Who discovered the nuclear reaction

Nuclear fission

The Nuclear fission generates energy for nuclear power plants and drives the explosion of nuclear weapons. In this post you will learn, among other things, what nuclear fission is and how it was discovered.

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Nuclear fission simply explained

There are 450 nuclear power plants in operation in 31 countries around the world. The share of nuclear energy in global electricity production was around 11% in 2019. This generation of energy is made possible by nuclear fission.

The Nuclear fission is a nuclear reaction in which a large nucleus breaks down into two smaller nuclei, releasing incredible amounts of energy.

More interesting fact: The nuclear fission of a single kilogram of uranium-235 releases the same amount of energy as the combustion of 3,000 tons of hard coal.

Nuclear fission can occur spontaneously in the case of very heavy atomic nuclei. For energy generation, however, the induced fission caused by bombardment with neutrons is more interesting. The nuclear fission of uranium, of which more than 50,000 tons are mined each year, is of the greatest importance.

What is a nuclear fission?

In the Nuclear fission it is a nuclear reaction. Generally are with one Nuclear reaction involved the particles of the nucleus of an atom. These particles are the protons and neutrons, too Nucleons are called and in their totality make up the atomic nucleus.

In principle, the following happens in a nuclear fission: A large, heavy nucleus breaks up into two or more smaller, lighter nuclei. That may sound interesting, but what are the benefits of nuclear fission and how can nuclear fission be triggered?

Nuclear fission come about

Spontaneous split

In the case of very heavy nuclei with an atomic number greater than 90, a spontaneous fission come. The nuclei disintegrate into smaller nuclei without the atomic nucleus being influenced in any way from outside. Spontaneous nuclear fission is a type of radioactive decay that produces one or more neutrons in addition to the smaller atomic nuclei. Except for the heaviest atomic nuclei, this process is very slow. The spontaneous fission of uranium-238, for example, has a half-life of about Years.

A typical example is the spontaneous fission of the Californium isotope Cf into the isotopes Xe and Ru and 4 neutrons. The reaction equation looks like this


Spontaneous nuclear fission is mainly used in research for the production of free neutrons.

Induced cleavage

Spontaneous nuclear fission has no practical use outside of research. Outside research is the induced nuclear fission of greater importance. This involves bombarding a large core with neutrons. If this atomic nucleus catches a neutron, it gets into an excited state. One possibility to leave this excited state is by splitting into smaller atomic nuclei.

This neutron-induced nuclear fission is basically possible for elements with an atomic number of 90 or more. The fission of uranium-235 plays an essential role for technical applications, for example for generating electricity in nuclear power plants. We will deal with this nuclear fission in detail later.

Fission benefits

Let us now turn to the question of the usefulness of nuclear fission. We restrict ourselves here and in the following sections to the case of induced cleavage.

If you weigh the heavy atomic nucleus and the neutron before the fission and the atomic nuclei and possible further neutrons after the fission, you will find that the total weight before the nuclear fission is greater than the total weight after the fission. Why this observation comes about is another question that we will answer later. More important is the fact that it happens. This difference in the masses is called Mass defect designated. In nuclear fission, one becomes the Mass defect proportional amount of energy released. And that is exactly what you make use of when generating energy in nuclear power plants.

Fission physics

In the case of nuclear fission, there is somehow a mass defect and thus a released energy proportional to this mass defect. In this section we explain why this mass defect occurs and how the released energy can be calculated.

Binding energy and mass-energy equivalence

Binding energy

If you try to break down an atomic nucleus into its individual components, you have to do a certain amount of work, because there is an attractive force between the nucleons. This work goes hand in hand with an energy that you have to muster with your muscular strength. And exactly this energy that you have to deliver in order to balance the attractive forces is called Binding energy.

It is an experimental finding that the binding energy with increasing nucleon number up to the iron isotope Fe increases and then decreases again. So the curve points to the isotope Feet a maximum. The slope is therefore positive on the left and negative on the right. The heavy atomic nuclei are of interest in nuclear fission. As a result, one tends to be on the right side of the maximum.

When a heavy atomic nucleus splits into two lighter nuclei, we move along the curve of the binding energy to the left, i.e. in the direction of the maximum. Since the slope is negative in this area, the values ​​of the binding energy decrease from left to right. But in the case of a nuclear fission, we move from right to left, thus the binding energy increases. The binding energy of the two nuclei after the split is therefore greater than the binding energy of the nucleus before cleavage. But what does this have to do with the observed mass defect?

In our contribution to the Nuclear fusion we explain to you how a larger binding energy can lead to a smaller mass. And that's exactly what happens here. Since the binding energy is greater after the cleavage, the mass of the two nuclei together is also smaller than the mass of the nucleus before the cleavage. This explains the observed mass defect.

Equivalence of mass and energy

So the mass after the split is somehow smaller than the mass before the split. In order to understand why energy is suddenly released in the process, let us use the most famous formula in physics: The Equivalence of mass and energy after Einstein. This equivalence is expressed in the formula


Here is the mass of an object and the speed of light. The letter stands for the Resting energy, i.e. the energy that the object possesses when it is not moving as a whole.

To illustrate this formula, imagine two analog clocks made up of exactly the same atoms and molecules. The difference is that one of the two analog clocks is ticking while the other is standing still. According to Einstein, the ticking clock has more mass than the stationary clock. How can that be? When the clock is ticking, the hands move. So you have kinetic energy. In addition, the friction between the gears increases the thermal energy. Furthermore, potential energy is stored in the spiral springs. All these forms of energy divided by the speed of light squared result in a mass, according to Einstein, and it is precisely this extra mass that makes the ticking clock weigh more than the stationary clock.

So mass and energy are the same except for one constant. For our nuclear fission this means the following: The mass after the fission is smaller, but so is the energy. Now energy is a conserved quantity. And it is precisely this “missing” energy that corresponds to the energy released. If we have the mass defect with denote, then results for the released energy


Nuclear fission uranium

In this section we look at the nuclear fission of uranium, which is important for energy production. We will introduce you to various splitting options and calculate the amount of energy released for a very specific split.

Nuclear fission uranium fission fragments

In general, the atomic nuclei after fission are called Fissure fragments designated. In most cases there are two split fragments. Very rarely, it can also split into three fragments.

But for a given atomic nucleus there is Not exactly one gap possibility. Frequently, however, the split occurs in a light atomic nucleus (mass number about 90) and a heavy atomic nucleus (mass number about 140).

As an example, we will show you three possible cleavage options for uranium-235:

  • the split into barium (Kr) and krypton (Kr)


  • splitting into rubidium (Rb) and cesium (Cs)


  • splitting into strontium (Sr) and xenon (Xe)


The neutrons produced during these fission can in turn induce nuclear fission, which in turn creates neutrons which can also induce nuclear fission and so on. This process of Chain reaction is used in nuclear power plants.

Nuclear fission uranium released energy

Let's use the fission into barium and krypton with the help of the Einstein relationship to calculate the amount of energy released during the nuclear fission of uranium-235. For this we need the atomic masses of uranium-235, neutron, barium-141 and krypton-92. These are





The atomic mass before the split is

and the atomic mass after fission

The mass defect thus arises to


So we get for the amount of energy released


Here we took advantage of the fact that

and taking advantage of the conversion between electron volts (eV) and joules (J)


That is the energy released for a Nuclear fission. The nuclear fission of just one kilogram of uranium-235 uses an amount of energy of around free, which corresponds to the burning of 3000 tons of hard coal.

Fission Discovery

The discovery of the neutron in 1932 by James Chadwick led to immediate implications for experimental nuclear physics. Since the neutron is electrically neutral, it can penetrate the atomic nucleus.

The first experiments that took advantage of this were by bombarding uranium with neutrons Enrico Fermi and his research colleagues in Rome in 1934. He discovered fission products in the process, but the results were viewed with skepticism.

The experiments were done through Otto Hahn, Lise Meitner and Fritz Strassmann repeated. The collaboration with Lise Meitner took place mainly by email, as she had to flee from Germany as a Jew at the time. Otto Hahn reported to Lise Meitner about his discovery of the barium fission fragment when uranium was bombarded with neutrons.

Meitner then convinced himself and Hahn that barium was the result of a process known today as nuclear fission. The results were published by Hahn and Strassmann in 1939. Meitner and her nephew Otto Frisch published a theoretical explanation of Hahn and Strassmann's observations also in 1939.

For this reason Otto Hahn and Fritz Strassmann count as those Explorer nuclear fission and Lise Meitner and Otto Frisch as the first to provide a theoretical explanation for this process. It was Otto Frisch who introduced the term nuclear fission. Otto Hahn originally used the term "uranium fission".