Energy is released when two light nuclei are fused together as the heavier fused nucleus is more tightly bound.
The it important to note that the energy released is very large per unit mass of material.
For the nuclei to fuse they must overcome their mutual electrostatic repulsion which is possible at temperatures between 108 K and 109 K. If a fusion reaction does occur then there enough energy released keep the reaction going. The challenges to sustainable fusion are reaching the appropriate temperatues and containing the plasma with out the plasma touching the containment vessel.
Nuclear Fission
Energy is released when a neutron causes the fission of a fissionable isotope such as 235U.
The energy released in this reaction is ~ 200 MeV split approximately. This is 45 million times greater energy per atom of fuel than would be released in a chemical reaction.
| • Kinetic Energy from fission fragments | 165 ± 5 MeV |
| • Prompt γ−ray energy | 7 ± 1 MeV |
| • Kinetic Energy of fission neutrons | 5 ± 0.5 MeV |
| • β rays from fission products | 7 ± 1 MeV |
| • α rays from fission products | 6 ± 1 MeV |
| • Neutrino's from fission products | 10 ± 1 MeV |
The release of fission neutrons is crucial as it allows the possibility of a chain reaction. Neutrons are uncharged so can approach a nucleus at low energies without being repulsed by the coulomb force. The cross-section of interaction is greater at low energies.
A reproduction constant k is defined as
k > 1 leads to a supercritical reaction which is used in a bomb
k < 1 leads to a subcritical reaction and the reactions dies out
k = 1 is known as critical and leads to a stable reaction
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