If, on the other hand, the regime is looking to build a weapon that could be launched quickly, then an implosion weapon is better. Implosion weapons work by packing explosives around a sphere of uranium-235. Detonating the explosives simultaneously will squeeze the sphere until it reaches a critical mass. To make it work requires precise timing and a hard-to-design neutron “detonator” that will give the weapon a kick at just the right time. But the advantages are that an implosion device uses less material and can fit atop a missile. Evidence collected by the International Atomic Energy Agency (IAEA) suggests that Iran has been working on implosion designs.
How hard is it to come up with a simple design on your own? "Not hard at all," says John Coster-Mullen. Coster-Mullen is a truck driver from Waukesha, Wisconsin who has devoted his life to reverse-engineering “Little Boy” and “Fat Man,” the first two nuclear weapons ever to be used. His designs are considered credible by the experts and depend only on a knowledge of physics, unclassified material about the bombs and the extensive open literature on nuclear chain reactions. It has taken Coster-Mullen 19 years to develop his drawings, but a small team with a physics and engineering background could probably do it in a matter of months.
Step 7: Manufacture
At this stage, the wannabe nuclear state may have its design and your enriched uranium-235, but it’s still not quite there yet. First the team of scientists has got to get the uranium out of gaseous form and into a metal. A simple recipe using water, hydrofluoric acid, and magnesium does the trick. With the metal ready, it has to be machined into the desired shape: either two halves of a sphere for an implosion weapon, or discs for a gun-type device. But they will have to take care not to work on too much at once, says James Acton, a physicist with the Carnegie Endowment for International Peace. "If you screw up at that stage there's a risk of a criticality accident," he says. It won't be a full nuclear blast, but the explosion will be powerful enough to destroy the workshop and probably give anyone working on the device a fatal dose of radiation.
Step 8: Develop a delivery system
A nuclear weapon is no good unless there is a way to deliver it to the enemy (ideally far away from the launch site). "States that love nuclear also love missiles," says Jeffrey Lewis. Short range missiles can be bought form rogue states like North Korea. But long range ballistic missile technology is tightly controlled, and trying to develop a missile is as or more difficult than building the weapon itself. Intercontinental ballistic missiles must be as light-weight as possible, while still being powerful and accurate, according to David Wright, a ballistic missile expert at the Union of Concerned Scientists. That makes it very hard to make them work, as demonstrated by the most recent North Korean launch failure in April. Alternatives include stealthy submarine-launched weapons and cruise missiles, which are even more complex, and bombers. That last option, while slow and vulnerable, is probably the best technological bet for an upstart nation.
Step 9: Test
Even if a country has built a simple gun-type weapon, it may find it valuable to conduct a nuclear test. These days, tests are done underground to contain radioactive fallout, so it will require digging a hole or finding an abandoned mineshaft that can be backfilled with rocks and gravel.
The result won't be a mushroom cloud, but it will still get noticed. The Comprehensive Test Ban Treaty Organisation in Vienna runs a sensitive global network of seismometers and radionuclide detectors that can pick up even the smallest nuclear explosions.