Moving from fission to fusion is the Holy Grail of nuclear energy


NUCLEAR FUSION, which promises the clean production of virtually limitless energy from readily available raw materials, is the Holy Grail of research that hopes to find a viable successor to the generation of energy from fossil fuels. However, developing a fusion reactor is proving to be a tough nut to crack and may take much longer than originally expected. The current state of play in nuclear fusion is described by Michael Moyer in the March edition of Scientific American.

Conventional nuclear power, nuclear fission, is based on the break-up (fission) of the heaviest naturally occurring element – uranium atoms. Nuclear fusion, on the other hand, means the joining together (fusion) of atoms of the lightest natural element – hydrogen.

Nuclear fusion is the process that takes place in our sun when hydrogen atoms fuse together to produce helium, releasing enormous amounts of energy in the process. In order to build a nuclear fusion power plant, therefore we must reproduce the enormously high temperature and pressure that exist in the interior of the sun, and these conditions must be safely maintained over long periods in the nuclear fusion power plant.

Two approaches are being used to achieve fusion. One relies on lasers and the other relies on heating a magnetically contained plasma.

The National Ignition Facility (Nif), a 13-year, $4 billion enterprise at Lawrence Livermore National Laboratory, California, will start fusion experiments later this year. The Nif will bombard pellets containing two heavy varieties of hydrogen (deuterium and tritium) with the world’s most powerful laser beam.

The laser energy will crush the pellet so forcefully that hydrogen fusion will occur, releasing much energy.

Fusion has been achieved before, but more energy was used to generate the lasers than was released during fusion. It is confidently expected that Nif will reach the point where fusion energy output exceeds the input energy.

The second major fusion facility, a $14 billion project in southern France, named Iter, is scheduled to be built in 2018, and to start deuterium-tritium fusion tests in 2026. Iter will heat hydrogen, using microwave radiation, to 150 million degrees, creating a highly mobile state of matter called a plasma (a sea of electrically charged atoms). Electrically charged particles are affected by a magnetic field.

The hot plasma will be contained by a magnetic field generated by superconducting magnets, and fusion will occur.

Unlike the intermittent laser bursts in Nif, it is hoped that the magnetic field will hold the plasma together for up to hundreds of seconds, producing a sustained burst of fusion.

So, that’s where we are now – trying to ignite the fusion process and keep it going for a short while.

But, remember, in a working nuclear fusion plant the fusion must be maintained continuously, year in year out. The core of the fusion plant must also be able to withstand extremely high temperatures year after year, and to withstand long-term bombardment from high-energy neutrons generated in the fusion process (this bombardment turns ordinary material brittle).

Another problem is to source a continuous supply of tritium, one of the two reactants in the fusion process. The other reactant, deuterium is available in limitless supply from sea-water. Tritium can be made in a conventional fission nuclear power plant at a rate of 2-3kg per year and at a cost of about $100 million (€74m) per kg.

However, a fusion plant will consume a kg of tritium per week, so to supply tritium from a fission plant is not a practical proposition.

The fusion plant must be designed to automatically generate a sufficient supply of its own tritium. In theory, this will be done by allowing the high-energy neutrons generated in fusion to bombard a surrounding blanket containing lithium. The neutrons will induce lithium to split into helium and tritium, and this tritium will replace the tritium used up in the fusion reaction.

Tritium generation must proceed with the greatest efficiency, otherwise the fusion process will wind down and stop. The formidable technicalities of tritium supply have yet to be worked out. We are surely a long way from building a fusion power plant.

At current rates of progress, the construction of the first demonstration nuclear fusion plant may not begin until around 2100.

But Nif director Edward Moses has proposed a compromise plan to develop a hybrid fission-fusion plant that could be connected to the national grid in 20 years – a laser inertial fusion engine (Life).

Only 5 per cent of the uranium that goes into a fission nuclear power plant gets used up before the fuel is withdrawn and stored as high level radioactive waste. Life would use neutrons from laser powered fusion to bombard this spent fission fuel, causing fission reactions and producing heat that would be used to generate electricity.

William Reville is associate prof of biochemistry and public awareness of science officer – see understandingscience.