Editing Fusion power (section)

== Material selection ==
{{Further|International Fusion Materials Irradiation Facility}}

Structural material stability is a critical issue.<ref name="Materials">{{Cite book|last=Roberts, J. T. Adrian |title=Structural Materials in Nuclear Power Systems|date=1981|publisher=Springer US|isbn=978-1468471960|location=Boston, MA|oclc=853261260}}</ref><ref>{{Cite web|date=September 9, 2021|title=Roadmap highlights materials route to fusion|url=https://www.theengineer.co.uk/fusion-materials/|access-date=September 17, 2021|website=The Engineer|language=en-US}}</ref> Materials that can survive the high temperatures and neutron bombardment experienced in a fusion reactor are considered key to success.<ref>{{Cite journal |last=Klueh |first=R. L. |title=Metals in the nuclear-fusion environment |journal=Materials Engineering |volume=99 |pages=39–42}}</ref><ref name="Materials" /> The principal issues are the conditions generated by the plasma, neutron degradation of wall surfaces, and the related issue of plasma-wall surface conditions.<ref>{{Cite thesis|title=Interaction of atomic hydrogen with materials used for plasma-facing wall in fusion devices |type=Doctorate |publisher=[A. Založnik]|date=2016|place=Ljubljana|language=en|first=Anže|last=Založnik|oclc = 958140759}}</ref><ref>{{Cite journal |last=McCracken |first=G. M. |date=1997 |title=Plasma surface interactions in controlled fusion devices |url=http://dx.doi.org/10.1088/0029-5515/37/3/413 |journal=Nuclear Fusion |volume=37 |issue=3 |pages=427–429 |doi=10.1088/0029-5515/37/3/413 |issn=0029-5515 |s2cid=250776874}}</ref> Reducing hydrogen permeability is seen as crucial to hydrogen recycling<ref>{{Citation|last=Mioduszewski|first=Peter|title=Hydrogen Recycling and Wall Equilibration in Fusion Devices |date=2000|url=http://dx.doi.org/10.1007/978-94-011-4331-8_23|work=Hydrogen Recycling at Plasma Facing Materials|pages=195–201|place=Dordrecht|publisher=Springer Netherlands|doi=10.1007/978-94-011-4331-8_23|isbn=978-0792366300|access-date=October 13, 2020}}</ref> and control of the tritium inventory.<ref name="permeation">{{Cite journal|last=Nemanič|first=Vincenc|date=2019|title=Hydrogen permeation barriers: Basic requirements, materials selection, deposition methods, and quality evaluation|journal=Nuclear Materials and Energy|volume=19|pages=451–457|doi=10.1016/j.nme.2019.04.001|issn=2352-1791|doi-access=free|bibcode=2019NMEne..19..451N }}</ref> Materials with the lowest bulk hydrogen solubility and diffusivity provide the optimal candidates for stable barriers. A few pure metals, including tungsten and beryllium,<ref>{{cite web|url=https://www.americanelements.com/fusion-energy|title=American Elements Creates Detection Window for EPFL Fusion Reactor|access-date=February 16, 2023|publisher=American Elements}}</ref> and compounds such as carbides, dense oxides, and nitrides have been investigated. Research has highlighted that coating techniques for preparing well-adhered and perfect barriers are of equivalent importance. The most attractive techniques are those in which an ad-layer is formed by oxidation alone. Alternative methods utilize specific gas environments with strong magnetic and electric fields. Assessment of barrier performance represents an additional challenge. Classical coated membranes gas permeation continues to be the most reliable method to determine hydrogen permeation barrier (HPB) efficiency.<ref name="permeation" /> In 2021, in response to increasing numbers of designs for fusion power reactors for 2040, the [[United Kingdom Atomic Energy Authority]] published the [https://www.royce.ac.uk/content/uploads/2021/09/UK_Fusion_Materials_Roadmap_Interactive.pdf UK Fusion Materials Roadmap 2021–2040], focusing on five priority areas, with a focus on tokamak family reactors:

* Novel materials to minimize the amount of activation in the structure of the fusion power plant;
* Compounds that can be used within the power plant to optimise breeding of tritium fuel to sustain the fusion process;
* Magnets and insulators that are resistant to irradiation from fusion reactions—especially under cryogenic conditions;
* Structural materials able to retain their strength under neutron bombardment at high operating temperatures (over 550 degrees C);
* Engineering assurance for fusion materials—providing irradiated sample data and modelled predictions such that plant designers, operators and regulators have confidence that materials are suitable for use in future commercial power stations.

=== Superconducting materials ===
[[File:SuperOX Wire Production from 2013 to 2021.png|thumbnail|upright=1.5|SuperOx was able to produce over 186 miles of YBCO wire in nine months for use in fusion reactor magnets, dramatically surpassing the company's previous production targets.]]

In a plasma that is embedded in a magnetic field (known as a magnetized plasma) the fusion rate scales as the magnetic field strength to the 4th power. For this reason, many fusion companies that rely on magnetic fields to control their plasma are trying to develop high temperature superconducting devices. In 2021, SuperOx, a Russian and Japanese company, developed a new manufacturing process for making superconducting [[YBCO]] wire for fusion reactors. This new wire was shown to conduct between 700 and 2000 Amps per square millimeter. The company was able to produce 186 miles of wire in nine months.<ref>Molodyk, A., et al. "Development and large volume production of extremely high current density YBa2Cu3O7 superconducting wires for fusion." ''Scientific Reports'' 11.1 (2021): 1–11.</ref>

=== Containment considerations ===
Even on smaller production scales, the containment apparatus is blasted with matter and energy. Designs for plasma containment must consider:

* A heating and cooling cycle, up to a 10&nbsp;MW/m<sup>2</sup> thermal load.
* [[Neutron radiation]], which over time leads to [[neutron activation]] and [[embrittlement]].
* High energy ions leaving at tens to hundreds of [[electronvolt]]s.
* [[Alpha particle]]s leaving at millions of [[electronvolt]]s.
* Electrons leaving at high energy.
* Light radiation (IR, visible, UV, X-ray).

Depending on the approach, these effects may be higher or lower than fission reactors.<ref name="Shin Kajita 2014">"Thermal response of nanostructured tungsten". Shin Kajita, et al., January 2014, Nuclear Fusion 54 (2014) 033005 (10 pp.)</ref> One estimate put the [[neutron radiation|radiation]] at 100 times that of a typical [[pressurized water reactor]].{{Citation needed|date=March 2014}} Depending on the approach, other considerations such as [[electrical conductivity]], [[Vacuum permeability|magnetic permeability]], and mechanical strength matter. Materials must also not end up as long-lived [[radioactive waste]].<ref name="Materials" />

=== Plasma-wall surface conditions ===
For long term use, each atom in the wall is expected to be hit by a neutron and displaced about 100 times before the material is replaced. These high-energy neutron collisions with the atoms in the wall result in the absorption of the neutrons, forming unstable isotopes of the atoms. When the isotope decays, it may emit [[alpha particles]], [[protons]], or [[gamma rays]]. Alpha particles, once stabilized by capturing electrons, form helium atoms which accumulate at grain boundaries and may result in swelling, blistering, or embrittlement of the material.<ref name="Shin Kajita 2014"/><ref>{{cite journal |last1=Gilbert |first1=Mark |title=Neutron-induced dpa, transmutations, gas production, and helium embrittlement of fusion materials |journal=Journal of Nuclear Materials |date=2013 |volume=442 |issue=1–3 |pages=S755–S760 |doi=10.1016/j.jnucmat.2013.03.085|arxiv=1311.5079 |bibcode=2013JNuM..442S.755G }}</ref>

=== Selection of materials ===
[[Tungsten]] is widely regarded as the optimal material for plasma-facing components in next-generation fusion devices due to its unique properties and potential for enhancements. Its low sputtering rates and high melting point make it particularly suitable for the high-stress environments of fusion reactors, allowing it to withstand intense conditions without rapid degradation. Additionally, tungsten's low [[tritium]] retention through co-deposition and implantation is essential in fusion contexts, as it helps to minimize the accumulation of this radioactive isotope.<ref>{{cite journal |last1=Neu |first1=R. |last2=Dux |first2=R. |display-authors=1|title=Tungsten: an option for divertor and main chamber plasma facing components in future fusion devices |journal=Nuclear Fusion |date=2005 |volume=45 |issue=3 |pages=209–218|doi=10.1088/0029-5515/45/3/007|bibcode=2005NucFu..45..209N |s2cid=56572005 }}</ref><ref>{{cite journal |last1=Philipps |first1=V. |last2=Noname |first2=N. |display-authors=1|title=Tungsten as material for plasma-facing components in fusion devices |journal=Journal of Nuclear Materials |date=2011 |volume=415|issue=1 |pages=S2–S9|doi=10.1016/j.jnucmat.2011.01.110|bibcode=2011JNuM..415S...2P }}</ref><ref>{{cite journal |last1=Neu |first1=R. |last2=Riesch |first2=J. |display-authors=1|title=Advanced tungsten materials for plasma-facing components of DEMO and fusion power plants |journal=Fusion Engineering and Design |date=2016 |volume=109-111 |pages=1046–1052|doi=10.1016/j.fusengdes.2016.01.027|bibcode=2016FusED.109.1046N |hdl=11858/00-001M-0000-002B-3142-7 |hdl-access=free }}</ref><ref>{{cite journal |last1=Coenen |first1=J.W.|title=Fusion Materials Development at Forschungszentrum Jülich |journal=Advanced Engineering Materials|date=2020 |volume=22 |issue=6 |pages=1901376|doi=10.1002/adem.201901376|doi-access=free }}</ref>

Liquid metals (lithium, [[gallium]], [[tin]]) have been proposed, e.g., by injection of 1–5&nbsp;mm thick streams flowing at 10&nbsp;m/s on solid substrates.{{Citation needed|date=March 2014}}

Graphite features a gross erosion rate due to physical and chemical [[sputtering]] amounting to many meters per year, requiring redeposition of the sputtered material. The redeposition site generally does not exactly match the sputter site, allowing net erosion that may be prohibitive. An even larger problem is that tritium is redeposited with the redeposited graphite. The tritium inventory in the wall and dust could build up to many kilograms, representing a waste of resources and a radiological hazard in case of an accident. Graphite found favor as material for short-lived experiments, but appears unlikely to become the primary [[plasma-facing material]] (PFM) in a commercial reactor.<ref name="Materials" /><ref>{{Cite journal|title= Plasma–surface interaction in the stellarator W7-X: Conclusions drawn from operation with graphite plasma-facing components|journal=Nuclear Fusion|date=December 2, 2021 |volume=62 |issue=1 |page=016006 |doi=10.1088/1741-4326/ac3508 |s2cid=240484560 |last1=Brezɩnsek |first1=S. |last2=Dhard |first2=C. P. |last3=Jakubowski |first3=M. |last4=König |first4=R. |last5=Masuzaki |first5=S. |last6=Mayer |first6=M. |last7=Naujoks |first7=D. |last8=Romazanov |first8=J. |last9=Schmid |first9=K. |last10=Schmitz |first10=O. |last11=Zhao |first11=D. |last12=Balden |first12=M. |last13=Brakel |first13=R. |last14=Butterschoen |first14=B. |last15=Dittmar |first15=T. |last16=Drews |first16=P. |last17=Effenberg |first17=F. |last18=Elgeti |first18=S. |last19=Ford |first19=O. |last20=Fortuna-Zalesna |first20=E. |last21=Fuchert |first21=G. |last22=Gao |first22=Y. |last23=Goriaev |first23=A. |last24=Hakola |first24=A. |last25=Kremeyer |first25=T. |last26=Krychowiak |first26=M. |last27=Liang |first27=Y. |last28=Linsmeier |first28=Ch |last29=Lunsford |first29=R. |last30=Motojima |first30=G. |display-authors=1 |doi-access=free }}</ref>

Ceramic materials such as silicon carbide (SiC) have similar issues like graphite. Tritium retention in silicon carbide plasma-facing components is approximately 1.5-2 times higher than in graphite, resulting in reduced fuel efficiency and heightened safety risks in fusion reactors. SiC tends to trap more tritium, limiting its availability for fusion and increasing the risk of hazardous accumulation, complicating tritium management.<ref>{{cite journal |last1=Mayer |first1=M. |last2=Balden |first2=M. |last3=Behrisch |first3=R. |title=Deuterium retention in carbides and doped graphites |journal=Journal of Nuclear Materials |date=1998 |volume=252 |issue=1 |pages=55–62 |doi=10.1016/S0022-3115(97)00299-7 |bibcode=1998JNuM..252...55M |url=https://www.sciencedirect.com/science/article/pii/S0022311597002997 }}</ref><ref>{{cite journal |last1=Koller |first1=Markus T. |last2=Davis |first2=James W. |last3=Goodland |first3=Megan E. |last4=Abrams |first4=Tyler |last5=Gonderman |first5=Sean |last6=Herdrich |first6=Georg |last7=Frieß |first7=Martin |last8=Zuber |first8=Christian |title=Deuterium retention in silicon carbide, SiC ceramic matrix composites, and SiC coated graphite |journal=Nuclear Materials and Energy |date=2019 |volume=20 |pages=100704 |doi=10.1016/j.nme.2019.100704 |bibcode=2019NMEne..2000704K |doi-access=free }}</ref> Furthermore, the chemical and physical sputtering of SiC remains significant, contributing to tritium buildup through co-deposition over time and with increasing particle fluence. As a result, carbon-based materials have been excluded from [[ITER]], [[DEMOnstration Power Plant|DEMO]], and similar devices.<ref>{{cite journal |last1=Roth |first1=Joachim |last2=Tsitrone |first2=E. |last3=Loarte |first3=A. |last4=Loarer |first4=Th. |last5=Counsell |first5=G. |last6=Neu |first6=R. |last7=Philipps |first7=V. |last8=Brezinsek |first8=S. |last9=Lehnen |first9=M. |last10=Coad |first10=P. |last11=Grisolia |first11=Ch. |last12=Schmid |first12=K. |last13=Krieger |first13=K. |last14=Kallenbach |first14=A. |last15=Lipschultz |first15=B. |last16=Doerner |first16=R. |last17=Causey |first17=R. |last18=Alimov |first18=V. |last19=Shu |first19=W. |last20=Ogorodnikova |first20=O. |last21=Kirschner |first21=A. |last22=Federici |first22=G. |last23=Kukushkin |first23=A. |title=Recent analysis of key plasma wall interactions issues for ITER |journal=Journal of Nuclear Materials |date=2009 |volume=390-391 |pages=1–9 |doi=10.1016/j.jnucmat.2009.01.037 |bibcode=2009JNuM..390....1R |hdl=11858/00-001M-0000-0026-F442-2 |url=https://www.sciencedirect.com/science/article/pii/S0022311509000506 |issn=0022-3115|hdl-access=free }}</ref> 

Tungsten's sputtering rate is orders of magnitude smaller than carbon's, and tritium is much less incorporated into redeposited tungsten. However, tungsten plasma impurities are much more damaging than carbon impurities, and self-sputtering can be high, requiring the plasma in contact with the tungsten not be too hot (a few tens of eV rather than hundreds of eV). Tungsten also has issues around eddy currents and melting in off-normal events, as well as some radiological issues.<ref name="Materials" />

Recent advances in materials for containment apparatus materials have found that certain ceramics can actually improve the longevity of the material of the containment apparatus. Studies on [[MAX phases]], such as titanium silicon carbide, show that under the high operating temperatures of nuclear fusion, the material undergoes a phase transformation from a hexagonal structure to a face-centered-cubic (FCC) structure, driven by helium bubble growth. Helium atoms preferentially accumulate in the Si layer of the hexagonal structure, as the Si atoms are more mobile than the Ti-C slabs. As more atoms are trapped, the Ti-C slab is peeled off, causing the Si atoms to become highly mobile interstitial atoms in the new FCC structure. Lattice strain induced by the He bubbles cause Si atoms to diffuse out of compressive areas, typically towards the surface of the material, forming a protective silicon dioxide layer.<ref>{{cite journal |last1=Su |first1=Ranran |title=Helium-driven element depletion and phase transformation in irradiated Ti3SiC2 at high temperature |journal=Journal of the European Ceramic Society |date=2023 |volume=43 |issue=8 |pages=3104–3111 |doi=10.1016/j.jeurceramsoc.2023.01.048}}</ref>

Doping vessel materials with iron silicate has emerged as a promising approach to enhance containment materials in fusion reactors, as well. This method targets helium embrittlement at grain boundaries, a common issue that arises as helium atoms accumulate and form bubbles. Over time, these bubbles coalesce at grain boundaries, causing them to expand and degrade the material's structural integrity. By contrast, introducing iron silicate creates [[nucleation]] sites within the metal matrix that are more thermodynamically favorable for helium aggregation. This localized congregation around iron silicate nanoparticles induces matrix strain rather than weakening grain boundaries, preserving the material’s strength and longevity.<ref>{{cite journal |last1=Myat-Hyun |first1=Myat |title=Tailoring mechanical and in vitro biological properties of calcium‒silicate based bioceramic through iron doping in developing future material |journal=Journal of the Mechanical Behavior of Biomedical Materials |date=2022 |volume=128 |doi=10.1016/j.jmbbm.2022.105122|pmid=35168129 }}</ref><ref>{{cite web |last1=Stauffer |first1=Nancy |title=More durable metals for fusion power reactors |url=https://news.mit.edu/2024/more-durable-metals-fusion-power-reactors-0819 |website=MIT News|date=August 19, 2024 }}</ref>