More than two years after the devastating Fukushima nuclear disaster, the largest nuclear accident since Chernobyl, the repercussions continue in Europe. Inevitably, the episode has prompted renewed debate in Brussels and EU countries about nuclear power.
For instance, Germany has decided to shut down its fission plants. Moreover, in a referendum, 95% of the Italian public opposed plans to restart a nuclear programme in the country. However, the United Kingdom has committed to building two fission power plants.
A danger in the polarised debate is that nuclear is often portrayed as a single, undifferentiated energy source. This is wrong and risks losing the opportunity to explore the role that new nuclear technologies can play.
The starting-point for debate should be the daunting energy problems that many countries face. With growing challenges to energy security, the range of energy sources must be broadened, with greenhouse-gas emissions reduced. There is also a pressing need to reduce pollution by coal and oil extraction and combustion.
So, what do new generations of nuclear fission, fusion and hybrid offer?
Fission: Modern power stations using fission, which harnesses energy from the radioactive decay of uranium and other fissile materials, are considerably safer than older ones such as Fukushima. This is because of stronger containment structures, more secure storage of spent fuel rods and emergency systems to prevent overheating.
As the supply of uranium is limited, there are controversial plans in some countries to construct fast breeder reactors to recycle waste and use the fuel more efficiently. However, there are proliferation dangers associated with the plutonium by-product.
Fission will only continue to be acceptable if the immediate risks of the systems are reduced. Despite improved safety, the rare, but catastrophic failures of operations such as Chernobyl cannot be dismissed. As Fukushima showed, there are also remaining risks from natural hazards and even aircraft crashes - plus the dangers of fission associated with the storage of waste for over 10,000 years.
Fusion: The principle of controlled thermonuclear fusion is to extract energy from processes similar to those occurring inside the sun, where hydrogen atoms are fused together to form helium. This is a 'clean' process with negligible long-lived radioactive waste.
However, because of the great size needed for a 'pure' fusion reactor and the unsolved problem of fabricating materials to withstand the materials' requirement, the development challenges are substantial and may take decades to overcome.
Hybrid: The long-term future of nuclear may therefore lie with combining nuclear fission (atoms splitting) and fusion (atoms merging) in a hybrid reactor. Indeed, governments, agencies and research institutes are already moving tentatively in this direction.
Hybrid could become a reality within the next two decades -- the International Atomic Energy Authority has started a project on conceptual development of steady state compact fusion neutron sources, and the Institute of Plasma Physics in China is planning to build a hybrid fusion proof-of-principle prototype experiment by 2025. International experiments are under way into the critical fusion parts of such a system.
The basic principle is that neutrons generated by fusion in the plasma core stimulate fission in the outer blanket that contains uranium or other fissile materials. Because there is relatively less energy extracted from the plasma than in pure fusion, continuous operation can be engineered more readily.
The fission is well below critical mass and only operates when there is a current flowing in the plasma, which can be switched off at a moment's notice. This is why the system is safer, especially in regions where earthquakes and tsunamis can occur.
A major advantage of hybrid reactors for countries without uranium is that it uses a wider range of fuels. And they do not produce the long-lived waste produced in fission because the high-energy neutron flux from the fusion process transmutates these into isotopes that decay over hundreds rather than tens of thousands of years.
Not only does this eliminate some nuclear-waste problems, it helps to rid the world of weapons-grade materials. Furthermore, if thorium is used, it cannot be converted into weapons-grade uranium.
While even modest-sized hybrid reactors could provide affordable and almost limitless energy, their power output can be controlled through the fusion process. Thus the operation is safe enough for a power station to be located even in countries prone to natural hazards. Moreover, the controllability would allow fusion-fission power to be used either as base load or more flexibly in combination with renewable energy, which is inherently more variable.
Many aspects of hybrid nuclear require further intense research. While workable hybrid technology is still some way off, timeframes could be accelerated with the right commitments from the public and private sectors.
Taken overall, this emerging hybrid option deserves wider understanding and support if we are going to have the maturity of debate we need about the role that nuclear can play in the future energy mix.