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{{Short description|Proposed uses of generating power from nuclear fission and fusion}}
'''Hybrid nuclear fusion–fission''' ('''hybrid nuclear power''') is a proposed means of generating [[Electrical power industry|power]] by use of a combination of [[nuclear fusion]] and [[Nuclear fission|fission]] processes.
The basic idea is to use high-energy [[fast neutron]]s from a fusion reactor to trigger fission in non-[[fissile material|fissile]] fuels like [[Uranium-238|U-238]] or [[Thorium-232|Th-232]]. Each neutron can trigger several fission events, multiplying the energy released by each fusion reaction hundreds of times. As the fission fuel is not fissile, there is no self-sustaining chain reaction from fission. This would not only make fusion designs more economical in power terms, but also be able to burn fuels that were not suitable for use in conventional fission plants, even their [[nuclear waste]].
In general terms, the hybrid is very similar in concept to the [[fast breeder reactor]], which uses a compact high-energy fission core in place of the hybrid's fusion core. Another similar concept is the [[accelerator-driven subcritical reactor]], which uses a [[particle accelerator]] to provide the neutrons instead of nuclear reactions.
==History==
The concept dates to the 1950s, and was strongly advocated by [[Hans Bethe]] during the 1970s. At that time the first powerful fusion experiments were being built, but it would still be many years before they could be economically competitive. Hybrids were proposed as a way of greatly accelerating their market introduction, producing energy even before the fusion systems reached [[Fusion energy gain factor|break-even]].{{sfn|Bethe|1979|p=48}} However, detailed studies of the economics of the systems suggested they could not compete with existing fission reactors.{{sfn|Barrett|Hardie|1980}}
The idea was abandoned and lay dormant until the continued delays in reaching break-even led to a brief revival of the concept around 2009.<ref name="hybrid">{{cite journal | author = Gerstner, E. | title = Nuclear energy: The hybrid returns | year = 2009 | journal = [[Nature (journal)|Nature]] | volume = 460 | issue = 7251| pages = 25–8 | pmid = 19571861|doi=10.1038/460025a| s2cid = 205047403 |url=http://www.nature.com/news/2009/090701/pdf/460025a.pdf| doi-access = free }}</ref> These studies generally concentrated on the [[nuclear waste]] disposal aspects of the design, as opposed to the production of energy.<ref>{{cite conference |url=http://web.mit.edu/fusion-fission/HybridsPubli/Hybrid_Fusion_Fission_Conference_A.pdf |title=Fusion-Fission Hybrid Conference |date=19 May 2009}}</ref> The concept has seen cyclical interest since then, based largely on the success or failure of more conventional solutions like the [[Yucca Mountain nuclear waste repository]]
Another major design effort for energy production was started at [[Lawrence Livermore National Laboratory]] (LLNL) under their [[LIFE (fusion)|LIFE]] program. Industry input led to the abandonment of the hybrid approach for LIFE, which was then re-designed as a pure-fusion system. LIFE was cancelled when the underlying technology, from the [[National Ignition Facility]], failed to reach its design performance goals.<ref>{{cite journal |first=Kirk|last=Levedahl |url=http://nnsa.energy.gov/sites/default/files/nnsa/07-13-inlinefiles/SSQ%20V3%20N1_Final_11june13.pdf |title=National Ignition Campaign Closure and the Path Forward for Ignition |journal=Stockpile Stewardship Quarterly |date=June 2013 |pages=4–5 |access-date=2020-02-10 |archive-url=https://web.archive.org/web/20170502160132/https://nnsa.energy.gov/sites/default/files/nnsa/07-13-inlinefiles/SSQ%20V3%20N1_Final_11june13.pdf |archive-date=2017-05-02 |url-status=dead}}</ref>
Apollo Fusion, a company founded by Google executive [[Mike Cassidy (entrepreneur)|Mike Cassidy]] in 2017, was also reported to be focused on using the subcritical nuclear fusion-fission hybrid method.<ref>{{cite magazine |magazine=Popular Mechanics |url=http://www.popularmechanics.com/science/energy/a25922/apollo-fusion-startup-googler-nuclear-power/ |title=Can 'Apollo Fusion' Bring Us Clean Nuclear Energy? |first=Avery |last=Thompson |date=3 April 2017}}</ref><ref>{{cite news |newspaper=Forbes |url=https://www.bloomberg.com/news/articles/2017-04-03/former-google-vice-president-starts-a-company-promising-clean-and-safe-nuclear-energy |title= Former Google Vice President Starts a Company Promising Clean and Safe Nuclear Energy  |first=Brad |last=Stone |date=3 April 2017}}</ref> Their web site is now focussed on their [[hall effect thruster]]s, and mentions fusion only in passing.<ref>{{cite web |title=Apollo Fusion |url=http://apollofusion.com}}</ref>
On 2022, September 9, Professor Peng Xianjue of the Chinese Academy of Engineering Physics announced that the Chinese government had approved the construction of the world's largest pulsed-powerplant - the Z-FFR, namely Z(-pinch)-Fission-Fusion Reactor- in Chengdu, Sichuan province. Neutrons produced in a Z-pinch facility (endowed with cylindrical symmetry and fuelled with deuterium and tritium) will strike a coaxial blanket including both uranium and lithium isotopes. Uranium fission will boost the facility's overall heat output by 10 to 20 times. Interaction of lithium and neutrons will provide tritium for further fueling. Innovative, quasi-spherical geometry near the core of Z-FFR leads to high performance of Z-pinch discharge. According to Prof. Xianjue, this will considerably speed up the use of fusion energy and prepare it for commercial power production by 2035.<ref>{{cite magazine |magazine=Eurasian Times|url=https://eurasiantimes.com/china-to-achieve-nuclear-fusion-energy-in-six-years/ |title=China Approves World's Largest Pulsed-Power Plant; Sets EyeOn Achieving Nuclear Fusion Energy In Six Years – TopScientist |first=Ashish  |last=Dangwal |date=16 September 2022}}</ref>{{sfn|Chu|2012}}{{sfn|Zhang|2012}}{{sfn|Zhenghong|2014}}
==Description==
===Fission basics===
Conventional [[fission power system]]s relies on a [[chain reaction]] of [[nuclear fission]] events that release two or three neutrons that cause further fission events. By careful arrangement and the use of various absorber materials, the system can be set in a balance of released and absorbed neutrons, known as [[critical mass|criticality]].<ref>{{cite web |url=https://www.nrc.gov/reading-rm/basic-ref/glossary/criticality.html# |title=Criticality |website=NRC}}</ref>
[[Natural uranium]] is a mix of several isotopes, mainly a trace amount of [[Uranium-235|<sup>235</sup>U]] and over 99% [[Uranium-238|<sup>238</sup>U]]. When they undergo fission, both of these isotopes release fast neutrons with an energy distribution peaking around 1 to 2&nbsp;MeV. This energy is too low to cause fission in <sup>238</sup>U, which means it cannot sustain a chain reaction. <sup>235</sup>U will undergo fission when struck by neutrons of this energy, so <sup>235</sup>U  a chain reaction. There are too few <sup>235</sup>U atoms in natural uranium to sustain a chain reaction, the atoms are spread out too far and the chance a neutron will hit one is too small. Chain reactions are accomplished by concentrating, or ''enriching'', the fuel, increasing the amount of <sup>235</sup>U to produce [[enriched uranium]],{{sfn|Brennen|2005|p=16}} while the leftover, now mostly <sup>238</sup>U, is a waste product known as [[depleted uranium]]. <sup>235</sup>U will sustain a chain reaction if enriched to about 20% of the fuel mass.{{sfn|Brennen|2005|p=19}}
<sup>235</sup>U will undergo fission more easily if the neutrons are of lower energy, the so-called ''[[thermal neutron]]s''. Neutrons can be slowed to thermal energies through collisions with a [[neutron moderator]] material, the easiest to use are the hydrogen atoms found in water. By placing the fission fuel in water, the probability that the neutrons will cause fission in another <sup>235</sup>U is greatly increased, which means the level of enrichment needed to reach criticality is greatly reduced. This leads to the concept of ''reactor-grade'' enriched uranium, with the amount of <sup>235</sup>U increased from just less than 1% in natural ore to between 3 and 5%, depending on the reactor design. This is in contrast to ''[[weapons-grade]]'' enrichment, which increases to the <sup>235</sup>U to at least 20%, and more commonly, over 90%.{{sfn|Brennen|2005|p=19}}
To maintain criticality, the fuel has to retain that extra concentration of <sup>235</sup>U. A typical fission reactor burns off enough of the <sup>235</sup>U to cause the reaction to stop over a period on the order of a few months. A combination of burnup of the <sup>235</sup>U along with the creation of neutron absorbers, or ''poisons'', as part of the fission process eventually results in the fuel mass not being able to maintain criticality. This burned-up fuel has to be removed and replaced with fresh fuel. The result is [[nuclear waste]] that is highly radioactive and filled with long-lived radionuclides that present a safety concern.<ref name=processing>{{cite web |url=https://world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/processing-of-used-nuclear-fuel |title=Processing of Used Nuclear Fuel |date= 18 December 2020 |website=World Nuclear Association}}</ref>
The waste contains most of the <sup>235</sup>U it started with, only 1% or so of the energy in the fuel is extracted by the time it reaches the point where it is no longer fissile. One solution to this problem is to [[Nuclear reprocessing|reprocess]] the fuel, which uses chemical processes to separate the <sup>235</sup>U (and other non-poison elements) from the waste, and then mixes the extracted <sup>235</sup>U in fresh fuel loads. This reduces the amount of new fuel that needs to be mined and also concentrates the unwanted portions of the waste into a smaller load. Reprocessing is expensive, however, and it has generally been more economical to simply buy fresh fuel from the mine.<ref name=processing/>
Like <sup>235</sup>U, <sup>239</sup>Pu can maintain a chain reaction, so it is a useful reactor fuel. However, <sup>239</sup>Pu is not found in commercially useful amounts in nature. Another possibility is to ''[[Breeder reactor|breed]]'' <sup>239</sup>Pu from the <sup>238</sup>U through [[neutron capture]], or various other means. This process only occurs with higher-energy neutrons than would be found in a moderated reactor, so a conventional reactor only produces small amounts of Pu when the neutron is captured within the fuel mass before it is moderated.<ref name=breeder>{{cite web |url=https://world-nuclear.org/information-library/current-and-future-generation/fast-neutron-reactors |title=Processing of Used Nuclear Fuel |date=  26 August 2021  |website=World Nuclear Association}}</ref>
It is possible to build a reactor that does not require a moderator. To do so, the fuel has to be further enriched, to the point where the <sup>235</sup>U is common enough to maintain criticality even with fast neutrons. The extra fast neutrons escaping the fuel load can then be used to breed fuel in a <sup>238</sup>U assembly surrounding the reactor core, most commonly taken from the stocks of depleted uranium. <sup>239</sup>Pu can also be used for the core, which means once the system is up and running, it can be refuelled using the <sup>239</sup>Pu it creates, with enough left over to feed into other reactors as well. This concept is known as a [[breeder reactor]].<ref name=breeder/>
Extracting the <sup>239</sup>Pu from the <sup>238</sup>U feedstock can be achieved with chemical processing, in the same fashion as normal reprocessing. The difference is that the mass will contain far fewer other elements, particularly some of the highly radioactive fission products found in normal nuclear waste.<ref name=breeder/>
===Fusion basics===
Fusion reactors typically burn a mixture of [[deuterium]] (D) and [[tritium]] (T). When heated to millions of degrees, the [[kinetic energy]] in the fuel begins to overcome the natural electrostatic repulsion between nuclei, the so-called [[coulomb barrier]], and the fuel begins to undergo fusion. This reaction gives off an [[alpha particle]] and a high energy [[neutron]] of 14&nbsp;MeV. A key requirement to the economic operation of a fusion reactor is that the alphas deposit their energy back into the fuel mix, heating it so that additional fusion reactions take place. This leads to a condition not unlike the chain reaction in the fission case, known as ''ignition''.<ref name=ignited>{{cite magazine |url=https://www.lle.rochester.edu/media/publications/lle_review/documents/v142/142_01_Alpha%20Heating.pdf |title=Alpha Heating and Burning Plasmas in Inertial Confinement Fusion |date=January–March 2015 |magazine=LLE Review |volume=142 |pages=77–82}}</ref>
Building a reactor design that is capable of reaching ignition has proven to be a significant problem. The first attempts to build such a reactor took place in 1938, and the first success was in 2022, 84 years later.<ref>{{cite tech report |url=https://lasers.llnl.gov/content/assets/docs/news/age_of_ignition_book.pdf |title=The Age of Ignition |publisher=LLNL |date=2023}}</ref> Even in that case, the amount of energy released was orders of magnitude less than the energy needed to operate the machine. A reactor that produces more electricity than is used to operate it, a condition known as ''engineering breakeven'', will require decades more work.<ref>{{cite web |url=https://www.iaea.org/topics/energy/fusion/faqs#:~:text=A%20prototype%20of%20a%20fusion,on%20funding%20and%20technical%20advancement. |title=When is electricity generated through fusion expected to be available? |website=IAEA |date=12 October 2016 }}</ref>
Additionally, there is an issue of fueling such a reactor. Deuterium can be obtained by the separation of hydrogen isotopes in seawater (see [[heavy water#Production|heavy water production]]). Tritium has a short half-life of just over a decade, so only trace amounts are found in nature. To fuel the reactor, the neutrons from the reaction are used to breed more tritium through a reaction in a ''blanket'' of [[lithium]] surrounding the reaction chamber.<ref>{{cite web |url=https://www.iter.org/mach/TritiumBreeding |title=Tritium Breeding |website=ITER}}</ref> Tritium breeding is key to the success of a D-T fusion cycle, and to date, this technique has not been demonstrated. Predictions based on computer modelling suggest that the breeding ratios are quite small and a fusion plant would barely be able to cover its own use. Many years would be needed to breed enough surplus to start another reactor.<ref name=jason>{{cite tech report |title=Tritium |url=https://irp.fas.org/agency/dod/jason/tritium.pdf |first=Desmond |last=McMorrow |publisher=MITRE Corporation |date=November 2011}}</ref>
===Hybrid concepts===
Fusion–fission designs essentially replace the [[Breeding blanket|lithium blanket]] of a typical fusion design with a blanket of fission fuel, either natural uranium ore or even nuclear waste. The fusion neutrons have more than enough energy to cause fission in the <sup>238</sup>U, as well as many of the other elements in the fuel, including some of the [[transuranic]] waste elements. The reaction can continue even when all of the <sup>235</sup>U is burned off; the rate is controlled not by the neutrons from the fission events, but by the neutrons being supplied by the fusion reactor.{{sfn|Bethe|1979|p=48}}
Fission occurs naturally because each event gives off more than one neutron capable of producing additional fission events. Fusion, at least in D-T fuel, gives off only a single neutron, and that neutron is not capable of producing more fusion events. When that neutron strikes fissile material in the blanket, one of two reactions may occur. In many cases, the kinetic energy of the neutron will cause one or two neutrons to be struck out of the nucleus without causing fission. These neutrons still have enough energy to cause other fission events. In other cases, the neutron will be captured and cause fission, which will release two or three neutrons. This means that every fusion neutron in the fusion–fission design can result in anywhere between two and four neutrons in the fission fuel.{{sfn|Bethe|1979|p=48}}
This is a key concept in the hybrid concept, known as ''fission multiplication''. For every fusion event, several fission events may occur, each of which gives off much more energy than the original fusion, about 11 times. This greatly increases the total power output of the reactor. This has been suggested as a way to produce practical fusion reactors even though no fusion reactor has yet reached break-even, by multiplying the power output using cheap fuel or waste.{{sfn|Bethe|1979|p=48}} However, many studies have repeatedly demonstrated that this only becomes practical when the overall reactor is very large, 2 to 3&nbsp;GWt, which makes it expensive to build.<ref>{{cite tech report |last1=Tenney |first1=F. |last2=Bathke |first2=C. |display-authors=1 |title=A Systems Study of Tokamak Fusion–Fission Reactors |url=http://www.ralphmoir.com/media/tenneyMerged.pdf |date=November 1978 |publisher=Princeton Plasma Physics Laboratory |pages=336-337}}</ref>
These processes also have the side-effect of breeding <sup>239</sup>Pu or <sup>233</sup>U, which can be removed and used as fuel in conventional fission reactors. This leads to an alternate design where the primary purpose of the fusion–fission reactor is to reprocess waste into new fuel. Although far less economical than chemical reprocessing, this process also burns off some of the nastier elements instead of simply physically separating them out. This also has advantages for [[nuclear non-proliferation|non-proliferation]], as enrichment and reprocessing technologies are also associated with nuclear weapons production. However, the cost of the nuclear fuel produced is very high and is unlikely to be able to compete with conventional sources.{{sfn|Barrett|Hardie|1980}}
===Neutron economy===
A key issue for the fusion–fission concept is the number and lifetime of the neutrons in the various processes, the so-called ''[[neutron economy]]''.
In a pure fusion design, the neutrons are used for breeding tritium in a lithium blanket. Natural lithium consists of about 92% <sup>7</sup>Li and the rest is mostly <sup>6</sup>Li. <sup>7</sup>Li breeding requires neutron energies even higher than those released by fission, around 5&nbsp;MeV, well within the range of energies provided by fusion. This reaction produces [[tritium]] and [[helium-4]], and another slow neutron. <sup>6</sup>Li can react with high or low energy neutrons, including those released by the <sup>7</sup>Li reaction. This means that a single fusion reaction can produce several tritiums, which is a requirement if the reactor is going to make up for natural decay and losses in the fusion processes.<ref name=jason/>
When the lithium blanket is replaced, or supplanted, by fission fuel in the hybrid design, neutrons that do react with the fissile material are no longer available for tritium breeding. The new neutrons released from the fission reactions can be used for this purpose, but only in <sup>6</sup>Li. One could process the lithium to increase the amount of <sup>6</sup>Li in the blanket, making up for these losses, but the downside to this process is that the <sup>6</sup>Li reaction only produces one tritium atom. Only the high-energy reaction between the fusion neutron and <sup>7</sup>Li can create more than one tritium, and this is essential for keeping the reactor running.<ref name=jason/>
To address this issue, at least some of the fission neutrons must also be used for tritium breeding in <sup>6</sup>Li. Every neutron that does is no longer available for fission, reducing the reactor output. This requires a very careful balance if one wants the reactor to be able to produce enough tritium to keep itself running, while also producing enough fission events to keep the fission side energy positive. If these cannot be accomplished simultaneously, there is no reason to build a hybrid. Even if this balance can be maintained, it might only occur at an economically infeasible level. For this reason, a variety of neutron releasing substances have been suggested as a way to multiply the number of neutrons available.<ref>{{cite journal |journal=Fusion Engineering and Design |date= September 2019 |pages=1569–1573 |first1=Vladimir  |last1=Khripunov |first2=Boris  |last2=Kuteev |first3=Alexey |last3= Zhirkin |url=https://www.sciencedirect.com/science/article/abs/pii/S0920379619303126 |title=Enhanced tritium production in fusion-fission hybrids for the external consumption|volume= 146 |doi= 10.1016/j.fusengdes.2019.02.130 }}</ref>
==Overall economy==
Through the early development of the hybrid concept, the question of overall economics appeared difficult to answer. A series of studies starting in the late 1970s provided a much clearer picture of the hybrid in a complete fuel cycle and allowed the economics to be better understood. These studies indicated there was no reason to build a hybrid.{{sfn|Barrett|Hardie|1980}}
One of the most detailed of these studies was published in 1980 by [[Los Alamos National Laboratory]] (LANL).{{sfn|Barrett|Hardie|1980}} They noted that the hybrid would produce most of its energy indirectly, both through the fission events in the reactor, and much more by providing <sup>239</sup>Pu to fuel other fission reactors. In this overall picture, the hybrid is essentially identical to the breeder reactor in the same fashion as the hybrid.{{sfn|Barrett|Hardie|1980|p=2}} Both require chemical processing to remove the bred <sup>239</sup>Pu, both presented the same proliferation and safety risks as a result, and both produced about the same amount of fuel. Since the bred fuel is the primary source of energy in the overall cycle, the two systems were almost identical in the end.{{sfn|Barrett|Hardie|1980|p=3}}
What was not identical, however, was the technical maturity of the two designs. The hybrid would require considerable additional research and development before it would be known if it could even work, and even if that were demonstrated, the result would be a system essentially identical to breeders which were already being built at that time. The report concluded:
<blockquote>The investment of time and money required to commercialize the hybrid cycle could only be justified by a real or perceived advantage of the hybrid over the classical FBR. Our analysis leads us to conclude that no such advantage exists. Therefore, there is not sufficient incentive to demonstrate and commercialize the fusion–fission hybrid.{{sfn|Barrett|Hardie|1980|p=3}}</blockquote>
==Rationale==
The fusion process alone currently does not achieve sufficient gain (power output over power input) to be viable as a power source. By using the excess neutrons from the fusion reaction to in turn cause a high-yield fission reaction (close to 100%) in the surrounding subcritical fissionable blanket, the net yield from the hybrid fusion–fission process can provide a targeted gain of 100 to 300 times the input energy (an increase by a factor of three or four over fusion alone).  Even allowing for high [[Energy conversion efficiency|inefficiencies]] on the input side (i.e. low laser efficiency in ICF and [[Nuclear fusion#Bremsstrahlung losses in quasineutral, isotropic plasmas|Bremsstrahlung losses]] in Tokamak designs), this can still yield sufficient heat output for economical electric power generation.  This can be seen as a shortcut to viable fusion power until more efficient pure fusion technologies can be developed, or as an end in itself to generate power, and also consume existing stockpiles of nuclear fissionables and waste products.
In the LIFE project at the Lawrence Livermore National Laboratory [[LLNL]], using technology developed at the [[National Ignition Facility]], the goal is to use [[Inertial confinement fusion#ICF mechanism of action|fuel pellet]]s of [[deuterium]] and [[tritium]] surrounded by a fissionable blanket to produce energy sufficiently greater than the input ([[laser]]) energy for electrical power generation.  The principle involved is to induce [[inertial confinement fusion]] (ICF) in the fuel pellet which acts as a highly concentrated point source of [[neutron]]s which in turn converts and fissions the outer fissionable blanket. In parallel with the ICF approach, the [[University of Texas]] at Austin is developing a system based on the [[tokamak]] fusion reactor, optimising for nuclear waste disposal versus power generation. The principles behind using either ICF or tokamak reactors as a neutron source are essentially the same (the primary difference being that ICF is essentially a point-source of neutrons while Tokamaks are more diffuse toroidal sources).
==Use to dispose of nuclear waste==
The surrounding blanket can be a [[fissile]] material (enriched uranium or [[plutonium]]) or a fertile material (capable of conversion to a fissionable material by neutron bombardment) such as [[thorium]], [[depleted uranium]] or [[spent nuclear fuel]].  Such [[subcritical reactor]]s (which also include [[particle accelerator]]-driven neutron [[spallation]] systems) offer the only currently-known  means of active disposal (versus storage) of spent nuclear fuel without reprocessing.  [[Nuclear fission product|Fission by-products]] produced by the operation of commercial light water nuclear reactors ([[LWR]]s) are long-lived and highly radioactive, but they can be consumed using the excess neutrons in the fusion reaction along with the fissionable components in the blanket, essentially destroying them by [[nuclear transmutation]] and producing a waste product which is far safer and less of a risk for [[nuclear proliferation]].  The waste would contain significantly reduced concentrations of long-lived, weapons-usable [[actinide]]s per gigawatt-year of electric energy produced compared to the waste from a LWR. In addition, there would be about 20 times less waste per unit of electricity produced.  This offers the potential to efficiently use the very large stockpiles of enriched fissile materials, depleted uranium, and spent nuclear fuel.
==Safety==
In contrast to current commercial fission reactors, hybrid reactors potentially demonstrate what is considered [[inherently safe]] behavior because they remain deeply [[subcritical]] under all conditions and decay heat removal is possible via passive mechanisms.  The fission is driven by neutrons provided by fusion ignition events, and is consequently not self-sustaining.  If the fusion process is deliberately shut off or the process is disrupted by a mechanical failure, the fission [[Damping ratio|damps out]] and stops nearly instantly. This is in contrast to the forced damping in a conventional reactor by means of control rods which absorb neutrons to reduce the [[neutron flux]] below the critical, self-sustaining, level.  The inherent danger of a conventional fission reactor is any situation leading to a [[positive feedback]], runaway, [[chain reaction]] such as occurred during the [[Chernobyl disaster]].  In a hybrid configuration the fission and fusion reactions are decoupled, i.e. while the fusion neutron output drives the fission, the fission output has no effect whatsoever on the fusion reaction, eliminating any chance of a positive feedback loop.
==Fuel cycle==
There are three main components to the hybrid fusion fuel cycle: [[deuterium]], [[tritium]], and fissionable elements.{{sfn|Bethe|1979}}  Deuterium can be derived by the separation of hydrogen isotopes in seawater (see [[heavy water]] production).  Tritium may be generated in the hybrid process itself by absorption of neutrons in lithium bearing compounds.  This would entail an additional lithium-bearing blanket and a means of collection. Small amounts of tritium are also produced by [[neutron activation]] in nuclear fission reactors, particularly when heavy water is used as a [[neutron moderator]] or coolant. The third component is externally derived fissionable materials from demilitarized supplies of fissionables, or commercial nuclear fuel and waste streams.  Fusion driven fission also offers the possibility of using [[thorium]] as a fuel, which would greatly increase the potential amount of fissionables available.  The extremely energetic nature of the [[fast neutrons]] emitted during the fusion events (up to 0.17 the speed of light) can allow normally non-fissioning <sup>238</sup>U to undergo fission directly (without conversion first to <sup>239</sup>Pu), enabling refined natural Uranium to be used with very low enrichment, while still maintaining a deeply subcritical regime.
==Engineering considerations==
Practical engineering designs must first take into account safety as the primary goal.  All designs should incorporate passive cooling in combination with refractory materials to prevent melting and reconfiguration of fissionables into geometries capable of un-intentional criticality.  Blanket layers of Lithium bearing compounds will generally be included as part of the design to generate Tritium to allow the system to be self-supporting for one of the key fuel element components.  Tritium, because of its relatively short half-life and extremely high radioactivity, is best generated on-site to obviate the necessity of transportation from a remote location.  D-T fuel can be manufactured on-site using Deuterium derived from heavy water production and Tritium generated in the hybrid reactor itself.  [[Spallation#Nuclear spallation|Nuclear spallation]] to generate additional neutrons can be used to enhance the fission output, with the caveat that this is a tradeoff between the number of neutrons (typically 20-30 neutrons per spallation event) against a reduction of the individual energy of each neutron.  This is a consideration if the reactor is to use natural Thorium as a fuel.  While high energy (0.17c) neutrons produced from fusion events are capable of directly causing fission in both Thorium and <sup>238</sup>U, the lower energy neutrons produced by spallation generally cannot.  This is a tradeoff that affects the mixture of fuels against the degree of spallation used in the design.
==See also==
* [[Subcritical reactor]], a broad category of designs using various external neutron sources including [[Spallation#Nuclear spallation|spallation]] to generate non-self-sustaining fission (hybrid fusion–fission reactors fall into this category).
* [[Muon-catalyzed fusion]], which uses exotic particles to achieve fusion ignition at relatively low temperatures.
* [[Breeder reactor]], a nuclear reactor that generates more fissile material in fuel than it consumes.
* [[Generation IV reactor]], next generation fission reactor designs claiming much higher safety, and greatly increased fuel use efficiency.
* [[Traveling wave reactor]], a pure fission reactor with a moving reaction zone, which is also capable of consuming wastes from LWRs and using depleted Uranium as a fuel.
* [[Liquid fluoride thorium reactor]], a fission reactor which uses molten thorium fluoride salt fuel, capable of consuming wastes from LWRs.
* [[Integral Fast Reactor]], a fission fast breeder reactor which uses reprocessing via electrorefining at the reactor site, capable of consuming wastes from LWRs and using depleted Uranium as a fuel.
* [[Aneutronic fusion]] a category of nuclear reactions in which only a small part (or none) of the energy released is carried away by energetic neutrons.
* [[Project PACER]], a reverse of this concept, attempts to use small fission explosions to ignite hydrogen fusion (fusion bombs) for power generation
* [[Cold fusion]]
* [[COLEX process (isotopic separation)]]
==References==
===Citations===
{{Reflist}}
===Bibliography===
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|last=Zhang |first=Y
|title=Dynamics of quasi-spherical Z-pinch implosions with mass redistribution and displacement modification
|date=2012
|journal= Phys. Plasmas
|pages=122704
|doi=10.1063/1.4771575
|volume=19
|issue=12
|doi-access=free
}}
*{{cite journal
|last=Zhenghong |first=L
|title=Conceptual design of Z-pinch driven fusion-fission hybrid power reactor (in Chinese)
|date=2014
|journal= High Power Laser and Particle Beams
|pages=100202
|doi=10.11884/HPLPB201426.100202
|volume=26
|issue=10
}}
==Further reading==
* {{cite web|title=Hybrid Fission-Fusion Reactor Initiated by a Laser|author1=A.P. Barzilov |author2=A.V. Gulevich |author3=O.F. Kukharchuk |author4=A.V. Zrodnikov |publisher=Institute of Physics & Power Engineering |location=Obninsk, Russia|url=http://www.tfd.chalmers.se/~valeri/Ajax/zrod.html}}
*{{cite web|title= Fusion–Fission Research Workshop: includes workshops final report, related white papers and publications. |author=MIT |url=http://web.mit.edu/fusion-fission/}}
==External links==
* [https://archive.today/20130203034753/http://www.springerlink.com/content/9803n15542276654/?p=74cac0c63374464fa71566f1a723c7c2&pi=10  Potential Role of Lasers for Sustainable Fission Energy Production and Transmutation of Nuclear Waste C.D. Bowman and J. Magill]
* [https://web.archive.org/web/20100527093012/https://lasers.llnl.gov/about/missions/energy_for_the_future/life/ Laser Inertial Fusion–Fission Energy (LIFE) Project] at the Lawrence Livermore National Laboratory,
* [http://www.utexas.edu/news/2009/01/27/nuclear_hybrid/?AddInterest=1284 Nuclear Fusion–Fission Hybrid Could Destroy Nuclear Waste And Contribute to Carbon-Free Energy Future] University of Texas at Austin
* [http://www.ralphmoir.com/fusion-fission/ Ralph Moir's fusion–fission hybrid page – contains many research papers about the topic]
*[http://www.itheo.org/ International Thorium Energy Organisation - www.IThEO.org]
{{DEFAULTSORT:Hybrid nuclear fusion-fission}}
[[Category:Nuclear technology]]
[[Category:Nuclear fusion]]
[[Category:Nuclear fission]]