Fusion
Harnessing nuclear fusion has been one of the dreams of the
nuclear age. The attraction of fusion stems from two facts:
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1. it relies on fuel that is cheap and abundant and 2. it produces virtually no radioactive waste. |
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What
is Fusion?
Fusion is where two light atomic nuclei combine to form a
heavier nucleus with the release of a large amount of energy.
An example is set out below:
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When the two particles fuse together their combined mass is less than the sum of separate masses originally. The mass defect is converted to energy – so of it binding energy, the rest released in the reaction.
In order that the two positively charged atoms fuse together, they must approach each other at very high speeds in order to overcome the coulomb repulsive forces between them. The high speeds could be achieved using particle acceleration, but the electrical energy that the resulted energy would yield.
Alternatively, the high speeds could be achieved by heating deuterium gas to extremely high temperatures – 10 to the power of 8k whereupon it becomes fully ionised and is called a plasma. Increasing the plasma’s temperature causes it to expand and so the density of particles falls and so the number of fusions (yielding energy) falls. The difficulty of confining the plasma in a small enough volume at a temperature of 10 to the power of 8k is the major stumbling block at present.
One possibility is to use magnetic fields surrounding a dough-nut shaped vacuum chamber (torus) to control the plasma. The problem of confining the plasma using magnetic field lines has been compared to confining jelly with rubber bands.
Suitable
conditions for fusions exist in the interior of stars. The enormous
heat output of the sun is thought to be due to the fusion of
hydrogen nuclei into helium. So far man’s only real success with
worthwhile energy yields from fusion is the hydrogen bomb.
Fusion research
The JET (Joint European Torus) in England has come closest to
demonstrating the feasibility of fusion. . It consists of a
doughnut-shaped vacuum chamber surrounded by magnets.
The aim is to heat hydrogen atoms to temperatures up to 10 times
higher than the centre of the sun, which causes them to break apart
and become a “plasma” ---a form of matter approached only by
lightning under normal circumstances on earth. Within this plasma,
atomic nuclei re-form to produce helium and in the process release a
great deal of energy.
What are the difficulties with fusion?
The great challenge of these experiments is confinement. Confining
plasma with lines of magnetic force has been compared to trying to
confine jelly with rubber bands. As the temperature increases,
impurities from the vessel wall enter the plasma, causing problems.