NEAR NUCLEAR FUSION. NUCLEAR FUSION
It begins with a series of introductory articles on energy production by nuclear fusion and the IFMIF-DONES project. The purpose is to bring these fields of scientific knowledge and technological development, both of which have such decisive social impact, closer to all those interested in understanding and learning about them.
What is nuclear fusion?
Nuclear fusion occurs when two atomic nuclei come together to form another nucleus, in many cases emitting a large amount of energy. In nature, reactions of this type arise in the core of the Sun and other stars, and the processes favored by the released energy are the reason why these stars are so bright.
How can two atomic nuclei be united, if both are positively charged?
We were always told that two electric charges of the same sign repel each other, but if two atomic nuclei collide with adequate energy, then the electrostatic repulsion (the so-called Coulomb repulsion) can be overcome so that they can get very close —less than 10-15 m from each other— because the so-called nuclear force, which is much stronger, prevails and the nuclei join. The fusion!
The figure shows a fusion reaction of a deuterium nucleus, 2H, and a tritium nucleus, 3H. In this process, initial energies of the order of 108 K are required and a helium nucleus, 4He, emerges together with a neutron, releasing (3.5 + 14.1 =) 17.6 MeV of energy.
Because of its ‘greater simplicity’ and abundance of hydrogen, this reaction is now being explored to generate electricity in the near future.
A bit of History
In 1917, even before nuclear fission reactions, Ernest Rutherford observed the first fusion reaction while radiating different gases with alpha particles (helium nuclei). This researcher saw that by irradiating nitrogen, quite energetic protons (hydrogen nuclei) appeared, according to the following scheme:
14N + 4He → 17O + 1H -1.2 MeV
This process, which unlike the previous one is endoergic —it absorbs energy—, surprised Rutherford, as it contradicted what he had observed up to then (the Coulomb barrier could only be overcome with enough energy). George Gamov unveiled this enigma in 1928, based on the new quantum mechanics and the tunnel effect.
In 1920, Arthur Eddington —on account of the precise measurements that Francis W. Aston had made one year earlier of the isotopic masses— postulated that these reactions were the stars energy source (just two decades later Hans Bethe published some possible mechanisms for these stellar reactions).
However, it was Rutherford’s assistant Mark Oliphant who, in 1934, began to systematically study the fusion reactions by bombarding deuterium with deuterium nuclei. This reaction, which is not the most frequent in stars, can occur in two ways:
2H + 2H → 3He + 1n + 3.3 MeV
2H + 2H à3H + 1p + 4.0 MeV These investigations were kept strictly secret, because, unfortunately, the first objective of thermonuclear fusion was of a military nature. Thus, in the United States Edward Teller and Stanislaw Ulam laid the theoretical foundations for building weapons based on nuclear fusion, and on November 1, 1952 the first hydrogen bomb, called Ivy Mike, designed by Richard L. Garwin, was detonated on Enewetak Atoll (in the Marshall Islands). This showed in a practical but terrible way that nuclear fusion was also possible on Earth and that it did indeed release an enormous amount of energy.
Where does the energy in nuclear fusion come from?
If the sum of the masses of the initial nuclei is greater than the mass of the nucleus formed after fusion, that difference in mass has been transformed into energy, according to Einstein’s formula of mass-to-energy equivalence:
ΔE = Δm · c2
which is released, distributed between the kinetic energy that animates the reaction products and the radiant energy. Consider that the fusion of one gram of deuterium-tritium would provide about 100 megawatt-hour (MW-h) of thermal energy.
Nuclear fusion in the stars
After our Sun formed, like other stars, there was a huge period during which hydrogen nuclei (protons) fused to form helium, and colossal energy was thrown off. It is known that in moderately large stars, this occurs mainly through the proton-proton reaction and that, when the hydrogen starts to be scarce in the interior of the star, helium fusion begins. All this according to the following scheme:
The reaction of deuterium and tritium
It was stated before that the most studied reaction to produce energy was the fusion of deuterium and tritium. This is true for several reasons.
The proton-proton reaction that takes place in stars is too slow to be used on Earth for industrial purposes. And, although there are other fusion reactions (see box below) that produce fewer neutrons than deuterium and tritium (and, consequently, less activation of wall materials, resulting in smaller radioactivity) and the resulting energy of which is of more direct use, they nevertheless present various technical problems:
- A lower energy gain for each individual process.
- Reaching significantly higher energies (temperatures) of the reagents.
- A lower availability of reagents.
In short, the most promising nuclear fusion reaction currently available to produce energy on Earth is that of deuterium (D) and tritium (T). For this reaction to occur, the following conditions must be met:
- That the reagents reach sufficient energy (temperature).
- That the density of particles is high enough.
- Let the time of confinement be sufficient.
In particular, these conditions (which are called Lawson’s) involve a temperature of about 150 megakelvins (ten times higher than at the Sun’s core) and a pressure of a few bars (several orders of magnitude less than at the inner of the Sun). At these technically achievable values, the D and the T are in plasma state (a gas of positive nucleus and negative electrons, produced by strong heating and electromagnetic fields) and the D-T reaction is much more likely than the first phase of the proton-proton reaction.
To use the D-T reaction as an energy source, an international collaboration emerged to develop fusion reactors by magnetic confinement of D-T plasma. Certainly, the minimum confinement time has not yet been reached (the facilities are still too small for this), but deuterium and tritium fusion has already been achieved for a short time in the JET experiment. With ITER, it is expected that more energy will be released than that used to heat the plasma. The first production of commercially usable electricity is foreseen through the DEMO experiment.
Other fusion reactions
The following table includes several fusion processes, with their reaction products and the energy released (when the same reagents trigger different products, the percentages of each competing route are given).