The use of nuclear energy arouses passions for and against. Firm bets on fusion, which together with renewables could be the energy source of the future, and bitter controversies about fission which, also together with renewables, is the energy source of the present. Angels and demons have a common heart: the nucleus of atoms.

Decades remain before the energy obtained by fusion enters our homes, while current nuclear power plants work exclusively by fission.

But what are fusion and fission? What are its fundamentals? Why do we yearn for one while holding the sword of Damocles over the other?

The heart shared by both is a physics of energies and masses that seem to volatilize. This paradox can be explained from two magnets that repel each other, and an afternoon of shopping at the neighborhood greengrocer.

Stubborn game, dilemma of geniuses

We have all tried in vain to join two magnets by poles of the same sign. But, if we are unable to join the positive poles of two rudimentary fridge magnets, how is it possible that positive charges as pure as protons form atomic nuclei without repelling each other?

The existence of nuclei formed only by united positive charges was so unthinkable that physicists at the beginning of the 20th century (no less than Einstein, Marie Curie, Bohr, Rutherford and many others) came to think that the nucleus also contained electrons (negative charge ) that compensated for the repulsion between protons. This hypothesis carried serious problems, but it seemed less crazy than that of a nucleus made up of positive little balls that attracted each other.

a mysterious guest

Soon after it was confirmed that the partners of the protons in the nucleus were not electrons but neutrons (without electrical charge). Things got complicated until, to everyone’s relief, an unexpected guest appeared: the strong nuclear interaction. This interaction is an attractive force that acts over extremely small distances and holds protons and neutrons (generically called nucleons) together in nuclei. It is so strong that it dominates the already strong repulsive force between protons.

Thus, inside each nucleus we find a giant canceling out another.

We don’t get the accounts

If we go down to the grocery store and buy ten oranges of 200 grams each, the weight will indicate a total mass of 2,000 grams. What’s more, if the greengrocer puts them in a bag and compacts them under pressure, the mass of the set is still 2,000 grams. But this obvious result with oranges, pears, and watermelons does not hold up in atomic nuclei.

In a hypothetical protoneríaif we buy six protons and six neutrons for build a carbon nucleus, the situation is different. The books say their masses add up to 12.0096 atomic mass units (u), a very small multiple of the kilogram. However, when our protonero trustworthy go into the back room and monte the nucleus, will come out with a mass cluster of 12.0000 u, that is, 0.0096 u less than it should. A 1.6 with 28 zeros in front if we express it in kilos.

The energy that is not lost

This fact is universal: the mass of nuclei is less than that of their separate components. The difference between the two is called the mass defect. But what is this loss due to? Is it the same for all elements? Is the protoner deceiving us?

Of course not. What happens is that, if the electrical repulsion is overcome, the strong nuclear interaction attracts a few nucleons and these caen being united in a nucleus, energy is released.

This is not surprising: when the fruit bowl drops an orange due to another attractive force such as gravity, energy is also released in the form of heat and noise when it hits the ground.

Since the nuclei of different elements have different sizes and number of nucleons, the energy that binds each one with the rest varies depending on the element. Thus, we need more energy to remove a proton from an iron nucleus than from a uranium one, which is why the former is much more stable than the latter.

Now, according to Einstein’s famous equation, E=mc2, mass and energy are the same thing. That means that the release of energy also translates into a decrease in mass as we saw when buying the carbon core in the proton.

This equation, which has sold more shirts than some soccer stars, says many other things and was conceived in a different context than nuclear energy, but that’s another story.

Mystery solved: the protonero did not deceive us in regret. Actually, the missing mass-energy has been released by joining the nucleons.

At the nuclear power plant: the key to fission

In nuclear power plants they break (fission) uranium or plutonium nuclei by throwing neutrons at them and various reactions can occur. In one of them a barium nucleus is produced, another one of krypton, 3 neutrons and electromagnetic radiation.

The mass defect of the process is negative and energy is released in the form of movement of the products and radiation. This energy is used to heat water, evaporate it and move a turbine producing electrical current.

In other words, the energy inextricably linked to the mass defect of different rupturing nuclei is harnessed to produce thermal and electrical energy. This is the operating principle of nuclear power plants. No more no less.

Fission or fusion? the difference beyond a letter

The mass defect of large, unstable nuclei implies a release of energy when they break apart, a process known as nuclear fission. Nuclear power plants are based on it, which produce more than 20% of Spain’s energy.

But we can also obtain energy if instead of breaking them, we join small nuclei like those of hydrogen. This process, called nuclear fusion, should provide us with energy from abundant, easily obtained elements and with practically innocuous residues.

The problem with fusion lies in the extraordinary technical complexity to control it, since it requires reactors capable of reaching temperatures of the order of 100 million degrees Celsius and withstanding extremely high levels of radiation. Unfeasible today.

As illustrative data, fusion and fission were described in the 1930s of the last century. In less than 10 years it was possible to control fission in a reactor, and before 20 it was used in the first nuclear power plant. On the other hand, 100 years later the first fusion plant will still not work.

An afternoon of shopping can illustrate what is behind fission and fusion, their relationship at the heart of atomic nuclei. Present and future in a change of letters.

Antonio Manuel Peña García, Professor of the Electrical Engineering Area, University of Granada

This article was originally published on The Conversation. Read the original.