Nuclear fusion-fission hybrid

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Hybrid nuclear fusion-fission (hybrid nuclear power) is a proposed means of generating power by use of a combination of nuclear fusion and fission processes. The concept dates to the 1950s, and was briefly advocated by Hans Bethe during the 1970s, but largely remained unexplored until a revival of interest in 2009, due to the indefinite delays in the realization of pure fusion.[1]

In the LIFE project at the Lawrence Livermore National Laboratory LLNL, using technology developed at the National Ignition Facility, the goal is to use fuel pellets 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 neutrons 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).

Rationale[edit]

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 inefficiencies on the input side (i.e. low laser efficiency in ICF and 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.

Unlike a conventional fission reactor, the fusion hybrid can consume almost all of the uranium fuel without enrichment or reprocessing. This has advantages for non-proliferation, as enrichment and reprocessing technologies are also associated with nuclear weapons production. The low fuel consumption, lack of need for enrichment, and small waste volumes also significantly reduce fuel cycle costs. However, the fusion equipment required will increase the construction cost of the reactor.

Use to dispose of nuclear waste[edit]

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 reactors (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. Fission by-products produced by the operation of commercial light water nuclear reactors (LWRs) 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 actinides 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[edit]

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 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.

Fuel cycle[edit]

There are three main components to the hybrid fusion fuel cycle: deuterium, tritium, and fissionable elements.[2] Deuterium can be derived by separation of hydrogen isotopes in sea water (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. 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 U-238 to undergo fission directly (without conversion first to Pu-239), enabling refined natural Uranium to be used with very low enrichment, while still maintaining a deeply subcritical regime.

Engineering considerations[edit]

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

See also[edit]

  • Subcritical reactor, a broad category of designs using various external neutron sources including 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

References[edit]

  1. ^ Gerstner, E. (2009). "Nuclear energy: The hybrid returns". Nature 460 (7251): 25–8. doi:10.1038/460025a. PMID 19571861. 
  2. ^ Bethe, Hans (May 1979). "The fusion hybrid". Physics Today: 44–51. ISSN 0031-9228. Retrieved 16 May 2012. 

Further reading[edit]

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