The dream of a future generation of reactors which use the spent fuel of today’s reactor fleet to power them has been around for many years. This dream may become a reality with proposed hybrid reactors under development, which have the potential to extract almost 100% of the energy present in nuclear fuel.
What was considered waste not so long ago is now being used to produce useful products, such as energy. The same could apply to nuclear “waste”. Radiation is after all a form of energy, and a way surely exists to capture all of this energy safely and convert it to more benign forms. The ultimate goal would be to extract all the energy from radioactive fuel and leave a non-radioactive residue. It may be possible to achieve this goal using hybrid nuclear technology.
A hybrid of any sort consists of the optimised combination of several different technologies. The nuclear hybrid reactor concept comprises a combination of a conventional fission reactor process and fusion reactor principles. The system would consist of a fusion reactor core in combination with a subcritical fission reactor. The results of the fusion reaction, which would normally be absorbed by the cooling system of the reactor, would feed into the fission section, and sustain the fission process. The concept is shown in Fig.1Some of the advantages claimed for the proposed system are:
Claims that the hybrid can increase the energy extracted from mined uranium by a factor of 100 have been made . The hybrid fission-fusion reactor is seen as a useful near-term commercial application of fusion while longer term research continues in pursuit of pure fusion power systems.
Nuclear fission reactors use fissile material. Fission reactors can be designed to handle naturally abundant U-235, as well as fuels described as slightly enriched (2 to 5% U-235), highly enriched (20 to 30% U-235), or fully enriched (>90% U-235) .
The waste problem arises from the nature of the fission reaction itself. The reaction starts when a neutron strikes the nucleus of a fissile isotope such as uranium-235 or plutonium-239, causing it to split apart. The result is a pair of lighter nuclei, a burst of energy and a number of new neutrons. The energy is extracted as heat. But to keep the fission reaction going, some of the newly created neutrons must collide with other fissile nuclei, causing them to split and to release still more neutrons in a chain reaction. This happens easily as long as the fuel is fresh and there is nothing else for the neutrons to hit. As the reactions continue, however, the fuel accumulates more fission-product nuclei, most of which absorb the neutrons without doing anything else. Eventually, so many neutrons are absorbed that the chain reaction can no longer sustain itself, at which point the fuel becomes waste, even though most of the original fissile material is still there .
There are widely differing estimates of the available uranium ore, but there is no dispute about one thing: a once-through nuclear economy based on light water reactors (LWRs) uses only about 1% of the available fuel. Fission or fusion breeders potentially use all of it. The question is whether the world is so well endowed with fuel that we can afford to discard 99% of it . Part of the challenge facing the nuclear industry is the storage of spent fuel rods and handling waste. Despite reprocessing of spent fuels to recover fissile material, the amount of nuclear “waste” is growing. Any viable technology that can use this waste material is a bonus.
The nuclear fusion reaction fuses atoms of an element to produce a new element plus prodigious amounts of energy. The process has its genesis in the hydrogen bomb, which fused hydrogen atoms to produce helium. To produce fusion requires very high temperatures of the order of millions of degrees Celsius, and this was achieved by a using a conventional nuclear bomb as the source. This device produced a short lived uncontrolled massive release of energy, and although it demonstrated that the reaction could be achieved, did not contribute much to the idea of fusion as a controllable source of energy.
Modern designs focus on controllable fusion and use. This reaction fuses two isotopes of hydrogen, deuterium (2H) and tritium (3H), to form helium and a neutron. The neutron produced is a useful component in activation fission reactions. The reaction requires the same extremely high temperatures and the material is contained in plasma confined by high magnetic fields. Several research units are currently in operation but none have yet achieved the state where the energy produced exceeds the energy required to drive the operation.
The hybrid reactor concept
The answer to the fission problem is supplementing the chain reaction with an independent source of neutrons, which must of necessity consume less energy than that produced by the fission process. With enough neutrons, much more of the uranium and plutonium in the fuel would be used up, and most of the long-lived radioactive fission products would also burn up, greatly reducing waste. And so the idea of the fusion/fission hybrid was born.In principle, building a hybrid was simply a matter of wrapping a blanket of fissile material around a fusion reactor, and letting the energy flow. In practice, though, the technology of the past was not up to the task. Faced with what seemed like insurmountable difficulties, the hybrid idea was shelved and physicists focused their efforts on the separate development of pure-fission and pure-fusion reactors. And except for a brief flurry of interest following the energy crisis of the late 1970s, when the Nobel laureate physicist Hans Bethe tried to drum up support for hybrids, the idea has stayed on the shelf – until now .
In the hybrid system the neutrons required to sustain the fission reaction in the fissile material are obtained from the fusion reaction. The fusion reaction depends on a supply of energy, which allows both fusion and fission reactions to be controlled. The fusion reaction is not self-sustaining, and once the source of energy is removed, the fusion reaction ceases, and the fission reaction is also shut down. The advantage of the hybrid system is that the energy produced by the fission reaction far exceeds that required to drive the fusion reaction. The fission reaction is deeply subcritical and is only sustained by the fusion reaction.
Since the only function of the fusion reaction is to produce neutrons to sustain the fission reaction, a fairly low level source is required. A key finding revealed that the needed 2H neutron source, which may be intermittent without interrupting overall plant power output, can be less than 1% of total plant power. This implies that fusion Q(eng) energy gains near 0,1 would be adequate . The fission core blanket would contain material which is fissile, i.e. capable of sustaining a nuclear reaction, derived from depleted uranium cores as well as fission products contained in the spent cores. The depleted cores still contain fissile material which could be used in a hybrid system.
In contrast to current commercial fission reactors, hybrid reactors potentially demonstrate what is considered inherently safe behaviour 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 rapidly and stops. 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 is a wide range of interest in the hybrid principle, and in addition to committed projects described in the following section, there are numerous theoretical designs based on hybrid reactor principles . The idea that fusion reactors could be used as neutron sources for many applications has resulted in the design of compact or “mini” fusion reactors with reduced performance requirements and reduced cost compared to energy producing fusion reactors.
Molten salt hybrid tokomak (MSHT) 
This concept design is based on the full sized Tokamak reactor known as the international experimental thermonuclear reactor (ITER), a reactor being built in France as part of an international project to develop fusion reactor technology. The ITER, which will be the largest Tokomak in the world, is designed to produce 500 MW of electricity, with an input power of 50 MW, giving a target gain figure of ten.
The proposal to combine the ITER Tokomak with a fission reactor comes from Evgeny Velikov . According to Velikov, nuclear power based on MSHT would allow possibilities in both reactor inventory generation (including fuel reprocessing) and “simple” power generation (without fuel reprocessing) to be divided between different groups of countries, that would provide nuclear power with complementary security features relative to weapons-grade material proliferation, and contribute to its international marketing flexibility. Velikov identifies the need to eliminate five vital risks if the nuclear renaissance is to become possible:
Eliminating these risks is possible using MSHT – molten salt (MS) hybrid (H) Tokamak (T) and a fuel cycle of innovative design/structure. This would be a modular system, self-withstanding against unprotected dangerous events, fed by thorium, or natural uranium in a dense subcritical blanket with essentially enhanced neutron balance and capability incinerating residual actinides and long life fission products.
Laser inertial fusion energy system (LIFE) 
This system is based on the pumped diode laser fusion reactor developed by Livermore National Laboratories. LIFE (laser inertial fusion energy) is an approach to pure fusion energy as well as a once-through, closed nuclear fuel cycle. A LIFE power plant comprises a solid-state laser system and a fusion target chamber which can be surrounded by a subcritical fission blanket (Fig.2). The balance of plant includes a fusion target factory, a heat exchanger, and other systems. The LIFE engine starts with a 10 to 20 MW diode-pumped solid-state laser system to provide about 1,4 MJ of energy. Deuterium-tritium fusion targets are injected at 10 to 15 times a second into the centre of a 2,5 m radius fusion chamber. The laser beams ignite the fusion targets to obtain energy gains of 25 to 35 and fusion yields of 35 to 50 MJ of energy, thereby creating 350 to 500 MW of fusion power (about 80 % in the form of fusion neutrons, with the rest of the energy in x-rays and ions). Each fusion target generates about 1019 neutrons.
The pulsed fusion reactions produce high-energy neutrons, which then bombard a spherical blanket containing a high-heat-capacity lithium based molten salt “charged” with fission fuel. The resulting fission reactions will produce additional energy that can be harvested for electricity production. Using depleted uranium or spent nuclear fuel from existing nuclear power plants in the blanket, the engine will be capable of burning the by-products of the current nuclear fuel cycle. Because the fusion neutrons are produced independently of the fission process, the fission fuel could be used without reprocessing. In this way, the engine may be able to consume nuclear waste as fuel, mitigate against further nuclear proliferation, and provide long term sustainability of carbon-free energy.
Livermore scientists believe that by combining the best aspects of fusion and fission, this fusion-fission solution could minimise concerns and drawbacks associated with the current nuclear energy fuel cycle. A LIFE engine could be designed to burn spent nuclear fuel and generate electricity while virtually eliminating all actinides. The reactor would have enormous fuel flexibility. The fission blanket could consist of natural uranium, spent nuclear fuel, depleted uranium, highly enriched uranium (rich in uranium-235), natural thorium, excess weapons plutonium, and other actinides and could extract nearly all of the energy from the fuel. Because a LIFE engine can extract virtually 100% of the energy content of its fuel (compared to about 1% of a typical nuclear power plant), the nuclear waste it does produce has significantly reduced concentrations of long-lived actinides .
Compact fusion neutron source (CFNS)
The primary purpose of these systems appears to be the processing of nuclear waste rather than the production of energy, although energy is produced during the process. There are several proposed designs for this system [6, 7], all of which are based on the use of fusion to produce neutrons to drive and sustain the associated fission reaction, rather than energy or heat. The requirement to only produce neutrons has led to the concept of a compact system design, with lower requirements, which is claimed to be easier to realise than a full size power producing fusion reactor. Many designs are based on the super-compact spherical Tokomak configuration, some of which are ten times smaller than the power producing model. The Texas design seems to be focused more on processing nuclear waste than producing electricity.
Other options considered are the use of the hybrid configuration to generate fissile material which is removed and used to power a conventional fission reactor. This concept necessitates the use of a liquid, possibly molten salt, to remove the fissile material generated. Several proposals based on this type of operation have been made, some centred on the concept of a nuclear energy park, where a single hybrid reactor could provide sufficient fuel for several conventional fission reactors .
 R Wooley: “An engineering study for a fusion-fission molten salt reactor”, Princeton plasma physics laboratory, www.pppl.gov/events/engineering-study-fusion-fission-hybrid-reactor
 E Gerstner: “The hybrid returns”, Nature, Vol 460, 2 July 2009.
 W Mannheimer: “The case for fission suppressed hybrid fusion”, www.aps.org/units/fps/newsletters/201104/manheimer.cfm?renderforprint=1
 E Velikov: “Future development of nuclear power and the role of the fusion neutron source”, NRC Kurchatov institute www.iter.org/doc/www/content/com/Lists/Stories/Attachments/1413/Hybrids.pdf
 A Heller: “Safe and sustainable energy with LIFE”, Lawrence Livermore National Laboratory, https://str.llnl.gov/AprMay09/moses.html
 University of Texas: “Nuclear fusion-fission hybrid could destroy nuclear waste and contribute to carbon-free energy future”, http://news.utexas.edu/2009/01/27/nuclear_hybrid
 MP Gryaznevich, et al: “Options for a steady state compact fusion neutron source” Transactions of fusion science and technology, Vol 61 January 2012, www.ccfe.ac.uk/assets/Documents/FS%26Tvol61p89.pdf
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