Reactions in the Uranium-235 fission

Episode#2: Nuclear fission

The second in-depth look at the Magnetic confinement fusion format: instructions for use

by Luca Longo
7 min read
by Luca Longo
7 min read

Magnetic confinement fusion: instruction for use is an original format by Eni that aims to explain various aspects related to a technology with enormous potential.

In Episode#1: Atoms and fundamental forces, we understood what lies inside the nucleus of an atom and learned to master concepts such as protons, neutrons and isotopes: we are now fully prepared to proceed on our journey to discover nuclear energy.
The two nuclear reactions we are going to talk about today are fission and fusion. We are very interested in them because they can both be used to produce energy; let us now try to understand what they are and how they differ. 


In terms of spelling there is only a different vowel and a consonant between them, but in reality these are two exactly opposite reactions in the atom. Nuclear fission occurs when a heavy nucleus breaks up and is split into two lighter ones, whose masses (when added together) are less than the original mass. To achieve this, we must choose an isotope of an atom; for example, the heaviest available on Earth, 235U. This isotope, remember, has its 92 protons, like all uranium atoms, but has "only" 235-92=143 neutrons.
When the 235U atom is hit by a neutron with appropriate kinetic energy, a new isotope is formed.
It is still Uranium, because we already know that when you change the number of neutrons in an atom you do not change its nature, but the nucleus itself is no longer 235U: it becomes 236U, which reminds us that there is an extra neutron in it.
236U, however, is terribly unstable: as soon as it forms, it falls apart, giving rise to a Barium 141Ba atom and a Krypton 92Kr atom. This process is called fission.
Barium and Krypton have 36 and 56 protons respectively. That makes a total of 92 and the proton count adds up. But if we add up their total mass, we see that it comes to 141+92=233. There are three neutrons missing, which have in fact broken away and shot off as three independent particles.
The overcoming of the strong fundamental forces holding the two Barium and Krypton nuclei and the three neutrons together also releases a lot of energy in the form of kinetic energy. But that's not all: if there are many other 235U atoms surrounding the one that broke, the three neutrons produced by its fission can hit them and break them ; each of them will release three neutrons that in turn... That's it: this is a chain reaction. It can only be stopped if we find a way to catch more stray neutrons than are formed and prevent them from hitting and breaking other 235U atoms.
The fission of a single atom of 235U releases 202 million electronvolts (a unit 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% 235U can generate energy equivalent to that produced by burning 300 to 3,000 kg of coal. The precise value depends on many technical factors, but in essence, for the same weight, between three hundred thousand and three million times more nuclear energy is released from the fission of a given mass of uranium than is released - in the form of chemical bonding energy - by burning an equal mass of coal.


In order to harness the energy contained in coal, it is burnt in thermal power plants, which also produces a lot of pollution in the extraction, transport, combustion and disposal of the smoke and ash.

To harness the energy contained in Uranium, a nuclear reactor is used which, from a certain point of view, is designed according to the usual paradigm of energy production technologies: the heat produced by either uranium fission or coal combustion is used to heat water in a boiler and generate high pressure steam. Steam expands and drives a turbine. A turbine is basically a rotor attached to a series of magnets and conducting wires and surrounded by other fixed magnets. The rotor is set into rotation, the variable magnetic field generates an electric field, which in turn moves electrical charges through the wires, producing electrical energy.
In a nuclear reactor, there is a container called core, which is used to heat water. It is made up of a number of cylinder holes into which bars are fitted. Some of them consist of a bundle of smaller cylinders where individual uranium pellets are aligned. Each pellet is a small cylinder about the diameter of a coin and 2-3 cm high. 


However, uranium rods would overheat due to the chain reaction between neutrons and 235U, which could cause the entire core to melt. We are not talking about nuclear fusion, which is an entirely different matter, but about the reactor's core melting and destroying the reactor due to the high temperature produced by fission.
To avoid this, other rods called control rods, which are made of neutron-absorbing materials, are inserted between one uranium rod and another. These can be inserted into the core or extracted by sliding them along the cylindrical holes. Usually Cadmium, Boron, Hafnium or Gadolinium are used, as these elements are very efficient in capturing stray neutrons.


When we want to lower the temperature of the reactor, we slide the control rods into the core. In this way, they absorb more neutrons produced by the uranium nuclei undergoing fission, preventing them from hitting other uranium nuclei. Viceversa, when we want to increase the reactor’s temperature, we simply lift the control rods out. This allows the neutrons released by fission to hit other uranium atoms, propagating the chain reaction. We must not overdo it if we want to avoid big trouble. In the event of a problem, all the control rods are lowered into the cylindrical holes. As a result, most of the neutrons are captured and the nuclear reaction stops.
In this way, the core acts as a very high-energy heat source, heating the high-pressure cooling water that flows between the rods (it's called the primary circuit) which in turn heats the water in the secondary circuit that exits the core and makes steam available. From here, electricity is produced through the turbine and generator.
In the fission process, not a single molecule of carbon dioxide is generated, but the reaction products (the nuclear waste), the core structure and other parts of the reactor that come into contact with the reaction environment become highly radioactive and must be handled very carefully and according to procedures set out by experienced personnel.
Fusion works in the exact opposite way and is used to release energy from the lightest elements... but that is another story and we'll talk about it another time.

The author: Luca Longo

Industrial chemist specialized in theoretical chemistry. He was a researcher for 30 years before moving on to Eni's scientific communication.


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