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Debriefing North Korean nuclear tests, 1 of 2: The physics

It was interesting to hear news of the recent (February 2013) North Korean nuclear test.

North Korea and weapons of mass destruction
North Korea and weapons of mass destruction (Photo credit: Wikipedia)

NK had apparently detonated an approximately 6-7 kiloton fission warhead some 1 km into the earth’s crust. Seismic waves, created by the explosion’s released power, were picked up by several geological monitoring stations.

Let’s review some of the physics and terminology that goes with nuclear weapons. Even though probably many of you consider nuclear arms as something strange and perhaps even frightening, the science and history behind the developments so far are very interesting.

Classical models about the very essence of our material existence (models without empiric background) were numerous. Ancient philosophers pondered about the subject and often promptly gave a “definite” answer, after which not much criticism was expected. Empirical testing was haphazard, if not lacking altogether. The models involved gods and deities, and were quite eccentric to modern scientists.

Nuclear physics is a branch of physics that deals with forces and interaction between the smallest known “classical” particles, the atoms – made up of a nucleus and an electron shell. Nuclear physics studies phenomena happening in around the femtometer scale.

The building blocks of an atom (electrons, neutrons, and protons) have properties that keep the atom stable under normal conditions. Stability means in this context that the atom is able to participate in chemical reactions (by the properties of its electron shell) but this type of interaction is completely different from an atomic reaction. A nuclear event happens when something disturbs the very core of an atom, the “nucleus”; such an event is a very special one, and requires relatively large amounts of energy and specific electromagnetic properties from the disturbing object. The splitting of an atomic nucleus has been explained as a bit similar what happens to a drop of water, if one adds just a little more water into it: the interaction forces can no longer sustain the drop, and it splits; the “camel’s back” has snapped. The atomic nucleus is held together by one of the four major forces, the “weak interaction”. Too much energy in the nucleus will break the harmony and the nucleus will emit two things; particles and energy. If the particles happen to be a neutron or many neutrons, and there’s a plenty of similarly behaving matter within the reach of those nuclea, then a chain reaction might happen. A nuclear explosion is just such a thing!

Albert Einstein knew of the potential of nuclear energy, but was quite skeptical about the possibility of practical use of it. He thought for a long time that releasing energy according to his famous formula (E=mc^2) would require such immense amounts of input energy that it would never prove economical (sensible). As a devout pacifist, he was astonished and frightened by the message brought by former student of his, Leo Szilárd, of the possibility of using atomic energy as the source of a very powerful weapon. Szilárd had been researching nuclear fission (a chain reaction) since the 1930s and had come up with a mechanism to initiate a powerful event in certain suitable “fissile materials“.

Difference between a chemical and a nuclear reaction

In chemistry it is the electron shell that defines interaction types between atoms and molecules that they compose. An exothermic reaction releases energy to the environment; it “heats up” its surrounding matter. Molecular structures (configuration of atoms and molecules in relation to each other) are broken so that the whole system ends up in a lower energy state – thus the difference (of energy) between the two states is dissipated.

For example, when in the combustion cycle of an otto engine, a mixture of air and fuel is ignited by a spark, the explosion releases energy – which is turned into mechanical energy by the pressure-driven piston moving back and forth in a cylinder. Thus the vehicle is given mechanical energy (velocity) by chemical reaction. The fuel is said to store the energy in chemical form.

Usually the nucleus (the “center” of an atom) is relatively stable. Stability means that the nucleus of an atom will not change its configuration of protons + neutrons in any “humanly” short time; ie. a stable nucleus has a very long half-life. (For more information on radioactivity and half life, here). The nucleus is quite isolated from the general “macro” environment of matter, ie. the half-life is not affected by ordinary happenings outside nucleus – like electron shell interactions with matter. The particles in the atom’s nucleus are tightly held together by forces very different from those of the other parts of the atom.

Atomic weapons blasts are a product of two different types: fission and fusion. The Hiroshima and Nagasaki bombs were fission bombs. Fission means like shooting a billiard ball into a pack of other balls, which then set forth a cascade of events: the secondary balls bounce into tertiary ones, and so on. If these balls are arranged in a specific formation, the total action pattern becomes very rapid and energetic. Albert Einstein had predicted the immense power set in the fabric of atomic interactions, but he was skeptical whether this power could be economically released. Einstein found out in 1939, by Leo Szilard visiting him in the Long Island, that a breakthrough had happened: the theory and practical appliance of chain reaction using neutrons to split the atomic nucleus.

Following, I will try and write the part 2/2, which will contain a bit more backgrounders:

  • Cold War: the strategic aftermath of World War II
  • Practical sides of building an atomic weapon, and barriers that make/made it expensive
An induced nuclear fission event involving ura...
An induced nuclear fission event involving uranium-235 (Photo credit: Wikipedia)

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