DEMOnstration Power Plant

DEMO (DEMOnstration Power Plant) 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" (FOAK) commercial power plant, sometimes referred to as PROTO.
While there is no clear international consensus on exact parameters or scope, the following parameters have been used as a baseline for design studies: DEMO should produce at least 2000 megawatts (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.
To achieve its goals, if utilizing a conventional tokamak design, 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, it was estimated in 2006 that DEMO could make fusion energy available by 2033, but design is now planned for the 2020s and proposed operations have now been delayed until as late as the 2050s. It is estimated that subsequent commercial fusion reactors could be built for about a quarter of the cost of DEMO. However, the ITER experience suggests that development of a multi-billion US dollar tokamak-based technology innovation cycle able to develop fusion power stations that can compete with non-fusion energy technologies is likely to encounter the "valley of death" problem in venture capital, i.e., insufficient investment to go beyond prototypes, as DEMO tokamaks will need to develop new supply chains and are labor intensive.
DEMO's place in the development of fusion power.
For some ITER consortium countries, DEMO may now be a phase rather than a specific ITER consortium machine and may see the collapsing of the DEMO and PROTO machines phases into multiple national multi-phase plants. The 2019 US National Academies of Sciences, Engineering, and Medicine 'Final Report of the Committee on a Strategic Plan for U. S. Burning Plasma Research' noted, "a large DEMO device no longer appears to be the best long-term goal for the U.S. program. Instead, science and technology innovations and the growing interest and potential for private-sector ventures to advance fusion energy concepts and technologies suggest that smaller, more compact facilities would better attract industrial participation and shorten the time and lower the cost of the development path to commercial fusion energy". Approximately two dozen private-sector companies are now aiming to develop their own fusion reactors within the DEMO roadmap timetable. The 3 October 2019 UK Atomic Energy announcement of its Spherical Tokamak for Energy Production (STEP) grid-connected reactor for 2040 suggests a combined DEMO/PROTO phase machine apparently to be designed to leapfrog the ITER timetable. China's proposed CFETR machine, a grid-connected gigawatt-generating reactor, overlaps the DEMO timetable. Despite these developments, in November 2020, an independent expert panel reviewed EUROfusion's design and R&D work on DEMO, and EUROfusion confirmed it was proceeding with the next step of its Roadmap to Fusion Energy, namely the conceptual design of DEMO in partnership with the European fusion community and industry, suggesting an EU-backed DEMO-phase machine that could formally bear the DEMO name. Meanwhile, the US appears to be working towards its own national demonstration fusion power plant on a cost-sharing basis.
Timeline.
The DEMO timeline has slipped several times, following slippage in the ITER timetable. The following timetable was presented at the IAEA Fusion Energy Conference in 2004 by Christopher Llewellyn Smith:
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.
This 2012 roadmap was intended to be updated in 2015 and 2019. The EFDA was superseded by EUROfusion in 2013. The roadmap was subsequently updated in 2018.
This would imply operations commencing sometime in the 2050s.
Technical considerations.
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.
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.
Once fusion has begun, high-energy neutrons at about 160,000,000,000 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 40,000,000,000 kelvins will remain behind (temporarily) to heat the plasma, and must make up for all the loss mechanisms (mostly bremsstrahlung X-rays from electron deceleration) which tend to cool the plasma rather quickly.
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.
Conceptual design.
All aspects of DEMO were discussed in detail in a 2009 document by the Euratom-UKAEA Fusion Association.
Four conceptual designs PPCS A,B,C,D were studied. Challenges identified included:
In the 2012 timeline the conceptual design should be completed in 2020.
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. 
PROTO.
PROTO was a proposal for a beyond-DEMO experiment, part of the 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 was only expected after DEMO, beyond 2050, and probably will not be the second part of a DEMO/PROTO experiment as it no longer appears in official documentation.