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Little Boy Fission Bomb dropped on Nagasaki

When the sub-atomic particles of an atom break apart they do so with great speed. Much of this speed (or Kinetic energy) is converted to heat. This heat can be harnessed and used in nuclear weapons.

To make this energy available it is necessary to start a chain reaction. A chain reaction occurs when the sub-atomic particles from one atom smashes into another atom causing this to break apart and so on.

In a nuclear weapon all the energy that is created by a chain reaction is released almost instantaneously, which creates a tremendous, devastating explosion.

Naturally occurring atoms do not explode in chain reactions. This is because they do not contain enough fissile atoms to sustain a chain reaction To create suitable atoms for the process to occur enriching must take place.  

Naturally occurring uranium 235 contains around 7 fissile atoms per thousand. This is not enough to sustain a chain reaction.

Enriched nuclear fuel contains more fissile atoms, thus they are better at sustaining a chain reaction. To enrich uranium, scientists add plutonium to it.  

 Fission creates energy by breaking apart atoms.

 Fusion works in the opposite way. It creates energy by joining atoms together.

Although atomic bombs which work by fission are devastating weapons (both the bombs dropped on Japan by America towards the end of WWII were fission bombs) fusion weapons (or thermonuclear weapons – to give them they proper name) are far more efficient. For example when 1 kg of uranium is converted into heat energy by fission it produces 6 times less energy than when 1kg of deuterium in converted to heat energy by fusion.

 Thermonuclear weapons produce energy in the same way in which the Sun produces energy. Fast moving sub atomic particles of deuterium (a type of hydrogen) smash into each other and bond together.

 In order for the particles to bond together they must be moving very rapidly. To make the sub-atomic particles move at the required speed they must be heated up. The temperature needed for this to occur is not found naturally occurring on the Earth. It is around 10,000,000 ^oc.

When deuterium atoms combine they eventually form helium. The effects of this are to create a total mass of helium which is less than the original mass of deuterium. This difference in mass is called the mass defect. It is this, according to Einstein’s Law, which creates the heat energy.

To create the ignition temperature needed to start a fusion reaction, scientists use a nuclear fission bomb within the thermonuclear weapon.

The neutron bombs differs from other types of nuclear weapon in as much as its primary lethal effects come from the radiation which it emits after detonation. The neutron bomb is constructed in such a way that the blast and heat effects are kept to a minimum. This weapon is useful if enemy personnel, not structures are the intended target.

Little Boy Fission Bomb dropped on Nagasaki

  Fission Bombs

  A fission bomb uses an element such as Uranium 235. When an atom split the amount of energy released in a very short period of time is immense . A pound of highly enriched uranium , if used in a nuclear bomb would create the same amount of energy as a million gallon of petrol . When you consider a pound of uranium is about the size of a cricket ball , and a million gallons of petrol would fill a cube (50ft by 50ft building), you can see just how much energy is available in a little bit of uranium.

In a fission bomb the fuel needs to be kept in two or more sub critical masses. At the time of detonation these masses must be combined . Free neutrons must be introduced to the super critical mass to start the fission. To bring the sub critical masses together to form a super critical mass 2 techniques are used.  These are :

  Gun triggered fission bombs and  implosion triggered fission bombs.

Gun Triggered Fission Bombs

The easiest way to bring 2 sub critical masses together to form a super critical mass is to fire one into the other, A “bullet” of u-235 is placed at one end of a tube. A sphere of sub critical mass is placed at the other end of the tube. After the bomb is released a pressure sensor determines the appropriate height for detonation. When the correct altitude is reached explosives fire the bullet along the barrel. The bullet strikes the sphere breaking a piece of foil which, in turn allows a small pellet of polonium and beryllium to begin emitting alpha particles . The alpha particles strike the beryllium which frees neutrons. These neutrons then initiate fission. This fission occurs inside a tamper, usually made from u-238. The purpose of which is to reflect liberated neutrons back into the fission core, increasing the efficiency of the reaction process. Once the chain reaction is initiated providing there are enough free neutrons to continue fission, the bomb explodes.  “Little Boy” was a gun triggered fission bomb. It was 1.5% efficient.

Implosion Triggered Fission Bombs

Although more complex to manufacture than gun triggered fission bombs, the implosion triggered fission bomb was developed by the Manhattan project scientists. It was found that such a device was 17% efficient, (over ten times more efficient than a gun triggered fission bomb). Fat Man was an implosion triggered fission bomb.

Although fission bombs were devastatingly effective, scientists soon realised that more efficient and hence, more powerful weapons, could be designed. A number of difficulties had to be over come before a fusion bomb could be built which actually worked. Research, conducted largely by Edmond Teller, over came these problems. Teller encased a fission bomb within a fusion bomb, thus creating sufficiently high temperatures and pressures for fusion of deuterium and tritium to occur causing vast amounts of heat and radiation to be emitted. Fusion bombs, as well as being more efficient, could also be made substantially smaller than fission triggered weapons. The smaller devices could then be used in artillery shells and smaller missiles fired from portable launchers. Thus, limited nuclear war could be waged.

The energy produced by fusion weapons is created in a similar manner to the way which the Sun creates it’s energy. The reactions which occur in both the Sun and fusion bombs are called Thermonuclear reactions.

The neutron bomb, also known as a enhanced radiation weapon (erw), is a small fission device. It's main purpose is to destroy living material, whilst causing only limited damage by blast or heat. 

 The primary effect of the weapon is emission of neutrons (hence the name). These are lethal to living organisms within the blast area. The design of the bomb is such, that physical structures are only effected over a much smaller distant than conventional H-bombs. 

These weapons are preferred by NATO as they limit, at least to a small extent, the amount of collateral damage caused.