Fusion energy still a pipe dream

 

NUCLEAR FUSION is the holy grail of power generation, promising the safe, clean generation of boundless energy from essentially limitless, cheap fuel – if we can ever get it to work. The timeline for the development of nuclear fusion has been lengthened several times, costs have ballooned and it seems unlikely that nuclear-fusion power will be available before the end of this century. The current situation is summarised by Geoff Brumfiel in the June edition of Scientific American.

Massive energy is released in nuclear fusion because some of the matter in the atomic nucleus is converted into energy, as in Einstein’s famous equation E=mc2, where E is energy, M is mass and C is the speed of light. Even the tiniest amount of matter is equivalent to an enormous amount of energy, because C2 is such a huge number. The nuclear fusion of 1g of fuel releases the same energy as burning 11 tonnes of coal.

The sun’s enormous energy is generated by nuclear fusion when hydrogen atoms fuse together to form helium. The plan for earthly nuclear fusion requires us to simulate the sun on Earth – an enormously difficult task. Fusion on Earth requires a temperature of 100,000,000 degrees Kelvin, 10 times hotter than the sun, whose huge gravitational field allows fusion to occur more easily.

Two forms of hydrogen, deuterium and tritium, will fuel nuclear-fusion power plants. Deuterium is easily extracted from sea water and tritium, a mildly radioactive form of hydrogen, can be made from lithium, a fairly common metal. Hydrogen atoms exist as a “plasma” of positively charged atomic nuclei at fusion temperatures. Fusion temperature is reached by heating the hydrogen with a mixture of microwaves, electricity and bombardment with particles. A viable fusion plant must generate much more energy from fusion than the input energy necessary to heat the fuel to fusion temperature. The energy released will heat water, raising high-pressure steam to turn a turbogenerator that generates electricity.

The principle of deriving fusion energy in this way was first demonstrated in the Joint European Torus (Jet) device at Culham, UK, which has been in operation since 1983. In 1997, a fusion power of 10 megawatts was sustained for 0.5 seconds and 65 per cent of the power expended to ignite the plasma was recovered through fusion. The 80 cubic metre Jet plasma is too small to produce a net energy gain. This will be the role of the International Thermonuclear Experimental Reactor (Iter) under construction in Cadarache, France, with a plasma volume of about 830 cubic metres. The Iter is designed to generate fusion power of 500 megawatts, 10 times the power needed to ignite the plasma.

Iter is a collaboration of seven partners – EU, US, Japan, Russia, China, India and South Korea. The EU provides 45.5 per cent of the funding and each other partner provides 9.1 per cent. Each partner constructs different components for Iter in its own country and ships them to France for incorporation into Iter. This arrangement is intrinsically awkward and dogs the project with delays and budgetary over-runs. Original construction costs were estimated at $5 billion (€3.9bn). This doubled in the mid-1990s and costs have now doubled again to $20 billion (€15.6bn).

Iter fusion will operate intermittently, up to 30 minutes at a time, because materials that can withstand continuous fusion conditions are not yet available. A programme to develop these materials, essential for commercial reactors, is also under way, but it will have to overcome formidable technical difficulties.

Iter will test the feasibility of a sustained fusion reaction and will then become a test nuclear fusion power plant. Following several delays it is hoped to build Iter by 2020, after which about 1,000 scientists and engineers will work on the device for 20 years. If Iter works, a demonstration reactor with all the functions of a power plant will be built by 2050 and tested for 10 to 20 years. Finally, it may be possible to start up full-scale nuclear fusion worldwide by 2100, but many things could lengthen this timeline. Fusion technology emits no warming CO2 gas but nuclear fusion will not be available to mitigate the effects of global warming this century.

Bringing the sun to Earth was never going to be easy, but work will continue because the dream of cheap, clean and virtually unlimited power is irresistible.


William Reville is an emeritus professor of biochemistry and public awareness of science officer at UCC. understandingscience.ucc.ie

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