The dream of obtaining electrical energy from the fusion of two atomic nuclei is a little closer


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The fusion of atomic nuclei releases large amounts of energy. It is the reaction that makes stars shine: two hydrogen nuclei come together and become helium, and in the process part of the mass is converted into energy. Is it possible to “tame” that reaction so that it ends up being a source of electrical energy for humanity in the future?

Nuclear physics tells us that the bonding of hydrogen nuclei is achieved when they are at hundreds of millions of degrees. Under these conditions, matter is not a solid, nor a liquid, nor a gas. Atoms are “decomposed” into their two components: nucleus -the one we want to unite- and electrons. Thus decomposed matter is volatile, and it is necessary to confine it in some form of container. In the ITER experimental nuclear fusion reactor, currently under construction in Cadarache, France, containment is achieved by powerful magnetic fields.

But there is another strategy: the so-called inertial confinement. In 1972, now half a century ago, the American physicist John Nuckolls proposed it as an idea in an article in Nature. By similar dates, Nobel laureate Nikolai Basov reached similar conclusions in the Soviet Union, and Robert Dautray, shortly after, in France.

Thus began an investigation that over five decades has achieved many advances by and for energy, but also for other areas of physics and technology, such as lasers themselves. But it is now, with recently published results, that one of the central ideas of inertial fusion has finally been demonstrated.

Laser pulses to create microsols

In fusion by inertial confinement, very small amounts of matter, just milligrams of hydrogen -specifically its isotopes deuterium and tritium- contained in capsules of millimeters, must reach the same conditions of temperature and density that occur in the Sun. How achieve it? The answer lies in a high-energy laser pulsed in nanoseconds (0.000000001 seconds).

The laser deposits its energy in the outer shell of the capsule with the hydrogen, causing the shell to expand. Due to the “rocket effect” –remember that in a rocket the gas goes out towards the ground and the vehicle rises towards the sky–, the rest of the mass of the target is quickly compressed inwards: an implosion. Once the temperature conditions are achieved in the center of the hydrogen, the nuclear fusion reactions will begin in it.

Inertial confinement fusion fuel microcapsule like those used in the NIF facility.

And now comes the important part of the result published on January 26 in Nature Y Nature Physics: shows that, as predicted 50 years ago, the kinetic energy of helium nuclei resulting from fusion reactions is deposited, by collisions, in the outermost zone with the denser hydrogen, heating it in turn and propagating from the interior to the exterior that thermal wave –as we see when throwing a stone into the water–.

Naturally we all understand that, without anything to keep “squeezing”, the matter will expand and will no longer be in the desired conditions. The lockdown time is only 0.1 nanoseconds (0.0000000001 seconds)!

But if we manage to repeat this mechanism ten times per second, aha! Then we will have enough energy and power to seriously consider an electric power plant.

Much cheaper to compress than heat

Now, why is this double step of “match” in the center and “propagation of the fire” outwards necessary? Because it is much “cheaper”, in terms of energy required, to compress than to heat the same material. This is the secret and the importance of achievement. With this theoretical argument, now validated by the new results, it makes sense to keep dreaming about this form of energy.

The article in Nature Physics collects the experiments and computational results carried out at the National Ignition Facility (NIF) of the Lawrence Livermore National Laboratory in the USA, together with other laboratories, which demonstrates 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 a few nanoseconds per pulse.

What has been published in January shows the propagation of burning in the experiments of August 2020 and February 2021. In August 2021 even higher energy values ​​were reached, but it is a result that still needs to be repeated.

But there is more. What is needed is for the process to be repeated continuously over time and throughout the life of the reactor. And for that the laser of that energy should be repetitive. Research is on it, along with the search for an optimization of the mechanism so that less laser energy is used.

pending challenges

Finally, there remain the challenges that are common to the two confinement options: the materials, the refrigeration systems and the reproduction of tritium (an isotope that does not exist in nature and that must be manufactured in situ). These are challenges that are being addressed, but whose time perspective is going to times above the year 2050 and beyond.

The question is: does it pay? The answer is yes. Although time took us to the 70s of this century (that is, a century after its proposal) its “inexhaustibility” of fuel (hydrogen), its safety and its reduction of waste leads us to the prospect of solving, in combination with other sources, the problem of energy that, if not, our grandchildren and great-grandchildren will talk about, and will talk at risk.

The other way to achieve electrical energy from nuclear fusion is through magnetic confinement, which will not yield immediate results either. ITER is an experimental facility not connected to the network that will allow the ignition and burning to be demonstrated and the testing of systems later applicable to the final reactor or DEMO. The start will be around 2025-2026 and its real operation towards the expected achievements, in 2035. DEMO is proposed in the European Union on the border of 2050-2060.

I conclude with a piece of news that is no less necessary to spread because it is known: Spain awaits final funding for the construction and operation of the IFMIF-DONES facility, which must demonstrate the viability of the materials proposed for the reactor structures.

This article was originally published in the Science Media Center Spain.

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