellow Harvard Law School in 2014. The author thanks James Brannan and John Burroughs for their valuable input on this article.


[1] North Korea: Four Ballistic Missiles Fired into Sea, BBC (Mar. 6, 2017),

[2] Daniel Horner & Kingston Reif, Lowering Nuclear Risks: An Interview With Former Defense Secretary William Perry, Arms Control Association (Jan./Feb. 2016),

[3] Nuclear Testing 1945 – Today, CTBTO Preparatory Commission, (last visited Mar. 9, 2017).

[4] Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water, Aug. 5, 1963, 480 U.N.T.S 44, [hereinafter Partial Test Ban Treaty], available at

[5] G. Venturini, Test-Bans and the Comprehensive Test Ban Treaty Organization, in Nuclear Non-Proliferation in International Law, Vol. I, 138 (J.L. Black-Branch & D. Fleck eds., 2014).

[6] Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water, United Nations Office for Disarmament Affairs, (last visited Mar. 13, 2017).

[7] Partial Test Ban Treaty, supra note 4, art. I, para. 1.

[8] Venturini, supra note 5, at 135.

[9] Partial Test Ban Treaty, supra note 4, art. I, para. 2.

[10] In spite of the openness of this type of clause, there are only few examples where states made use of them. See Frederic L. Kirgis, North Korea’s Withdrawal From The Nuclear Nonproliferation Treaty, ASIL Insights (Jan. 24, 2003),

[11] Treaty Between The United States of America and The Union of Soviet Socialist Republics on the Limitation of Underground Nuclear Weapon Tests art. I, U.S.-Russ., July 3, 1974, 13 ILM 906, available at

[12] See, in particular, Article 1, para 1 of the Tlatelolco Treaty (nuclear-weapon-free zone for Latin America); Article 3, para 1(c) of the Bangkok Treaty (Southeast Asia); Article 5(a) of the Pelindaba Treaty (Africa); and Article 5(a) of the Semipalatinsk Treaty (Central Asia).

[13] The Antarctic Treaty art. 5, Dec. 1, 1959, 402 UNTS 71.

[14] For  the  purposes  of  the NPT,  a nuclear weapon state is one that has manufactured and exploded a nuclear weapon or other nuclear explosive device prior to Jan. 1, 1967.

[15] Protocol No. 3 to the South Pacific Nuclear Free Zone Treaty art. 1, Apr. 14, 1997, 1971 UNTS 482.

[16] G.A. Res. 50/245, Comprehensive Nuclear-Test-Ban Treaty (Sept. 17, 1996).

[17] Venturini, supra note 5, at 145.

[18] CTBTO Preparatory Commission, (last visited Mar. 9, 2017).

[19] Status of Signature and Ratification, CTBTO Preparatory Commission, (last visited Mar. 9, 2017).

[20] Venturini supra note 5, at 148. As a result, states such as the United States that have not ratified this treaty are nevertheless legally bound.

[21] See Joint Statement on the Comprehensive Nuclear Test-Ban Treaty by China, France, the Russian Federation, the United Kingdom, and the United States of America (Sept. 15, 2016), available at The Security Council also references this statement in S.C. Res. 2310 (Sept. 23, 2016).

[22] A proposal by Argentina, Ecuador, and Uruguay introducing a delay of not more than twelve months was rejected During the Vienna Conference. U.N. Conference on the Law of Treaties, 131, A/CONF.39/1 l/Add.2 (1971).

[23] The first of these conferences was held in Vienna in 1999 and the last one in New York, on Sept. 29, 2015.

[24] See, e.g., Daniel Rietiker, Some Thoughts on Article VI NPT and Its Customary Nature, Arms Control Law (June 10, 2014),

[25] See, for this kind of treaty, Vienna Convention on the Law of Treaties art. 60 § 2(c), May 23, 1969, 1155 UNTS 331, 8 ILM 679.

[26] George Bunn, The Status of Norms against Nuclear Testing, The Nonproliferation Review, Winter 1999, at 20.

[27] See Importance of CTBT Stressed at NPT Prepcom, CTBTO Preparatory Commission, (last visited Mar. 9, 2017).

[28] Venturini, supra note 5, at 151.

[29] Bunn, supra note 26, at 29.

[30] Id.

[31] See S.C. Res. 2270 (Mar. 2, 2016).

[32] S.C. Res. 2310 (Sept. 23, 2016).

[33] John Carlson, Nonproliferation Benefits of India Deal Remain Elusive, Arms Control Today (June 2015),