The Iran nuclear deal, formally known as the Joint Comprehensive Plan of Action (JCPOA), which was signed in Vienna on July 14, 2015, will come up for its next ninety-day review by the White House next month. In view of the fact that President Trump has named as his new national security adviser John Bolton, who seems to be even more hawkishly opposed to the agreement than Trump himself, one worries about its future (at least as it involves this country). For this reason, I wish to try to review what will be lost if the JCPOA is not renewed; this will require me to elucidate some elementary nuclear weapons physics.

The first thing I would like to point out does not involve any physics. The first paragraph of the agreement says that “Iran reaffirms that under no circumstances will Iran ever seek, develop or acquire nuclear weapons.” Note the “ever.” There is no sunset clause here. Next, we will come to the body of the agreement—after a brief course.

All nuclear weapons involve fission. This is also true of the so-called hydrogen bomb, which gets much of its energy from nuclear fusion but is triggered by a fission primary. Thus, if one can keep a country from obtaining fission weapons, one has solved the problem of proliferation. Fission is the splitting of a heavy nucleus like that of uranium or plutonium into usually two nuclei, usually of substances somewhere in the middle of the periodic table. The first observed case was that of uranium, which split into barium and krypton. These nuclei have a smaller total mass and this mass difference is, according to the equation E=mc2, available as energy—for explosions, for example. Neutrons are also emitted, and these can cause further fissions and, under the right circumstances, a chain reaction.

To create a nuclear weapon, two things are required: a design and a sufficient amount of “fissile” material. It is my belief that the Iranians have the design. One limitation of the JCPOA is that there is no provision for verifying this. Unless the Iranians tell us, or unless they come up with a bomb, we don’t know. But there are two reasons why I think they have the design. The first is the likelihood that they bought a design for a Chinese device from the Pakistani metallurgist and nuclear proliferator A.Q. Khan. But the second reason is more important because it has larger implications.

I use Pakistan as an example. With fewer than six physicists and computer facilities that were inferior to what any high school student now has, the Pakistanis designed their nuclear weapons using open-source information. Iran has a very sophisticated nuclear program and I feel quite certain they have followed a similar path. This means, I believe, that if they ever had enough fissile material, they could produce a bomb in very short order. Therefore, the focus must be on restricting this material; to that end, I turn to the JCPOA.

There are two potential bomb fission materials: uranium 235 and plutonium 239. This is because these isotopes are “fissile.” Any isotope can be fissioned if it is bombarded by neutrons above a threshold energy, but fissile isotopes can be fissioned by neutrons of any energy. This is what makes a fission bomb work. For this reason, the entire issue boils down to the production of these isotopes. They require completely different production methods. Plutonium is produced in reactors; uranium is mined. I will deal with the plutonium first because it is in dealing with this issue that the JCPOA has been particularly successful.

All reactors produce plutonium as a byproduct. The fuel elements are made predominantly with uranium 238 and a small percentage of uranium 235, which is responsible for the fission. If uranium 238 absorbs a neutron, it can become uranium 239, which decays into neptunium 239, which in turn decays in the fissile isotope plutonium 239. It is possible to design a reactor to maximize this sequence—and this is what the Iranians did. The reactor was located in Arak, an industrial city about 160 miles south-west of Tehran. The Iranians never allowed consistent inspections of this installation, and they maintained the fiction that it was for producing medical isotopes—the kinds that are used in the treatment of cancer. The agreement is very clear about the disposition of this reactor:

Iran will redesign and rebuild a modernized heavy water research reactor in Arak, based on agreed conceptual design, using fuel enriched [with uranium 235] up to 3.67 percent, in a form of international partnership which will certify the final design. The reactor will support peaceful nuclear research and radioisotope production for medical and industrial purposes.

There is every reason to be certain that Iran has abided by this clause of the agreement, and it is one of the JCPOA’s triumphs—although it is also a bit of a sideshow.

This is because the real Iranian nuclear weapons program was based on uranium. The uranium that comes out of a mine is over 99 percent uranium 238, with a tiny admixture of uranium 235. The object of any weapons program is to enhance this smaller percentage to the point where it comprises at least 90 percent. This is achieved by processing that has been given the name “enrichment”—though I regard this as a poor choice since it seems to imply some sort of chemical process. In fact, enrichment is a mechanical process that takes advantage of the tiny difference in mass between the two isotopes. The method of choice is the centrifuge.

Iran’s centrifuge program is a triumph of its technology. The Iranians did buy and then reverse-engineer a fairly primitive prototype from Pakistan, but they have vastly improved upon it. Centrifuges work in “cascades,” which are intricately designed so that the product of one set is fed into the next. This is, to say the least, a non-trivial technology that the Iranians have mastered on their own. It involves the synchronization of thousands of centrifuges. What the agreement has to say about all this is quite complex and is spelled out in a twenty-nine-page annex to it. There are two important centrifuge enrichment locations Fordow and Natanz. Here is what the agreement says about the Fordow installation:

Iran will convert the Fordow facility into a nuclear, physics and technology centre. International collaboration including in the form of scientific joint partnerships will be established in agreed areas of research. 1044 IR-1 centrifuges in six cascades will remain in one wing at Fordow. Two of these cascades will spin without uranium and will be transitioned, including through appropriate infrastructure modification, for stable isotope production. The other four cascades with all associated infrastructure will remain idle. All other centrifuges and enrichment-related infrastructure will be removed and stored under IAEA [International Atomic Energy Agency] continuous monitoring…

The Natanz facility will be allowed to continue limited enrichment, up to 3.67 percent. The total amount of any enriched uranium the Iranians will be allowed to store at any one time is 300 kilograms. Any excess would have to be sold on the international market. The agreement specifies that this provision holds for fifteen years. There are stipulations covering the extent that the Iranians may try to improve the design of their centrifuges. The indications are that the Iranians have been taking advantage of what is permitted and have some vastly improved designs.

The IAEA has certified that—some minor violations aside—the Iranians have implemented the agreement. What happens if there is no agreement? The IAEA inspectors would leave the country and the program would restart at full bore. Most experts estimate that it would be a matter of months before the Iranians built their first bomb. The degree of chaos and danger this would introduce to the Middle East region and beyond is hard to overstate.

One cannot say with any certainty whether the countries that will remain in the agreement will be able to constrain the Iranians, who have made absolutely clear that they will not agree to any modification of it. The notion that President Trump has of somehow getting a “better deal” is delusional. There is no better deal. The Iranians have everything they need to make nuclear weapons—including uranium. The JCPOA is our best, and perhaps our only, chance of preventing Iran from getting the bomb.

An earlier version of this essay misstated that it is the mass of smaller nuclei, of barium and krypton, created by fission from uranium atoms that is available for explosive energy, according to the equation E=mc2; it is rather the difference in mass between the original uranium nucleus and the smaller nuclei of barium and krypton that is available as energy. The essay has been updated accordingly.