DEMO

DEMO (DEMOnstration Power Station) is a proposed nuclear fusion power station that is intended to build upon the ITER experimental nuclear fusion reactor. The objectives of DEMO are usually understood to lie somewhere between those of ITER and a "first of a kind" commercial station. While there is no clear international consensus on exact parameters or scope, the following parameters are often used as a baseline for design studies: DEMO should produce at least 2 gigawatts of fusion power on a continuous basis, and it should produce 25 times as much power as required for breakeven. DEMO's design of 2 to 4 gigawatts of thermal output will be on the scale of a modern electric power station.^[1]

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. It is estimated that subsequent commercial fusion reactors could be built for about a quarter of the cost of DEMO.^[2][3]

Timeline

The following timetable was presented at the IAEA Fusion Energy Conference in 2004 by Christopher Llewellyn Smith:^[2]

• Conceptual design is to be complete by 2017
• Engineering design is to be complete by 2024 (after input from ITER D-T tests, and data from IFMIF - both delayed as of 2016)
• The first construction phase is to last from 2024 to 2033
• The first phase of operation is to last from 2033 to 2038
• The station is then to be expanded and updated (e.g. with phase 2 blanket design)
• The second phase of operation is to start in 2040

In 2012 European Fusion Development Agreement (EFDA) presented a roadmap to fusion power with a plan showing the dependencies of DEMO activities on ITER and IFMIF.^[4]

• Conceptual design to be complete in 2020 ^[4]:63
• Engineering design complete, and decision to build, in 2030
• Construction from 2031 to 2043
• Operation from 2044, Electricity generation demonstration 2048

This 2012 roadmap was intended to be updated in 2015 and 2019,^[4]:49 but EFDA was superseded by FusionForEnergy (F4E).

Technical design

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 resonant state which splits to form in turn a helium nucleus (an alpha particle) and a high-energy neutron.

2
1H
+ 3
1H
→ 4
2He
+ 1
0n
+ 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, where quantum-tunneling effects permit the parent nuclei to "fuse" together into the resonant state. In principle, some fusion reactions will occur also when beams of deuterons and tritons are accelated and made to collide head-on, the way particle physics colliders work. However, the probability of elastic scattering (simple deflection of the beams, which will tend to defocus the beams) is much larger than the fusion cross section (the probaility for the nuclei to fuse). Therefore, much more energy would be used to accelerate the beams, than whatever energy would arise from the fusion reactions. The "trick" is to form a quasi-maxwellian distribution for the deuterons and the tritons, at very high temperatures, where the nuclei in the "tail" of the maxwellian undergo fusion, while the continuous elastic collisions among the other nuclei (as said, the majority of the events) will not alter the state of the plasma.
• 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 kelvins will flood out of the plasma along with X-rays, neither being affected by the strong magnetic fields. 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 kelvins 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 electric 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.

All aspects of DEMO were discussed in detail in a document by the Euratom-UKAEA Fusion Association.^[5]

Radioactive waste

While fusion reactors like ITER and DEMO will produce neither transuranic nor fission product wastes, which together make up the bulk of the nuclear wastes produced by fission reactors, some of the components of the ITER and DEMO reactors will become radioactive due to neutrons impinging upon them. It is hoped that plasma facing materials will be developed so that 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. Development of these materials is the prime purpose of the International Fusion Materials Irradiation Facility. 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.^[6]

PROTO

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, beyond 2050, and may or may not be a second part of DEMO/PROTO experiment.^[7]

References