The Second World War dragged on for six years, took a toll of nearly 8 crore human lives and yet an end was nowhere in sight. The Hiroshima and Nagasaki atom bombs killed about 168,000 people in two swift strokes and ended the War in six days flat!
There have been many great scientists in history. Some have been elevated to the status of poster boys of science. Ask even a layman and chances are that he would have heard the name of Einstein. Ask about his work, however, and they draw a blank. On the other hand, there is absolutely nothing in the world, which has affected mankind as deeply as nuclear weapons! Everybody knows them. Since WWII, one thing that has been the focus of national pride, as well as the deepest fears of mankind, is nuclear weapons. For all times to come until doomsday, international politics on which the very survival and well-being of the human race depend has been influenced by and shall be conducted under the shadow of nuclear weapons.
Most people do not know that if there is one man, who is single-handedly responsible for the design of nuclear weapons, it is the relatively less-known scientist Dr John von Neumann (born Neumann János Lajos). If I have to pick the greatest of them all geniuses of all times in history, it will not be the likes of Newton or Einstein; it will be none but Neumann.
Neumann was born in 1903 in the then Austro-Hungarian Empire in a noble family, receiving the best and the most elite education of the era and the place. He was a mathematical prodigy but was persuaded by his family to take a degree in chemical engineering as they thought that with the boom of the chemical industry in post-WWI Germany, the financial prospects in chemical engineering would be better. However, destiny had different things in mind. He went on to take a PhD in mathematics in 1927 and like many other scientists of Europe in that era, migrated to the USA in 1933.
Twelve years later, he was to solve a problem in applied physics, for which a fellow physicist Dr Hans Bethe working in the Manhattan Project, himself a Nobel Laureate, openly used to say and has placed it on record that the problem had simply ‘defeated’ the galaxy of distinguished scientists in the Project. Neumann cleared a seemingly insurmountable hurdle towards the development of nuclear bombs. His design still drives the core (called Primary) of modern hydrogen bombs also, the most powerful of which (the Tsar Bomba of USSR) was some 3000 times more powerful than the Nagasaki bomb designed by him.
Unlike most other ‘specialist’ scientists, he was a polymath, with great knowledge of European history and literature. As if to confirm that strange are the ways of God, this genius succumbed to cancer at just 54 years of age. Even on his deathbed, he used to entertain his brother by reciting by heart the first few lines of each page of Goethe’s Faust. As he lay dying, he was still unsure of whether he had done enough important work in his life!
Demystifying the science of atom bomb
The atom bomb or nuclear bomb (nuke) is a device which derives its explosive power from a phenomenon known as nuclear fission. Now what is fission? You know that atoms consist of nuclei (plural of nucleus) at their core and electrons outside. Now what makes up the nuclei? There are sub-atomic particles called protons and neutrons which make up the nuclei of atoms.
Now what we do is take some heavy elements like Uranium-235 (i.e., uranium atom of atomic mass 235) or Plutonium-239 (i.e., plutonium atom of atomic mass 239) and bombard them with neutrons. These heavy nuclei are not very stable and they break into two fragments of roughly equal size—this is called nuclear fission. Besides them, a couple of neutrons are also released.
The whole process entails the release of a large amount of energy. Why? It so happens that the combined mass of the two nuclei formed and the neutrons are very slightly less than the mass of the uranium or plutonium nucleus. So where does the mass go? It is actually converted into energy by the famous mass-energy equivalence relationship of Einstein, E = mc2. The material that can undergo fission is called fissile material. The first step in making the bomb is enrichment or increasing the percentage of the fissile isotope. Now if somehow the neutrons could hit other nuclei, they would break them too. The neutrons produced by their fission would break still more nuclei and so on. Thus a chain reaction could take place which would grow exponentially and enormous amounts of energy would be released in an extremely short time—that is, you get a nuclear explosion!
So what is the problem in making a Nuclear Bomb?
Sounds fairly simple? In practice, it turns out to be extremely complex. What happens is that if you have but a small quantity of the fissile material, chances are that most of the neutrons would escape without doing anything useful—after all, they are extremely small particles. If you have a sizeable quantity then chances are that they would collide with some nucleus before they are able to escape. This critical quantity of a fissile material that would ensure an explosion by a chain reaction is known as its critical mass. Less than that, the material is said to be a sub-critical mass. In order to make a bomb which you could explode at will, an assembly of fissile material must be brought from a sub-critical to a supercritical state extremely suddenly.
If you wish to make a bomb, you must ensure that the sub-critical pieces do not fly apart as a result of the energy produced in the first moment and thus end the progression of the chain reaction prematurely. You have to keep the supercritical mass together for about a microsecond. It is not easy because the tremendous energy released tends to blow everything apart with tremendous force. Also, you must ensure that before you want to explode, stray neutrons should not be creating any mischief.
The majority of the technical difficulties of designing and manufacturing a fission weapon are based on the need to both reduce the time of assembly of a supercritical mass to a minimum and reduce the number of stray (pre-detonation) neutrons to a minimum.
‘Little Boy Bomb’ that destroyed Hiroshima (August 6)
There are basically two techniques for assembling a supercritical mass. Broadly speaking, in the ‘gun-assembly’ type, we bring two sub-critical masses together forcefully by hitting them together. This was the design of the Little Boy bomb that was dropped on Hiroshima. The problem with the Little Boy design is that it could use only highly enriched uranium. It used 64.1 kg of HEU and only 1.09 kg (1.7%) of that underwent fission. The rest was blown away by its own explosion and wasted. The design was never tested because there was only that much uranium in the USA! Uranium bombs cannot be made more powerful beyond a practical limit.
‘Fat man bomb’ that devastated Nagasaki (August 9)
We cannot use plutonium in a gun-assembly type because plutonium has a contamination of Pu-240 and thus there are a large number of stray neutrons in plutonium. It is likely to result in ‘pre-detonation’, that is, the background neutrons would start the fission process even before the fissile material has reached the desired configuration. This would result in the weapon fizzling out to a dramatically reduced yield.
The critical mass of compressed fissile material decreases as the inverse square of the density is achieved. So you could take a mass of fissile material and if you could compress it suddenly from all sides, its density would increase to such an extent that it would become a supercritical mass. This means that the only way out of using plutonium was to very quickly compress a sub-critical mass into a supercritical one—compressing the metal to nearly 2.5 times its density. Compressing from all sides is called ‘implosion’.
Now this implosion was a serious problem. What happens is that under such enormous pressures, even a metal starts flowing like a liquid and liquid has a tendency of squirting out. It is like squeezing a bunch of grapes in your palm—some juice will certainly leak out. Manhattan scientists had been trying huge quantities of explosives placed around plutonium in various ways. But explosive force produced by an explosion is dissipated in 360 degrees and nothing worked. It was extremely frustrating.
God said let Neumann be, and there was the bomb
For achieving a sudden but uniform compression such that no plutonium is allowed to squirt out, Neumann devised something called ‘explosive lenses’. As you know, a lens can focus light. An explosive lens focuses explosive force. He argued that light bends in glass because light waves have different velocities in air and in glass. Similarly, the detonation wave produced by an explosive must bend if it encounters another explosive, which has a different detonation velocity. An optical lens can focus light waves because it bends them suitably because of the shape given to it. Similarly, for the explosive lens, Neumann calculated that in order to convert a spherically expanding shock wave front into a spherically converging one, the boundary shape must be a paraboloid; similarly, to convert a spherically diverging front into a flat one, the boundary shape must be a hyperboloid, and so on.
In an implosion-type nuclear weapon, polygonal lenses are arranged around the spherical core of the bomb and it actually looks like a football. Fabricating explosive lenses is also a challenge in both the chemistry of explosives and high-precision mechanical engineering. The bomb dropped on Nagasaki used 32 lenses, while more efficient bombs would later use 92 lenses. They can make about 25% of the fissile material undergo fission!
On July 16, 1945, it was the implosion design that was tested in the so-called Trinity Shot and, which famously reminded Robert Oppenheimer, the Director of the Los Alamos Laboratory of a quotation from Bhagwad Gita.
Plutonium bombs can be made very small and the smallest of them, the W54 warhead on Davy Crockett’s recoilless rifle weighed just 23.1 kg and measured just 27.3 x 40 cm. The Fat Man, in comparison, weighed 4.7 tons and was 10.8 x 5 feet!
Also Read: The Hiroshima and Nagasaki bombings
Even great hydrogen bombs use Neumann’s invention
A hydrogen bomb makes use of a phenomenon called nuclear fusion. They got the popular name of the hydrogen bomb because initially, the nuclei used were isotopes of hydrogen. You could think of it as the reverse of nuclear fission. In this, some light nuclei are fused together to make the nucleus of a heavier element—a very high amount of energy is released in this process too. What happens is that the mass of the nucleus of the heavy element formed by fusion is very slightly less than the combined mass of the nuclei that went into fusion and the difference appears as energy, just as it happens in fission.
Stars like our sun derive their energy from essentially the same process, though infinitely slower. There the enormous gravitational force fuses them. But, how do we do that for a bomb in a flash? The problem was solved by using the enormous heat generated in the process of nuclear fission. Thus for a hydrogen bomb, the plutonium bomb devised by Neumann in its core (called Primary) is used as a ‘trigger’. Nuclear fusion generates more energy than nuclear fission and thus hydrogen bombs are more powerful than atom bombs.