It is now a year since the devastating Tohoku earthquake and tsunami triggered the Fukushima disaster, the largest nuclear accident since Chernobyl. Inevitably, the episode has prompted renewed debate about nuclear power.
Since Fukushima, important developments have taken place. Germany has decided to shut down its fission plants. Moreover, in a referendum, 95 per cent of the Italian public opposed plans to restart a nuclear programme in the country.
However, the United States (where public support for nuclear power is growing) and the United Kingdom have committed to building two fission power plants. Meanwhile, China is continuing with a massive programme of nuclear power reactor construction, and Turkey has re-committed to building its first nuclear power plant. South Africa is also reportedly soon to announce an expansion of its nuclear power programme.
A danger in the occasionally polarised international 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 in the energy mix.
Indeed, those who seek to write nuclear off completely are missing what could be remarkable developments on the horizon with hybrid technologies potentially reshaping the way we think about nuclear energy.
The starting point for debate should be the daunting energy problems 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 (which continue to cause more deaths per year than nuclear power has in its entire history).
Renewables are a key part of the solution, but no country can be sure of the reliability of energies such as wind or solar in twenty to fifty years, given changing climatic conditions. Relying on neighbouring countries for power also carries risks.
There seems to be no alternative but to include nuclear in the energy mix for the foreseeable future. So, what do new generations of 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. Future developments will greatly reduce the volumes of radioactive waste produced.
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.
In the United States, President Barack Obama's budget request for the magnetic fusion programme for fiscal year 2013 was $400 million dollars. However, the overall US domestic programme was stripped back to increase funding for the International Thermonuclear Experimental Reactor (ITER) project whose budget, $26 billion dollars, has increased by a factor of 3 in the last 5 years.
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 fusion was first proposed by the American Nobel laureate Hans Bethe to enable more widely available reserves of nuclear fuels other than uranium, such as thorium, to be used. 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 underway 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 (which could include nuclear waste). 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 transmutes these into isotopes that decay over a hundred 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; current collaboration between groups in Russia, China, the United States, South Korea and Britain needs to involve more countries. 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.
* Lord Julian Hunt, a former fusion technology researcher, is a visiting professor at Delft University of Technology. Graham O'Connor is a former senior scientist at ITER.