Enriched uranium is uranium that has gone through a technological process to increase the proportion of the uranium-235isotope. As a result, natural uranium is divided into enriched uranium and depleted uranium.
Natural uranium contains three isotopes of uranium: uranium-238 (99.2745%), uranium-235 (0.72%) and uranium-234 (0.0055%). The isotopeuranium-238 is a relatively stable isotope, incapable of an independent nuclear chain reaction, unlike the rare uranium-235. Currently, uranium-235 is the main fissile material in the chain reactions of nuclear reactors and nuclear weapons technologies. However, for many applications, the fraction of the uranium-235 isotope in natural uranium is small and the nuclear fuel preparation generally includes the uranium enrichment stage.
Reasons for uranium enrichment
The nuclear chain reaction implies that at least one neutron of the uranium atom formed by the decomposition will be captured by another atom and, consequently, will cause its decomposition. In a first approximation, this means that the neutron must hit the uranium-235 atom before it leaves the reactor. This means that the composition of the nuclear fuel with uranium must be sufficiently compact so that the probability of finding the next uranium atom for a neutron is high enough. But as reactions take place inside the reactor, uranium-235 burns gradually, reducing the probability of a collision of a neutron and a uranium-235 atom. Consequently, the low proportion of uranium-235 in nuclear fuel requires:
- A larger reactor volume for the neutron's travel to be longer;
- A larger proportion of the reactor volume must be occupied by nuclear fuel to increase the probability that a neutron will collide with a uranium atom;
- It is often necessary to recharge the fuel to maintain a given bulk density of uranium-235 in the nuclear reactor;
- A high proportion of uranium-235 high in spent fuel.
In the process of improving nuclear technology, optimal economic and technological solutions were found that required an increase in the content of uranium-235 in the fuel, that is, the enrichment of uranium.
In nuclear weapons, the task of enrichment is almost the same: it requires that in the extremely short time of a nuclear explosion, the maximum number of uranium-235 atoms find their neutron, their decomposition and their release energy. For this, we need the maximum possible mass density of uranium-235 atoms, which can be achieved with limited enrichment.
The degree of enrichment of uranium Natural uranium with a content of uranium-235 of 0.72% is used in some power reactors (for example, in Canadian CANDU), in reactors that produce plutonium (for example, A-1) .
Uranium with a content of uranium-235 up to 20% is called low enriched. Uranium with an enrichment of 2-5% is now widely used in power reactors around the world. Uranium enriched up to 20% is used in research and experimental reactors.
Uranium with a content of uranium-235 above 20% is called highly enriched or weapon. At the dawn of the nuclear age, several types of nuclear weapons based on uranium-based weapons were built with an enrichment of around 90%. Highly enriched uranium can be used in a thermonuclear weapon. In addition, highly enriched uranium is used in nuclear power reactors with a long-term fuel campaign (ie with rare refills or without recharging at all), for example, in spacecraft reactors or on-board reactors.
Depleted uranium with a content of uranium-235 0.1-0.3% remains in the landfills of the enrichment industry. It is widely used as armor piercing projectile shells for artillery shells due to the high density of uranium and the cost of depleted uranium. In the future, it is proposed to use depleted uranium in fast neutron reactors, where uranium-238, which is not compatible with the chain reaction, can be transmuted into plutonium-239, which is compatible with the chain reaction. The resulting MOX fuel can be used in traditional thermal neutron power reactors.
Technology to obtain enriched uranium
Many methods of isotope separation are known. Most methods are based on different masses of atoms from different isotopes: 235 is slightly lighter than 238 due to the difference in the number of neutrons in the nucleus. This manifests itself in different inertia of the atoms. For example, if you make the atoms move in an arc, the heavy ones will tend to move along a radius larger than the light ones.
Electromagnetic and aerodynamic methods are based on this principle. In the electromagnetic method, uranium ions are accelerated in the accelerator of elementary particles and twisted in a magnetic field. In the aerodynamic method, the gaseous uranium compound is blown through a special nozzle snail. A similar principle in gas centrifugation: A gaseous uranium compound is placed in a centrifuge, where inertia causes heavy molecules to concentrate near the wall of the centrifuge. The methods of thermal diffusion and gas diffusion use the difference in the mobility of molecules: gas molecules with a light uranium isotope are more mobile than heavy ones. Therefore, they penetrate more easily into the small pores of the special membranes with gas diffusion technology. In the thermal diffusion method, the less mobile molecules are concentrated in the colder lower part of the separation column, displacing the more mobile ones towards the hot upper part. Most separation methods work with uranium gaseous compounds, most often with UF 6.
Many of the methods tried to be used for the industrial enrichment of uranium, but at present almost all uranium enrichment facilities are based on gas centrifugation. Along with centrifugation, the method of gas diffusion was widely used in the past.
At the dawn of the nuclear age, electromagnetic, thermal diffusion and aerodynamic methods were used. Today, centrifugation demonstrates the best economic parameters for enriching uranium. However, research is being done on promising separation methods, for example, laser isotope separation.