Last night I watched my recording of Tuesday’s BBC2 Horizon programme on the progress of the development of fusion power(if you missed it is available to watch on iPlayer for a few days) . The programme was presented by Professor Brian Cox who has presented previous Horizon programmes on such topics as the structure and meaning of time. He has an unusual style which works quite well.
The basis of the programme was that we want to replace fossil fuel with other forms of energy production to prevent/slow global warming. It showed that it would not be feasible to replace fossil fuels entirely using even a mixture of ‘alternative energy’ sources. Therefore some other energy source would be required. Could that be fusion energy - the energy of the stars and the hydrogen bomb? The hydrogen bomb shows we can generate huge quantities of energy but controlling the reaction is the key problem to solve.
The main questions the programme attempted to answer were: as fusion energy does not involve the release of (much) carbon dioxide, “could it provide the world with ‘clean’ power?” and secondly: “if so, could it do so in time to stop/limit global warming?”. If the answers were “yes “or at least “very likely”, then perhaps the world should put the vast majority of its investment currently going into wind power, solar power etc. into fusion power instead.
Quite a bit of the programme concentrated on the basic facts of what nuclear fusion energy is and the history of its discovery and use in the atomic bomb and that it forms the basis of the energy produced by the sun. This left little time to go into any detail on the current state of development of fusion for peaceful energy production.
The raw material for fusion energy (deuterium - heavy form of hydrogen) is abundant in the oceans as ‘heavy water’ and the end product of fusion is helium which is benign. Potentially a fusion reactor could produce all the worlds energy needs with little pollution.
Unfortunately, of course, it is not as easy as that. The main problem is that the nuclear reaction only happens at extremely high temperatures or lower temperatures but extremely high pressures like those at the centre of sun. In these conditions the deuterium exists as a plasma which reacts violently with any part of a container it comes into contact with which is destroyed literally in a flash. To avoid this,the deuterium has to be contained in some other way.
In most experimental systems rather than pure deuterium, a mixture of deuterium and tritium (an even heavier form of hydrogen but not usefully available in nature) is used. This mixture is easier to ‘ignite’, ie it is easier to start a chain fission nuclear reaction with the mixture than with pure deuterium.
The method currently in use in most of the worlds experimental reactors is containment by a very strong toroidal (donut shaped) magnetic field within the reactor . This field is intended to keep the plasma away from the inside surface of the physical container. The most successful toroidal configuration so far has been the tokamak.
However, so far, the systems have only proved to be stable for short times of a few seconds. They also use huge amounts of electrical energy to make the magnetic field strong enough. The latest system (built in South Korea) is showing promise. By using superconducting magnets the amount of electrical power required is drastically reduced and the system seems to offer a more stable containment than before. The worlds most advanced tokamak reactor called ITER is being build in France by a international consortium. This planned to produce 5 to 10 times more energy than it uses and to hold/control the plasma for 8 minutes. However it will not be designed to actually produce any electricity. It should be operational in 2018.
The other approach is to ‘fire’ an intense burst of photons (light) evenly on all sides of a small pellet of deuterium and tritium. In the programme Professor Fox visited the huge experimental system in USA where an immense laser produces a light beam with an energy of 500 terawatts . This is more than than the USA as a whole uses continuously, however the pulse lasts only an extremely short time. This pulse of light squeezes the pellet extremely rapidly causing the temperature and pressure of the fuel at the centre of the pellet to reach conditions for a chain fission reaction to start.
The resulting burst of energy is greater than the energy used for the laser but unfortunately is also so large it immediately destroys the apparatus that held the pellet. A lot of time is then spent setting up the experiment for the next pulse. Even if that technological problem is solved, the time it takes the laser to get back up to power is also significant and this would also have to be solved so that a continuous series of pulses of energy could be produced forming the basis of a practical power station.
The conclusion was that it was looking more likely fusion powered energy production could be achieved. Whether it would become practical and, very importantly from the point of view of global warming, how quickly that might be was still too early to say. Guesses ranged from 15 to 50 years.
It certainly doesn’t look like we’ll be having domestic fusion power devices down the garden or in the cellar anytime soon!