Over the weekend, the New York Times carried a rather disturbing report that the International Atomic Energy Agency [IAEA] thinks Iran has amassed the critical mass of knowledge necessary to build an atomic bomb;
Senior staff members of the United Nations nuclear agency have concluded in a confidential analysis that Iran has acquired “sufficient information to be able to design and produce a workable” atom bomb.
The IAEA report is hedged about, as reports from governmental agencies are wont to be, with cautions that the information is tentative and subject to further investigation and confirmation. The details that have surfaced so far, however, are troubling.
Readers will recall that the United States dropped two atomic bombs on Japan, at Hiroshima and Nagasaki, helping to bring about the end of World War II in the Pacific. What is not so well known is that the two bombs were of different designs, and used different fissile materials. The Hiroshima bomb, dubbed “Little Boy”, was a gun-type device using enriched uranium as the fissile material. The Nagasaki bomb, “Fat Man”, used plutonium in an implosion-type design.
A gun-type design is the more primitive of the two types. Essentially, two pieces of enriched uranium, each of less than critical mass, are placed at opposite ends of a tube (the “gun barrel”); typically, one piece will be shaped like a hollow cylinder that fits over the other piece like a sleeve. The two are rapidly rammed together by detonation of an explosive charge behind one of the uranium masses, resulting in a super-critical mass, a chain reaction, and an explosion. A successful bomb requires ~25 kg of highly-enriched uranium (80% U-235); this type of design generally is not usable with plutonium. This type of device tends to be inefficient, in terms of the energy produced for the amount of fissile material used.
An implosion-type bomb, such as Fat Man, is more difficult to design and build. It uses a hollow sphere of fissile material, usually plutonium, which is compressed to super-critical mass by the detonation of a surrounding layer of shaped-charge explosives. It is necessary to compress the plutonium as a sphere; if it becomes misshapen, some material is likely to be ejected, resulting in what is termed a “fizzle”, which may spread a quantity of radioactive (and extremely toxic) plutonium around, without producing a very large explosion.
Gun-type designs have usually been the principal concern as far as terrorism is concerned: they require less expertise to put together. A crude gun-type bomb, which could easily fit into a truck (the barrel of the Hiroshima device was ~2 meters long), might not be efficient, but might be attractive as a terrorist weapon. This is why some experts place such emphasis on securing available stockpiles of fissile material.
Countries with nuclear weapons programs universally use implosion-type designs, because of their greater efficiency, and because that efficiency allows smaller and lighter weapons to be built:
Implosion designs, compact by nature, are considered necessary for making nuclear warheads small and powerful enough to fit atop a missile.
Both the Times article, and a report in Wired magazine, suggest that the IAEA has evidence that Iran has been working on high-explosive technology, as a prelude to building implosion-type warheads that could be used on a missile.
Mastering the design of high-explosive lenses is crucial to the design of an implosion device like the Trinity gadget or the Nagasaki atomic bomb. In addition, the IAEA has also probed Iran’s interest in ballistic missile design. In the latest board report, the agency reiterated “the need to hold discussions with Iran on the engineering and modelling studies associated with the re-design of the payload chamber [for a new ballistic missile] … to exclude the possibility that they were for a nuclear payload.”
This does not sound good. A country run by religious lunatics is not one that should be allowed to have nuclear weapons.