DEMO (DEMOnstration Power Plant) is a proposed nuclear fusion power plant that is intended to build upon the expected success of the ITER experimental nuclear fusion reactor. Whereas ITER's goal is to produce 500 megawatts of fusion power for at least 500 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 breakeven, DEMO's goal is to produce 25 times as much power. DEMO's 2 to 4 gigawatts of thermal output will be on the scale of a modern electric power plant. Also notably, DEMO is intended to be the first fusion reactor to generate electrical power. Earlier experiments, such as ITER, merely dissipate the thermal power they produce into the atmosphere as steam.
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 available by 2033. Subsequent commercial fusion reactors could be built for nearly a quarter of the cost of DEMO if things go according to plan.
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. 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 will produce their own tritium, dispensing with the fission reactor currently used for this purpose.
PROTO is a beyond DEMO experiment, part of European Commission long-term strategy for research of fusion energy. PROTO would act as a prototype power station, taking in any remaining technology refinements, and demonstrating electricity generation on a commercial basis. It is only expected after DEMO, meaning a post-2050 timeline, and may or may not be a second part of DEMO/PROTO experiment. This might possibly make PROTO the first commercial nuclear fusion power plant in the world.
The following timetable was presented at the IAEA Fusion Energy Conference in 2004 by Prof. Sir Chris Llewellyn Smith. These dates are conceptual and as such are subject to change.
- Conceptual design is to be complete by 2017
- Engineering design is to be complete by 2024
- The first 'Construction Phase' is to last from 2024 to 2033
- The first phase of operation is to last from 2033 to 2038
- The plant is then to be expanded/updated
- The second phase of operation is to last from 2040 onwards
How the reactor will work
The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. When deuterium and tritium fuse, the two nuclei come together to form a helium nucleus (an alpha particle) and a high-energy neutron.
- + + + 17.6 MeV
DEMO will be constructed once designs which solve the many problems of current fusion reactors are engineered. These problems include: containing the plasma fuel at high temperatures, maintaining a great enough density of reacting ions, and capturing high-energy neutrons from the reaction without melting the walls of the reactor.
- The activation energy for fusion is very large because the protons in each nucleus strongly repel one another; they are both positively charged. In order to fuse, the nuclei must be within 1 femtometre (1 10 15 metres) of each other, which is achievable using very high temperatures.
- DEMO, a tokamak reactor, requires both dense plasma and high temperatures for the fusion reaction to be sustained.
- High temperatures give the nuclei enough energy to overcome their electrostatic repulsion. This requires temperatures in the region of 100,000,000 C, and is achieved using energy from various sources, to include ohmic heating (from electric currents induced in the plasma), microwaves, ion beams, or neutral beam injection.
- Containment vessels melt at these temperatures, so the plasma is to be kept away from the walls using magnetic confinement.
Once fusion has begun, high-energy neutrons at about 160,000,000,000 C will flood out of the plasma along with x-rays, neither being affected by the strong magnetic fields (see neutron flux). Since neutrons receive the majority of the energy from the fusion, they will be the reactor's main source of thermal energy output. The ultra-hot helium product at roughly 40,000,000,000 C will remain behind (temporarily) to heat the plasma, and must make up for all the loss mechanisms (mostly bremsstrahlung x-rays from electron collisions) which tend to cool the plasma rather quickly.
- The tokamak containment vessel will have a lining composed of ceramic or composite tiles containing tubes in which warm liquid lithium metal will flow, cooling the lining.
- Lithium readily absorbs high-speed neutrons to form helium and tritium, becoming hot in the process.
- This increase in temperature is passed on to another (intermediate) coolant, possibly (pressurized) liquid water in a sealed, pressurized pipe.
- Heat from the intermediate coolant will be used to boil water in a heat exchanger.
- Steam from the heat exchanger will be used to drive turbines and generators, to create electrical current.
- Waste heat energy in excess of the generated electrical energy is dumped into the environment.
- Helium byproduct is the 'ash' of this fusion, and will not be allowed to accumulate too much in the plasma.
- Carefully measured amounts of deuterium and tritium are added back into the plasma and heated.
- The lithium is processed to remove the helium and tritium, with the balance recycled to collect more heat and neutrons. (Only a tiny amount of lithium is consumed.)
The DEMO project is planned to build upon and improve the concepts of ITER. Since it is only proposed at this time, many of the details, including heating methods and the method for the capture of high-energy neutrons, are still undetermined.
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