The New York Times | Jeremy Bernstein: On Oct. 23, 1966, China conducted a nuclear test with a device that was flown on a Dongfeng missile. It produced a yield of 12 kilotons of equivalent TNT � a little less than what destroyed Hiroshima. Like the Hiroshima bomb, its fissile material was highly enriched uranium. However, its design was different. It used implosion to assemble the critical mass by compressing the sphere that was made of the fissile material.


This became the universal design because of the speed at which a critical explosive mass can be assembled. The Hiroshima bomb developed a critical mass more primitively by shooting one subcritical piece into another using chemical explosives. The Chinese use of implosion employed the same technique used in the plutonium bomb dropped on Nagasaki. The precise design of this Chinese weapon is known because in late 2001 or early 2002, it was sold by the Pakistani proliferator A. Q. Khan to the Libyans, who subsequently turned it over to the C.I.A. The question is: Did Mr. Khan also sell it to Iran?


The likelihood is that he did. And that is a powerful reason why President Trump must not back out of the nuclear deal with Iran when he is required to recertify the deal in October, as he has strongly hinted that he intends. If the deal is scrapped, Iran, nuclear design in hand, will be able to produce its first nuclear weapon in just months.


This will be possible because Iran�s infrastructure for bomb building is already in place. In 1989, Mr. Khan began shipping centrifuge components to Iran. These were so called P1 centrifuges � an early Pakistan design. Later, when these centrifuges were inspected by the International Atomic Energy Agency, they were found to contain traces of highly enriched uranium, which must have come from the Pakistani program.


There is a limited amount of enrichment that a single centrifuge can produce, no matter how long it spins its charge of uranium hexafluoride gas. But when 164 centrifuges are arranged in cascades, with one stage of enrichment feeding into the next, enough uranium can be enriched to produce an atomic bomb. The Iranians, reverse engineering the technology they bought from Mr. Khan, not only manufactured their own centrifuges, but also mastered the cascade technology.


But did Mr. Khan sell them the bomb design itself? It seems highly likely that he did, though it has never been confirmed. By 1989 Mr. Khan was running what was essentially a supermarket for nuclear technology. It was a moneymaking operation, done certainly with the knowledge of the Pakistan government. (When in the mid-1990s Mr. Khan began trading with the North Koreans, providing centrifuges for rockets, the goods were carried in Pakistani military aircraft.) He was trying to sell anything he could to anyone willing to buy, pocketing the money. So it is likely that from 1989 on, the Iranians had this design.


Among other things, the Joint Agreement with Iran, the seven-party nuclear deal, forbids designing, developing, fabricating, acquiring or using detonation systems suitable for a bomb, unless it is for non-nuclear purposes and subject to monitoring.


This provision is enforceable, but Iran will not allow I.A.E.A. inspection of the sensitive military complexes where such systems might be developed, such as at Parchin, a military base near Tehran suitable for testing high explosives. There is a suspicion that the Iranians did research on implosion there. The Trump administration argues that this refusal is clear evidence that the nuclear pact is useless and should be scrapped. My conclusion is just the opposite.


If indeed, as seems likely, Iran already has the design for a nuclear weapon, it need only amass enough fissile material to complete the bomb. This is often described as the �breakout time.� Roughly speaking, a bomb needs about 28 kilograms of 90 percent enriched uranium, or about eight kilograms of plutonium. The Iranians are allowed to enrich only a total of 300 kilograms of low-enriched uranium, while you would need about 3,000 kilograms to produce enough highly enriched uranium to make a bomb. The Iranians also tried to produce plutonium in a reactor designed for that purpose near the city of Arak. Under the conditions of the nuclear pact this reactor is being completely reconfigured so it cannot be used for producing plutonium. This is one of the successes of the treaty.


If the deal is scrapped, the Iranians could resume their centrifuge enrichment program, which is now being carefully monitored. Iran�s stock of highly enriched uranium could then grow, possibly within a few months, to provide enough for a weapon.


If the Trump administration does not recertify the agreement when it comes around in just a few weeks, the rest of the signers may attempt to maintain the treaty. But without the United States, the Iranians have suggested that they will consider the provisions no longer binding. Shouldn�t the dire situation in North Korea be enough of a headache already that we do not want to deal with another nuclear armed country? One can only hope that Mr. Trump sees it that way.