Nuclear fusion: why we are talking about a turning point

The Financial Times (Ft) was the first today to announce a important step forward in obtaining clean energy from nuclear fusion. The American newspaper talks about “net energy gain”a result that would have been obtained in the laboratories of the government research center Lawrence Livermore National Laboratory in California. Using the method of inertial confinement fusion (which we will see below), the scientists would have managed to obtain about 2.5 Megajoules of energy, having supplied “only” 2.1 to the reactor. It would be there first time that we manage to obtain by a controlled nuclear fusion reaction more energy than supplied. The people (defined by the FT as “people aware of the preliminary results of a recent experiment”, and whose names are not known) who released this information also pointed out that these data are still being analysed. We’ll have to wait until tomorrow for the Press conference issued by the US Department of Energy, during which the official information will be released. Meanwhile, let’s see why we are so interested in learning how to reproduce in a reactor what the stars have been doing for billions and billions of years.

The stars know it

Nuclear fusion is the reverse process of nuclear fissionthe one we already know how to use to generate energy through processes which, unfortunately, produce considerable quantities of radioactive waste. There controlled nuclear fusionon the other hand, would allow the production of large quantities of “green” energy. In nuclear fusion, as the name indicates, two light atoms are driven to fuse their nuclei, creating a heavier one. But what is the gain and what are the difficulty? Let’s start with the second question. The nucleus of atoms is made up of neutral particles (neutrons) and others that are positively charged (protons). Two nuclei manage to interact and therefore a “merge” only to very short distancesto which the electrostatic repulsion forcesdue to the approach of particles with the same charge, manage to be won by the so-called “nuclear forces”, which allow the nucleus of each atom to stick together. And here is the difficulty: to overcome the repulsive forces it is necessary supply a huge amount of energy to the systemmaking it reach very high temperatures. The stars know this, since it is precisely nuclear fusion processes that last billions of years that keep them lit (luckily for us!). In that case, it is the mass that comes in handy: the enormous mass of stars causes the gravitational force inside them to be very high and this makes possible the collision and subsequent fusion of the nuclei of the atoms contained in them. Going back to our question, the energy gain comes from the fact that when the two nuclei fuse, the mass of the final nucleus is slightly less compared to the sum of the masses of the starting nuclei. Given that – as Antoine-Laurent de Lavoisier had already intuited in 1700 and as the famous E=mc² of Einstein – “nothing is created, nothing is destroyed, everything is transformed”, what happens is that the “lost” mass during the merger turns into energy liberated.

Inertial confinement fusion

Scientists at the Lawrence Livermore National Laboratory use gods very high energy laser beams to ignite nuclear fusion reactions within a reactor containing a sphere consisting of a mixture of tritium and deuterium in a solid state. Tritium and deuterium are two isotopes of hydrogen and contain in their nucleus, respectively, a proton and a neutron or a proton and two neutrons. The nucleus of hydrogen, on the other hand, contains only one proton. The fusion between the tritium and deuterium nuclei, which causes the formation of helium, is the “easiest” to happen because it requires a smaller amount of energy than that required for the fusion of two hydrogen nuclei. The very high energy laser beams hitting the sphere cause theevaporation of the surface layer and its compression with consequent heating. This triggers the much desired fission reactions and the consequent release of energy. Of course, every step of this complicated process has its challenges and we’ll have to wait for tomorrow to have more details and discover, perhaps, how this group of scientists managed for the first time in history to obtain more energy output than that supplied to the reactor through the beams of laser beams.