When a uranium atom is split into or more atoms of medium relative atomic mass, there is a decrease in mass and a corresponding release of energy. The packing fraction curve shows that there would be a similar result if two light nuclei could be combined together to give a heavier one. Combination or fusion of two nuclei is not, however, at all easy. They are both positively charged and repel each other. At high temperatures, however, nuclei may have sufficient energy to enable them to overcome the repulsive forces and to unite. This is the very principle behind the manufacture of hydrogen bomb.
1. Stellar Energy: Von Weizsacker and Bethe proposed, in 1938, that the vast amount of energy emitted by the sun and by stars might originate from a process involving nuclear fusion.
The modern view is that there are two cycles, one producing the energy in very hot stars, and the other taking place in cooler stars, such as the sun.
The overall result is that four protons combine to give one helium nucleus, with the emission of γ-rays and two positrons
12C appears to act in the nature of a catalyst. The energy liberated by such a cycle of changes is in agreement with the energy liberated by hot stars, and each stage of the cycle can be simulated in the laboratory.
In cooler stars, such as the sun, the cycle of changes is thought to be
2. The Hydrogen Bomb: The temperature attained when 235U or plutonium atoms undergo fission in an explosive chain reaction are high enough to initiate fusion, or thermonuclear, reaction involving bombs, in effects, a 235U or plutonium fission explosion acts as a detonator for a fusion reaction.
Mixtures containing tritium, 3H, are easiest to explode but suffer from the disadvantage that tritium, which has to be manufactured by neutron bombardment of lithium-6
Is not stable. Deuterium is, therefore, probably the main component of a hydrogen bomb. If reaction b could made to take place completely, 1 kg of deuterium would provide as much energy as 2865000 kg of coal.
The advantage, if that be the right word, of hydrogen bomb is that unlimited amounts of hydrogen isotope mixtures can be detonated as the size of a single bomb is not limited by critical-size restrictions as it is in a uranium bomb. Bombs much more powerful than uranium bombs have, in fact, been exploded.
3. Controlled Thermonuclear Fusions: Efforts to control a useful thermonuclear fusion reaction have not yet been successful. If they ever are, a new source of energy will become available.
Bombardment of heavy water with accelerated deuterons does lead to the reaction,
For neutrons are emitted, but very few fusions take place, and more energy has to be put into the accelerated deuterons than is obtained from the reaction.
For successful fusion reactions, which would liberate more energy than is needed to initiate them, it seems likely that temperatures of the order of 108 K will have to be achieved, and no constructional material will withstand such temperatures. The problem may not, however, be insoluble, and several hopeful lines are being investigated.
In one approach a current is passed through a gas in a tube, squeezing the gas into a narrow column along the axis of the tube. This happens because magnetic forces pinch the gas into the centre. An additional magnetic field along the axis of the tube, provided by an external electromagnet, also helps to keep the gas on the axis of the tube, and away from the walls. It is hoped, in this way, to be able to heat deuterium gas, out of contact with any material which might melt, by passing very high current through it. None of the experiments have yet been fully successful, but if they can be developed, or some alternative method of heating devised, deuterium, which is relatively cheap and relatively abundant, and may be the fuel of the future.