It must be obvious to everyone now that Iran is attempting to produce a nuclear weapon. The claim of peaceful pursuit of nuclear power is rendered implausible by the concentrated efforts to increase the enrichment of uranium — far beyond what might be required for nuclear fuel. In fact, Iran probably has enough medium-enriched uranium to make a crude bomb already, even though it has not developed the technology to produce technically “weapons-grade” uranium.
I used to think that plutonium presented the biggest risk of nuclear weapons proliferation, because there’s so much of it lying about. Plutonium doesn’t exist naturally, but it’s a by-product of the nuclear power industry. Some reactors are designed to “burn” plutonium, but for the most part it’s just stockpiled. The UK alone has over 100 tonnes, or 17,000 times the amount that went into the atomic bomb dropped on Nagasaki.
The Manhattan Project — the Allied programme to develop nuclear weapons in the Second World War — began working on three separate atomic bomb designs. In the end, one was abandoned during development, and the other two were used in the Trinity test in New Mexico and the attacks on Hiroshima and Nagasaki in Japan.
The two working designs were code-named “Fat Man” and “Little Boy”. The former was the type detonated in Trinity and then dropped on Nagasaki. It was the more complex bomb, consisting of a spherical arrangement of plutonium which was “imploded” inwards by explosives. “Little Boy”, on the other hand, was a so-called “gun” bomb, where two pieces of nuclear material, this time uranium, were shot together like a bullet hitting a target. The design was judged to be so simple that a test was unnecessary: the device was dropped immediately on Hiroshima.
The one which didn’t get made — it was called “Thin Man” — was also a gun design, but a plutonium gun. The Manhattan Project found that plutonium for bombs was in much better supply than uranium, even though plutonium is an artificial element, not found in nature. Plutonium was created by irradiating natural uranium, and then separating out the plutonium product by chemical means (different elements react differently). To make weapons-grade uranium, however, uranium-235 had to be separated out from uranium-238. These are the same element, differing only in nuclear properties, and can’t be separated by chemistry. Separation was very difficult with the technology of the time.
However, calculations showed that plutonium was too reactive to be used in a gun-type bomb, unless the “gun barrel” was made impractically long. The basis for all atomic bombs is assembling a critical mass of nuclear material, in which a rapid chain reaction takes place, releasing a large amount of energy. However, a chain reaction can begin before the critical mass is properly squeezed together, in which case it will blow the nuclear material apart, terminating the reaction. In the case of the plutonium gun, the scientists couldn’t work out how to get the two sub-critical pieces of plutonium to contact quickly enough in a weapon which could feasibly be put on a bomber aircraft.
The Fat Man design separated the plutonium into more, smaller pieces, so that the chance of a premature chain reaction was avoided. However, the engineering of the imploding explosive device was very precise and complicated, requiring major industrial resources and the best work of some exceptional scientists and engineers.
The difficulty of making an implosion bomb makes it a challenge, even given the resources of a state government, and it is likely that in most cases the simpler, gun-type design will be chosen. Since a plutonium gun is also impractical, enriched, weapons-grade uranium must be acquired. For uranium enriched to 85% or more of uranium-235, you need about 60kg (preferably in several pieces) for a critical mass for a simple weapon. (The comparable figure for plutonium is just 11kg, and lots of plutonium is stockpiled, since it’s a by-product of nuclear power generation). But if you have a lot of uranium, you can make a bomb with material enriched to only 20%.
According to the International Atomic Energy Authority, Iran is researching multi-point explosive capabilities, the technology for an implosion plutonium bomb. However, the scale of their uranium enrichment programme is evidence that they are pursuing the simpler uranium gun bomb first. This would enable them to detonate a static nuclear “test”, long before they had the ability to make a weapon.
Yesterday, a resolution was introduced jointly by China, France, Germany, Russia, the United States, and the United Kingdom to the IAEA Board of Governors. It criticized Iran for defying UN Security Council resolutions to suspend uranium enrichment, and called on Iran to allow inspections of evidence that it is pursuing weapons technology. The resolution was passed, with 31 votes for, 1 against and 3 abstentions. In light of previous events, it seems unlikely that Iran will comply.