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.
Another similar concept is the accelerator-driven subcritical reactor, which uses a particle accelerator to provide the neutrons instead of nuclear reactions.
Hybrids were proposed as a way of greatly accelerating their market introduction, producing energy even before the fusion systems reached break-even.
[2] 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.
[3] These studies generally concentrated on the nuclear waste disposal aspects of the design, as opposed to the production of energy.
[4] 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 program.
LIFE was cancelled when the underlying technology, from the National Ignition Facility, failed to reach its design performance goals.
[5] Apollo Fusion, a company founded by Google executive Mike Cassidy in 2017, was also reported to be focused on using the subcritical nuclear fusion-fission hybrid method.
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.
This leads to the concept of reactor-grade enriched uranium, with the amount of 235U increased from just less than 1% in natural ore to between 3 and 5%, depending on the reactor design.
A combination of burnup of the 235U 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.
The result is nuclear waste that is highly radioactive and filled with long-lived radionuclides that present a safety concern.
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.
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.
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.
[22] 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.
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.
[25] Both require chemical processing to remove the bred 239Pu, both presented the same proliferation and safety risks as a result, and both produced about the same amount of fuel.
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.
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.
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.
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.
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.
This offers the potential to efficiently use the very large stockpiles of enriched fissile materials, depleted uranium, and spent nuclear fuel.
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.
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.