Building a nuclear weapon is a long, complex and dangerous task that requires lots of fissile material, advanced knowledge, specialised laboratories and a deep treasure chest.
A nuclear bomb works when a sufficient quantity of uranium or plutonium isotopes is brought together, causing neutrons to collide with atoms and break them apart, releasing more neutrons that then whack into other atomic nuclei and so on, in a vast, sudden release of energy.
But transforming the theory of chain reaction into reality is difficult and expensive.
”Nuclear weapons are not simple devices, contrary to frequent assertions that one could be built in a garage or basement,” says the Bulletin of the Atomic Scientists, published by a US anti-proliferation group, the Educational Foundation for Nuclear Science.
Problem No. 1: getting enough highly enriched uranium-235, either by stealing it or buying it from a renegade state, or enriching it from commercial uranium using gaseous diffusion or gas centrifuge –techniques that are time-consuming, costly and risky. An alternative isotope is plutonium-239, a man-made element that is derived by changing the atomic structure of uranium-238 in a nuclear reactor.
But plutonium is viciously toxic and many thousands of times more radioactive than uranium, and thus would pose a serious threat to the backroom bomber who wants to handle and shape it.
The basic nuclear weapon is the Hiroshima bomb, Little Boy: a ”gun-type” device in which two masses of uranium are placed into a tube and fired into each other, achieving critical mass, the microsecond when the chain reaction is triggered.
Such weapons are considered reliable but are large and unwieldy, which makes it a problem for delivering them, and from the physics point of view, they are not terribly efficient.
They also require lots of uranium: about 55 kilograms, or enough to fill about eight soft-drinks cans.
A more advanced design is the so-called implosion weapon, which requires far less fissile material, usually plutonium, which is shaped into a sphere known as a ”pit.”
A ring of high explosives surrounding the ”pit” is then detonated, compressing it in a supercritical mass. The Nagasaki bomb, called Fat Man, was just such a device, and required around six kilograms of plutonium, although modern US and European weapons require less and can be made into portable designs called ”suitcase” bombs.
But getting the design right is technically very difficult, for the sphere has to be machined to exactly the right shape and the location and timing of the explosive charges have to be perfect to ensure critical mass.
Some of the equipment, such as the machining tools, can be bought commercially. But others, such as centrifuges and gas membranes to enrich uranium to weapons-grade, are highly specialised and could not be bought discreetly by a renegade organisation.
”Modern nuclear weapons are extremely complex and (entail) neutron initiators to get the reaction going even more efficiently, and small amounts of tritium, an isotope of hydrogen, to ?boost? the power of the weapon,” the Bulletin says. In addition to materials and equipment, a country or terrorist group seeking to make a nuclear weapon, rather than simply steal one, has to have an array of specific skills.
They would have to have a design, and knowledge of engineering, nuclear physics, radiation, high explosives and electrical circuitry.
”As many as a dozen highly-skilled scientists and engineers would be necessary,” says the US think tank, the Council on Foreign Relations. ”Would-be bomb makers would (also) require money, a base from which to work, and specialised equipment.” – Sapa-AFP