الانشطار النووي - الطاقة النووية .. الفيزياء

Nuclear Fission - Nuclear Energy .. Physics

Nuclear Fission

The fission of the nucleus of heavy atoms is a method of producing energy through nuclear reactions. In 1939, Otto Hahn discovered that bombarding materials containing uranium with neutrons is sufficient to give small amounts of light elements that are formed as a result of the fission of atoms.

It then became certain that the energy resulting from fission is about 2 × 10-11 joules, but if we know that one kilogram of uranium contains approximately 2.5 × 1024 atoms, it becomes clear to us that the fission of this entire quantity is capable of giving 7.5 × 1031 joules.

For comparison, it is sufficient to know that one kilogram of coal gives only 4 × 1071 joules when it burns completely.
This means that a ton of uranium gives the same amount of energy as two million tons of coal. Efforts began to combine to produce energy from the atom after it became clear, especially during the last world war, that its nucleus constitutes an important source of energy.

Natural uranium consists of three isotopes: every 100,000 atoms of this natural metal contain 6 atoms of the 234U isotope, 720 of the 235U isotope, and the rest are all 238U atoms. It is worth noting that when a 238U atom collides with a fast neutron (with a speed exceeding 17 × 610 m/s), it splits into two parts, each part forming a real nucleus.
If it collides with a slow-speed neutron, it captures it and gives off the isotope 239U, which is characterized by its instability and rapid decay into a new element, neptunium-239U (239-Nb), and a beta particle (electron). This last element in turn decays into plutonium-239 after giving off another beta particle.

Both 239Blu and 235U, which are rare, gain their importance from the fact that they can be fissioned by neutrons, no matter how fast they are (because they are both fissionable materials) and from the fact that this fission is accompanied by the giving off of one, two, or three neutrons, which in turn contribute to other fissions that together form what is called the continuous reaction. Since fission takes only a millionth of a second, a few kilograms of 235U and 239Blu disappear very quickly, causing a devastating explosion.

We can store the elements 235U 239Blu safely, but in small quantities (only a few kilograms) because they allow the neutrons to escape from them and avoid the continuous reaction. The smallest mass in which the continuous reaction occurs is called a critical mass. If we exceed it, the space allocated to a unit of volume decreases and the neutrons are able to remain inside the material, launching a continuous reaction. The atomic bomb is nothing but two critical masses of the elements 235U and 239Blu, to which a specific mechanism is added that suddenly combines them, forming a single mass larger than the critical mass.