Remarks by Daryl Kimball, Executive Director, October 9, 2026

For a briefing organized by the Federal Ministry for European and International Affairs of the Republic of Austria and the Princeton University Program on Science and Global Security on the margins of the 1st Committee of the 81st General Assembly

The CTBT is one of the most successful nuclear nonproliferation agreements in the history of the nuclear age. It brought an end to a dangerous, deadly era that involved more than 2,000 nuclear explosions, more than half of which were conducted by the United States.

This year marks the 30th anniversary of the conclusion of the treaty.

Today, the treaty has 188 signatures and near-universal support; no state is openly violating the treaty, which bans any and all nuclear explosions, which the five nuclear-armed members of the NPT (the N5) all understand to be any nuclear test that which produce a self-sustaining fission chain reaction.

Today, the treaty's International Monitoring System (IDC) and International Data Center (IDC) are operating provisionally, but they are operating and are more effective than envisioned by the treaty's negotiators.

But the de facto global nuclear test moratorium and the CTBT are facing unprecedented new challenges.

Not only have nine key states failed to ratify the treaty, which has held up its formal entry into force, but there are new U.S. accusations that China and Russia have engaged in very low-yield testing. Those accusations have not been independently confirmed.

Nevertheless, in response, President Donald Trump has threatened to resume U.S. nuclear testing "on an equal basis" for the first time since 1992.

Even if China might have conducted a nuclear test explosion as alleged in 2020, two nuclear wrongs don't make a right. A resumption of nuclear testing by any state, especially the United States, would be a strategic disaster.

If Trump were to try to do so and could overcome the substantial U.S. public and Congressional opposition to renewed testing, it would likely set off a chain reaction of nuclear test explosions by others and blow a massive hole in the fabric of the already tattered nonproliferation system.

Rather than open the door to a new era of nuclear testing, the U.S. and the other NPT nuclear-armed states, with some help from nonnuclear weapons states, should try to work together to:

• reaffirm their support for the CTBT and its entry into force (EIF), and pending EIF,
• develop and implement new technical measures to ensure no state is conducting clandestine low-yield nuclear explosions pending the formal entry into force of the treaty.

Allow me to elaborate a bit further on some key issues.

What Are the Challenges to Detecting and Deterring Low-Yield Nuclear Test Explosions?

The CTBTO’s existing International Monitoring System (IMS) provides powerful nuclear test monitoring capabilities, but because entry into force has been delayed because the United States and eight other states have failed to ratify the Treaty, we don’t have the option of short-notice, on-site inspections to help detect and deter illicit nuclear testing.

As a result, it is possible that very low-yield nuclear explosions could potentially escape detection, or if there is a suspicious, “low-yield” event, it may be impossible for the International Monitoring System to determine with confidence whether suspicious seismic events were due to a nuclear explosion, a chemical high explosion or earthquake activity.

This is the situation we are now in. Earlier this year, the United States alleged that China of conducted a very low-yield nuclear test explosion at its former nuclear test site at Lop Nur in June of 2020.

Despite the assertions of the United States, the available data from the CTBTO’s monitoring system and other civilian seismic networks about the seismic event on that date are inconclusive.

This is why it is so important for nuclear armed states to engage in technical discussions to develop voluntary confidence building measures to ensure ongoing activities at their former test sites are fully compliant with their obligations under the CTBT.

This is not a new idea. In fact, this is an idea that civil society proposed as far back as the third CTBT Article XIV Conference on Facilitating Entry Into Force in 2005. The Arms Control Association and other civil society experts recommended that the “nuclear weapon states should implement confidence-building … measures at their sites” to ensure they are not currently engaged in prohibited activities. At the time, the suggestion was dismissed by some states as premature.

Then in 2023, the head of the U.S. National Nuclear Security Administration, Jill Hruby proposed a commonsense variation on the same concept. She suggested the United States, Russia, and China could work together and with others "to develop a regime that would allow reciprocal observation with radiation detection equipment at each other’s subcritical experiments to allow confirmation that the experiment was consistent with the CTBT. We have several well-considered technical ideas on how this could be effective.”

Then a May 2026 P3 statement (issued by France) picked up on the concept. In their statement, France, the UK, and the United States called for: "multilateral strategic stability talks to promote mutual transparency and adopt concrete risk reduction measures, such as establishing a ballistic missile and space launch notification mechanism or pursuing confidence-building measures regarding nuclear explosive test monitoring, including increasing the ability to detect tests of any yield towards ensuring nuclear weapons states adhere to a zero-yield standard.”

The P5 group reportedly met this week in New York, and yesterday U.S. A/S Christopher Yeaw offered his ideas on the subject of "Nuclear Testing" at a U.S.-sponsored side event in this room.

In our view, the Nuclear Five should focus their energy on addressing practical issues and solutions though serious technical talks:

• To deal with the possibility of very low-yield hydronuclear tests, confidence building measures could involve installing instruments at experimental facilities to ensure that no subcritical experiment produced a self-sustaining nuclear chain reaction by measuring neutron output.
• To deal with the possibility of multi-ton yield nuclear explosions, deployment of additional monitoring stations on a voluntary basis, operated by an independent entity or by the CTBTO, on or nearer to former nuclear test ranges in Russia, China, and the United States could augment the CTBTO's system and effectively lower the threshold for detection.

What is Considered Prohibited and Permitted Under the CTBT?

As we think about ways to better detect and deter clandestine nuclear test explosions, it is important to understand what the CTBT permits and what is prohibited.

The scope of the CTBT and the unilateral moratoria announced by former nuclear testing states after the CTBT was opened for signature in 1996 is well-established. Article I prohibition on any and all nuclear explosions, which according to an official U.S. State Department Fact Sheet, are "nuclear tests that produce a self-sustaining, supercritical chain reaction of nuclear energy."¹

As China's lead CTBT negotiator, Amb. Sha Zukang at the Conference on Disarmament said on March 28, 1996:

“China is committed to concluding a CTBT which prohibits any nuclear-weapon test explosion at any place and in any environment. Proceeding from this guiding principle, the Chinese delegation proposed at the outset of the negotiations its scope text prohibiting any nuclear-weapon test explosion which releases nuclear energy.

The phrase ‘release of nuclear energy’ was intended to distinguish the scope of the CTBT from that of the PTBT and TTBT and to define the scope of the CTBT with more precise and scientific language. The Chinese delegation has always held that the scope of the CTBT should exclude any threshold.

After two years of negotiations, most countries have reached a common understanding on the phrase “any nuclear-weapon test explosion” in the scope article. That is to say, the future CTBT will without any threshold prohibit any nuclear-weapon test explosion.”

According to a related U.S. State Department fact sheet, all of the other N5 delegations made statements that align with this explanation of the scope of the CTBT, which the basis of the nuclear testing moratoria they each now observe.²

How Real Are the U.S. Threats to Resume Nuclear Testing?

President Trump’s vague directive from October 30, 2025, to resume U.S. nuclear explosive testing “on an equal basis” appears to be all about political retribution and his belief that some other nuclear-armed state has conducted a nuclear test in violation of a treaty that his administration does not support. Since his threat was issued, the State Department accused China of conducting a clandestine nuclear test explosion in 2020. Since Trump’s threat was issued, however, the White House and the NNSA have not been able or willing to clarify what kind of nuclear weapons tests Trump is talking about.

Since 1992 when Congress mandated that the United States halt nuclear testing, the U.S. nuclear weapons labs have confirmed year after year that there is no technical or military reason for renewed U.S. nuclear test explosions. More recently and since Trump made his nuclear testing threat, the heads of the National Nuclear Security Administration and U.S. Strategic Command have reaffirmed that there is no technical or military basis to restart nuclear explosive testing.

Nevertheless, an internal NNSA planning document from February indicates that the agency plans to “Execute the President’s directive with respect to the testing of the U.S. nuclear deterrent” before the end of 2028.

What are their options? The U.S. Department of Energy and the U.S. nuclear laboratory directors have reported that "a limited nuclear test explosion to signal the readiness of the U.S. nuclear deterrent or to respond to another Nation’s test could be conducted in 6 to 10 months.”

A fully instrumented test to address a potential problem in the stockpile (i.e. one that incorporates experiments to derive information about weapons performance and/or effects) would take longer, perhaps up to 24-36 months. Tests to develop a new nuclear weapons capability might take up to 60 months. 

The 1974 Threshold Test Ban Treaty requires a 200‐day notification of a test.

The likely site of any such test would be the former nuclear test site in Nevada, now known as the Nevada National Security Site, where weapons scientists perform experiments at the Principal Underground Laboratory for Subcritical Experimentation (PULSE) facility.

If the Trump administration chose to conduct a very low-yield hydronuclear test explosion, the PULSE facility could be a candidate site for such a test.

My assessment is that Trump's threat to resume U.S. nuclear explosive testing is real, but the administration's ability to do so is limited due to opposition at home and around the globe.

What Can Be Achieved Technically Through Very Low-Yield Nuclear Tests?

In 2002, the U.S. National Academy of Sciences (NAS) reviewed what information states with various levels of prior nuclear weapon testing experience might plausibly gain regarding nuclear weapon design and performance from tests at different explosive yields, including low supercritical yields. The table below is reproduced from that report.³

Purposes and Plausible Achievements for Testing at Various Yields

The NAS analysis suggests that very-low yield nuclear test explosions (i.e. <10 tons TNT equivalent) and even lower yield hydronuclear test explosions provide no or very limited value for new nuclear warhead design, especially for states with significant nuclear testing experience like the United States, Russia, and China.

Bottom Lines

Even if there are serious concerns about China possibly conducting a prohibited nuclear explosive test on June 22, 2020, U.S. threats to resume testing are counterproductive and naive response. Two nuclear wrongs do not make a right.

I would add that public accusations of Chinese treaty violations are not conducive to achieving desired and necessary results, nor do factually challenged criticisms of the CTBT itself.

Instead, states should seek to clarify and resolve compliance concerns in a professional manner in accordance with Article VI.C of the CTBT, which provides that state parties “should, whenever possible, make every effort to clarify and resolve, among themselves or with or through the Organization, any matter which may cause concern about possible non-compliance with the basic obligations of this Treaty.”

To conclude, we recommend that the five major nuclear-armed states should:

• formally recommit to the CTBT and its entry into force,
• support full funding to facilitate the effective operation of the IMS and the IDC, the CTBTO's global monitoring system to help detect and deter cheating, and
• work together to augment existing verification and monitoring tools by engaging in serious technical talks on new, voluntary, reciprocal confidence-building measures to ensure no state is conducting clandestine nuclear explosions as prohibited by the CTBT.

Thanks for your attention.

 1. "Scope of the Comprehensive Nuclear Test-Ban Treaty," Fact Sheet, Bureau of Arms Control, Verification, and Compliance, https://2009-2017.state.gov/t/avc/rls/212166.htm 

2. Key P-5 Public Statements on CTBT Scope," Fact Sheet, Bureau of Arms Control, Verification, and Compliance, https://2009-2017.state.gov/t/avc/rls/173945.htm

3. National Academy of Sciences, Technical Issues Related to the Comprehensive Nuclear Test Ban Treaty, 2002, https://www.nap.edu/catalog/10471/technical-issues-related-to-the-comprehensive-nuclear-test-ban-treaty