JIIA Strategic Comments (2026-5) Allegations of a Chinese Nuclear Test

Papers in the "JIIA Strategic Commentary Series" are prepared mainly by JIIA research fellows to provide comments and policy-oriented analyses of significant international affairs issues in a readily comprehensible and timely manner.

Introduction

The United States has contended that China conducted a nuclear explosion test on 22 June 2020, and China has countered by denouncing this claim as "groundless". While uncertainties surround this matter, the pertinent issues are outlined below from the perspective of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), based on currently available public information.

The US allegation

US Under Secretary of State for Arms Control and International Security Thomas DiNanno asserted at the 6 February Conference on Disarmament in Geneva that China had conducted nuclear explosive tests on 22 June 2020. These tests allegedly employed "decoupling" technology, a method designed to make seismic waves generated by the explosion harder to detect, and Under Secretary DiNanno cited this as an attempt to conceal purported low-yield nuclear tests¹  at the Lop Nur test site in western China.

On 17 February, US Assistant Secretary of State for Arms Control and Non-Proliferation Christopher Yeaw revealed at the Hudson Institute that "seismic signals measuring magnitude 2.75 were detected on 22 June 2020 near the Lop Nur nuclear test site in western China". He supplemented the initial claim by presenting analytical results of the detected seismic event as evidence² .

China has rejected these US accusations, stating: "We firmly oppose the US's false claims"; "The US's allegations of suspected nuclear explosions are entirely without basis"; and "The US is fabricating a pretext for resuming nuclear testing³" .

Statement by Executive Secretary Floyd

Dr Robert Floyd, Executive Secretary of the Provisional Technical Secretariat of the CTBT Organisation in Vienna, issued a statement dated 6 February regarding the series of US assertions, declaring: "The International Monitoring System (IMS) stations established under the CTBT did not detect any event consistent with the characteristics of a nuclear weapon test explosion on 22 June 2020, and subsequent detailed analysis has not changed this result" .

Dr Robert Floyd, Executive Secretary of the Provisional Technical Secretariat of the CTBT Organisation in Vienna, issued a statement dated 6 February regarding the series of US assertions, declaring: "The International Monitoring System (IMS) stations established under the CTBT did not detect any event consistent with the characteristics of a nuclear weapon test explosion on 22 June 2020, and subsequent detailed analysis has not changed this result" .

Analytical results of seismic waves

Although the CTBT IMS is capable of detecting explosions equivalent to or greater than 500 tonnes of TNT, the magnitude of the two seismic events was significantly smaller (described by Executive Secretary Floyd as being in the hundreds of tonnes, and analysed by the Norwegian institute NORSAR as being in the range of 3 to 15 tonnes). NORSAR pointed out that, as these events had only been detected by a seismic station in Kazakhstan, it was not possible to identify the exact location of the events. Furthermore, NORSAR determined that the characteristics of the two seismic events were not specific to explosions and did not rule out the possibility that they were small natural earthquakes . This is consistent with the statement made by Executive Secretary Floyd on 17 February.

It should be noted that, under the CTBT, the final judgement of the nature of any event (for example, whether it was a man-made explosion or a natural earthquake) rests with the State Parties (Part I, Paragraph 18, of the Protocol), and there is no provision for calculating the yield of detonations.

Had the CTBT been in force

It is worth noting here that, in his statement on 6 February, Executive Secretary Floyd indicated that the CTBT has mechanisms to address smaller-scale nuclear explosions (but these would only be operational after the CTBT entered into force). Although not explicitly named, these mechanisms seem to refer to the "consultation and clarification" provided in Paragraphs 29 to 33 of Article IV, the "on-site inspections" referred to in Paragraphs 34 to 67 of Article IV, and the "confidence-building measures" mentioned in Paragraph 68 of Article IV. These are stipulated as part of the CTBT's verification regime in addition to the IMS.

In cases such as the suspected nuclear explosion in question, consultation and clarification between the US and China, between the US and the CTBT Organization, or through the CTBT Organization would likely elucidate any concerns and seek to resolve them. Alternatively, the US might request an on-site inspection if there were substantial grounds to believe a nuclear explosion had occurred. Should an on-site inspection be requested, an international inspection team convened following the necessary procedures would then proceed to collect evidence of a nuclear explosion in the vicinity of the epicentre of the detected seismic event. As a confidence-building measure, China might also offer pre-notification of any intended explosions exceeding 300 tonnes of TNT equivalent, thereby potentially preventing suspicions such as those arising in this instance. In any case, to reiterate, consultation and clarification, on-site inspections and confidence-building measures will not be invoked until the CTBT’s entry into force.

Implications of the US’s claims

It remains unclear why Executive Secretary Floyd initially stated that no CTBT IMS monitoring stations had detected events indicative of an explosion, and then announced over ten days later that two events had been detected – albeit with the cause of the events remaining unknown – and that their locations had been specified. Considering that Dr Floyd issued his latter statement on the same day that the United States raised suspicions of nuclear testing by China, some form of communication may have taken place between the US and the CTBTO Provisional Technical Secretariat.

With regard to the seismic events asserted by the United States this time, there has of yet been no announcement of the detection of radionuclides that would constitute direct evidence of nuclear explosion testing. It is thus not entirely clear why the US feels confident in asserting that nuclear testing using decoupling technology was conducted, based solely on the detection of seismic events originating near the Lop Nur nuclear test site.

It is worth recalling that President Donald Trump on 30 October 2025 "instructed the Department of War to commence nuclear weapons testing on an equal basis with other countries" . China claims that the US assertions are merely a pretext for resuming nuclear testing but, at this stage, the connection between the US’s assertions and its resumption of nuclear testing is unclear. Were the US to resume nuclear explosion testing, the moratorium on nuclear testing (the voluntary suspension of nuclear explosion tests) that has been maintained among nuclear weapon states could collapse. This is evident from the fact that, immediately after President Trump's instruction to resume nuclear testing, Russian Defence Minister Andrey Belousov proposed that preparations for nuclear testing should proceed and stated that nuclear testing could be carried out at short notice at the Novaya Zemlya nuclear test site in the Arctic Ocean . Heretofore it has been expected that the entry into force of the CTBT would trigger a domino effect whereby one non-ratifying state after another would ratify the treaty if the US did so. Depending on future developments, however, there is a concerning possibility that the opposite could happen, leading to a cascade of nuclear testing.

This analysis presented is that of the author and does not represent the official views of any associated institution.

1. “Statement to the Conference on Disarmament”, U.S. Department of State, 6 February 2026, https://www.state.gov/releases/under-secretary-for-arms-control-and-international-security-affairs/2026/02/statement-to-the-conference-on-disarmament.
2. 「米高官 “2020年 中国が爆発伴う核実験実施”と改めて主張 [US Official Reiterates Claim that China Conducted Explosive Nuclear Test in 2020]」, NHK, 19 February 2026, https://news.web.nhk/newsweb/na/na-k10015055601000.
3. 「6年前の『中国の秘密核実験』証拠浮上…米『われわれも再開する』 [Evidence Emerges of 'China's Secret Nuclear Test' Six Years Ago... US 'We Will Resume Ours Too]」,中央日報 [The Chosun Ilbo], 19 February 2026, https://japanese.joins.com/JArticle/345022, https://japanese.joins.com/JArticle/345023.
4.  “Statement by Robert Floyd, Executive Secretary of the CTBTO”, CTBTO Preparatory Commission, 6 February 2026, https://www.ctbto.org/resources/for-the-media/press-releases/statement-robert-floyd-executive-secretary-ctbto-2026-02-06.
5.  “Statement by Robert Floyd, Executive Secretary of the CTBTO”, CTBTO Preparatory Commission, 17 February 2026, https://www.ctbto.org/resources/for-the-media/press-releases/statement-robert-floyd-executive-secretary-ctbto-2026-02-17.
6.  “USA publicly alleged that China conducted a low-yield nuclear test”, NORSAR, 19 February 2026, https://www.norsar.no/nyheter/usa-publicly-alleged-that-china-conducted-a-low-yield-nuclear-test.
7.  Donald J. Trump@realDonaldTrump, Truth, 30 October 2025, https://truthsocial.com/@realDonaldTrump/posts/115460423936412555.
8. 「核実験再開？米ロ核戦力強化へ [Nuclear tests to resume? US and Russia strengthen nuclear forces]」, NHK, 14 November 2025, https://news.web.nhk/newsweb/na/na-k10014976661000.