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{{Short description|Type of nuclear fission reactor}}
[[File:Laufwellenreaktor.gif|thumb|Numeric simulation of a TWR. Red: uranium-238, light green: plutonium-239, black: fission products. Intensity of blue color between the tiles indicates neutron density]]

A '''traveling-wave reactor''' ('''TWR''') is a proposed type of [[nuclear reactor|nuclear fission reactor]] that can convert [[fertile material]] into usable fuel through [[nuclear transmutation]], in tandem with the burnup of fissile material. TWRs differ from other kinds of [[Fast-neutron reactor|fast-neutron]] and [[breeder reactor]]s in their ability to use fuel efficiently without [[uranium enrichment]] or [[nuclear reprocessing|reprocessing]], {{dubious|Dubious|date=September 2020}} instead directly using [[depleted uranium]], [[natural uranium]], [[thorium]], [[spent nuclear fuel|spent fuel]] removed from [[light water reactor]]s, or some combination of these materials. The concept is still in the development stage and no TWRs have ever been built.

The name refers to the fact that fission remains confined to a boundary zone in the reactor core that slowly advances over time. TWRs could theoretically run self-sustained for decades without refueling or removing spent fuel.

==History==
Traveling-wave reactors were first proposed in the 1950s and have been studied intermittently. The concept of a reactor that could breed its own fuel inside the reactor core was initially proposed and studied in 1958 by [[Savely Moiseevich Feinberg]], who called it a "breed-and-burn" reactor.<ref>S. M. Feinberg, "Discussion Comment", Rec. of Proc. Session B-10, ICPUAE, United Nations, Geneva, Switzerland (1958).</ref> Michael Driscoll published further research on the concept in 1979,<ref>M. J. Driscoll, B. Atefi, D. D. Lanning, "An Evaluation of the Breed/Burn Fast Reactor Concept", MITNE-229 (Dec. 1979).</ref> as did [[Lev Feoktistov]] in 1988,<ref>L. P. Feoktistov, "An analysis of a concept of a physically safe reactor", Preprint IAE-4605/4, in Russian, (1988).</ref> [[Edward Teller]]/[[Lowell Wood]] in 1995,<ref>E. Teller, M. Ishikawa, and L. Wood, "[http://www.nucleonica.com/wiki/Articles/Teller/Teller.pdf Completely Automated Nuclear Reactors for Long-Term Operation" (Part I)], ''Proc. of the Frontiers in Physics Symposium'', American Physical Society and the American Association of Physics Teachers Texas Meeting, Lubbock, Texas, United States (1995) ; Edward Teller, Muriel Ishikawa, Lowell Wood, Roderick Hyde, John Nuckolls, "[http://www.nucleonica.com/wiki/Articles/Teller/ICENES96.htm Completely Automated Nuclear Reactors for Long-Term Operation II : Toward A Concept-Level Point-Design Of A High-Temperature, Gas-Cooled Central Power Station System (Part II)]", Proc. Int. Conf. Emerging Nuclear Energy Systems, ICENES'96, Obninsk, Russia (1996) UCRL-JC-122708-RT2.</ref> [[Hugo van Dam]] in 2000<ref>H. van Dam, "The Self-stabilizing Criticality Wave Reactor", ''Proc. Of the Tenth International Conference on Emerging Nuclear Energy Systems (ICENES 2000)'', p. 188, NRG, Petten, Netherlands (2000).</ref> and [[Hiroshi Sekimoto]] in 2001.<ref>H. Sekimoto, K. Ryu, and Y. Yoshimura, "CANDLE: The New Burnup Strategy", ''Nuclear Science and Engineering'', 139, 1–12 (2001).</ref>

The TWR was discussed at the Innovative Nuclear Energy Systems (INES) symposiums in 2004, 2006 and 2010 in Japan where it was called "CANDLE" Reactor, an abbreviation for ''Constant Axial shape of Neutron flux, nuclides densities and power shape During Life of Energy production''.<ref>as proposed by Sekimoto in 2001 and 2005 published in Progress in Nuclear Energy</ref> In 2010 Popa-Simil discussed the case of micro-hetero-structures,<ref>"advanced Nuclear Reactor from Fiction to Reality", by Popa-Simil, published in the INES-3 proceeding</ref> further detailed in the paper "Plutonium Breeding In Micro-Hetero Structures Enhances the Fuel Cycle", describing a TWR with deep burnout enhanced by plutonium<ref>{{Cite web|url=http://www.ans.org/meetings/file/201|title=Plutonium Futures Plutonium Breeding In Micro-Hetero Structures Enhances the Fuel Cycle|last=L. Popa_Simil|first=Liviu|website=Plutonium Futures 2010|access-date=2018-03-06|archive-date=2020-01-21|archive-url=https://web.archive.org/web/20200121082926/http://www.ans.org/meetings/file/201|url-status=dead}}</ref> fuel channels and multiple fuel flow. In 2012 it was shown that fission<ref>{{Cite web|url=https://www.researchgate.net/publication/241122307|title=Enhanced Singular Wave Reactor for Surface Power|last=L. Popa-Simil}}</ref> waves are a form of bi-stable reaction diffusion phenomenon.<ref>A.G. Osborne, G.D. Recktenwald, M.R. Deinert, "Propagation of a solitary fission wave", Chaos, 22, 0231480 (2012).</ref> It has also been shown that fission waves can be stable, unstable or undergo a Hopf birfurcation depending on thermal feedback.<ref>{{Cite journal |last1=Osborne |first1=Andrew G. |last2=Deinert |first2=Mark R. |date=October 2021 |title=Stability instability and Hopf bifurcation in fission waves |journal=Cell Reports Physical Science |language=en |volume=2 |issue=10 |pages=100588 |doi=10.1016/j.xcrp.2021.100588|bibcode=2021CRPS....200588O |doi-access=free }}</ref> Irradiation damage has been shown to be an obstacle to the use of conventional materials in wave reactors, but in 2012 it was shown that fuel enrichment can be used to reduce this problem<ref>Osborne, AG, MR Deinert (2012): Neutron damage reduction in a traveling wave reactor.  ''Proceedings of Physor 2012'', Knoxville, TN, April 15–20, 2012.</ref> and this was confirmed again in 2019.<ref>{{Cite journal |last1=Keckler |first1=Chris |last2=Fratoni |first2=Massimiliano |last3=Greenspan |first3=Ehud |date=2020-11-01 |title=Sensitivity and Uncertainty Analysis of Neutron Spectrum and DPA in a B&B Core |url=https://www.tandfonline.com/doi/full/10.1080/00295639.2020.1715688 |journal=Nuclear Science and Engineering |language=en |volume=194 |issue=11 |pages=1079–1088 |doi=10.1080/00295639.2020.1715688 |bibcode=2020NSE...194.1079K |s2cid=214281608 |issn=0029-5639}}</ref>

No TWR has yet been constructed, but in 2006 [[Intellectual Ventures]] launched a spin-off named [[TerraPower]] to model and commercialize a working design of such a reactor, which later came to be called a "traveling-wave reactor". TerraPower has developed TWR designs for low- to medium- (300 MWe) as well as high-power (~1000 MWe) generation facilities.<ref>K. Weaver, C. Ahlfeld, J. Gilleland, C. Whitmer and G. Zimmerman, "Extending the Nuclear Fuel Cycle with Traveling-Wave Reactors", Paper 9294, ''Proceedings of Global 2009'', Paris, France, September 6–11, (2009).</ref> [[Bill Gates]] featured TerraPower in his 2010 [[TED (conference)|TED]] talk.<ref>{{cite video
|url=http://www.ted.com/talks/lang/eng/bill_gates.html|title=Innovating to zero!
|people=[[Bill Gates]]
|access-date=2010-07-13
|publisher=[[TED (conference)|TED]]
}}</ref>

In 2010 a group from TerraPower applied for patent EP 2324480 A1 following WO2010019199A1 "Heat pipe nuclear fission deflagration wave reactor cooling". The application was deemed withdrawn in 2014.<ref>{{Citation|title = Heat pipe nuclear fission deflagration wave reactor cooling|url = https://patents.google.com/patent/EP2324480A4|access-date = 2015-10-14}}</ref>

In September 2015 TerraPower and [[China National Nuclear Corporation]] (CNNC) signed a memorandum of understanding to jointly develop a TWR. TerraPower planned to build a 600 MWe demonstration Plant, the TWR-P, by 2018–2022 followed by larger commercial plants of 1150 MWe in the late 2020s.<ref>World Nuclear News  http://www.world-nuclear-news.org/NN-TerraPower-CNNC-team-up-on-travelling-wave-reactor-25091501.html</ref> However, in January 2019 it was announced that the project had been abandoned due to technology transfer limitations placed by the [[First presidency of Donald Trump|Trump administration]].<ref>{{cite news |url=https://www.caixinglobal.com/2019-01-05/nuclear-power-trial-in-china-will-not-proceed-101366789.html |title=Nuclear Power Trial in China Will 'Not Proceed' |date=5 January 2019 |newspaper=Caixin |first1=Chen |last1=Xuewan |first2=Mo |last2=Yelin |first3=Jason |last3=Tan |first4=Tao |last4=Ziwei}}</ref>

==Reactor physics==
Papers and presentations on TerraPower's TWR<ref>R. Michal and E. M. Blake, "John Gilleland: On the traveling-wave reactor", ''Nuclear News'', pp. 30–32, September (2009).</ref><ref name="Wald">{{cite magazine | last = Wald | first = M. | title = 10 Emerging Technologies of 2009: Traveling-Wave Reactor | magazine = MIT Technology Review | access-date = April 12, 2018 | date = February 24, 2009 | url = http://www2.technologyreview.com/article/412188/tr10-traveling-wave-reactor/ | archive-date = March 16, 2014 | archive-url = https://web.archive.org/web/20140316110927/http://www2.technologyreview.com/article/412188/tr10-traveling-wave-reactor/ | url-status = dead }}</ref><ref name="Gilleland">{{Cite conference | author = Gilleland, John| title = TerraPower, LLC Nuclear Initiative | publisher = University of California at Berkeley, Spring Colloquium | date = 2009-04-20 | url = http://www.nuc.berkeley.edu/node/1077 | access-date = April 12, 2018 | url-status = dead | archive-url = https://web.archive.org/web/20090731023254/http://www.nuc.berkeley.edu/node/1077 | archive-date = July 31, 2009 }}</ref> describe a pool-type reactor cooled by liquid sodium. The reactor is fueled primarily by depleted uranium-238 "fertile fuel", but requires a small amount of enriched uranium-235 or other "fissile fuel" to initiate [[nuclear fission|fission]]. Some of the fast-spectrum neutrons produced by fission are absorbed by neutron capture in adjacent fertile fuel (i.e. the non-fissile depleted uranium), which is "bred" into [[plutonium]] by the nuclear reaction:

:<math>\mathrm{^{238}_{\ 92}U + \,^{1}_{0}n \;\rightarrow\; ^{239}_{\ 92}U \;\rightarrow\; ^{239}_{\ 93}Np + \beta \;\rightarrow\; ^{239}_{\ 94}Pu + \beta}</math>

Initially, the core is loaded with fertile material, with a few rods of fissile fuel concentrated in the central region. After the reactor is started, four zones form within the core: the depleted zone, which contains mostly fission products and leftover fuel; the fission zone, where fission of bred fuel takes place; the breeding zone, where fissile material is created by neutron capture; and the fresh zone, which contains unreacted [[fertile material]]. The energy-generating fission zone steadily advances through the core, effectively consuming fertile material in front of it and leaving spent fuel behind. Meanwhile, the heat released by fission is absorbed by the molten sodium and subsequently transferred into a closed-cycle aqueous loop, where electric power is generated by steam turbines.<ref name="Wald"/>

==Fuel==
TWRs use only a small amount (~10%) of enriched uranium-235 or other fissile fuel to "initiate" the nuclear reaction. The remainder of the fuel consists of natural or depleted uranium-238, which can generate power continuously for 40 years or more and remains sealed in the reactor vessel during that time.<ref name="Gilleland"/> TWRs require substantially less fuel per kilowatt-hour of electricity than do [[light-water reactor]]s (LWRs), owing to TWRs' higher fuel burnup, energy density and [[thermal efficiency]]. A TWR also accomplishes most of its reprocessing within the reactor core. Spent fuel can be recycled after simple "melt refining", without the chemical separation of plutonium that is required by other kinds of breeder reactors. These features greatly reduce fuel and waste volumes while enhancing proliferation resistance.<ref name="Wald"/>

Depleted uranium is widely available as a feedstock. Stockpiles in the United States currently contain approximately 700,000 metric tons, which is a byproduct of the [[nuclear enrichment|enrichment]] process.<ref>United States Department of Energy, [http://web.ead.anl.gov/uranium/mgmtuses/storage/index.cfm "Depleted UF<sub>6</sub> Inventory and Storage Locations"] {{Webarchive|url=https://web.archive.org/web/20090827060702/http://web.ead.anl.gov/uranium/mgmtuses/storage/index.cfm |date=2009-08-27 }}.  Accessed October 2009.</ref> TerraPower has estimated that the [[Paducah Gaseous Diffusion Plant|Paducah enrichment facility]] stockpile alone represents an energy resource equivalent to $100 trillion worth of electricity.<ref name="Gilleland"/> TerraPower has also estimated that wide deployment of TWRs could enable projected global stockpiles of depleted uranium to sustain 80% of the world's population at U.S. per capita energy usages for over a millennium.<ref>L. Wood, T. Ellis, N. Myhrvold and R. Petroski, "Exploring The Italian Navigator's New World: Toward Economic, Full-Scale, Low Carbon, Conveniently-Available, Proliferation-Robust, Renewable Energy Resources", 42nd Session of the Erice International Seminars on Planetary Emergencies, Erice, Italy, 19024 August (2009).</ref>

In principle, TWRs are capable of burning spent fuel from LWRs, which is currently discarded as radioactive waste. Spent LWR fuel is mostly low enriched uranium (LEU) and, in a TWR fast-neutron spectrum, the neutron absorption cross-section of fission products is several orders of magnitude smaller than in a LWR thermal-neutron spectrum. While such an approach could actually bring about an overall reduction in nuclear waste stockpiles, additional technical development is required to realize this capability.

TWRs are also capable, in principle, of reusing their own fuel. In any given cycle of operation, only 20–35% of the fuel gets converted to an unusable form; the remaining metal constitutes usable fissile material. Recast and reclad into new driver pellets without chemical separations, this recycled fuel can be used to initiate fission in subsequent cycles of operation, thus displacing the need to enrich uranium altogether.

The TWR concept is not limited to burning uranium with [[plutonium-239]] as the "initiator" in a <sup>238</sup>U–<sup>239</sup>Pu cycle, but may also burn [[thorium]] with [[uranium-233]] as the "initiator" in a <sup>232</sup>Th–<sup>233</sup>U cycle.<ref>{{Cite journal | doi = 10.3390/en4091337| title = Traveling Wave Reactor and Condition of Existence of Nuclear Burning Soliton-Like Wave in Neutron-Multiplying Media| journal = Energies| volume = 4| issue = 12| pages = 1337| year = 2011| last1 = Rusov | first1 = V. D. | last2 = Linnik | first2 = E. P. | last3 = Tarasov | first3 = V. A. | last4 = Zelentsova | first4 = T. N. | last5 = Sharph | first5 = I. V. | last6 = Vaschenko | first6 = V. N. | last7 = Kosenko | first7 = S. I. | last8 = Beglaryan | first8 = M. E. | last9 = Chernezhenko | first9 = S. A. | last10 = Molchinikolov | first10 = P. A. | last11 = Saulenko | first11 = S. I. | last12 = Byegunova | first12 = O. A. | doi-access = free }}</ref>

==Traveling wave vs. standing wave==
The breed-burn wave in [[TerraPower]]'s TWR design does not move from one end of the reactor to the other<ref name="Ellis">{{Cite conference |author1=T. Ellis |author2=R. Petroski |author3=P. Hejzlar |author4=G. Zimmerman |author5=D. McAlees |author6=C. Whitmer |author7=N. Touran |author8=J. Hejzlar |author9=K. Weaver |author10=J. Walter |author11=J. McWhirter |author12=C. Alhfeld |author13=T. Burke |author14=A. Odedra |author15=R. Hyde |author16=J. Gilleland |author17=Y. Ishikawa |author18=L. Wood |author19=N. Myrvold |author20=W. Gates III | title = Traveling-Wave Reactors: A Truly Sustainable and Full-Scale Resource for Global Energy Needs | publisher = American Nuclear Society, Summer Meeting | date = 2010-06-14 | url = http://terrapower.com/uploads/docs/ICAPP_2010_Paper_10189.pdf | access-date = April 12, 2018 |archive-url=https://web.archive.org/web/20170109170150/http://terrapower.com/uploads/docs/ICAPP_2011_Paper_11199.pdf |archive-date=January 9, 2017}}</ref> but gradually from the center out. Moreover, as the fuel's composition changes through nuclear transmutation, fuel rods are continually reshuffled within the core to optimize the neutron flux and fuel usage over time. Thus, instead of letting the wave propagate through the fuel, the fuel itself is moved through a largely stationary burn wave.  This is contrary to many media reports,<ref name="nyt1">{{Cite news | author = M. Wald| title = Developer of Novel Reactor Wins $35 Million Infusion | newspaper = The New York Times | date = 2010-06-14 | url = https://www.nytimes.com/2010/06/15/business/energy-environment/15nuke.html | access-date = June 15, 2010}}</ref> which have popularized the concept as a candle-like reactor with a burn region that moves down the length of a fuel section. By replacing a static core configuration with an actively managed "standing wave" or "[[soliton]]", however, TerraPower's design avoids the problem of cooling a moving burn region. Under this scenario, the reconfiguration of fuel rods is accomplished remotely by robotic devices; the containment vessel remains closed during the procedure, with no associated downtime.

==References==
{{Reflist|30em}}

==Further reading==
*[[Arjun Makhijani]] (September 2013), "[http://ieer.org/wp/wp-content/uploads/2013/09/TravelingWaveReactor-Sept20131.pdf Traveling Wave Reactors: Sodium-cooled Gold at the End of a Nuclear Rainbow?]", [[Institute for Energy and Environmental Research]].

==External links==
*[http://intellectualventureslab.com/?page_id=532 "TerraPower Reactors Approach the Nuclear Ideal"] by [[Intellectual Ventures]] Lab.
*[http://earth2tech.com/2010/02/15/terrapower-how-the-travelling-wave-nuclear-reactor-works/ TerraPower: How The Traveling Wave Nuclear Reactor Works] {{Webarchive|url=https://archive.today/20130122032956/http://earth2tech.com/2010/02/15/terrapower-how-the-travelling-wave-nuclear-reactor-works/ |date=2013-01-22 }}, inc 4 diagrams from IV.
*[http://www.terrapower.com/ Terrapower.com company website]
*{{cite journal|arxiv=1207.3695|last1= Rusov|first1= V. D.|title= On some fundamental peculiarities of the traveling wave reactor|last2=  Tarasov|first2= V. A.|last3=  Sharf|first3= I. V.|last4=  Vaschenko|first4= V. M.|last5=  Linnik|first5= E. P.|last6=  Zelentsova|first6= T. N.|last7=  Beglaryan|first7= M. E.|last8=  Chernegenko|first8= S. A.|last9=  Kosenko|first9= S. I.|last10=  Smolyar|first10= V. P.|year= 2012|doi=10.1155/2015/703069|bibcode= 2012arXiv1207.3695R|volume=2015|journal=Science and Technology of Nuclear Installations|pages=1–23|doi-access= free}}

{{Nuclear fission reactors}}

[[Category:Nuclear power reactor types]]