Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus (in contrast with fission power). In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested as an increase in temperature of the reactants. Fusion power is a primary area of research in plasma physics.
The term is commonly used to refer to potential commercial production of net usable power from a fusion source, similar to the usage of the term "steam power." The leading designs for controlled fusion research use magnetic (tokamak design) or inertial (laser) confinement of a plasma, with heat from the fusion reactions used to operate a steam turbine which in turn drives electrical generators, similar to the process used in fossil fuel and nuclear fission power stations.
Fusion power is believed to have significant safety advantages over current power stations based on nuclear fission. Fusion only takes place under very limited and controlled circumstances, and requires a constant feed of new fuel to maintain the reaction, so the cessation of active fuelling or simple changes to the control system quickly shuts down fusion power reactions.
By comparison, fission reactors only require that there is sufficient fuel within a small enough space, and are subject to catastrophic failures that self-maintain the reaction, notably a meltdown. In a fusion reactor there is no possibility of runaway heat build-up or large-scale release of radioactivity, little or no atmospheric pollution, the power source comprises light elements in small quantities, the waste products are short-lived in terms of radioactivity.
Fusion powered electricity generation was initially believed to be readily achievable, as fission power had been. However the extreme requirements for continuous reactions and plasma containment led to projections being extended by several decades. In 2010, more than 60 years after the first attempts, commercial power production is still believed to be unlikely before 2050.
As of July 2010, the largest experiment by means of magnetic confinement has been the Joint European Torus (JET). In 1997, JET produced a peak of 16.1 megawatts (21,600 hp) of fusion power (65% of input power), with fusion power of over 10 MW (13,000 hp) sustained for over 0.5 sec. In June 2005, its successor, ITER, was announced by the seven parties involved in the project - the United States, China, the European Union (EU), India, Japan, the Russian Federation, and South Korea. ITER is designed to produce ten times more fusion power than the power put into the plasma over many minutes; for example 50 MW of input power to produce 500 MW of output power. ITER is currently under construction in Cadarache, France. DEMO is intended as the next generation of research from ITER, and to be the first reactor demonstrating sustained net energy-producing fusion on a commercial scale. It has been proposed to begin construction of DEMO in 2024.
Inertial (laser) confinement, which was for a time seen as more difficult or infeasible, has generally seen less development effort than magnetic approaches. However this approach made a comeback following further innovations, and is being developed at both the United States National Ignition Facility as well as the planned European Union High Power laser Energy Research (HiPER) facility. As of 2010 heating to 3.3 million Kelvin was achieved and in October 2010 the first integrated ignition test was announced to have been completed successfully with the 192-beam laser system firing over a million joules of ultraviolet laser energy into a capsule filled with hydrogen fuel. Fusion ignition tests are to follow.
The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The deuterium-tritium fusion rate peaks at a lower temperature (about 70 keV, or 800 million kelvin) and at a higher value than other reactions commonly considered for fusion energy.The basic concept behind any fusion reaction is to bring two or more nuclei close enough together so that the residual strong force (nuclear force) in their nuclei will pull them together into one larger nucleus. If two light nuclei fuse, they will generally form a single nucleus with a slightly smaller mass than the sum of their original masses (though this is not always the case). The difference in mass is released as energy according to Albert Einstein's mass-energy equivalence formula E = mc2. If the input nuclei are sufficiently massive, the resulting fusion product will be heavier than the sum of the reactants' original masses, in which case the reaction requires an external source of energy. The dividing line between "light" and "heavy" is iron-56. Above this atomic mass, energy will generally be released by nuclear fission reactions; below it, by fusion.
Fusion between the nuclei is opposed by their shared electrical charge, specifically the net positive charge of the protons in the nucleus. 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 favoured as a fusion product. As a consequence, most fusion reactions combine isotopes of hydrogen ("protium", deuterium, or tritium) to form isotopes of helium (3He or 4He).
The reaction cross section, denoted σ, is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei. If the reactants have a distribution of velocities, as is the case in a thermal distribution within a plasma, then it is useful to perform an average over the distributions of the product of cross section and velocity. The reaction rate (fusions per volume per time) is <σv> times the product of the reactant number densities:
Perhaps the three most widely considered fuel cycles are based on the D-T, D-D, and p-11B reactions. Other fuel cycles (D-3He and 3He-3He) would require a supply of 3He, either from other nuclear reactions or from extraterrestrial sources, such as the surface of the moon or the atmospheres of the gas giant planets. The details of the calculations comparing these reactions can be found here.
Several drawbacks are commonly attributed to D-T fusion power:
In a production setting, the neutrons would be used to react with lithium in order to create more tritium. This also deposits the energy of the neutrons in the lithium, which would then be cooled to remove this energy and drive electrical production. This reaction protects the outer portions of the reactor from the neutron flux. Newer designs, the advanced tokamak in particular, also use lithium inside the reactor core as a key element of the design. The plasma interacts directly with the lithium, preventing a problem known as "recycling". The advantage of this layout was demonstrated in the Lithium Tokamak Experiment.
Assuming complete tritium burn-up, the reduction in the fraction of fusion energy carried by neutrons is only about 18%, so that the primary advantage of the D-D fuel cycle is that tritium breeding is not required. Other advantages are independence from limitations of lithium resources and a somewhat softer neutron spectrum. The price to pay compared to D-T is that the energy confinement (at a given pressure) must be 30 times better and the power produced (at a given pressure and volume) is 68 times less.
The term is commonly used to refer to potential commercial production of net usable power from a fusion source, similar to the usage of the term "steam power." The leading designs for controlled fusion research use magnetic (tokamak design) or inertial (laser) confinement of a plasma, with heat from the fusion reactions used to operate a steam turbine which in turn drives electrical generators, similar to the process used in fossil fuel and nuclear fission power stations.
Fusion power is believed to have significant safety advantages over current power stations based on nuclear fission. Fusion only takes place under very limited and controlled circumstances, and requires a constant feed of new fuel to maintain the reaction, so the cessation of active fuelling or simple changes to the control system quickly shuts down fusion power reactions.
By comparison, fission reactors only require that there is sufficient fuel within a small enough space, and are subject to catastrophic failures that self-maintain the reaction, notably a meltdown. In a fusion reactor there is no possibility of runaway heat build-up or large-scale release of radioactivity, little or no atmospheric pollution, the power source comprises light elements in small quantities, the waste products are short-lived in terms of radioactivity.
Fusion powered electricity generation was initially believed to be readily achievable, as fission power had been. However the extreme requirements for continuous reactions and plasma containment led to projections being extended by several decades. In 2010, more than 60 years after the first attempts, commercial power production is still believed to be unlikely before 2050.
As of July 2010, the largest experiment by means of magnetic confinement has been the Joint European Torus (JET). In 1997, JET produced a peak of 16.1 megawatts (21,600 hp) of fusion power (65% of input power), with fusion power of over 10 MW (13,000 hp) sustained for over 0.5 sec. In June 2005, its successor, ITER, was announced by the seven parties involved in the project - the United States, China, the European Union (EU), India, Japan, the Russian Federation, and South Korea. ITER is designed to produce ten times more fusion power than the power put into the plasma over many minutes; for example 50 MW of input power to produce 500 MW of output power. ITER is currently under construction in Cadarache, France. DEMO is intended as the next generation of research from ITER, and to be the first reactor demonstrating sustained net energy-producing fusion on a commercial scale. It has been proposed to begin construction of DEMO in 2024.
Inertial (laser) confinement, which was for a time seen as more difficult or infeasible, has generally seen less development effort than magnetic approaches. However this approach made a comeback following further innovations, and is being developed at both the United States National Ignition Facility as well as the planned European Union High Power laser Energy Research (HiPER) facility. As of 2010 heating to 3.3 million Kelvin was achieved and in October 2010 the first integrated ignition test was announced to have been completed successfully with the 192-beam laser system firing over a million joules of ultraviolet laser energy into a capsule filled with hydrogen fuel. Fusion ignition tests are to follow.
Fuel cycle
Fusion between the nuclei is opposed by their shared electrical charge, specifically the net positive charge of the protons in the nucleus. 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 favoured as a fusion product. As a consequence, most fusion reactions combine isotopes of hydrogen ("protium", deuterium, or tritium) to form isotopes of helium (3He or 4He).
The reaction cross section, denoted σ, is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei. If the reactants have a distribution of velocities, as is the case in a thermal distribution within a plasma, then it is useful to perform an average over the distributions of the product of cross section and velocity. The reaction rate (fusions per volume per time) is <σv> times the product of the reactant number densities:
- ƒ = (½n)2 <σv> (for one reactant)
- ƒ = n1n2 <σv> (for two reactants)
Perhaps the three most widely considered fuel cycles are based on the D-T, D-D, and p-11B reactions. Other fuel cycles (D-3He and 3He-3He) would require a supply of 3He, either from other nuclear reactions or from extraterrestrial sources, such as the surface of the moon or the atmospheres of the gas giant planets. The details of the calculations comparing these reactions can be found here.
D-T fuel cycle
The easiest (according to the Lawson criterion) and most immediately promising nuclear reaction to be used for fusion power is:- 2 1D + 3 1T → 4 2He + 1 0n
- 1 0n + 6 3Li → 3 1T + 4 2He
- 1 0n + 7 3Li → 3 1T + 4 2He + 1 0n
Several drawbacks are commonly attributed to D-T fusion power:
- It produces substantial amounts of neutrons that result in induced radioactivity within the reactor structure.
- Only about 20% of the fusion energy yield appears in the form of charged particles (the rest neutrons), which limits the extent to which direct energy conversion techniques might be applied.
- The use of D-T fusion power depends on lithium resources, which are less abundant than deuterium resources. However, lithium is relatively abundant on earth.
- It requires the handling of the radioisotope tritium. Similar to hydrogen, tritium is difficult to contain and may leak from reactors in some quantity. Some estimates suggest that this would represent a fairly large environmental release of radioactivity.
In a production setting, the neutrons would be used to react with lithium in order to create more tritium. This also deposits the energy of the neutrons in the lithium, which would then be cooled to remove this energy and drive electrical production. This reaction protects the outer portions of the reactor from the neutron flux. Newer designs, the advanced tokamak in particular, also use lithium inside the reactor core as a key element of the design. The plasma interacts directly with the lithium, preventing a problem known as "recycling". The advantage of this layout was demonstrated in the Lithium Tokamak Experiment.
D-D fuel cycle
Though more difficult to facilitate than the deuterium-tritium reaction, fusion can also be achieved through the reaction of deuterium with itself. This reaction has two branches that occur with nearly equal probability:-
2H + 2H → 3T + 1H → 3He + n
Assuming complete tritium burn-up, the reduction in the fraction of fusion energy carried by neutrons is only about 18%, so that the primary advantage of the D-D fuel cycle is that tritium breeding is not required. Other advantages are independence from limitations of lithium resources and a somewhat softer neutron spectrum. The price to pay compared to D-T is that the energy confinement (at a given pressure) must be 30 times better and the power produced (at a given pressure and volume) is 68 times less.
D-3He fuel cycle
A second-generation approach to controlled fusion power involves combining helium-3 (3He) and deuterium (2H). This reaction produces a helium-4 nucleus (4He) and a high-energy proton. As with the p-11B aneutronic fusion fuel cycle, most of the reaction energy is released as charged particles, reducing activation of the reactor housing and potentially allowing more efficient energy harvesting (via any of several speculative technologies). In practice, D-D side reactions produce a significant number of neutrons, resulting in p-11B being the preferred cycle for aneutronic fusion.p-11B fuel cycle
If aneutronic fusion is the goal, then the most promising candidate may be the Hydrogen-1 (proton)/boron reaction:- 1H + 11B → 3 4He
