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Most of us have some degree of knowledge as to what fusion is… that it could solve the energy problems of the world and that star trek/wars loves it. What I aim to do in this post however,  is give you a deeper understanding of how humans can harness this furnace of stellar energy, for glorious benefit, and also unspeakable evil.

What is fusion

Fusion is the process by which two small atomic nuclei are forced together to form one larger nucleus. The nucleus is the tiny centre of an atom, less than a billionth of the volume, yet comprising 99.99% of the atoms mass, famously described as a fly in a cathedral. As nuclei get more massive, starting with hydrogen, the binding energy increases, until iron is reached, element number 26, then the trend reverses. The result of this is that energy can be released by starting with a small nucleus (usually H) and moving toward Fe, or coming down from a huge nucleus (U, Pu) toward iron. The former process is fusion (joining), and the latter fission, (splitting).

binding-e-graphFig 1.1, the binding energy of the known elements

Much greater energy is liberated through fusion than fission, (a substantial portion of its mc^2 *) than with fission. However, there is a problem. fission is easy, it only requires that enough of a large unstable nucleus such as uranium be present in a small space and a reaction will occur. However, if this were to occur with hydrogen, the Hindenburg would have made a MUCH larger boom. The difference arises in the nuclear process. Fusion requires the two nuclei to get very close together, and each nucleus is a huge source of positive charge as it contains at least one proton. As anyone who has used magnets will know, like charges repel. This repulsion must be overcome by allowing the atoms to smash together at enormous speeds. Temperature is just a macroscopic term for the average speed of atoms or molecules in a substance. Thus, to fuse nuclei, the fusing material must be hot. Very hot. Under normal conditions, at least 10^6 Kelvin. This can be achieved but is very hard to control, which is why sci-fi loves “cold fusion”, an  improbable concept with current technology. Experiments have shown that fusion is viable but fusion power stations are a long way down the pipeline.

Fusion power is especially desirable as it can be generated from pure water, and produces little or no radioactive waste, in contrast to current nuclear power stations, whose reactors consume hazardous fissile materials and release tonnes of highly radioactive sludge which must be disposed of. Our only current use for fusion is in weaponry, a field in which it excels. The largest fusion weapon ever detonated was the Soviet Czar, a 100 Mt warhead which was scaled down to 50 Mt by using a lead damper in place of the recommended uranium. This has now been shown to be an important modification, as a 100 Mt blast would have had grave consequences for life in the wider area around the test site.

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Fig 1.2, The mushroom cloud of the Czar from the air… The cloud measured 110 km high by 40 wide.

The huge temperatures required to fuse deuterium and tritium, (heavier isotopes of hydrogen which contain more neutrons) are achieved using a fission primary, a conventional fission charge which wraps around the fusion based centre. The fission bomb detonates first and generates temperatures of two million Kelvin, which ignite a fusion reaction in the core of the device. the result is a huge blast which can equal the chemical energy of a cube of TNT 600m to a side, and with a mass of one billion kilograms. the surrounding area, for many kilometres is flattened and the soil vapourised, leaving bare polished rock. even outside the killzone, flash burns, flying glass lacerations and radiation exposure would kill many. These weapons have such unearthly power that they detected around the world as a moderate earthquake. No human being has ever been intentionally killed using fusion, and the day on which that changes, may well be the last day of civilized human existence for many years, possibly forever.

  • The mc^2 of a fuel is the maximum energy obtainable from that amount of matter. It is given, in joules (J) by the mass in kg, multiplied by the square of the speed of light in m/s (3*10^8 ).