The second in-depth study of the format on new energy forms

In the first episode: ‘Atoms and forces, the origin of magnetic fusion‘we have understood what is hidden inside the nucleus of an atom and to master concepts such as protons, neutrons and isotopes: now we are perfectly equipped to proceed in our journey to discover nuclear energy.

The two nuclear reactions we are talking about today are fission and fusion. They interest us a lot because you can use both of them for produce energynow let’s try to understand what they are and how they stand out.

At the lexical level only one vowel and one consonant change, but in reality it is two exactly opposite reactions of the atom.

There nuclear fission happens when a heavy core breaks down and is split into two lighter ones, whose masses (if added together) do not reach the original mass. To obtain this result, we must choose an isotope of a specific atom, iThe heaviest available on Earth: the 235U This isotope, we recall, has its 92 protons, like all Uranium atoms, but it has “only” 235-92 = 143 neutrons.

When the atom of 235U collides with a neutron, a new atom is formed.

It is always Uranium, because we already know that when the number of neutrons of an atom is changed, its nature does not change, but the nucleus itself is no longer 235U: becomes 236U to remind us that there is an extra neutron inside.

L’ 236U is though terribly unstable: as soon as it forms it breaks up giving rise to a Barium atom 141Ba and to an atom of Krypton ninety twoKr. This process is called fission.

Barium and Krypton have 36 and 56 protons respectively. In total it is 92 and the proton count comes back. But if we add their total mass we see that we get to 141 + 92 = 233. Three neutrons are missing, which in effect broke off and shot off as three independent particles.

The beauty is that this breakdown of the strong fundamental forces that hold together the two nuclei of Barium and Krypton and the three neutrons it also releases a lot of energy. But not only: if around the atom of 235U that fell apart if many others were found, the three neutrons produced by its fission go to hit other atoms of 235U which in turn break and release three neutrons each which in turn …

Here you are: this is a chain reaction. It can only be stopped if we find ways of catching more stray neutrons than are being formed, and preventing them from hitting and breaking other atoms of 235U.

The fission of a single atom of 235U frees 202 million Electron Volts (which is a unit of measurement used in electromagnetism and chemistry to measure the work done by an electron to cross a potential difference of 1 Volt). One gram of Uranium containing only 3% of 235U can develop one equivalent energy to that produced by burning from 300 to 3,000 kg of coal.

The precise value depends on many technical factors, but in substance, for the same weight, we release from three hundred thousand to three million times more energy from the fission of a certain mass of uranium than that which we release by burning an equal mass of coal.

To use the energy contained in coal, it is burned in thermal power plants, also producing a lot of pollution in the extraction, transport, combustion and disposal of fumes and ashes.

To utilize the energy contained in Uranium, a nuclear reactor that… from a certain point of view it is the same as before. In fact, the heat produced by the fission of uranium or by the combustion of coal is used to heat water in a boiler and generate steam under pressure.

This vapor expands and causes one to move turbine. A turbine is nothing more than a rotor attached to a series of magnets and conducting wires and surrounded by other fixed magnets. The rotor is put into rotation, the moving magnetic field generates an electric field which in turn passes electricity through the wires producing electricity.

In a nuclear reactor for heating water there is a container called core. This consists of a series of cylindrical holes within which there are some bars. Each of these bars is composed of a bundle of cylinders smaller where the singles have been aligned pellets of Uranium. Each pellet is a small cylinder with a diameter of a dime and about 2-3 cm high.

But the uranium bars, left to themselves, would overheat due to the fission of the 235U and would cause the core to melt. We are not talking about nuclear fusion – which is quite another thing – but precisely the core that melts by destroying the reactor due to the high temperature produced by the fission.

To avoid this, between one Uranium bar and the next, other bars are inserted – called control bars – made of neutron absorbing materials. These can be inserted inside the core or extracted by sliding them along the cylindrical holes. They are usually used Cadmium, Boron, Hafnium or Gadolinium which are very efficient in capturing stray neutrons.

When you want decrease the reactor temperature, the control rods are lowered into the core. Thus, these absorb more neutrons produced by the Uranium nuclei that undergo fission, preventing them from hitting other Uranium nuclei.

When, on the other hand, you want increase the reactor temperaturejust lift the control rods avoiding that the neutrons released by the fission are captured but allowing them to go and hit other Uranium atoms by propagating the chain reaction.

Don’t overdo it if you want to avoid creating big trouble. In case of problems, all the control rods are lowered into the cylindrical holes, most of the neutrons are thus captured and the nuclear reaction stops.

So, the core acts as a very high energy heat source, heats the high pressure cooling water flowing between the bars (primary circuit) and this in turn heats that of the secondary circuit that comes out of the core and makes available the steam. From here, through the turbine and generator, electric current is produced.

In the fission process, not even a molecule of carbon dioxide is generated, but the reaction products (slag), the core and other parts of the reactor that come into contact with the reaction environment become highly radioactive And they must be handled very carefully to avoid problems.

The mergerinstead, it works exactly the other way around and it serves to release energy from the lighter elements … but this is another story and it will have to be told another time …

Magnetic confinement fusion
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The post Nuclear fission in a few (simple) words first appeared on Technology Review Italia.

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