Reality → Energy → Nuclear energy → Fusion
The Sun, like other stars, is a natural fusion reactor producing helium from hydrogen. One of the more promising ways to replicate the process on Earth appears to be the fusion of the two hydrogen isotopes deuterium and tritium [1] . Their fusion could yield significantly more energy than the fission of uranium while producing much less nuclear waste. To ignite and sustain the fusion reaction, a huge barrier of atomic and nuclear repulsion must be overcome [2] . Worldwide, unsuccessful experiments [3] have been conducted since more than sixty years, when ZETA and the first tokamak became operational. Now, the international ITER project [4] aims at becoming a forerunner of an industrial-scale demo fusion power plant before the end of this century [5] . Given the imponderability of fusion power and a lengthy transition to 'green' energy, power generation by established nuclear fission will probably have to be maintained in the interim [6] .
Of several possible alternatives, the fusion of deuterium and tritium requires the lowest ignition temperature. The energy produced would be several times higher than from nuclear fission. Deuterium constitutes 0.016 % of the hydrogen contained in water. Tritium, a radioactive hydrogen isotope with short (13 years) half-life, does not occur naturally but can be produced from lithium through neutron bombardment (the lithium nucleus absorbs a neutron and then decays into helium and tritium, a process that would also occur in a fusion reactor).
To produce helium, the hydrogen atoms have to be ionized (stripped of electrons) and the nuclei must be brought together against their repelling electromagnetic force to expose them to the strong force which then causes fusion. To ignite the process, the deuterium-tritium mixture has to be heated to a plasma of at least 100,000,000 kelvin (fusion of other elements would require up to several billion kelvin).
An incomplete list of past fusion research is presented in a Wikipedia timeline. The largest undertaking so far has been the National Ignition Facility of the Lawrence Livermore Lab in California. The facility aimed at demonstrating the feasibility of achieving nuclear fusion with energy gain. To ignite the reaction, 192 powerful lasers (world’s most powerful laser system) were focussed on a small target chamber holding a tiny hydrogen sample (see How NIF works). The facility failed to achieve ignition (see final review). In recent years, also several smaller privately financed projects have been initiated (e.g., see General Fusion and Lockheed Martin). All projects struggle on two fronts: to achieve ignition temperature, and to confine the hot plasma.
ITER (International Thermonuclear Experimental Reactor) is a large-scale international test facility under construction in southern France (see virtual tour). The first plasma test run is presently expected 2027 (11 years later than the original estimate). The project cost is presently estimated to exceed €20 billion (4 times higher than originally estimated). The goal is to demonstrate that the controlled fusion process can yield 10 times more energy than it consumes (no electricity will be delivered). The project will operate the world’s largest tokamak, a doughnut-shaped vacuum vessel that confines the plasma by magnetic fields (see plasma heating and confinement). Research at the Joint European Torus (JET), a precursor of ITER, is still ongoing. Also, experiments with smaller spherical tokamaks (e.g., NSTX and MAST) as well as a stellarator (see Wendelstein 7-X) are accompanying the ITER development. It is uncertain, whether any of these projects will lead to a demonstration project for a fusion power plant.
Two crucial parts of ITER's tokamak, directly exposed to untested heat and radiation, are the blanket and the divertor. If tests of all components of the supercomplex machine can be successfully completed during the late 2020s, fusion experiments could be started in the 2030s and, if successful, the way could be open for a demo fusion power plant, operational by mid-century and, if successful again, the first industrial fusion power plant(s) by the end of the century (possibly, in the opinion of some experts, based on China's CFETR project).
Hydro power, by far the largest renewable energy source, faces significant constraints for further growth. Other largely CO2-free renewables (wind, solar, geothermal) presently account for only 6 % of world electricity generation and cannot fully replace nuclear and fossil fuel power within the near future. As part of bridging the gap there is some recent discussion of using thorium instead of uranium for nuclear energy, in particular in connection with India's dire energy situation. However, the use of thorium would not circumvent controversial reprocessing and would require huge investments that utility companies are unable and unwilling to meet.