Nuclear Fission
Atomic power was “predicted” by Albert Einstein in his 1905 paper on the Theory of Relativity. This postulated that mass and energy are related to each other. This relationship is described by the world’s best known equation:
e = mc2 where e=energy in ergs, m=mass in grams and c=speed of light (3x1010cm/sec)
At the time, Einstein had no idea that the conversion of mass into energy would ever be practical or even possible. Certainly the reverse process – converting energy into mass – has never been achieved.
Fission reactors are almost always fueled with uranium. This element (along with many others) is unstable, spontaneously disintegrating into smaller atoms. When this happens, the total mass of the atoms and subatomic fragments formed is very slightly less than the mass of the original uranium atom. The lost mass appears as energy (heat) in accordance with Einstein’s equation.
There are several ways for the uranium atom to decay or disintegrate. In one sequence, alpha particles (helium nuclei) and beta particles (electrons) are lost in a series of 11 steps. Each step results in an unstable atom until eventually, a stable isotope of lead is formed. In this sequence, one gram of uranium 238 breaks down to lead and alpha particles with a total mass loss of 0.225 milligrams, or 0.056%. Because the speed of light (3x1010cm/sec), a huge number, is squared in Einstein’s equation, the energy released is immense!
The energy yield from the decay of one gram of uranium 238 is 2.5x1017ergs or 21 million kilojoules. By comparison, burning one gram of gasoline supplies only 50 kilojoules!
The spontaneous decay or fission (disintegration) of natural uranium is a rare event. The main isotope of uranium, 238U, has a half-life of 4.5×109 years. This means that there was twice as much uranium on earth 4.5billion years ago than there is now. The missing amount gradually disintegrated. (Science is sure of this because the ore sample contains the stable isotopes as evidence of past disintegration!) The other naturally occurring isotope of uranium, 235U, disintegrates far more rapidly. Its half-life is 7×108, or 700million years. Relatively rapid decay has made 235U rare. That is why naturally occurring uranium is 99.3% 238U and 0.7% 235U. The 235U has mostly vanished!
Every time a uranium atom decays or disintegrates, a tiny amount of mass is converted into heat. On its own, the decay of uranium is so slow that no detectable heat is produced. However, the process can be hurried – and must be if useful energy is to be obtained from it. Separating uranium from its rocky ore concentrates it. When a uranium atom (or another radioactive atom) decays, subatomic particles are usually released. If such a particle hits another uranium atom, it too may disintegrate – like a window hit by a baseball. Packing uranium atoms closer together, increases the probability that another uranium nucleus will be hit.
In the case of 235U, a “hit” by a neutron causes it to disintegrate, releasing 3 neutrons. These 3 have the potential to cause 3 more disintegrations . . . etc. This is the “chain reaction” on which the “atom bomb” is based. In a bomb, the chain reaction is “allowed” to run out of control. In a power reactor, the chain reaction must be well controlled by absorbing some of the neutrons. (A “hydrogen bomb” simply adds a fusion (below) nuclear reaction to the uranium fission reaction to create a truly awesome explosion – as if the atom bomb wasn’t enough!)
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