DEMO

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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 will be on the scale of a modern electric power plant.

To achieve its goals, DEMO must be 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 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].

[edit] How the reactor will work

See also: nuclear fusion

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

${}^{2}_{1}\mbox{H} + {}^{3}_{1}\mbox{H} \rightarrow {}^{4}_{2}\mbox{He} + {}^{1}_{0}\mbox{n} + 17.6 \mbox{ MeV}$

There are three technological barriers that DEMO will attempt to overcome; that first the nuclei need to be convinced to fuse, second that the burning plasma produced by this ignition will need to be contained and controlled, and third that the energy liberated needs to be somehow utilised.

The activation energy for fusion is so high because the protons in each nucleus will tend to strongly repel one another, as they each have the same positive charge. Nuclei must be within 1 femtometre (1 x 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 - fusion occurs.
At these tempeatures, any containment vessel would melt, so the plasma needs to be kept away from the walls, using magnetic confinement - plasma contained.

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, it is they that will be the fusion reactor's source of energy output.

The tokamak containment vessel will have a thick inner-coating of liquid lithium.
Lithium readily absorbs high speed neutrons to form further helium and tritium.
The helium and tritium rejoin the plasma, and the remaining lithium suffers a subsequent temperature increase.
This increase in temperature is passed onto (pressurized) liquid water in a sealed pipe. The presence of a localised hotspot in the pipe sets up a convection current.
The convection current will be used drive the turbine of a generator, to create an electrical current - useful energy.

[edit] References

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