What US officials are alleging

Earlier this month, Undersecretary of State Thomas DiNanno said that the U.S. believes China carried out “yield-producing” nuclear explosive tests, including one on June 22, 2020, at the Lop Nur test site in northwestern China.

Last week, Assistant Secretary Christopher Yeaw gave more details. He pointed to a seismic signal at 09:18 UTC, arguing it looked more like an explosion than an earthquake or mining activity. U.S. officials also accused China of using “decoupling,” which is a technique to try to hide the seismic signature of a test.

The administration has also made similar accusations against Russia, but the charges against China are more specific, naming a date, place, and method.

The timing here matters. These allegations emerged the same week New START expired, leaving the U.S. and Russia with no binding limits on strategic weapons for the first time in decades. Secretary of State Marco Rubio has argued that bilateral U.S.-Russia frameworks are now “obsolete” and that China must be included in any future agreement, despite the fact that China’s arsenal is much smaller than Russian and American stockpiles. Pointing the finger at China for secretly testing conveniently supports that narrative and gives the U.S. cover to walk away from its own restraint.

U.S. intelligence reportedly has assessed that this alleged test is part of a broader Chinese campaign to develop a new generation of nuclear weapons, including low-yield tactical nuclear weapons.

What the experts say

The administration’s charge clashes with what the world’s central technical authority for detecting nuclear explosive tests says.

CTBTO Executive Secretary Robert Floyd explained that for the date in question, the organization’s monitoring system “did not detect any event consistent with the characteristics of a nuclear weapon test explosion at that time,” and that subsequent detailed analysis had not changed that conclusion.

CTBTO sensors picked up two small seismic events 12 seconds apart at the specified time and date, but they were way too weak to count as likely nuclear explosions (the system’s threshold is about 500 tons of TNT). With the available data, the CTBTO said it can’t confidently say what caused the events.

Floyd pointed to what is missing: the CTBT's strongest verification tools, including short-notice on-site inspections, can only be activated once the treaty enters into force. The U.S. has signed the CTBT but never ratified it. Neither has China. Russia signed and ratified it but later withdrew.

Norway’s NORSAR, an independent seismic monitoring institution, also checked the data and reached a similar conclusion. It found a small event at the same time, but said it “cannot confirm or refute a nuclear test” from the available seismic data, and that the event could have been a small natural earthquake.

In short, we’re on the verge of restarting nuclear testing, even though the public technical record can’t settle the question, and the verification tools aren’t available because the treaty that created them isn’t in force.

What the US should do instead

If Washington really has a strong case that China tested a nuclear weapon, it should rally international support to hold Beijing accountable. If the evidence isn’t solid enough to share, it’s not solid enough to break decades of nuclear testing restraint.

There are clear steps forward. The U.S., China, and Russia should ratify the CTBT so that the world can actually use the verification tools designed for situations just like this.

With the Non-Proliferation Treaty Review Conference coming up in April, Washington could push for a political declaration by the five major nuclear-armed states (the U.S., Russia, China, France, and the UK), reaffirming the testing moratorium. U.S. officials are already meeting with delegations from these countries for nuclear arms control talks. Reaffirming the test ban would give the U.S. the moral authority to demand openness from others.