The fusion of atomic nuclei releases large amounts of energy. It is the reaction that makes stars shine: two hydrogen nuclei come together and turn into helium, and in that process, a part of the mass is converted into energy. Is it possible to ‘tame’ that reaction so that it becomes a future source of electrical energy for humanity?
Nuclear physics tells us that the union of hydrogen nuclei is achieved when they are hundreds of millions of degrees hot. Under these conditions, matter is neither solid, liquid, nor gas. Atoms are ‘decomposed’ into their two components: nucleus – the one we want to unite – and electrons. In this decomposed state, matter is volatile, and it needs to be confined in some form of container. In the nuclear fusion experimental reactor ITER, currently under construction in Cadarache (France), containment is achieved through powerful magnetic fields.
However, there is another strategy: so-called inertial confinement. In 1972, half a century ago, American physicist John Nuckolls proposed it as an idea in a Nature article. Around the same time, Nobel laureate Nikolai Basov in the Soviet Union reached similar conclusions, and shortly after, Robert Dautray in France.
Thus began a research that over five decades has achieved many advances for energy, but also for other areas of physics and technology, such as lasers. But it is now, with the results published last week, that one of the central ideas of inertial fusion has finally been demonstrated.
In inertial confinement fusion, very small amounts of matter, just milligrams of hydrogen – specifically its deuterium and tritium isotopes – contained in millimeter-sized capsules, must reach the same temperature and density conditions as in the Sun. How to achieve this? The answer lies in a high-energy laser with nanosecond pulses (0.000000001 seconds).
The laser deposits its energy in the outer layer of the hydrogen-containing capsule, causing the expansion of that layer. Through the ‘rocket effect’ – remember that in a rocket the gas goes down and the rocket goes up – the rest of the target mass quickly compresses inwards: an implosion. Once the temperature conditions in the center of the hydrogen are achieved, nuclear fusion reactions begin.
Published on January 26 in Nature and Nature Physics are results demonstrating that, as predicted 50 years ago, the kinetic energy of the helium nuclei produced by fusion reactions is deposited, through collisions, in the outermost region with the densest hydrogen, heating it in turn and propagating from the inside out that thermal wave – as we see when throwing a stone into water.
The confinement time lasts only 0.1 nanoseconds! But if I manage to repeat that mechanism ten times per second, voilà!: then I have enough energy and power to seriously consider a plant for generating electrical energy.
Now, why is this double step of ‘match in the center and propagation of fire’ outward necessary? Because it is much ‘cheaper’, in terms of energy required, to compress than to heat the same matter. This is the secret and importance of the achievement. With this theoretical argument, now validated by the new results, it makes sense to continue dreaming of this form of energy.
The article in Nature Physics presents the experiments and computational results carried out at the Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) in the US in conjunction with other laboratories, proving after 50 years that this mechanism is a reality. NIF is a laser with 2 megajoules of energy in each pulse distributed in 192 beams and with some nanoseconds of pulse.
What is now published demonstrates the burn propagation in the experiments of August 2020 and February 2021. In August 2021, even higher energy values were achieved, but this result still needs to be repeated.
But there is more. What is needed is continuous repetition of the process over time and during the reactor’s lifetime. And for that, the high-energy laser should be repetitive. Research is underway, alongside the search for an optimization of the mechanism to use less laser energy.
Finally, there are challenges common to both confinement options: materials, cooling systems, tritium reproduction (an isotope not found in nature and that needs to be produced on-site). These challenges are being addressed, but their timeframe extends beyond 2050 and beyond.
The other way to achieve electrical energy from nuclear fusion is magnetic confinement, which will also not yield immediate results. ITER is an experimental facility not connected to the grid that will demonstrate ignition and burning, and test systems subsequently applicable to the final reactor or DEMO. Operation is expected to start around 2025-2026, with real achievements towards the projected goals by 2035. DEMO is planned in the European Union around 2050-2060.
I conclude with news that, despite being well-known, is important to be shared: Spain is awaiting final funding for the construction and operation of the IFMIF-DONES facility, which will demonstrate the viability of proposed materials for reactor structures.
José Manuel Perlado Martín is an emeritus professor of Nuclear Physics and president of the Guillermo Velarde Nuclear Physics Institute (IFN-GV) at the Polytechnic University of Madrid (UPM).
via: MiMub in Spanish
