الانشطار المتحكم به - الطاقة النووية .. الفيزياء

Controlled fission - Nuclear energy .. Physics

Controlled fission

The energy of the nuclear reaction in the atomic bomb is released within a fraction of a second. Therefore, this type of reaction is characterized by its difficulty of control and its incompatibility with the peaceful uses of nuclear energy. The nuclear reactor in power plants is necessary to provide the station with energy continuously and reliably. It is secured by two means: the thermal reactor and the fast reactor.

In the thermal reactor, moderators are used to slow down the speed of the neutrons resulting from the fission reaction, whose task is limited to splitting every atom that collides with it from the 235 U atoms, no matter how fast they are.

Since natural uranium contains only 72 235 U atoms in every 10,000 of its atoms, most collisions occur with atoms of the uranium-238 isotope, which is abundant in natural uranium and is characterized by absorbing fast neutrons. It is worth noting that the neutrons produced by the reaction are also fast neutrons, so it becomes very difficult to prolong the continuous reaction in natural uranium. However, if the speed of the neutrons (thermal neutrons) is reduced, their energy becomes similar to the thermal energy resulting from the vibration of the surrounding atoms, which increases the number of fissions within the uranium-235 atoms, since the other isotope 238U absorbs only a small number of neutrons.

In order to slow down the neutrons and make them thermal neutrons, materials called moderators (or tamperers) are added to the fuel that surround it. They are made of light materials that cannot capture neutrons, but rather slow down their speed so that, after several successive collisions, they become thermal neutrons. The most important types of moderators are graphite or (raw carbon) and heavy water (made of deuterium instead of hydrogen - DO 2O). As for the fast reactor, moderators are not used, but rather the number of neutron collisions with fissionable atoms is increased, by enriching the natural uranium fuel with additional atoms of 239 Blue or 235 U. In this way, the fast neutrons produce a gradual reaction that maintains its continuity on its own, but it is not as fast as the reaction that occurs in the bowels of the bomb itself. In this type of reactor, the core is surrounded by a cover of natural uranium, and some neutrons can escape from inside it. Others, due to the presence of some favorable conditions, become captive to the 238U atoms that seize it to form plutonium atoms. These reactors are called fast reactors because of the abundance of uranium that they produce in quantities that exceed their need to enrich the fuel inside them.

The nuclear reaction in these two types is controlled by neutron-absorbing rods that enter the core of the reactor and are called control rods. They contain cadmium, hafnium, and boron and are inserted or withdrawn automatically to ensure that the reaction rate does not change.

(Schematic of a fast reactor)
(Atomic power plant connected to a water desalination plant)

Most of the energy of the ongoing reaction is converted into heat generated from the huge amount of energy present in the post-fission nuclei (fission materials).

These materials then collide with other fuel atoms and the walls of the reactor, leading to an increase in their energy and a rise in their temperature. This heat is extracted from the core of the reactor by a liquid or gas for cooling. Then this liquid is taken after it becomes hot and used in different ways to produce the steam needed to operate turbines and electricity generators, as happens in known power plants.