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Professor of Tecnun-School of Engineering and researcher of CEIT, University of Navarra
Nuclear fusion is presented as an extraordinarily attractive energy source . It is a clean energy that has almost unlimited resources and does not generate highly radioactive waste.
Last December the U.S. Energy department announced in a press conference that the team at the National Ignition Facility (NIF (Taxpayer Identification Number)) at Lawrence Livermore National Laboratory had obtained more energy from nuclear fusion than was used to activate it. This is an important scientific milestone achieved by the pathway known as inertial confinement fusion.
Nuclear fusion is presented as an extraordinarily attractive energy source . It is a clean energy (it does not produce CO₂) that has practically unlimited resources and does not generate highly radioactive waste for thousands of years as in the case of uranium fission. Nuclear fusion can therefore be considered the energy of the future, as it is capable of generating high-power energy to replace current fossil fuel-based thermal power plants.
Star energy on Earth
Nuclear fusion is the energy of the stars, very close to our life on planet Earth, which depends on one of them: the Sun. The dream is to make the energy of the stars a reality on Earth. The process, however, cannot be the same, because inside a star fusion occurs extremely slowly.
To make nuclear fusion a reality on our planet, we must start with hydrogen isotopes such as deuterium (composed of a proton and a neutron) and tritium (a proton and two neutrons) which, when fused, give rise to a helium nucleus and a neutron, both at very high energies.
Returning to the NIF (Taxpayer Identification Number) experiment, in this case the fusion of deuterium and tritium was produced by applying a pulse of energy produced by 192 concentrated lasers to a tiny capsule containing the cooled fuel.
Specifically, an energy of 2.05 megajoules was delivered by lasers, which resulted in the compression of the capsule making possible the fusion of the deuterium and tritium inside and producing an energy of 3.15 megajoules, i.e. an energy gain of 154%. For the first time in history, ignition (more fusion energy than is contributed) was obtained by inertial confinement.
Limitations of inertial fusion
However, despite the undeniable milestone achieved, the news has to be taken with caution for several reasons. On the one hand, to produce a laser pulse such as the one at employee to obtain ignition requires around 300 megajoules of energy from network, since the efficiency of the lasers used is very low leave, less than 1%.
On the other hand, in inertial confinement fusion it is not clear how the neutron energy is harnessed to produce electrical power. In fact, ignition has taken so long to arrive because that is not the only task of NIF (Taxpayer Identification Number), which also supports the U.S. nuclear weapons program.
In addition, inertial fusion is a pulsed technology. It would require about 10 laser pulses per second instead of one pulse per day, requiring millions of capsules per day, which is a huge technological challenge .
Magnetic confinement fusion
Another way to obtain energy from nuclear fusion is magnetic confinement. In this concept, a gas of deuterium and tritium is heated in a toroidal vessel to temperatures on the order of 150 million Degrees to give the gas particles enough energy to overcome the repulsion due to the positive charges of the nuclei.
At such temperatures, the electrons separate from the nuclei, so that you no longer have a gas composed of neutral atoms, but a gas of charged particles, also called a plasma. A plasma can be confined by magnetic fields, thus avoiding the contact of the highly energetic particles with the vessel wall.
When the fusion reaction of deuterium and tritium takes place, helium is generated, which is confined by the magnetic field and thus remains in the plasma, providing it with its energy. Thus, the plasma temperature is self-maintaining with the fusion reaction.
The neutrons, on the other hand, are not confined by the magnetic field and transfer their energy to a coolant located immediately behind the wall exposed to the plasma. The coolant in turn transfers it to a steam exchanger, this to a turbine and this to a generator, so that electrical energy is produced.
Large nuclear fusion projects
The most immediate step in magnetic confinement fusion is the ITER reactor (International Thermonuclear Experimental Reactor), one of the world's largest technology projects, involving the European Union, USA, Russia, Japan, China, India and Korea.
ITER, located in southern France and due to be completed by the end of 2025, will be the world's largest magnetic confinement device. Operation with a deuterium-tritium plasma will begin in 2035, with energy gains of 1,000% expected with a fusion power of 500 megawatts, which, however, will not power the network, but will pave the way for future reactors that will.
In Europe, the next step will be DEMO, a fusion demonstration reactor where energy gains of 2,500% are expected with a fusion power output of 2,000 megawatts, of which 300-500 megawatts will be contributed to network. Construction of DEMO is expected to start around 2040, so that operation will be in the 2050s.
The biggest challenges facing DEMO are the development of the materials exposed to the fusion plasma, which have to withstand high thermal loads under a neutron flux, as well as obtaining tritium inside the reactor. In any case, ITER will be a definitive step on the road to nuclear fusion energy production.