A nuclear bomb can annihilate an enemy in a flash but, perhaps more importantly, also allow a state to flex its muscle on the world stage.
So far, just eight countries have officially detonated nuclear weapons but others are suspected of owning them or trying to develop them. Iran is currently top of that list, despite the country’s continued protestations that its nuclear programme is for peaceful purposes only.
Although the technology has been around for decades, becoming a nuclear nation is no easy task. It can involve smuggling, deception and years of hard work, not to mention the threat of swift retribution from other world powers.
So, what does it take to build a nuclear bomb? And why do few countries bother? BBC Future lays out the challenge and peril of any country wishing to undertake its very own nuclear weapons programme...
Step 1: Choose your isotope
Any aspiring nuclear nation will first need to gather its scientists, engineers and technicians. This team will know all things nuclear work on a simple principle: when a heavy nucleus of an atom splits, it converts a tiny amount of mass into pure energy. That energy can be used to treat cancer, generate electricity or level a city. Nuclear blasts are triggered through an uncontrolled chain reaction in a large block of material, where each new split causes more splits, releasing more energy. Fortunately, most radioactive materials cannot sustain chain reactions.
The most common isotopes that can are uranium-235 and plutonium-239. Plutonium-239 is the stuff of choice for the big boys - like Russia and the US - but it doesn't exist in nature. To get it, countries need to make it inside a nuclear reactor, and nuclear reactors are hard to hide, says Jeffrey Lewis of the blog Arms Control Wonk. Stealth is what is needed in the early days of a nuclear programme, so uranium is the natural choice for most countries.
Step 2: Get some uranium
Sounds tricky, but its actually the easiest step in the entire chain. Uranium is mined commercially all over the world and is sold in a powder form called “yellow cake". Some countries, like Iran, choose to go it alone and mine and process its own yellow cake. Others simply go to the world's biggest supplier - Kazatomprom, Kazakhstan's nationalized nuclear power company, which exported nearly 20,000 tonnes of uranium in 2011, according to the World Nuclear Association.
Unfortunately, buying in bulk has its drawbacks. Kazatomprom's uranium is mostly uranium-238, a naturally occurring isotope that will not sustain any nuclear reactions at all. Just 0.710% is the uranium-235 needed, yet the simplest nuclear weapon requires about 50kg of 90% pure uranium-235. As most countries will want two or three for testing and redundancy, it requires around 150kg (330lbs) of uranium-235, or around 20 tonnes or so of yellow cake.
On delivery it needs to be stored, which means a country need to think about building a nuclear complex. Iran stores it at the Isfahan facility, south of Tehran, whilst Iraq’s former leader Saddam Hussein kept his at Tuwaitha, south of Baghdad.
Step 3: Begin processing
Saddam's yellow cake was useless, but then so is everyone’s. To get useful uranium-235, a team will need to first separate the isotopes. It's trickier than it sounds because chemically speaking, uranium-235 and uranium-238 are identical. The only way to separate them is by their mass (238 has three more neutrons and is therefore a tiny bit heavier).
The most efficient technique is to spin uranium inside a centrifuge, but spinning uranium powder will make a mess. To get yellow cake into a more useful gaseous form, a team of researchers in white coats follow a simple recipe: heat it, to burn off impurities, then expose it to hydrogen fluoride to make uranium tetrafluoride. Heat the uranium tetrafluoride again in a kiln filled with fluorine gas, and with a bit of luck out will come gaseous uranium hexafluoride.
