The Fusion Reaction

Another kind of reaction giving even more energy is the nuclear fusion.

For many years scientists have wondered what the source of energy of the Sun or other stars is. If the classic combustion were the source, then our Sun would go out very soon. In 1938 Hans Albrecht Bethe worked the problem out - the mechanism supplying energy to the Sun is the nuclear fusion.

What does the reaction consist in?
Let's measure the mass of a helium nucleus. And now let's measure the mass of two free protons and two free neutrons. The result is puzzling - the helium nucleus weights less than its components weighted separately. And what has happened to the missing mass? (Doesn't this question remind you of something?). Well, as two protons and two neutrons combine together creating a helium nucleus, much energy is emitted. And as you know from the Einstein equation energy and mass are connected with each other. And so in the nuclear fusion some mass changes into energy.

And just thanks to such reaction stars produce enough energy to shine for billions of years. Inside them light nuclei combine producing nuclei of heavier elements. During that process huge amounts of energy are emitted. In the early Universe (not long after the Big Bang) there were only two kinds of elements subsisting: hydrogen and helium. First stars were created and inside them the fusion reaction started. Then the first stars died ejecting atoms of heavier elements in outer space. New stars were born, and planets (created of heavier elements) rotate around them. On one of these planets a human being is living, and their body is built of atoms synthesised inside stars.

The nuclear fusion reaction can proceed only in the medium of very high temperature and such pressure. In such conditions the atoms are strongly ionised and they form hot plasma.

According to Bethe the cycle of fusion proceeds as follows:

where: l is the neutrinos, 0+1e is the positrons.
Ultimately from these six equations we get:

Where Q is the emitted energy.
Let's now see how the mass balance looks like:
• before the reaction:

• and after the reaction:

So there is ~0.0273 u less than before. The energy of ~25,7 MeV was emitted.
Another well-known cycle of the nuclear transformation is the proton-proton chain also referred to as the deuterium cycle:

It produces the energy of ~26.2 MeV.

Since the Second World War researches on producing nuclear fusion on the Earth have been made. Unfortunately to start such reaction incredibly high temperature is needed. So to get energy one has to supply it first. That is not a problem when we don't need to control the reaction - the hydrogen bombs that use nuclear fusion were created. But still nobody managed to construct a power plant that would produce electricity by fusion. The works still go on.

On November 9th, 1991 in England there was an experiment conducted, in which the reaction of deuterium and tritium synthesis was achieved and maintained for 2 hours. The produced power was equal to about 1MW.