Des scientifiques battent le record de la quantité d’énergie produite lors d’une réaction de fusion contrôlée et soutenue

Les réacteurs à fusion magnétique contiennent du plasma super chaud dans un récipient en forme de beignet appelé tokamak.

La fusion nucléaire a franchi une étape importante grâce à de meilleures parois de réacteur – cette avancée technique se dirige vers les réacteurs du futur.

Des scientifiques d’un laboratoire en Angleterre ont battu le record de la quantité d’énergie produite lors d’une réaction de fusion contrôlée et soutenue. La production de 59 mégajoules d’énergie en cinq secondes lors de l’expérience Joint European Torus – ou JET – en Angleterre a été qualifiée de “percée” par certains médias et a suscité beaucoup d’enthousiasme parmi les physiciens. Mais une ligne commune concernant la production d’électricité par fusion est qu’elle est “toujours dans 20 ans”.

Nous sommes un physicien nucléaire et un ingénieur nucléaire qui étudie comment développer la fusion nucléaire contrôlée dans le but de produire de l’électricité.

Le résultat de JET démontre des avancées remarquables dans la compréhension de la physique de la fusion. Mais tout aussi important, cela montre que les nouveaux matériaux utilisés pour construire les parois internes du réacteur à fusion ont fonctionné comme prévu. Le fait que la nouvelle construction du mur ait aussi bien fonctionné est ce qui distingue ces résultats des jalons précédents et élève la fusion magnétique d’un rêve à une réalité.

Diagramme de fusion deutérium-tritium

Les réacteurs à fusion écrasent deux formes d’hydrogène ensemble (en haut) pour qu’elles fusionnent, produisant de l’hélium et un électron à haute énergie (en bas).

Fusionner des particules ensemble

La fusion nucléaire est la fusion de deux noyaux atomiques en un seul noyau composé. Ce noyau se décompose alors et libère de l’énergie sous la forme de nouveaux atomes et particules qui s’éloignent rapidement de la réaction. Une centrale à fusion capturerait les particules qui s’échappent et utiliserait leur énergie pour produire de l’électricité.

Il existe plusieurs façons de contrôler en toute sécurité la fusion sur Terre. Nos recherches se concentrent sur l’approche adoptée par JET – utiliser de puissants champs magnétiques pour confiner les atomes jusqu’à ce qu’ils soient chauffés à une température suffisamment élevée pour qu’ils fusionnent.

Le combustible des réacteurs actuels et futurs est composé de deux isotopes différents de l’hydrogène – ce qui signifie qu’ils ont un seul proton, mais un nombre différent de neutrons – appelés deutérium et tritium. L’hydrogène normal a un proton et aucun neutron dans son noyau. Le deutérium a un proton et un neutron tandis que le tritium a un proton et deux neutrons.

Pour qu’une réaction de fusion réussisse, les atomes de combustible doivent d’abord devenir si chauds que les électrons se libèrent des noyaux. Cela crée[{” attribute=””>plasma – a collection of positive ions and electrons. You then need to keep heating that plasma until it reaches a temperature over 200 million degrees Fahrenheit (100 million Celsius). This plasma must then be kept in a confined space at high densities for a long enough period of time for the fuel atoms to collide into each other and fuse together.

To control fusion on Earth, researchers developed donut-shaped devices – called tokamaks – which use magnetic fields to contain the plasma. Magnetic field lines wrapping around the inside of the donut act like train tracks that the ions and electrons follow. By injecting energy into the plasma and heating it up, it is possible to accelerate the fuel particles to such high speeds that when they collide, instead of bouncing off each other, the fuel nuclei fuse together. When this happens, they release energy, primarily in the form of fast-moving neutrons.

During the fusion process, fuel particles gradually drift away from the hot, dense core and eventually collide with the inner wall of the fusion vessel. To prevent the walls from degrading due to these collisions – which in turn also contaminates the fusion fuel – reactors are built so that they channel the wayward particles toward a heavily armored chamber called the divertor. This pumps out the diverted particles and removes any excess heat to protect the tokamak.

JET Magnetic Fusion Experiment

The JET magnetic fusion experiment is the largest tokamak in the world. Credit: EFDA JET

The walls are important

A major limitation of past reactors has been the fact that divertors can’t survive the constant particle bombardment for more than a few seconds. To make fusion power work commercially, engineers need to build a tokamak vessel that will survive for years of use under the conditions necessary for fusion.

The divertor wall is the first consideration. Though the fuel particles are much cooler when they reach the divertor, they still have enough energy to knock atoms loose from the wall material of the divertor when they collide with it. Previously, JET’s divertor had a wall made of graphite, but graphite absorbs and traps too much of the fuel for practical use.

Around 2011, engineers at JET upgraded the divertor and inner vessel walls to tungsten. Tungsten was chosen in part because it has the highest melting point of any metal – an extremely important trait when the divertor is likely to experience heat loads nearly 10 times higher than the nose cone of a space shuttle reentering the Earth’s atmosphere. The inner vessel wall of the tokamak was upgraded from graphite to beryllium. Beryllium has excellent thermal and mechanical properties for a fusion reactor – it absorbs less fuel than graphite but can still withstand the high temperatures.

The energy JET produced was what made the headlines, but we’d argue it is in fact the use of the new wall materials which make the experiment truly impressive because future devices will need these more robust walls to operate at high power for even longer periods of time. JET is a successful proof of concept for how to build the next generation of fusion reactors.

ITER Fusion Reactor Diagram

The ITER fusion reactor, seen here in a diagram, is going to incorporate the lessons of JET, but at a much bigger and more powerful scale. Credit: Oak Ridge National Laboratory, ITER Tokamak and Plant Systems

The next fusion reactors

The JET tokamak is the largest and most advanced magnetic fusion reactor currently operating. But the next generation of reactors is already in the works, most notably the ITER experiment, set to begin operations in 2027. ITER – which is Latin for “the way” – is under construction in France and funded and directed by an international organization that includes the U.S.

ITER is going to put to use many of the material advances JET showed to be viable. But there are also some key differences. First, ITER is massive. The fusion chamber is 37 feet (11.4 meters) tall and 63 feet (19.4 meters) around – more than eight times larger than JET. In addition, ITER will utilize superconducting magnets capable of producing stronger magnetic fields for longer periods of time compared to JET’s magnets. With these upgrades, ITER is expected to smash JET’s fusion records – both for energy output and how long the reaction will run.

ITER is also expected to do something central to the idea of a fusion powerplant: produce more energy than it takes to heat the fuel. Models predict that ITER will produce around 500 megawatts of power continuously for 400 seconds while only consuming 50 MW of energy to heat the fuel. This mean the reactor produced 10 times more energy than it consumed – a huge improvement over JET, which required roughly three times more energy to heat the fuel than it produced for its recent 59 megajoule record.

JET’s recent record has shown that years of research in plasma physics and materials science have paid off and brought scientists to the doorstep of harnessing fusion for power generation. ITER will provide an enormous leap forward toward the goal of industrial scale fusion power plants.

Written by:

  • David Donovan – Associate Professor of Nuclear Engineering, University of Tennessee
  • Livia Casali – Assistant Professor of Nuclear Engineering, Zinkle Faculty Fellow, University of Tennessee

This article was first published in The Conversation.The Conversation

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