In 1945 the Second World War in the Pacific ended following the nuclear bombings of the Japanese cities of Hiroshima and Nagasaki. Only days after the attacks, Japan officially surrendered, ending the war on September 2nd. The bombing of Japan was the end result of the Manhattan Project, a US-led research project into the development of nuclear weapons, running from 1942 to 1946. However, the idea that energy can be extracted from the centre of an atom has a longer history, and even Ireland plays a part in the story.

In this article I will explain some of the history and science behind nuclear fission and energy. Then I will tell you how to build an atomic bomb.

The concept of the atom has been around since ancient Greece, when Democritus imagined that the Universe was “in reality atoms and void”, but scientific experimentation revealing the nature of atoms and their components took place in the 19th and 20th Centuries. What most of us picture to be an atom came largely from the work of Ernest Rutherford and Niels Bohr in 1911 and 1922, respectively.

Rutherford probed the structure of gold atoms by using extremely thin sheets of gold foil bombarded with streams of alpha particles (two protons and two neutrons, identical to the nucleus of a helium atom). Most of the alpha particles pass right through the gold foil, but some were deflected slightly, and even less bounced right back in the direction they came from. From these results, Rutherford established three main things:

  1. The gold foil was mostly empty space (most particles passed through).
  2. The atoms contained electrons (they nudged the alpha particles off course, like a magnet can move a metal nail).
  3. The nucleus of the atom was extremely small and extremely dense (a small number of alpha particles bounced back).

Niels Bohr took Rutherford’s findings and, combining them with the discoveries of other scientists, developed a model of the structure of the atom; a small nucleus with electrons circling it in certain, precise orbits.

The Bohr model of the atom

The Bohr model of the atom

In 1905 Albert Einstein published a series of papers on different aspects of physics, all of which marked turning points in contemporary physics. One of the papers outlined his Theory of Special Relativity, and contained the famous equation E = mc².

E = mc² is the mass-energy equivalence equation and describes how mass and energy are effectively the same thing. is the speed of light, has a value of 3 x 108 metres per second. Squaring this number gives , which is 9 x 1016 m2/s(actually it’s slightly less, but we can ignore the small error here for now).

E is measured in Joules, and m is measured in kilograms. To get the equivalent energy of an object, simply multiply its mass by c2. For example, a mass weighing 2kg has an equivalent energy of 18 x 1016 Joules. Similarly, if you add energy to something, you increase its effective mass.

The mass-energy equivalence is extremely important in nuclear fission. If you split the nucleus of an atom into, say, two halves, the total sum of the masses of the fragments is less than the mass of the original nucleus. That is to say, mass is lost during the splitting process. Where does it go? You guessed it: it turns to pure energy.

A photo I took of Walton's accelerator in TCD

A photo I took of Walton’s accelerator in TCD

Ernest Walton was born in Waterford in 1903, and died in 1995 in Belfast at the age of 91. He got his formal training in physics from Trinity College Dublin where he was awarded his bachelor’s and master’s degree in physics in the mid-1920s. After this, he was offered a PhD studentship in Trinity College Cambridge in the UK where he met fellow student physicist John Cockcroft.

John Cockcroft and Ernest Walton collaborated in their work and built a particle accelerator to fire a stream of protons at the nucleus of a lithium atom. When the protons embedded themselves inside the atom, the nucleus became unstable, and broke apart into two helium atoms. Cockcroft and Walton were the first people to split the atom, and won a Nobel prize in 1951 for this achievement.

If you ever happen to be in the physics department of Trinity College Dublin, be sure to keep an eye out for a part of Walton’s particle accelerator on display.

Nuclear fission – the breaking apart of atomic nuclei – usually happens with heavier elements such as uranium. These materials are already close to ‘breaking point’, and sometimes their atoms do decay to become more stable by releasing energy and subatomic particles. This is radioactivity. To artificially trigger a disintegration event, the atom must be pushed over the edge, so that the strong nuclear force that holds the nucleus together is overcome by the electromagnetic force, which causes the particles inside the nucleus to repel each other and break apart the atom.

When an atom splits, it releases more subatomic particles. For example, if a neutron enters the nucleus of uranium-235, it becomes unstable uranium-236. The atom of U-236 then splits, creating two new elements, plus a smattering of more neutrons. These neutrons can then continue on and trigger yet more reactions in other uranium-235 atoms.

However, to build an operational atomic bomb or a nuclear reactor, there must be enough fissile material to keep the chain reaction going by itself. In the case of uranium-235, this critical mass is 52kg. Atomic weapons use a number of pieces of material under the critical mass, and then quickly bring them together to push it above the critical mass, so that their neutrons travel into each other and set off a self-sustaining nuclear reaction.

The thing about nuclear weapons is that there are no plans normally available: any country – or group of allied countries – that wants to build an atomic bomb pretty much has to discover the best way to build one on their own. The physics behind the nuclear fission process remains the same in all cases, but superpowers have spent years and vast amounts of money on research and development to find out how to build the most efficient bomb. Under the direction of Major General Leslie Groves, Julius Robert Oppenheimer was the theoretical physicist who developed the first nuclear bomb for the United States. The Manhattan Project cost the equivalent of $24billion (in 2014 terms) and employed about 130,000 people.

The bombs that fell on Japan at the end of the Second World War were the products of the Manhattan Project, and used two different designs: the Nagasaki bombing used an implosion-type bomb, where sub-critical uranium is compressed by explosives to increase its density and bring it above the criticality point, and the Hiroshima bombing used a very simple design called a gun-type bomb, where one sub-critical hemisphere of uranium was shot at another, creating a sphere above critical mass.


The design of a gun-type bomb is scarily simple, and anyone with secondary school-level physics and an imagination can easily build one. Simply fire one piece of uranium at another one, and you have a nuclear explosion.

To build a gun-type bomb you will, of course, need over 52kg of uranium; any less and your atomic bomb will not be able to sustain a reaction. A 52kg sphere of uranium has a diameter of 17cm: this is important to note, as the gun-type bomb works like a gun (surprise, surprise), and you will need an artillery weapon or a tank that suits this calibre.

The gun may need some minor modifications. After you form two sub-critical hemispheres of uranium with an extremely hot furnace and maybe a few other metal-milling tools – ideally around 32kg each – you will need to securely bolt one into the muzzle of the gun. The second hemisphere will need to be embedded in a projectile, such as a large bullet, that will fire from the gun’s chamber and head straight for the first hemisphere. When the two hemispheres make contact, there must be a mechanism in place to keep them locked together. If they bounce apart, the nuclear reaction will not sustain itself.

To detonate, simply pull the gun’s trigger.

Why use a gun at all? In order for a nuclear blast to take place, the two hemispheres must be brought together rapidly. Otherwise, neutrons from the uranium will slowly enter the hemispheres and cause a “fizzle”. This atomic bomb would not have the energy of the more carefully designed weapons of WWII or the Cold War, but the resulting blast from a gun-type weapon like this would be enough to cause catastrophic destruction to a city.

That is how to build an atomic bomb.