What is nuclear fusion reaction list any two advantages?

Answer 1

Nuclear fusion is the process of joining two atomic nuclei (two sets of protons and neutrons) together. If the nuclei are smaller than iron or nickel nuclei, there is a net release of energy (more energy comes out of the new nucleus than was used to make it). If the nuclei are larger than that, there is a net loss of energy (it takes more energy to combine the nuclei than you get back out).

Nuclear fusion is the process that powers stars and is what allows them to resist their own enormous gravity - they exist in a precarious equilibrium between being crushed and blowing themselves apart. In small stars like the sun the elements being fused are hydrogen isotopes, sometimes using carbon, nitrogen and oxygen as catalysts. The end product is helium and several other sub-atomic particles. Larger stars can fuse heavier elements - up to iron and nickel.

Once the star has an iron core, fusion stops and the star collapses under its own weight. The outer layers bounce off the core and destroy the star in a supernova. In these events temperatures and pressures rise high enough for all the remaining heavy elements to be produced.

Nuclear fusion on earth is achievable and several experimental reactors have produced power from fusion of hydrogen isotopes. Most notably, JET (Joint European Torus) in Oxfordshire, England, produced a world record of 16MW of fusion power for (I believe) six seconds.

Currently being built, ITER in southern France has been designed to produce 500MW of power for 50MW input. Assembly of the reactor buildings is due to start in 2015 and the first plasma is due in 2019. If it is successful, a follow-up plant called DEMO will attempt to commercialise fusion power.

Fusion is regarded as a cleaner source of energy than either fossil fuels (since there are no CO2 emissions) or conventional nuclear fission (no long-lived highly radioactive waste) but is not without problems. Foremost amongst those is the fact that even though the only waste product is helium gas, the reactor walls are badly damaged by the fast neutrons produced in the reaction and will need replacement every so often. The walls will become 'activated' by the neutron bombardement and will themselves need to be treated as radioactive waste.

Fusion power as currently envisaged has another advantage - the fuels can be 'bred' within the reactor and extracted from seawater. Furthermore, since fusion is a nuclear process rather than chemical, the energy output per gram of fuel is orders of magnitude greater: one deuterium nucleus and one tritium nucleus together mass just 8.35x10^-27kg and produce 17.6MeV of energy. That means a total of 8.35kg of fuel will produce 2.8x10^15J or 2.8 million gigajoules of energy. In contrast, the same mass of oil would only produce 390 megajoules of energy (1 GJ = 1000 MJ).

I recommend Google and wikipedia for more information. Many of the pages are quite good as well as accessible.

Konrad Harradine