Editing Fusion power (section)

== Fuels ==
The fuels considered for fusion power are mainly the heavier isotopes of hydrogen—[[deuterium]] and [[tritium]]. Deuterium is abundant on earth in the form of [[semiheavy water]], and historically co-produced at [[Hydroelectricity|hydroelectric power stations]]. Tritium, decaying with a half-life of 12 years, must be produced. Fusion reactor concepts assume as a component a proposed lithium "[[breeding blanket]]" technology surrounding the reactor.<ref>{{cite web |title=Thermal Discrete Element Analysis of EU Solid Breeder Blanket Subjected to Neutron Irradiation |url=https://hal.science/hal-02356062v1/file/1406.4199.pdf |website=HAL archives ouvertes |publisher=Fusion Science and Technology |access-date=March 24, 2024}}</ref>  [[Helium-3]] is a more speculative fuel, which must be mined extraterrestrially or produced by other nuclear reactions. The protium–boron-11 reaction is extremely speculative, but minimizes neutron radiation.<ref name="AtzeniMeyer-ter-Vehn2004">{{cite book |first1=Stefano |last1=Atzeni |first2=Jürgen |last2=Meyer-ter-Vehn |date=June 3, 2004 |title=The Physics of Inertial Fusion: BeamPlasma Interaction, Hydrodynamics, Hot Dense Matter |url={{google books |plainurl=y |id=BJcy_p5pUBsC}} |publisher=OUP Oxford |pages=12–13 |isbn=978-0191524059}}</ref>

=== Deuterium, tritium ===
{{main article|Deuterium–tritium fusion}}
[[File:Deuterium-tritium fusion.svg|thumb|upright=0.8|Diagram of the [[D+T|D-T]] reaction]]
The easiest nuclear reaction, at the lowest energy, is D+T:

:{{nuclide|Deuterium|link=yes}} + {{nuclide|Tritium|link=yes}} → {{nuclide|Helium|link=yes}} (3.5 MeV) + {{SubatomicParticle|10neutron|link=yes}} (14.1 MeV)

This reaction is common in research, industrial and military applications, usually as a neutron source. [[Deuterium]] is a naturally occurring [[isotope]] of hydrogen and is commonly available. The large mass ratio of the hydrogen isotopes makes their separation easy compared to the [[uranium enrichment]] process. [[Tritium]] is a natural isotope of hydrogen, but because it has a short [[half-life]] of 12.32 years, it is hard to find, store, produce, and is expensive. Consequently, the deuterium-tritium fuel cycle requires the breeding of tritium from [[lithium]] using one of the following reactions:

:{{SubatomicParticle|10neutron}} + {{nuclide|Lithium|6}} → {{nuclide|Tritium}} + {{nuclide|Helium}}
:{{SubatomicParticle|10neutron}} + {{nuclide|Lithium|7}} → {{nuclide|Tritium}} + {{nuclide|Helium}} + {{SubatomicParticle|10neutron}}

The reactant neutron is supplied by the D-T fusion reaction shown above, and the one that has the greatest energy yield. The reaction with <sup>6</sup>Li is [[exothermic reaction|exothermic]], providing a small energy gain for the reactor. The reaction with <sup>7</sup>Li is [[endothermic reaction|endothermic]], but does not consume the neutron. Neutron multiplication reactions are required to replace the neutrons lost to absorption by other elements. Leading candidate neutron multiplication materials are [[beryllium]] and [[lead]], but the <sup>7</sup>Li reaction helps to keep the neutron population high. Natural lithium is mainly <sup>7</sup>Li, which has a low tritium production [[Neutron cross section|cross section]] compared to <sup>6</sup>Li so most reactor designs use [[breeding blanket]]s with enriched <sup>6</sup>Li.

Drawbacks commonly attributed to D-T fusion power include:

* The supply of neutrons results in [[neutron activation]] of the reactor materials.<ref>{{Cite book|last1=Velarde|first1=Guillermo |title=Nuclear fusion by inertial confinement: a comprehensive treatise|last2=Martínez-Val|first2=José María|last3=Ronen|first3=Yigal|date=1993|publisher=CRC Press|isbn=978-0849369261|location=Boca Raton; Ann Arbor; London|language=en|oclc=468393053}}</ref><sup>:242</sup>
* 80% of the resultant energy is carried off by neutrons, which limits the use of direct energy conversion.<ref>{{cite journal |last1=Iiyoshi |first1=A |last2=Momota |first2=H. |last3=Motojima |first3=O. |display-authors=etal |date=October 1993 |title=Innovative Energy Production in Fusion Reactors |url=http://www.nifs.ac.jp/report/nifs250.html |url-status=dead |journal=National Institute for Fusion Science NIFS |pages=2–3 |bibcode=1993iepf.rept.....I |archive-url=https://web.archive.org/web/20150904055903/http://www.nifs.ac.jp/report/nifs250.html |archive-date=September 4, 2015 |access-date=February 14, 2012}}</ref>
* It requires the [[radioisotope]] tritium. Tritium may leak from reactors. Some estimates suggest that this would represent a substantial environmental radioactivity release.<ref>{{Cite web|title=Nuclear Fusion : WNA – World Nuclear Association|url=https://www.world-nuclear.org/information-library/current-and-future-generation/nuclear-fusion-power.aspx|access-date=October 11, 2020|website=www.world-nuclear.org}}</ref>

The [[neutron flux]] expected in a commercial D-T fusion reactor is about 100 times that of fission power reactors, posing problems for [[plasma facing material|material design]]. After a series of D-T tests at [[Joint European Torus|JET]], the vacuum vessel was sufficiently radioactive that it required remote handling for the year following the tests.<ref>{{cite journal|last=Rolfe|first=A. C.|title=Remote Handling JET Experience|journal=Nuclear Energy|date=1999|volume=38|issue=5|page=6|url=http://www.iop.org/Jet/fulltext/JETP99028.pdf|access-date=April 10, 2012|issn=0140-4067}}</ref>

In a production setting, the neutrons would react with lithium in the breeding blanket composed of lithium ceramic pebbles or liquid lithium, yielding tritium. The energy of the neutrons ends up in the lithium, which would then be transferred to drive electrical production. The lithium blanket protects the outer portions of the reactor from the neutron flux. Newer designs, the advanced tokamak in particular, use lithium inside the reactor core as a design element. The plasma interacts directly with the lithium, preventing a problem known as "recycling". The advantage of this design was demonstrated in the [[Lithium Tokamak Experiment]].

=== Deuterium ===
[[File:Deuterium Deuterium Fusion Cross Section.png|thumbnail|upright=1.7|Deuterium fusion cross section (in square meters) at different ion collision energies]]

Fusing two deuterium nuclei is the second easiest fusion reaction. The reaction has two branches that occur with nearly equal probability:

:{{nuclide|Deuterium}} + {{nuclide|Deuterium}} → {{nuclide|Tritium}} + {{nuclide|Hydrogen}}
:{{nuclide|Deuterium}} + {{nuclide|Deuterium}} → {{nuclide|Helium|3}} + {{SubatomicParticle|10neutron}}

This reaction is also common in research. The optimum energy to initiate this reaction is 15&nbsp;keV, only slightly higher than that for the D-T reaction. The first branch produces tritium, so that a D-D reactor is not tritium-free, even though it does not require an input of tritium or lithium. Unless the tritons are quickly removed, most of the tritium produced is burned in the reactor, which reduces the handling of tritium, with the disadvantage of producing more, and higher-energy, neutrons. The neutron from the second branch of the D-D reaction has an energy of only {{convert|2.45|MeV|abbr=on}}, while the neutron from the D-T reaction has an energy of {{convert|14.1|MeV|abbr=on}}, resulting in greater isotope production and material damage. When the tritons are removed quickly while allowing the <sup>3</sup>He to react, the fuel cycle is called "tritium suppressed fusion".<ref>{{Cite journal |last1=Sawan |first1=M. E. |last2=Zinkle |first2=S. J. |last3=Sheffield |first3=J. |date=2002 |title=Impact of tritium removal and He-3 recycling on structure damage parameters in a D–D fusion system |url=http://dx.doi.org/10.1016/s0920-3796(02)00104-7 |journal=Fusion Engineering and Design |volume=61–62 |pages=561–567 |doi=10.1016/s0920-3796(02)00104-7 |bibcode=2002FusED..61..561S |issn=0920-3796}}</ref> The removed tritium decays to <sup>3</sup>He with a 12.5 year half life. By recycling the <sup>3</sup>He decay product into the reactor, the fusion reactor does not require materials resistant to fast neutrons.

Assuming complete tritium burn-up, the reduction in the fraction of fusion energy carried by neutrons would be 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 lithium resources and a somewhat softer neutron spectrum. The disadvantage of D-D compared to D-T is that the energy confinement time (at a given pressure) must be 30 times longer and the power produced (at a given pressure and volume) is 68 times less.{{Citation needed|date=November 2014}}

Assuming complete removal of tritium and <sup>3</sup>He recycling, only 6% of the fusion energy is carried by neutrons. The tritium-suppressed D-D fusion requires an energy confinement that is 10 times longer compared to D-T and double the plasma temperature.<ref>J. Kesner, D. Garnier, A. Hansen, M. Mauel, and L. Bromberg, ''Nucl Fusion'' 2004; 44, 193</ref>

=== Deuterium, helium-3 ===
A second-generation approach to controlled fusion power involves combining [[helium-3]] (<sup>3</sup>He) and [[deuterium]] (<sup>2</sup>H):

:{{nuclide|Deuterium}} + {{nuclide|Helium|3}} → {{nuclide|Helium}} + {{nuclide|Hydrogen}}

This reaction produces <sup>4</sup>He and a high-energy proton. As with the p-<sup>11</sup>B [[aneutronic fusion]] fuel cycle, most of the reaction energy is released as charged particles, reducing [[neutron activation|activation]] of the reactor housing and potentially allowing more efficient energy harvesting (via any of several pathways).<ref name="Advanced Fuels">{{Cite journal|last=Nevins|first=W. M.|date=March 1, 1998|title=A Review of Confinement Requirements for Advanced Fuels|url=https://doi.org/10.1023/A:1022513215080|journal=Journal of Fusion Energy|language=en|volume=17|issue=1|pages=25–32|doi=10.1023/A:1022513215080|bibcode=1998JFuE...17...25N|s2cid=118229833|issn=1572-9591}}</ref> In practice, D-D side reactions produce a significant number of neutrons, leaving p-<sup>11</sup>B as the preferred cycle for aneutronic fusion.<ref name="Advanced Fuels" />

=== Proton, boron-11 ===
Both material science problems and non-proliferation concerns are greatly diminished by [[aneutronic fusion]]. Theoretically, the most reactive aneutronic fuel is <sup>3</sup>He. However, obtaining reasonable quantities of <sup>3</sup>He implies large scale extraterrestrial mining on the Moon or in the atmosphere of Uranus or Saturn. Therefore, the most promising candidate fuel for such fusion is fusing the readily available protium (i.e. a [[proton]]) and [[boron]]. Their fusion releases no neutrons, but produces energetic charged alpha (helium) particles whose energy can directly be converted to electrical power:

:{{nuclide|Hydrogen}} + {{nuclide|Boron|11}} → 3 {{nuclide|Helium}}

Side reactions are likely to yield neutrons that carry only about 0.1% of the power,<ref>{{Cite book |title=Emerging nuclear energy systems 1989: proceedings of the Fifth International Conference on Emerging Nuclear Energy Systems, Karlsruhe, F.R. Germany, July 3–6, 1989|date=1989|publisher=World Scientific |editor=von Möllendorff, Ulrich |editor2=Goel, Balbir |isbn=981-0200102|location=Singapore|oclc=20693180}}</ref><sup>:177–182</sup> which means that [[neutron scattering]] is not used for energy transfer and material activation is reduced several thousand-fold. The optimum temperature for this reaction of 123 keV<ref>{{Cite journal|last1=Feldbacher|first1=Rainer|last2=Heindler|first2=Manfred|date=1988|title=Basic cross section data for aneutronic reactor |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment|volume=271|issue=1|pages=55–64|doi=10.1016/0168-9002(88)91125-4|bibcode=1988NIMPA.271...55F|issn=0168-9002}}</ref> is nearly ten times higher than that for pure hydrogen reactions, and energy confinement must be 500 times better than that required for the D-T reaction. In addition, the [[power density]] is 2500 times lower than for D-T, although per unit mass of fuel, this is still considerably higher compared to fission reactors.

Because the confinement properties of the tokamak and laser pellet fusion are marginal, most proposals for aneutronic fusion are based on radically different confinement concepts, such as the [[Polywell]] and the [[Dense Plasma Focus]]. In 2013, a research team led by [[Christine Labaune]] at [[École Polytechnique]], reported a new fusion rate record for proton-boron fusion, with an estimated 80&nbsp;million fusion reactions during a 1.5 nanosecond laser fire, 100 times greater than reported in previous experiments.<ref>{{cite web|url=http://www.livescience.com/40246-new-boron-method-nuclear-fusion.html|title=Nuclear Fusion: Laser-Beam Experiment Yields Exciting Results|website=LiveScience.com|date=October 8, 2013}}</ref><ref>{{cite web|url=http://www.fusenet.eu/node/575|title=Record proton-boron fusion rate achieved – FuseNet|website=www.fusenet.eu|access-date=November 26, 2014|archive-url=https://web.archive.org/web/20141202062802/http://www.fusenet.eu/node/575|archive-date=December 2, 2014|url-status=dead}}</ref>