Nuclear Fusion Power

Fusion power refers to power generated by nuclear fusion reactions.   In this kind of reaction, two light atomic nuclei fuse together to form a heavier nucleus and release energy.   Most design studies for fusion power plants involve using the fusion reactions to create heat, which is then used to operate a steam turbine, similar to most coal-fired power stations as well as fission-driven nuclear power stations.   A fusion reactor will heat plasma to temperatures which are ten times those in the core of the sun.   Harnessing such extremes in an engineered "bottle" will take many decades, and ultimately may not be practical.   The largest current experiment, JET, has resulted in fusion power production slightly less than the power put into the plasma, maintaining an output of 16 MW for a few seconds.   In June 2005, the construction of the experimental reactor ITER, designed to produce several times more fusion power than the power put into the plasma over many minutes, was announced.   The production of net electrical power from fusion is planned for DEMO, the next generation experiment after ITER. ^{Wiki n.p.}

The basic concept behind any fusion reaction is to bring two or more atoms very close together, close enough that the strong nuclear force in their nuclei will pull them together into one larger atom.   If two light nuclei fuse, they will generally form a single nucleus with a slightly smaller mass than the sum of their original masses.   The difference in mass is released as energy according to Einstein's mass-energy equivalence formula E = mc˛.   If the input atoms are sufficiently massive, the resulting fusion product will be heavier than the reactants, in which case the reaction requires an external source of energy.   The dividing line between "light" and "heavy" is iron. Above this atomic mass, energy will generally be released in nuclear fission reactions, below it, in fusion. ^{Wiki n.p.}

Fusion between the atoms is opposed by their shared electrical charge, specifically the net positive charge of the nuclei.   In order to overcome this electrostatic force, or "Coulomb barrier", some external source of energy must be supplied.   The easiest way to do this is to heat the atoms, which has the side effect of stripping the electrons from the atoms and leaving them as bare nuclei.   In most experiments the nuclei and electrons are left in a fluid known as a plasma.   The temperatures required to provide the nuclei with enough energy to overcome their repulsion is a function of the total charge, so hydrogen, which has the smallest nuclear charge therefore reacts at the lowest temperature.   Helium has an extremely low mass per nucleon and therefore is energetically favored as a fusion product.   Therefore, most fusion reactions combine isotopes of hydrogen ("protium", deuterium, or tritium) to form isotopes of helium (3He or 4He). ^{Wiki n.p.}

The D-T fuel cycle:   D + T ----> 4He + n

Deuterium is a naturally occurring isotope of hydrogen and as such is universally available.   The large mass ratio of the hydrogen isotopes makes the separation rather easy compared to the difficult uranium enrichment process.   Tritium is also an isotope of hydrogen, but it occurs naturally in only negligible amounts due to its radioactive half-life of 12.34 years.   Consequently, the deuterium-tritium fuel cycle requires the breeding of tritium from lithium using one of the following reactions: ^{Wiki n.p.}

n + 6Li ---> T + 4He

n + 7Li ---> T + 4He + n

Other fuel cycles are possible.

The natural product of the fusion reaction is a small amount of helium, which is completely harmless to life and does not contribute to global warming.   Of more concern is tritium, which, like other isotopes of hydrogen, is difficult to retain completely.   During normal operation, some amount of tritium will be continually released.   There would be no acute danger, but the cumulative effect on the world's population from a fusion economy could be a matter of concern.   The 12 year half-life of tritium would at least prevent unlimited build-up and long-term contamination. ^{Wiki n.p.}

The large flux of high-energy neutrons in a reactor will make the structural materials radioactive.   The radioactive inventory at shut-down may be comparable to that of a fission reactor, but there are important differences.   The half-life of the radioisotopes produced by fusion tend to be less than those from fission, so that the inventory decreases more rapidly.   Furthermore, there are fewer unique species, and they tend to be non-volatile and biologically less active.   Unlike fission reactors, whose waste remains dangerous for thousands of years, most of the radioactive material in a fusion reactor would be the reactor core itself, which would be dangerous for about 50 years, and low-level waste another 100.   By 300 years the material would have the same radioactivity as coal ash.   Some material will remain in current designs with longer half-lives. ^{Wiki n.p.}

Large-scale reactors using neutronic fuels (e.g. ITER) and thermal power production (turbine based) are most comparable to fission power from an engineering and economics viewpoint.   Both fission and fusion power plants involve a relatively compact heat source powering a conventional steam turbine-based power plant, while producing enough neutron radiation to make activation of the plant materials problematic.   The main distinction is that fusion power produces no high-level radioactive waste, although activated plant materials still need to be disposed of. ^{Wiki n.p.}

It is far from clear whether nuclear fusion will be economically competitive with other forms of power.   The many estimates that have been made of the cost of fusion power cover a wide range, and indirect costs of and subsidies for fusion power and its alternatives make any cost comparison difficult.   The low estimates for fusion appear to be competitive with but not drastically lower than other alternatives.   The high estimates are several times higher than alternatives. ^{Wiki n.p.}

Fusion power has many of the benefits of long-term renewable energy sources with no greenhouse gas emissions as well as some of the benefits of such relatively finite energy sources as hydrocarbons and nuclear fission.   Like these currently dominant energy sources, fusion could provide very high power-generation density and uninterrupted power delivery. ^{Wiki n.p.}

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