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DEMO (DEMOnstration Power Plant) is a proposed nuclear fusion power plant that is intended to build upon the expected success of the ITER (originally an acronym for International Thermonuclear Experimental Reactor) nuclear fusion power plant. Whereas ITER's goal is to produce 500 million watts of fusion power for at least 400 seconds, the goal of DEMO will be to produce at least four times that much fusion power on a continual basis. Moreover, while ITER's goal is to produce 10 times as much power as is required for ignition, DEMO's goal is to produce 25 times as much power. DEMO's 2 gigawatts of thermal output will be on the scale of a modern electric power plant.

To achieve its goals, DEMO must have linear dimensions about 15% larger than ITER and a plasma density about 30% greater than ITER. As a prototype commercial fusion reactor DEMO could make fusion energy (which does not produce the global warming or pollution of fossil fuel, nor the long-lived radioactive waste of fission energy) available within 20 years. Subsequent commercial fusion reactors could be built for nearly a quarter of the cost of DEMO, if things go according to plan ^{[1] [2]}.

While fusion reactors like ITER and DEMO will not produce transuranic wastes, some of the components of the ITER and DEMO reactors will become radioactive due to neutrons impinging upon them. It is hoped that careful material choice will mean that the wastes produced in this way will have much shorter half lives than the waste from fission reactors, with wastes remaining harmful for less than one century. The process of manufacturing tritium currently produces long-lived waste, but both ITER and DEMO, it is hoped, will produce their own tritium, dispensing with the fission reactor currently used for this purpose.

How the reactor will work

The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power.

See also: nuclear fusion and fusion power

When deuterium and tritium fuse, two nuclei come together to form a helium nucleus (an alpha particle), and a high energy neutron.

There are three problems that DEMO must solve: getting the nuclei to fuse, containing the resulting plasma, and capturing the liberated energy.

• The activation energy for fusion is so high because the protons in each nucleus will tend strongly to repel one another, as they each have the same positive charge. Nuclei must be within 1 femtometre (1 × 10⁻¹⁵ metres) of each other to fuse - achieved by high temperatures.
• High temperatures give the nuclei enough energy to overcome their electrostatic repulsion. This requires temperatures in the region of 100,000,000 °C by using energy from microwaves and ion beams.
• At these temperatures, any containment vessel would melt, so the plasma needs to be kept away from the walls, using magnetic confinement.

Once fusion has begun, high energy neutrons will pour out of the plasma, not affected by the intense magnetic fields (see neutron flux). Since it is the neutrons that receive most of the energy from fusion, they will be the fusion reactor's source of energy output.

• The tokamak containment vessel will have a lining composed of ceramic or composite tiles containing tubes in which liquid lithium will flow.
• Lithium readily absorbs high speed neutrons to form helium and tritium.
• The lithium is processed to remove the helium and tritium.
• The deuterium and tritium are added in carefully measured amounts to the plasma.
• This increase in temperature is passed onto (pressurized) liquid water in a sealed, pressurized pipe.
• The hot water from the pipe will be used to boil water under lower pressure in a heat exchanger.
• The steam from the heat exchanger will be used to drive the turbine of a generator, to create an electrical current - useful energy.

References

1. ^ Beyond ITER. The ITER Project. Information Services, Princeton Plasma Physics Laboratory. Retrieved on 2006-11-11.
2. ^ Overview of EFDA Activities. EFDA. European Fusion Development Agreement. Retrieved on 2006-11-11.

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