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 basic idea is to use high-energy fast neutrons from a fusion reactor to trigger fission in non-fissile fuels like U or Th. 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 break-even. However, detailed studies of the economics of the systems suggested they could not compete with existing fission reactors.
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. These studies generally concentrated on the nuclear waste disposal aspects of the design, as opposed to the production of energy. 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. 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.
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. Their web site is now focussed on their Hall-effect thrusters, and mentions fusion only in passing.
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.
Description
Fission basics Conventional fission power systems rely 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 criticality.
Natural uranium is a mix of several isotopes, mainly a trace amount of 235U and over 99% 238U. When they undergo fission, both of these isotopes release fast neutrons with an energy distribution peaking around 1 to 2 MeV. This energy is too low to cause fission in 238U, which means it cannot sustain a chain reaction. 235U will undergo fission when struck by neutrons of this energy, so 235U sustains a chain reaction. There are too few 235U 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 235U to produce enriched uranium, while the leftover, now mostly 238U, is a waste product known as depleted uranium. 235U will sustain a chain reaction if enriched to about 20% of the fuel mass.
235U will undergo fission more easily if the neutrons are of lower energy, the so-called thermal neutrons. 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 235U 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 235U 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 235U to at least 20%, and more commonly, over 90%.
To maintain criticality, the fuel has to retain that extra concentration of 235U. A typical fission reactor burns off enough of the 235U to cause the reaction to stop over a period on the order of a few months. 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 reactor 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.
The waste contains most of the 235U it started with, only 1% or so of the energy in the fuel has been extracted by the time it reaches the point where it is no longer fissile. One solution to this problem is to reprocess the fuel, which uses chemical processes to separate the 235U (and other non-poison elements) from the waste, and then mixes the extracted 235U 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.
Like 235U, 239Pu can maintain a chain reaction, so it is a useful reactor fuel. However, 239Pu is not found in commercially useful amounts in nature. Another possibility is to breed 239Pu from the 238U 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.
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 235U 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 238U assembly surrounding the reactor core, most commonly taken from the stocks of depleted uranium. 239Pu can also be used for the core, which means once the system is up and running, it can be refuelled using the 239Pu it creates, with enough left over to feed into other reactors as well. This concept is known as a breeder reactor.
Extracting the 239Pu from the 238U 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.
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 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.
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. 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.
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). Tritium has a short half-life of 12.3 years, 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. 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 cover its own use. Many years would be needed to breed enough surplus to start another reactor.
Hybrid concepts Fusion–fission designs essentially replace the 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 238U, as well as many of the other elements in the fuel, including some of the transuranic waste elements. The reaction can continue even after all of the 235U 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.
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 one neutron, and that neutron cannot produce 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.