Science

Nuclear fusion: when can we (finally) “bottle up the sun”?

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Will nuclear fusion one day be successful or will it remain forever, as critics claim, “an energy of the future”? Impossible to decide. In laboratories seeking to reproduce on Earth the physical reactions that operate at the heart of the Sun, new perspectives seem to emerge. In late August, scientists at the National Ignition Facility (NIF) in Livermore, California, said they had made a big step forward. They generated 8 times more energy than before by focusing 192 lasers on a hydrogen pellet the size of a peppercorn. A few days later, MIT experts in Boston responded by announcing that they had managed to produce a magnetic field of record value, a key element in containing the cloud of ultra-hot particles from a future reactor.

For NIF physicists, there’s no question: the industry is going through a historic moment, comparable to the Wright brothers’ first powered flight, which precipitated the advent of modern aviation. However, most fusion researchers are more cautious. At best, “bottling the sun” and using it to supply the world with electricity will take several decades. There is even a risk that this highly advantageous technology – no CO2 emissions, no long-lived radioactive waste, and no Chernobyl or Fukushima-type accidents – will never be successful, as the challenges to be faced remain immense.

“There are two ways to do fusion, explains Dominique Grenêche, professor at the National Institute of Nuclear Sciences and Techniques (INSTN). We can use lasers – we speak of inertial fusion – or confine a plasma – atoms without an electron – in a magnetic field.” Thanks to the NIF’s work, the first method has just become more efficient, as the researchers recovered almost as much energy as they injected using lasers. “It is a remarkable performance, although it will have to be confirmed, estimates Guy Laval, physicist member of the Academy of Sciences. Unfortunately, this does not erase the serious disadvantages suffered by the sector.” First, laser studies are funded by the military, whose objectives take precedence over the rest. “This technology allows, above all, to validate the simulation calculations of an H-bomb explosion”, specifies Dominique Grenêche. Therefore, we are far from being a civil application. Furthermore, the lasers used are not yet efficient enough. Before projecting their energy onto a target, they must be “charged”, which requires great effort. “Less than 1% of the energy is conserved, the rest is dissipated. As long as there is such a loss, we have no hope that a reactor will work with lasers,” says Guy Laval.

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Inflate an air chamber that has a large leak.

Therefore, according to several experts, the most advanced route for a commercial reactor is still magnetic fusion, the technology used by Iter or MIT. But again, there are still many obstacles to overcome. The principle is to heat a plasma to 150 million degrees and confine it so that the atoms fuse together. This is done in a tokamak, a donut-shaped chamber. The problem ? Any plasma heated to high temperature causes instabilities. “The particles quickly leave the center of the plasma, which leads to a decrease in heat. As we want to keep it at a very high level, this requires adding much more energy,” explains Guy Laval. “A tokamak confines very poorly,” confirms Thiéry Pierre, CNRS research director. In a machine like Iter, the time it takes for the particles to travel to the walls does not exceed 3 seconds. If we had to take a picture, it would be like stubbornly inflating an inner tube with a large leak. ”

However, in terms of energy, having a good performance is essential. Iter’s designers aim for an amplification factor of 10: for an input equivalent to 50 megawatts, the machine should generate 500. Thanks to optimization, they hope to get closer to this result before the process is finished. The decade. Especially since Iter is based on parameters considered conservative. “In the future it will be possible to turn the controls a little more to explore new regimes,” says Guy Laval. But will it be enough? If one day we want to connect a machine of this type to the electrical network, it will have to do it at least 30 or 40 times better than the current prototype. “At the moment, we really don’t see how to do it. Personally, I do not believe that electricity of this type can be produced on a large scale before the end of the century ”, estimates Thiéry Pierre. Especially since other unknowns remain to be raised, such as fuels. In the short term, tritium (one of Iter’s two fuels along with deuterium) will be purchased in Canada. But Iter will also have to produce it on his site near Cadarache. However, the tests have not yet started.

Trompe-l’oeil’s progress on the side of start-ups

By comparison, competing startups appear to be going much faster, announcing functional facilities by 2030. Much more compact machines thanks to advances in magnetic fields. “But beware of unsustainable promises,” warns Jean Jacquinot, former director of the Joint European Torus (JET) experimental reactor and Iter’s scientific advisor. If the size is reduced, the loads borne by the walls become unbearable. Hence the sizing of our tokamak. Competing machines, at the moment, do not retain water in terms of heat dissipation. “New fusion players will continue to make their contribution to physics. The elements they develop could be integrated into Demo, the successor to Iter, once the prototype has shown that the fusion principle works.

But all this will take a long time. Too long, for some observers. “Iter was developed in the late 1980s. It is not suitable for the next generation of plasmas,” laments Thiéry Pierre. The magnetic fields designed by MIT, for example, are already three times stronger than those of the international program, but it seems impossible to foresee a transfer. “The management of the project was not strict enough at the beginning. A scientific and technical audit open to researchers who are not part of the small and very closed world of fusion would have been very useful. But it could never have happened.”, Laments Yves Farge. , physicist and member of the Academy of Technologies. “Given the backlog, it might be good to relaunch other solutions, such as fast neutron reactors which, for the same amount of uranium, release much more energy than current power plants and have proven their efficiency in the past,” suggests Dominique. Grenêche.

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For their part, the defenders of Iter intend to stay the course. “There are many unknowns, admits one of them. For example, a political event could lead to the closure of tritium producing plants in Canada ”. The tokamak path could also turn out less promising, or even fade in 2050 compared to other less expensive solutions (photovoltaics, green hydrogen, batteries). “But if we stopped this program today, we would not have the answer to this fundamental question: is it possible to reproduce on earth the energy of the heart of the stars?” Asks André Grosman, deputy director of the Institute. From research on magnetic confinement fusion (IRFM) of the CEA, before claiming: “75% of the reactor is built, it would be a bit ridiculous not to go all the way.” Jean Jacquinot agrees: “Let’s stop saying that the Iter project is expensive. Its budget does not exceed that of the London Olympics! Energy is, without a doubt, the main problem of the 21st century. If we really want to develop Carbon-free energy sources, much more will have to be invested! ”

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