How Nuclear Bombs Work

Given North Korea’s recent saber rattling and bomb photos it might be a good idea to give a rundown of how nuclear weapons work. While this article will not cover thermonuclear weapons or specifics on certain types, it will give you a good idea of what the North Koreans are up to and what a nuclear weapon actually is.

Keep in mind that this overview is very general and only covers the two basic designs.

Gun Method

There are two ways to create a nuclear bomb. In the gun method, conventional explosives (W) propel a uranium projectile into a smaller uranium target (H). After firing, the two pieces merge very quickly to form a supercritical mass.

“Supercritical” means that there is enough nuclear material to sustain an increasingly powerful nuclear chain reaction. In this state the material is primed to explode. All that you need to do is add a neutron source.

The polonium-beryllium initiators in this picture(G) were the neutron sources used for the early bombs. They initiate the nuclear chain reaction by providing neutrons at the point when the mass reaches supercriticality.

The neutrons hit the U-235 atoms, splitting them and releasing more neutrons. When an atom is split some of it’s mass is transformed into energy. Because energy is mass times the the speed of light squared(E=MC2) turning even a small amount of Uranium into energy can release enough energy to level a city.

A tamper (F,I) around the Uranium target reflects neutrons back into the reaction, increasing the efficiency and reducing the amount of fissile material needed to achieve critical mass. As far as nuclear bombs go, the more neutrons flying around inside the chain reaction, the more efficient, powerful and light the bomb can be made.

Picture_1

Advantages of the gun method

The gun method was used in the construction of the Mark 1 “Little Boy” nuclear bomb dropped on Hiroshima and the six bombs built by South Africa before they gave up their nuclear weapons. It required no testing prior to use. The tube (N) used is basically an artillery tube with one end sealed off. The explosive charge is simple cordite gun powder and does not require a complex fire control system.

Downfalls of the gun method

While Gun design is simple, it does have downfalls. First of all, it requires several dozen kilograms of Pure U-235. A rare isotope that represents less than one percent of natural uranium. Refining it is extremely difficult, requiring thousands centrifuges to distill the substance and billions of dollars in industrial investment.Therefore, only U-235 is a suitable material.

Plutonium cannot be used in a gun type weapon because it releases too many neutrons and would detonate before the two pieces were totally locked together. The result would be a fizzle; a premature or failed detonation with greatly reduced explosive power.

Another Issue is that the fission itself is fairly inefficient. Of all 64 kg of 80% enriched Uranium present in the Mark 1 bomb, very little actually underwent fission. The overwhelming majority was dispersed by the explosion. As the U.S and the Soviet Union both pursued implosion based bombs this problem was never rectified. The Mark 11 nuclear bomb was the last air dropped gun type weapon in U.S service and it’s maximum yield was only 30 kt.

The implosion method

The alternative is called the implosion method. All modern nuclear devices use this method. It involves compressing a Uranium or Plutonium sphere into a supercritical mass using specially arrayed explosives that surround the core. The detonators (A) are set to explode simultaneously, creating a perfectly symmetrical and focused shock-wave that that “implodes” the hollow fissile core (H) into supercriticality.

Like the gun design the implosion mechanism also uses a tamper (M) to reflect neutrons and a Polonium-Beryllium initiator to provide an initial neutron source to ignite the chain reaction.

Early weapons like the one shown below used several dozen detonators(A) to achieve the symmetrical compression of the core. Later weapons would massively improve on this with designs that required a few as two detonators.

 

 

Disadvantages of the implosion design

 

Because of their mechanical complexity and the extreme tolerances needed for the explosive lenses and firing sequence implosion bombs require high precision to manufacture and defects can result in a fizzle. The North Korean nuclear test in 2006 was suspected to be such a fizzle. It had a yield of less than a kiloton.

Advantages of the implosion design

There is a reason all modern nuclear bombs use the implosion mechanism as a building block. It is more efficient and requires much less fissile material than the gun type weapons .Even the original Mark 3 “Fat Man” nuclear bomb used only 11 kg of plutonium. Compare this to the 64 kg used in ‘Little Boy.”

If enough effort is put into their development, implosion bombs can shed much of their mass and can be made as small as a 155 mm artillery shell. The high explosive lenses can be replaced with a more lightweight designs. With the addition of nuclear boosting, implosion devices can be made even smaller and the yield can be raised significantly. Even without the use of large scale thermonuclear fusion(an H-bomb).

These are the basics of nuclear weapons. A starting point for any nation that aspires to be a nuclear weapons state. So far nine nations have met this basic threshold and sustained nuclear programs. North Korea is the newest member of this club.

 


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