This article is taken from the monthly journal Sciences et Avenir – La Recherche No. 900 of February 2022.
“An almost inexhaustible, safe source of energy with low environmental impact. The promise, presented as a slogan on the website of the experimental reactor Iter, currently under construction in Saint-Paul-Durance (Bouches-du-Rhone), seems extremely attractive. And it is not for nothing that 35 ITER partner countries will pay 20 billion euros for this gigantic experiment. At the same time, a lot of work is underway in the world to master thermonuclear fusion, including by start-ups.
Technology at the experimental stage
Because on paper this type of reaction has huge advantages. The fusion of deuterium and tritium nuclei, two isotopes of hydrogen (formed from a proton, but also from one and two neutrons, respectively), releases four times more energy than when fissioning uranium into an equal mass of fuel, and eight million times more, than burning oil. It does not pose a risk of runaway or meltdown of reactors, therefore, there is no danger of an accident like the one that occurred at Fukushima (Japan). Fuel is present in abundance in the earth’s crust or oceans. And the main product of the reaction is helium, a non-toxic inert gas. Mostly, “Unlike conventional nuclear processes, fusion does not produce long-lived highly radioactive waste. ” insists Jérôme Boucalossy, director of CEA’s magnetic confinement fusion research institute in Saint-Paul-le-Durance.
This article is taken from the monthly journal Sciences et Avenir – La Recherche #901 of March 2022.
“An almost inexhaustible, safe source of energy with low environmental impact. The promise, presented as a slogan on the website of the experimental reactor Iter, currently under construction in Saint-Paul-Durance (Bouches-du-Rhone), seems extremely attractive. And it is not for nothing that 35 ITER partner countries will pay 20 billion euros for this gigantic experiment. At the same time, a lot of work is underway in the world to master thermonuclear fusion, including by start-ups.
Technology at the experimental stage
Because on paper this type of reaction has huge advantages. The fusion of deuterium and tritium nuclei, two isotopes of hydrogen (formed from a proton, but also from one and two neutrons, respectively), releases four times more energy than when fissioning uranium into an equal mass of fuel, and eight million times more, than burning oil. It does not pose a risk of runaway or meltdown of reactors, therefore, there is no danger of an accident like the one that occurred at Fukushima (Japan). Fuel is present in abundance in the earth’s crust or oceans. And the main product of the reaction is helium, a non-toxic inert gas. Mostly, “Unlike conventional nuclear processes, fusion does not produce long-lived highly radioactive waste. ” insists Jérôme Boucalossy, director of CEA’s magnetic confinement fusion research institute in Saint-Paul-le-Durance.
But this technology is only at the experimental stage. At Itera, which is set to demonstrate its viability on a large scale in a 23,000-tonne reactor, the most vigorous reactions will not take place until 2035. Then it will take decades to control all phenomena (turbulence, energy release, etc.). If they see the light of day, the first industrial plants will not be built until the second half of the 21st century. What types of nuclear waste would then be produced by such reactors?
The first category will be generated by very energetic neutrons emitted during nuclear fusion. Captured by the walls of the reactors, these particles would turn certain atoms (cobalt, copper, nickel, tungsten, etc.) into radioactive isotopes. However, strong shielding will keep this phenomenon within the first meter of the fence. The materials that are destined to become radioactive will essentially be a vacuum chamber in which fusion reactions will take place, as well as an element called a “divertor” located on the floor of the vacuum chamber, which will allow the extraction of, in particular, helium. At Itera, these two elements would weigh 5,000 and 500 tons, respectively. Bulky, this waste will only be very mild to moderate radioactive. And their “half-life” (the duration of the disappearance of half of the radioactivity) for most will not exceed ten years, and for some of them at most several decades. “IN In the case of Iter, most of the waste will be stored at the Andra sites. (National Agency for Radioactive Waste Management, ed.)the rest are stored near the reactor until their radioactivity is reduced “, says Jérôme Bukalossi.
New Tritium Cleanup Techniques Under Study
The second type of waste is more difficult to deal with. This concerns tritium, an element that is not very radioactive but is highly mobile, which can create contamination problems. Very rare in its natural state, it will be produced by the reactors themselves by reacting neutrons with lithium. However, of all the tritium injected into the vacuum chamber, only a very small fraction will fuse with the deuterium. The rest of this gas will penetrate through the walls of the reactors, especially its first centimeters. For Itera, this waste will weigh just under 500 tons at the end of the experiment. Again, they should be stored in situ until they are harmless, as tritium has a half-life of only 12 years.
Research is underway to reduce the amount of tritium in the waste. Thus, the processing technologies under study will allow the extraction and processing of most of them by heating (up to about 1000°C) or even melting. “Research is also focused on so-called penetration barriers, emphasizes Jérôme Bukalossi. Consisting of a set of extremely thin layers, they block the tritium and prevent it from mixing with water in cooling systems and thus contaminating it. “Questions at the heart of European research projects (TRANSAT and soon TITANS) to best anticipate waste management from Iter, which will start production in 2035. Because the chances of a successful merger will also depend on this on the industrial scale.
Suspended hopes of transmutation
The transformation of active radionuclides over hundreds of thousands of years into stable or short-lived elements is the principle of transmutation. An attractive solution, but it is struggling to see the light. The idea is to cause the neutron to be absorbed by the nucleus of the radionuclide in order to induce a fission reaction that would lead to the formation of less radioactive elements. But according to the Institute for Nuclear Security Research (IRSN), the performance of the operation is low. “With the current state of knowledge, transmutation is not a sufficient solution for long-lived waste management.”, slice Igor Le Bars, director of security expertise at IRSN. But there is still hope: in 2019, the Frenchman Gerard Mourou proposed using super-powerful lasers for waste processing, which brought him the 2018 Nobel Prize in Physics. A track that will take decades of research…