Nuclear Fission v Nuclear Fusion

"Now I am become Death, the destroyer of worlds."  J. Robert Oppenheimer

David M Jones

A couple of years ago, I started to read a hefty, but very readable book entitled: ‘The Making of the Atomic Bomb’.  As I waded through page after page relating to the development of the science and physics that would eventually bring about the birth of the nuclear age, I marvelled at how difficult it was - and no doubt, still is - for mankind to create that which occurs naturally in nature.

The logistics of bringing about an environment in which a nuclear chain-reaction can begin seemed hardly feasible.  Many scientists considered the task might be physically impossible.  The conditions required for a nuclear chain-reaction to occur must be akin to those that exist only in the stars. 

My own curiosity eventually took me off on a tangent and I started to do a little research.  Our earthly scientists sought how to split uranium atoms to bring about nuclear fission; I soon discovered this was not the process which occurs in the stars.  On the contrary, stars, including our sun, create energy as a by-product of nuclear fusion!   What was the difference I wondered?  A little more reading revealed that our atom bomb and its fission reaction is achieved through splitting uranium atoms by means of an implosion.  That implosion results in a central core of fissionable material being placed under tremendous pressure – squeezing the metal core to less than half its previous volume.  Under those conditions a fission chain-reaction is possible.  This, I assumed, must at least compare briefly to natural conditions occurring under massive gravitational pressures.

FUSION

For fusion to work, extremely high energies are needed to fuse the nuclei together. This is needed to overcome the electrical repulsion (known as the coulomb barrier) between two positively charged nuclei, so that they get close enough to have the strong nuclear force bind the nuclei. This nuclear force has an effective range of around 10^-15 meters, which is why fusion occurs most easily in stars, where a high density and temperature environment exists. The density and temperature are the primary factors in determining the probability of the nucleons fusing in the star... Most of the energy generated within the Sun is created from a sequence of reactions that "burns" hydrogen into helium, known as the proton-proton reaction. (Team Thinkquest, 1998)

The article goes on to describe how, in our sun, the reaction occurs in the innermost region, where density is increased to one-hundred times the density of water on Earth.  At this density, temperatures soar to approximately fifteen million K (27,000,000 degrees F).  Hydrogen atoms are stripped of their electrons – creating plasma of free electrons and protons, the nuclei of the hydrogen.  Under these conditions hydrogen is converted to helium.  The resulting helium amalgam is smaller in mass than the original (hydrogen) free matter – the excess of this process being given off as heat and light.  That description, I hasten to add, is a gross over-simplification on my part to meet the restrictions of this short article.

Fundamentals: the Sun as a Star

Our Sun is by far the largest object in our solar system, containing more than 99% of solar system’s total mass. Observations of other stars indicate that the Sun is fairly "normal": it has a mass, luminosity and temperature that is somewhere in the middle-to-low end of the observed spectrum. It is also one of about 100 billion similar objects in the Milky Way.  Its characteristics are hard to grasp by earthly values, with a mass of 2 x 10³⁰ kg, an atmospheric temperature of 5500 ^oC and a luminosity of 4x10²⁰ megawatts.

The Sun is mainly composed of hydrogen and helium (~75% and ~25% by mass, respectively), with traces of heavier elements synthesized by past generations of stars in the solar neighbourhood.  These heavier elements are the main constituents of the inner terrestrial planets in the solar system; the Jovian planets have compositions almost identical to the Sun itself.

The proximity of the Sun to the Earth allows scientists to study phenomena in the solar atmosphere that are too small or too faint to be observed in even the nearest star to our own.  (The Curious Team, 1997 - 2010)

Armed with the knowledge that fusion occurs naturally, and the process of fission was ‘accidentally discovered’ by radio-chemists Otto Hahn and Fritz Strassmann in 1938 – whilst in the course unrelated experiments - I wondered where fission might occur spontaneously in nature. 

Unbelievably, it appears that 1.5 billion years ago - here on Earth - a natural nuclear fission reaction took place.  The site of this natural reaction was discovered 1972, in Oklo, Gabon, Africa - the area pictured above.  Scientists estimate the fission reaction continued on-and-off for hundreds of thousands of years!  Whilst it was active, this natural process produced nuclear waste similar to the wastes produced by the man-made nuclear fission reactors of today

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References:

Cohen, G. A. (1976, July 1). A Natural Fission Reactor. Retrieved January 8, 2010, from Scientific American: http://www.ans.org/pi/np/oklo/.

Rhodes, R. (1986). The making of the atomic bomb. New York: Simon & Schuster.

Team, T. C. (1997 - 2010). Ask an astronomer web site. Retrieved January 8, 2010, from Cornell University: http://curious.astro.cornell.edu/sun.php#questions.

Team, T. 9. (1998). Atomic alchemy basic fusion. Retrieved January 7, 2010, from Thinkquest 98: http://library.thinkquest.org/17940/index.html.