FusionNuclear fusion occurs when lighter elements, which have a lower binding energy per nucleon, fuse together to form a larger nucleus with a greater binding energy per nucleon. The difference in binding energy appears as heat. The nucleus formed has greater nuclear stability than the nuclei which fused to form it. All stars on the main sequence produce energy from the fusion of hydrogen to helium in a three step two-body sequence of reactions.
1H + 1H --> 2D + e- + ne 2D + 2D --> 3He + g 3He + 3He --> 4He + 1H + 1H The details of this sequence are not required for IB examination purposes. The reaction occurs in stars on the main sequence once their internal temperature reaches about 106 K due to gravitational collapse. Other nuclear fusion reactions can proceed in stars if their internal temperature is higher. Carbon formation or "helium burning" begins at about 100x106 K.
This reaction may proceed through an unstable beryllium intermediate:
24He --> 8Be 8Be + 4He --> 12C Beryllium is far less common in the universe than is hydrogen, helium, or carbon.
The stellar nuclear fusion reactions require very temperatures. The energy released by a nuclear fission bomb can provide sufficient energy to effect the easier fusion reactions of the heavier isotopes of hydrogen 2D (deuterium) and 3T (tritium) to helium:
22D --> 4He 2D + 3T --> 4He + n + 17.6 MeV These reactions have been used in thermonuclear weapons (hydrogen bombs). They release vast amounts of energy.
Attempts have been made for many years to construct fusion generators using these reactions under controlled conditions. In spite of very large amounts of development money, especially in the US and the former USSR, no controlled fusion reaction has yet produced net energy.
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