Most people know what nuclear bombs are. They have either seen their ferocity in video clips and photos of above ground tests or the infamous detonations over Hiroshima or Nagasaki at the end of World War II. However, few know how they are made or what makes them explode with such destructive power.

Amazingly, the mechanics and inner workings of nuclear bombs are openly available in books and scientific papers as well as on the Internet. But what can be found in open sources provide the reader with too much detail and use technical lingo, which could be hard to digest. Here, I will try to cut through most of the gibberish and make it more understandable.
Even highly simplified descriptions of nuclear bombs requires a cursory review and knowledge of basic physics-let's call it nuclear physics 101, and start by describing "nuclear fission."

Fission refers to a process at the atomic level where the nucleus of an atom splits releasing energy. This is the basic principle behind nuclear reactors that produce electricity as well as nuclear bombs. In nuclear reactors, fission is controlled and occurs in an orderly manner to generate heat, which is used to boil water, which makes steam that runs turbines that produce electricity. In a nuclear bomb, the same fission is accelerated and allowed to happen in an uncontrolled manner releasing enormous energy in a fraction of a second, which simply detonates.

A frequently asked question is: "Can a nuclear power plant explode?" The answer is: Never. Nuclear power plants are not designed to explode. In case of a malfunction, where fission gets out of control, the core nuclear material overheats causing a meltdown of the fuel, which releases radioactivity wreaking havoc with the environment. This is what happened at the Three Mile Island facility, which is one of the reasons why they build these giant concrete domes at nuclear power plants-to prevent any radioactivity from escaping.

Fission occurs best in two metals: uranium and plutonium. Like most other metals uranium is mined. But plutonium does not exist naturally anywhere; it must be created. Let's review the benchmarks in the nuclear fuel cycle. It all starts at a uranium mine. Does Iran have any uranium mines? Yes. Huge deposits have been discovered in Saghand in the Yazd province where the uranium ore deposit covers 100-150 square kilometers, with reserves estimated at 3,000-5,000 tons of uranium oxide.

Once uranium ore is dug out of the ground, it is milled into fine dust and then leached with acid to separate out the uranium. It is actually uranium oxide and it is called Yellow Cake.

The uranium (U) found in nature consists of two types (known as isotopes) one has 235 neutrons and the other 238, which are denoted as U235 and U238 respectively. The U235 variety is what is needed for fission, energy production and in nuclear bombs. The heavier U238 is useless. The problem is that the uranium ore is less than one percent U235 and not usable unless its concentration is increased. This is what is known as 'enrichment.'

We need mildly enriched uranium-where the U235 concentration is increased to about 5%-to power nuclear reactors. Highly enriched uranium-with 90% or more concentration of the lighter isotope-is necessary for atomic bombs. See "Uranium Enrichment" for more information about this process. The highly enriched uranium (HEU) is at first in gas form. Then metallic uranium, which is now weapons' grade, is extracted from the gas and fabricated into bomb cores.

It is critical to understand that a country in search of nuclear bombs doesn't necessarily need a nuclear reactor. All they need is the uranium ore and the enrichment facility. Then why has Iran paid a billion dollars to Russia to build a reactor for them in Bushehr, one might ask. There are two reasons: first, it offers their nuclear weapons program a 'peaceful use' cover and keeps the IAEA-International Atomic Energy Agency-off their backs. Second, they need a reactor to create plutonium, which makes even more powerful bombs.

In a reactor, the fuel rods-filled with low-grade enriched uranium-are grouped in clusters and placed in an airtight water tank. The energy released by the controlled fission inside the fuel rods heats the water that eventually produces electricity. During the slow fission, some of the uranium atoms absorb neutrons and transmute into plutonium. Once the fuel rods are depleted, they can be reprocessed to extract the plutonium, which is a relatively simple chemical process.

It is remarkable that very little highly enriched uranium is needed to make a nuclear bomb. You only need to have enough HEU to sustain a rapid chain reaction. To better understand this, let's look at the fission equation:


When a neutron hits a uranium atom, the impact shatters the nucleus causing fission, which releases energy plus two or three new neutrons. If these extra neutrons find new atoms to shatter, a chain reaction will start. If not, they escape into the atmosphere and the process fizzles out. So, there's a minimum amount of fissionable material necessary to start a chain reaction, which is called the supercritical mass. It's about 11 pounds of plutonium or 35 pounds of highly enriched uranium-roughly the size of a large grapefruit!

Click here for an animation video of nuclear fission.

How is such a powerful bomb made out of so little material? The simplest nuclear bomb design, pioneered in the Manhattan Project during World War II and subsequently dropped on Hiroshima, consists of a gun type mechanism that fires one piece of U235 at another piece of U235, instantly creating supercritical mass.

Suppose you take a grapefruit-size amount of HEU, cut it in half, place each half in a tube about a foot apart, then place a hundred pounds of high explosives-such as C4 or Semtex-behind each half and then detonate them simultaneously, you'll have a crude nuclear explosive device.

It's that simple. However, actually building it, making it detonate on demand, maximizing its yield and delivering it to the target, one and all, poses formidable technical challenges. No one knows for sure whether the Iranians have the necessary knowhow.

How destructive are these weapons? The smallest nuclear bomb yields the explosive power of 15,000-30,000 tons of dynamite. How you get such power from a grapefruit-sized ball of uranium? The answer lies in Albert Einstein's famous law of physics: E=MC².

(Source: Mushroom in the Sand.)