DEMO

Encyclopedia : D : DE : DEM : DEMO

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DEMO is a proposed experimental nuclear fusion reactor. It will build upon the research that is to be conducted by ITER, only that it will be at the scale of a commercial reactor. It may or may not generate electricity for a public grid, depending on its success.

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.

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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 (compressed) 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.

Atomic nucleus | Nuclear fusion | Nuclear power | Nuclear reactor | Timeline of nuclear fusion Plasma physics | Magnetohydrodynamics | Neutron flux | Fusion energy gain factor | Lawson criterion
Methods of fusing nuclei
Magnetic confinement: Tokamak - Spheromak - Stellarator - Reversed field pinch - Field-Reversed Configuration - Levitated Dipole Inertial confinement: Laser driven - Z-pinch - Bubble fusion Cold fusion: Muon-catalyzed fusion Inertial electrostatic confinement: Farnsworth–Hirsch Fusor Pyroelectric fusion
Fusion experiments
Magnetic confinement devices ITER (International) | JET (European) | JT-60 (Japan) | Large Helical Device (Japan) | EAST (China) | T-15 (Russia) | DIII-D (USA) | TFTR (USA) | NSTX (USA) | NCSX (USA) | Alcator C-Mod (USA) | LDX (USA) | PACER (USA) | H-1NF (Australia) | MAST (UK) | START (UK) | DEMO (Commercial) Inertial confinement devices NIF (USA) | Nova laser (USA) | OMEGA laser (USA) | Shiva laser (USA) Z machine (USA) See also: International Fusion Materials Irradiation Facility

 

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