Nuclear Fusion: A Historic Breakthrough

Caterina Riconda, professor of physics at Sorbonne University in the Laboratory for the Use of Intense Lasers, and Pierre Morel, associate professor at the Laboratory of Plasma Physics, discuss these unprecedented results.

How does nuclear fusion differ from fission currently used in nuclear power plants?

Caterina Riconda: Nuclear fusion is the process that occurs at the heart of stars. Unlike fission, which aims to break a heavy nucleus into lighter nuclei, fusion assembles light nuclei to form a heavier nucleus. The easiest elements to combine are two forms of hydrogen, deuterium and tritium, which produce a helium nucleus and a neutron. Fission and fusion have in common that they release enormous energy, almost a million times greater than the energy released during combustion in an engine.

Pierre Morel: In addition to the production of many radioactive elements, one of the major problems of fission is the control of the reaction. To break a nucleus, it must be bombarded with neutrons: a first neutron hits the nucleus, the fission of which produces another neutron. From one neutron, we get two which will be able to break two nuclei again, then four, then sixteen, etc. We thus obtain a chain reaction which is the principle of an atomic bomb. In a nuclear reactor, this chain is controlled, but the fuel must be constantly cooled, and a deficiency can lead to nuclear disasters such as Chernobyl or Fukushima.

What are the advantages?

P. M.: Fusion is a safe process: if the conditions are not ideal, the reaction stops immediately; it is therefore impossible to have a reaction that gets carried away. In addition, the reaction removes an unstable radioactive element, tritium, to obtain a very energetic neutron and helium, an inert element quite rare on Earth.

Even if the wall of the reactor risks becoming slightly radioactive in the long term with the bombardment of neutrons, this radioactivity has nothing to do, in terms of lifespan, with that caused by a fission reaction. From the point of view of waste safety and cleanliness, fusion is therefore clearly more advantageous.

What are the conditions to cause fusion?

C. R.: You need extreme conditions, in particular temperatures of the order of several hundred million degrees. This heat allows the positively charged nuclei to overcome the electrical repulsion that pushes them away and to fuse, releasing enormous amounts of energy. Under these conditions, matter becomes a plasma, a fairly dense and fully charged gas.

P. M.: The higher the temperature, the greater the agitation of the nuclei and the greater the probability that they will meet. The three conditions for obtaining fusion are therefore: temperatures and a sufficiently high number of particles as well as a sufficiently long confinement time.

What are the different ways to meet these conditions?

P. M.: There are different techniques, the best known of which are inertial confinement and magnetic confinement. For fusion to be profitable, it must release more energy than is needed to cause it and in particular to heat the plasma. For this, either the density is greatly increased with inertial confinement, or the energy confinement time is greatly increased with magnetic confinement.

C. R.: Inertial confinement is the method studied by the NIF: the scientists used very powerful lasers to heat and compress the deuterium/tritium mixture until their atoms fused. This method makes it possible to reach very high temperatures and densities (up to 1000 times the solid density), but the confinement times are very short, of the order of ten nanoseconds. A similar installation, but for the moment less energetic, exists in France: the Laser MegaJoule (LMJ)  (in French).

Magnetic confinement is used as part of the International Thermonuclear Experimental Reactor project, ITER. The positively charged plasma particles are contained in a ring-shaped magnetic cage called a tokamak. The confinement time is longer, but the density remains relatively low.

Why are the results published by the NIF considered a "historic breakthrough" in the field of nuclear fusion?

C. R.: Even though these two methods have seen great advances in the past, this is the first time that we have succeeded in producing more energy than that absorbed by the plasma. This is a theoretical breakthrough: scientists have succeeded in creating, in a controlled manner, conditions of density and temperature that do not exist anywhere on Earth. We already knew how to cause nuclear fusion, but not a self-sustaining reaction as was the case in December 2022. However, there is still a long way to go for nuclear fusion to become a solution for energy production.

What are the technological challenges to overcome to have an operational fusion reactor?

C. R.: Currently, the energy gain is at the plasma level, but not yet at the global engineering level. Furthermore, during the NIF experiment, only a few shots per week were carried out. However, an operational reactor should have a frequency of around 10 shots per second, every day of the year. Another problem: the walls of the reactor subjected to a constant bombardment of neutrons will have to keep sufficient mechanical properties for very long times.

P. M.: As far as the magnetic confinement fusion route is concerned, we also have to think about the treatment of the alpha particles which remain confined, how to renew the fuels in the tokamaks, the recovery of neutrons to heat the water and train a turbine or the production of tritium in situ and its reinjection into the fusion reaction, etc. There is still a lot of work to be done before the efficiency of fusion can be sufficiently improved. After a first plasma in 2025, we will gradually increase in power to obtain by 2035 an energy released 10 times greater than the energy injected into the machine.

France plays a major role in nuclear fusion research, doesn't it?

C. R.: Many of the issues mentioned above will be studied on ITER, the international experimental reactor built in France to prove the feasibility of controlled fusion, and on the LMJ. France is one of the European countries that has invested the most in plasma and fusion research. Admittedly, the historic breakthrough of December 2022 took place in California at the NIF, but it is above all an international research project for which several French scientists worked hand in hand with American colleagues. Sébastien Le Pape, director of the Laboratory for the Use of Intense Lasers (LULI) in which I work, also participated, in 2022, in a publication that is the basis of the progress of the NIF.