Some in the world are engaged by the charm offensive of Iranian President Hassan Rouhani, and hearing his protestations about having no intention of ever creating a nuclear weapon. But in fact, Iran has been developing its warhead for some 15 years. That design is now near completion.
Compare it to gunpowder. To use gunpowder, you stuff it into a bullet, load it into a rifle and then find a marksman. Iran has nearly mastered all those steps but in nuclear terms.
Four technological achievements are key to completing Tehran’s nuclear weapon: 1) accretion of enough nuclear materials, highly enriched to weapon’s grade, that is about 90 percent; 2) machining that material into metal for a spheroid warhead so it can fit into a missile nose cone for detonation; 3) a trigger mechanism to initiate the atomic explosion at the precise moment of missile re-entry; and, of course, 4) a reliable rocket delivery system to carry such a weapon.
Start with the nuclear material. Experts estimate that a single bomb would require approximately 25 kilograms of highly enriched uranium, or HEU, that has been boosted to concentrations of at least 90 percent. Much of Iran’s nuclear enrichment remains at 3.5 and 20 percent levels. But the numbers are deceiving. Enriching to 3.5 percent is 75 percent of the task of reaching weapons-grade. Once Iran has reached 20 percent, it has gone 90 percent of the distance. Today, Iran possesses enough nuclear material for a fast “breakout” that would finish the job, creating enough for five or 10 bombs, in about six weeks.
Second, that HEU must be metalized and shaped into a dense spheroid compact enough to fit into a missile nose cone. Iran has mastered the metallurgy using other high-density metals such as tungsten, which has been detonated in a special chamber to measure explosive character.
Third, the spheroid must be detonated. Iran’s warhead design employs a R265 shock generator hemisphere drilled with 5 mm boreholes filled with the explosive compound PETN. When triggered with precision, the PETN array can cause a massive synchronized implosion. That will fire an internal exploding bridgewire, which will in turn actuate an embedded neutron initiator to detonate the atomic reaction — and the mushroom cloud. This sequence of devices has been assembled and tested. Iran has more than 500 exploding bridgewires on hand.
Fourth, the warhead must be delivered. The Shahab-3 missile nose cone is large enough to accommodate the warhead. The outer radius of the R265 shock generator-wrapped warhead is 550 millimeters, less than the estimated payload chamber diameter of about 600 millimeters. Most of all, the Iranian military has selected the Shabab-3 not only because it possesses a range of 1,200 kilometers, but because it can be detonated in an airburst some 600 meters off the ground on re-entry. The height of 600 meters was used in the Nagasaki explosion. Such a bomb cannot be crashed into the ground, but detonated while still airborne. Iran has a small fleet of Shahab-3 missiles.
Hence, Iran’s metronomic accretion of enriched nuclear material is not just an ambiguous physics undertaking that should not worry the West. It is part and parcel of a nuclear plan that the international community must be determined to address.
Edwin Black is the author of the award-winning IBM and the Holocaust, and the forthcoming book Financing the Flames.