Guest essay by Eric Worrall
Building an energy producing nuclear fusion reactor remains elusive, but some companies are re-considering an old idea – combining nuclear fusion with nuclear fission in a single reactor, to overcome the disadvantages of both.
Fusion-fission hybrids: nuclear shortcut or pipe dream?
While nuclear fusion’s key milestones remain elusive, could fusion-fission hybrid reactors represent the best of both worlds? Start-up Apollo Fusion aims to make this complex concept a commercial reality, but formidable obstacles remain.
The fusion-fission hybrid concept
Is pure fusion truly only a matter of years away? Opinions vary due to the formidable technical challenges that remain to be solved. But while the likes of ITER, the National Ignition Facility and a host of start-ups and academic labs around the world continue to hammer away at the fusion chestnut, a decades-old alternative concept that combines fusion and fission has resurfaced.
The idea of a fusion-fission hybrid reactor has existed since the early 50s, with the earliest reference attributed to Russian nuclear physicist Andrei Sakharov. The fusion-fission hybrid concept is envisaged as a system that balances the advantages and disadvantages of the two nuclear generation paradigms; fission creates large amounts of energy per reaction, while fusion creates less energy per reaction but can generate abundant neutrons without the need for a chain reaction.
A fusion-fission hybrid reactor, then, would use a fusion reactor to provide neutrons to an encapsulating ‘blanket’ of fissile materials, so fusion is essentially used as a stable fuel source for traditional fission-based energy generation.
What are the advantages of such a hybrid system? For a start, using fusion-derived neutrons to feed fission reactions would massively expand the fuel available to run plants. Conventional fission reactors require one specific isotope of uranium, U-235 (or plutonium-239), which constitutes only 1% of raw uranium deposits, to drive the fission chain reaction. By using fusion as a fuel, a hybrid reactor would be able to use any uranium isotope while capitalising on the higher energy output of fission.
So with fusion feeding fission, a plant could theoretically operate more cleanly and efficiently, massively reducing waste and proliferation concerns while providing a way to use fusion even if positive net energy has not been achieved. In terms of safety, proponents say the concept would be inherently meltdown-proof because it operates in subcritical conditions and the fission would not be self-sustaining.
Using a nuclear fusion reactor to stimulate fission in suboptimal fuel is an old idea. To date the concept has languished, because it doesn’t seem to offer any advantages over a more conventional breeder reactor. A report in 1980 by the Los Alamos Scientific Laboratory concluded that there was no point exploring fusion / fission hybrid designs, because they offer no advantage over “conventional” breeder designs.
Summary: The future of nuclear power rests in the hands of a diverse group of decision makers whose motives and methods vary greatly.
In some respects, the two long-term cycles are similar. Each would probably be equally likely to win licensing approval and public acceptance.
In other respects, the advantage could belong to either cycle, depending on who the decision maker is. For instance, if the next generation of reactors is to be manufactured by private industry and operated by utilities, the fast breeder reactor cycle would be preferred. If, on the other hand, the federal government becomes the manufacturer and operator of fissile breeders, the hybrid would have the advantage.
The crucial difference between these two cycles is one of readiness. Whereas the fast breeder will probably be a commercial technology in the near future, the fusion-fission hybrid has yet to be proven scientifically feasible. A decision to commit federal funds for the demonstration and commercialisation of the hybrid would have to be based on a conviction that the hybrid is vastly superior to the LMFBR as a breeder of fissile fuel.
Although the hybrid is indeed superior in some respects, it also has some drawbacks. Furthermore, as is always the case with an untested concept, there is the possibility that unforeseen problems will emerge as the technology becomes better understood.
In the face of an already commercialized fast breeder reactor, there is not sufficient incentive, in our opinion, to demonstrate and commercialize the fusion-fission hybrid
Both fission / fusion hybrids and breeder reactors derive the bulk of their energy from burning nuclear waste or other low grade fuel, by bombarding the low grade fuel with a blizzard of neutrons. The difference between the two is how the neutrons are produced – a conventional breeder reactor uses a normal fission core to produce the neutrons, while the fission / fusion reactor uses a nuclear fusion reaction to irradiate the low grade fuel.
Fission / fusion hybrids might be safer. There is no risk of the fusion core suffering a fission reactor style meltdown, because any failure of a critical component immediately kills the fusion reaction. The low grade fuel blanket surrounding the fusion core would still have to be carefully designed for safety, to address risks such as loss of coolant, but without the fusion reaction stimulating the burn, it should immediately start to cool in the event of a major failure.
The other intriguing possibility is Fission / fusion hybrids could potentially be made very small. Desktop size electrostatic confinement fusion reactors have been available for decades, they are sold commercially as neutron sources. Desktop fusion reactors cannot produce net energy, and may never be able to do so, so by themselves they are not useful as a power source. But the fusion component of a fission / fusion hybrid reactor does not have to achieve breakeven by itself – it is entirely acceptable for the fusion component to be a net energy drain on the system, providing the fission component more than covers the energy lost to the fusion component. It would be intriguing to discover just how small you could make a fission / fusion hybrid reactor.
Commercialisation of fusion / fission hybrids might also potentially spur pure fusion development. There would be a strong commercial incentive to improve the design of the fusion component of the system, which might improve understanding of how to control pure fusion plasmas to the point that energy producing fusion plasmas become viable.
On the downside – a fission / fusion hybrid is still a fission reactor, so any attempt to build a fission / fusion hybrid is likely to attract all the usual green outcry.