An expert examines radionuclide station RN20 in Beijing, China. CTBTO Preparatory Commission
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Beneath the oceans, on distant islands, in barren deserts, on icy hillsides, and at hundreds of other spots around the globe, special sensors are sniffing the air, measuring ground motion, watching for a particular kind of light, and listening for unique sounds. Their function is to pick up the telltale sign of a nuclear explosion, and according to a scientific report released in Washington on March 30, they can now do it very well.

The sensors, deployed at more than 260 sites under the supervision of an international organization based in Vienna, are singly or collectively able to discern the distinctive traits of such blasts anywhere in the world, down to a level of explosive force “well below” the equivalent of 1000 tons of TNT, or a fraction of the force of the bomb that destroyed Hiroshima, a panel of the National Research Council told the White House in its report.

U.S.-owned intelligence gear deployed around the globe and on satellites can do even better, the report said, without disclosing how much. Its overall message was that if the United States decides to join a global treaty banning nuclear tests — a goal professed by many U.S. officials since the treaty was completed in 1996 — it would not have to worry about militarily-significant, undetected cheating by others.

That conclusion is welcome news to the Obama administration, which has endorsed the treaty but opted not to press for its ratification this year because of Republican roadblocks. Acting Under Secretary of State Rose Gottemoeller has repeatedly described this year as a good moment for skeptical, conservative lawmakers to learn more about the treaty, and the new, 204-page report by one of the country’s most respected scientific panels is meant to be their basic textbook.

The report — an unclassified version of a restricted study principally conducted by nine experts with government clearances — was conceived as an update of a similar analysis by the council in 2002, and its presentation at the National Academy of Sciences offices in Washington was uniformly upbeat. Among the authors is a former head of one of the U.S. nuclear weapon laboratories, a former head of the U.S. Strategic Command, and a former administrator of the National Nuclear Security Administration under President George W. Bush.

Due to scientific advances, the ability to detect nuclear blasts “is now actually better” than the council predicted it would be at this point, said the chairwoman, Ellen D. Williams, a specialist in nanoscience who now oversees scientific work at BP. And while the science of detection has been advancing by leaps and bounds — utilizing improved computational power and radioactive xenon noble-gas monitoring techniques, among other things — the science of hiding or muffling nuclear tests in caves and large holes has essentially been static since the late 1950’s, according to the panel.

The notion of trying to conduct a hidden nuclear blast is not as far-fetched as it might seem. Israel and South Africa collaborated on such a blast, over a remote portion of the South Atlantic Ocean, in 1979, according to some U.S. experts. Neither country has ever admitted it.

But the NRC panel said experienced nuclear powers such as China or Russia would gain nothing of value by conducting extremely small, hidden tests, while other states would “face serious costs, practical difficulties … and uncertainties” in trying to conceal such tests today. In any event, such achievements “would not require the United States to return to testing,” because the United States has or could readily produce more capable nuclear weapons based on existing designs and its long experience, the report said.

It added a cautionary note: Terrorists or others could potentially assemble a working bomb — without ever testing it — using fissile materials and design knowledge they grab from others. But the answer to that would also not be a return to U.S. testing, the panel said.

The international monitoring group, formally known as the Comprehensive Test Ban Treaty Organization, gets a hefty portion of it funding from Washington, even though the United States is not a treaty partner. But it runs on a total of $100 million, or roughly the cost of a single U.S. F15E fighter plane. The panel stressed that even though its capabilities cannot match those of the U.S. government, the evidence it picks up can be shared with others — unlike what the U.S.-owned systems glean. It has “a competent and dedicated staff that is operating well,” the panel said.

Imagine that.

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R. Jeffrey Smith worked for 25 years in a series of key reporting and editorial roles at The Washington...