Nuclear Power Plant Isar, Germany

Spent nuclear fuel pool

Turbine of a nuclear plant

Nuclear Fission

Nuclear Fission

Nuclear fission is the physical-chemical reaction through which the nucleus of an atom is split. In the main interest of fission reactions is that through this operation a large amount of energy is obtained. According to the nuclear energy definition, atomic energy is the energy contained in the nucleus of an atom and the energy obtained is thermal energy, energy in the form of heat.

The other form of exploitation is through nuclear fusion reactions. In this case, the process is inverse, fusing two different nuclei forming a single atomic nucleus.

After the fission of the atomic nucleus we obtain several fragments with a mass almost equal to half of the original mass plus two or three neutrons. It is remarkable the release of these two or three neutrons because they are what will allow a new reaction in another atom.

The sum of the masses of these fragments is less than the original mass. This 'lack' of masses (about 0.1 percent of the original mass) has been converted into energy according to the Einstein equation (E = mc 2 ). In this equation E corresponds to the energy obtained, ma the mass of which we speak and c is a constant, that of the speed of light: 299,792,458 m / s 2 . The energy obtained comes in the form of heat; it is, therefore, thermal energy.

Nuclear fission can occur when a nucleus of a heavy atom captures a neutron (induced fission), or it can occur spontaneously due to the instability of the isotope (spontaneous fission).

The material used as nuclear fuel has a very unstable atomic structure. Uranium and plutonium are generally used. The characteristics of these atoms is that they are very heavy, with a large number of protons with a positive charge in the nucleus. By having so many protons with a positive charge to the nucleus it is very difficult to maintain the links of forces to keep them together. For this reason, the collision with a single neutron is sufficient to destabilize the entire structure and break.

Nuclear chain fission reactions

Diagram of a chain of fission nuclear reactions

A chain reaction is a process by which neutrons that have been released in a first nuclear fission produce additional fission in at least one more nucleus. This atomic nucleus fissures and releases more neutrons in turn, giving the opportunity for the process to repeat itself.

These chain reactions can be controlled or uncontrolled. The controlled reactions would be the nuclear reactions produced in the nuclear reactor of a nuclear power plant in which the objective is to generate electrical energy in a constant and balanced manner. The uncontrolled nuclear reactions occur in the case of nuclear weapons in which the goal is to generate a large amount of energy in an instant.

If in each nuclear fission reaction caused by a neutron two more neutrons are released, then the number of fissions doubles in each generation. In this case, in 10 generations there are 1,024 fissions and in 80 generations approximately 6 x 10 23  fissions.

Critical mass

Critical mass is the minimum amount of fissionable material for a nuclear chain reaction to be maintained.

Although two to three neutrons occur in each nuclear fission, not all neutrons are available to continue the fission reaction; some are lost. If the neutrons released by each nuclear reaction are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining and will stop.

The amount of critical mass of a fissionable material depends on several factors: physical properties, nuclear properties, its geometry and its purity.

A sphere has the minimum possible surface area for a given mass, and therefore minimizes neutron leakage. If we also skirt the fissile material with a neutron reflector, much less neutrons are lost and the critical mass is reduced.

Although uranium can be found naturally, the uranium used in nuclear reactors is what is called enriched uraniumEnriched uranium is uranium that has been subjected to a treatment to make it a more unstable isotope. It is called isotopes atoms of the same element, whose nuclei have a different number of neutrons and, therefore, differ in mass number.

Controlled nuclear fission

Scheme on how the released neutrons are controlled to control the chain fission reaction

To maintain a sustained control of nuclear reaction, for every 2 or 3 neutrons released, only one must be allowed to collide with another uranium nucleus. If this ratio is less than one then the reaction will die, and if it is larger it will grow out of control (an atomic explosion).

Neutron absorption elements are used to control the amount of free neutrons in the reaction space. Most nuclear power reactors are controlled by means of control rods made of a material that has the property of absorbing free neutrons, for example, boron or cadmium.

In addition to the need to capture neutrons, neutrons often have a lot of kinetic energy (they move at high speed). These fast neutrons are reduced through the use of a moderator, such as heavy water and running water. Some nuclear reactors use graphite as a moderator, but this design has several problems. Once fast neutrons have slowed down, they are more likely to produce more nuclear fission or be absorbed by the control rods.

Spontaneous nuclear fission

In the reactions of spontaneous nuclear fission the absorption of a neutron is not necessary 

The rate of spontaneous nuclear fission is the probability per second that a given atom is fired spontaneously - that is, without any external intervention. Plutonium 239 has a very high rate of spontaneous fission compared to the rate of spontaneous fission of uranium 235. In certain isotopes of uranium, and especially plutonium, they have such an unstable atomic structure that they spontaneously fission.

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References

Last review: March 15, 2019