The scale of energy released through a fusion reaction (measured in millions of electron-volts - MeV) is around a million times greater than that of the energy released through the breaking of chemical bonds - the bonds between atoms (measured in eV), which is the source of energy for all non-photosynthetic heterotrophic life on Earth - such as Animals and Mushrooms, and that, which is in contained in fossil fuels.
Fusion energy, does not (currently) happen on Earth, but human made fission energy does.
Fusion - the fusing (or synthesis) of light atoms (i.e., hydrogen) into larger, heavier atoms (i.e., helium), releases energy. The final product (i.e., helium) is slightly more stable, and has slightly less mass than the sum of the original two atoms (i.e., two hydrogen atoms); this slight loss of mass has been converted into energy (E = mc2). 'Light' (as in low mass) atoms can be defined as those lighter than Iron (Fe56).
Fission - the splitting of heavy atoms (i.e., uranium and plutonium) into smaller, lighter atoms (i.e., barium and krypton), generating energy. As with fusion, the final product is slightly more stable, and the two products (i.e., barium and krypton) are also very slightly lighter than the original single nucleus (i.e., uranium 235); this slight loss of mass has also been converted into energy (E = mc2). 'Heavy' (as in high mass) atoms can be defined as heavier than Iron (Fe56).
Nuclear Power Stations use the fission reaction to release energy from heavy atoms, which basically, in turn, converts water into steam, turning a turbine, that spins a generator, that creates electricity (the same basic process for all forms of electricity production, except photovoltaics).
The fact that both fusion and fission generate energy may not, at first sight, seem obvious: if splitting larger atoms into smaller atoms, produces energy, then it could be logical to assume that fusing atoms together in larger atoms, would require energy; however, depending (mostly) on whether the nuclei are light or heavy, this is not the case.
Binding EnergyBinding energy is the energy needed to separate a nucleus into its constituent nucleons (protons and neutrons). A system of distinct, unconnected protons and neutrons has a larger mass than an atom of equivalent bound nucleons. The binding energy of a nucleus is equal to the amount of energy released when forming a new nucleus or nuclei through fission or fusion.
Within a nucleus there is an interaction between the electrostatic force repelling the protons - wanting to explode the nucleus apart, and the strong nuclear force - wanting to bind the nucleus tightly together. The electrostatic force can act across the entire nuclei, but the strong nuclear force can only act across the distance of around one nucleon.
In a very small (light) atom, with only one proton like hydrogen (H1), the repulsive electrostatic forces are very low, and the binding strong nuclear forces are, correspondingly, much higher. In a very large (heavy) atom, such as Uranium (U238), the opposite is true - the electrostatic forces are much higher, due to the large number of protons (and neutrons); and the strong nuclear force across the larger nucleus, is correspondingly much lower (as it is only 'strong' over very short distances).
Nuclei that contain more than 209 nucleons are all very unstable, and are subject to spontaneous decay. Atoms with a number of nucleons around 60, such as Iron and Nickel have a balance between these two counter-acting forces (electrostatic force and strong nuclear force) - and can be considered the most stable elements ('Goldilocks' elements) - as they are the most tightly bound.
All elements lighter than Iron and Nickel release energy when they fuse - and become more stable through fusion. By adding nucleons to iron, the product becomes heavier (not lighter), and so more mass means more energy is needed to be added to the system, and so energy is not released. All elements heavier than Iron and Nickel release energy when they break apart - and become more stable through fission. These heavier elements ‘prefer’ to loose nucleons, particularly during impact with other elements, releasing energy, as the products become lighter.
The amount of energy released by one fusion reaction (the proton-proton-chain example) is not as great as by one fission reaction (of uranium 235), however, if considered by energy per mass, then fusion releases far more energy then fission. For example, deuterium (2H) releases 275 million kcal of energy per gram, compared to 20 million kcal of energy per gram of Uranium 235; and also keeping in mind that fossil fuel is limited to about 10 kcal of energy per gram.
As mentioned in the a blog on Nucleosynthesis, binding energy also helps explain why Iron and Nickel are the last elements to be made in Stars (through fusion), as they are stable, they need energy from outside (like all the other following heavier elements) to fuse them with other elements to create the rest of the periodic table.
Fission vs Fusion as an Energy Resource
The debates over nuclear fissions continued use for energy are multi-faceted and polarised, even within the environmental community; and across state lines - i.e., France currently produces over 77% of it's electricity from Nuclear Energy, and Germany, produces less than 14%, with older reactors being decommissioned.
The basic negatives for using fission power is the radioactive waste (which will basically exist forever); it's links to atomic weapon production; when it goes wrong it has huge, devastating effects; the large bodies of water which are required are heated and it changes local ecosystems; and it's power stations are very expensive. The positives are mainly that it can produce a large amount of centralised, continuous, base-load energy (from a relatively small amount of material), and doesn't produce any carbon dioxide (or others toxins - except the radioactive waste, which can be contained) during energy production. However, nuclear power plant construction, and the extraction of uranium or plutonium and other elements, all use fossil fuels, and so the overall CO2 reduction is open to debate. Pro-nuclear can also point out that fossil fuels (taken as the alternative), such as coal and 'natural' gas, also contain radioactive elements, which are not contained, and when burnt can be released in the fly ash, as with the soot, that can be both carcinogenic, darken land surfaces (such as snow) reducing the albedo effect (reflectivity) of Earth, and contain heavy metals such as lead and mercury for example.
Ever since the 1930's, many scientists have also been working on trying to make fusion energy possible on Earth - but as yet, none have succeeded. Although, at least one company - Tri Apha Energy (TAE) in California, USA, seems to be getting closer in recent years, with a hydrogen-boron reactor; it's potential is said to be three to four times more energy per mass of fuel than fission, with virtually no waste, and no risk of melt-downs - watch this space (on Earth)…