Nuclear fusion–fission hybrid: Difference between revisions

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,. butAs 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.

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 2000s, when 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>

In a pure fusion design, the neutrons are used for breeding tritium in a lithium blanket. Natural lithium consists of about 92% ⁷Li-7 and the rest is mostly ⁶Li-6. ⁷Li-7 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. ⁶Li-6 can react with high or low energy neutrons, including those released by the ⁷Li-7 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 ⁶Li-6. One could process the lithium to increase the amount of ⁶Li-6 in the blanket, making up for these losses, but the downside to this process is that the ⁶Li-6 reaction only produces one tritium atom. Only the high-energy reaction between the fusion neutron and ⁷Li-7 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 ⁶Li-6. Every oneneutron 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 aan economically infeasible level. thatFor isthis economicallyreason, infeasiblea 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>

Through the early development of the hybrid concept, the question of overall economics appeared difficult to handleanswer. 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 appeared to indicateindicated 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}} Their studyThey noted that the hybrid would produce most of its energy indirectly, both through the fission events in its ownthe reactor, and much more by providing Pu-²³⁹Pu to fuel conventionalother fission reactors. In this overall picture, the hybrid is essentially identical to the [[breeder reactor]], which uses fast neutrons from plutonium fission to breed more fuel in a fission blanket in largely the same fashion as the hybrid.{{sfn|Barrett|Hardie|1980|p=2}} Both require chemical processing to remove the bred Pu-²³⁹Pu, both presented the same proliferation and safety risks as a result, and both produced about the same amount of fuel. Since thatthe 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 end result would be a system essentially identical to breeders which were already being built at that time. The report concluded:

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.

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. [[ActivationNuclear 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.

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, completely eliminating any chance of a positive feedback loop.

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 sea waterseawater (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 ²³⁸U-238 to undergo fission directly (without conversion first to ²³⁹Pu-239), enabling refined natural Uranium to be used with very low enrichment, while still maintaining a deeply subcritical regime.

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 ²³⁸U-238, the lower energy neutrons produced by spallation generally cannot. This is a tradeoff whichthat affects the mixture of fuels against the degree of spallation used in the design.

*{{cite techreporttech report

[[Category:Nuclear fusion|Hybridtechnology]]