Uranium enrichment is one of the key steps in creating nuclear weapons. Only a certain type of uranium works in nuclear reactors and bombs.
Separating this type of uranium from a more widespread variety requires great engineering skill, despite the fact that the technology necessary for this has been around for decades. The task is not to figure out how to separate uranium, but to build and run the equipment necessary to complete this task.
Uranium atoms, like element atoms found in nature in a variety, are called isotopes. (Each isotope has a different number of neutrons in its nucleus.) Uranium-235, an isotope that makes up less than 1 percent of all natural uranium, provides fuel for nuclear reactors and nuclear bombs, while uranium-238, an isotope that makes up 99 percent natural uranium, has no nuclear use.
Uranium Enrichment Degrees
A nuclear chain reaction implies that at least one neutron from the decay of a uranium atom will be captured by another atom and, accordingly, will cause its decay. In a first approximation, this means that the neutron must "stumble" on the 235 U atom before leaving the reactor. This means that the design with uranium should be compact enough so that the probability of finding the next uranium atom for the neutron is high enough. But as the 235 U reactor operates, it gradually burns out, which reduces the likelihood of a neutron meeting the 235 U atom, which forces them to lay a certain margin of this probability in the reactors. Accordingly, the low proportion of 235 U in nuclear fuel necessitates:
- a larger reactor volume so that the neutron is in it longer
- a larger proportion of the reactor volume should be occupied by fuel in order to increase the likelihood of a collision of a neutron and a uranium atom,
- more often it is required to reload fuel to fresh in order to maintain a given bulk density of 235 U in the reactor,
- a high proportion of valuable 235 U in spent fuel.
In the process of improving nuclear technology, economically and technologically optimal solutions were found that required an increase in the content of 235 U in the fuel, that is, uranium enrichment.
In nuclear weapons, the enrichment task is almost the same: it is required that in an extremely short time of a nuclear explosion, the maximum number of 235 U atoms find their neutron, decay, and release energy. For this, the maximum possible bulk density of atoms 235 U is required, which is achievable with the ultimate enrichment.
Uranium Enrichment Degrees [edit |
The key to separation
The key to their separation is that the uranium-235 atoms weigh slightly less than the uranium-238 atoms.
In order to separate the tiny amount of uranium-235 that is present in every natural sample of uranium ore, engineers first convert the uranium into gas using a chemical reaction.
Then the gas is introduced into a centrifuge tube in a cylindrical shape the size of a person or more. Each tube rotates on its axis at incredibly high speeds, pulling heavier uranium-238 gas molecules to the center of the tube, leaving lighter uranium-235 gas molecules closer to the edges of the tube where they can be sucked out.
Each time the gas is rotated in a centrifuge, only a small amount of uranium-238 gas is removed from the mixture, so the pipes are used in series. Each centrifuge pulls out a little uranium-238, and then transfers the slightly purified gas mixture to the next pipe, etc.
Uranium gas conversion
After the separation of gaseous uranium-235 at many stages of centrifuges, engineers use a different chemical reaction to convert uranium gas back to solid metal. This metal can later be formed for use in either reactors or bombs.
Since each step only cleans the mixture of uranium gas by a small amount, countries can only afford to run centrifuges that are designed to the highest level of efficiency. Otherwise, the production of even a small amount of pure uranium-235 becomes prohibitively expensive.
And the design and manufacture of these centrifuge tubes requires a certain level of investment and technical know-how beyond the reach of many countries. Pipes require special types of steel or mixtures that withstand significant pressure during rotation, must be completely cylindrical and made by specialized machines that are difficult to build.
Here's an example of a bomb that the United States dropped on Hiroshima. It takes 62 kg of uranium-235 to make a bomb, according to “building an atomic bomb” (Simon and Schuster, 1995).
The separation of these 62 kg from nearly 4 tons of uranium ore occurred in the world's largest building and used 10 percent of the country's electricity. “It took 20,000 people to build the facility, 12,000 people operated the facility, and in 1944 equipping it cost more than $ 500 million.” That's about $ 7.2 billion in 2018.
Why is enriched uranium so terrible?
Uranium or weapons-grade plutonium is dangerous in its pure form for one simple reason: from them, with a certain technical base, an explosive nuclear device can be made.
The figure shows a schematic representation of a simple nuclear warhead. Billets 1 and 2 of nuclear fuel are inside the shell. Each of them is one of the parts of the whole ball and weighs slightly less than the critical mass of the weapon metal used in the bomb.
When the TNT detonating charge is detonated, the uranium ingots 1 and 2 are combined into one, their total mass surely exceeds the critical mass for this material, which leads to a nuclear chain reaction and, consequently, to an atomic explosion.
It would seem nothing complicated, but in reality this, of course, is not so. Otherwise, there would be an order of magnitude more countries with nuclear weapons on the planet. Moreover, the risk of such dangerous technologies falling into the hands of sufficiently powerful and developed terrorist groups would greatly increase.
The trick is that only very rich powers with developed scientific infrastructure are able to enrich uranium, even with the current development of technology. Even more difficult, without which the atomic device would not work, separate the 235 and 238 uranium isotopes.
Uranium Mines: Truth and Fiction
In the USSR, at the philistine level, there was a hypothesis that doomed criminals work in uranium mines, thus expiating their guilt before the party and the Soviet people. This, of course, is not true.
Uranium mining is a high-tech mining industry, and it’s unlikely that anyone would have admitted to working with sophisticated and very expensive equipment and inveterate killers with robbers. Moreover, the rumors that uranium miners necessarily wear a gas mask and lead underwear are also nothing more than a myth.
Uranium is mined in mines sometimes up to a kilometer deep. The largest reserves of this element are found in Canada, Russia, Kazakhstan and Australia. In Russia, one ton of ore produces an average of about one and a half kilograms of uranium. This is by no means the largest indicator. In some European mines, this figure reaches 22 kg per ton.
The radiation background in the mine is about the same as on the border of the stratosphere, where civilian passenger aircraft are being patched.
Enrich uranium begins immediately after mining, directly near the mine. In addition to metal, like any other ore, uranium contains waste rock. The initial stage of enrichment comes down to sorting the cobblestones raised from the mine: those rich in uranium and poor. Literally every piece is weighed, measured by machines and, depending on the properties, sent to a particular stream.
Then a mill comes into play, grinding the uranium-rich ore into fine powder. However, this is not uranium, but only its oxide. Getting pure metal is the most complicated chain of chemical reactions and transformations.
However, it is not enough just to isolate pure metal from the starting chemical compounds. Of the total uranium contained in nature, 99% is occupied by the isotope 238, and its 235th counterpart is less than one percent. Separating them is a very difficult task, which not every country can solve.
Gas diffusion enrichment method
This is the first method by which uranium was enriched. It is still used in the USA and France. Based on the difference in density of 235 and 238 isotopes. Uranium gas released from the oxide is pumped under high pressure into a chamber separated by a membrane. Atoms 235 of the isotope are lighter, therefore, from the received portion of heat they move faster than “slow” uranium atoms 238, respectively, more often and more intensively beat against the membrane. According to the laws of probability theory, they are more likely to get into one of the micropores and be on the other side of this membrane.
The effectiveness of this method is small, because the difference between the isotopes is very, very small. But how to make enriched uranium usable? The answer is applying this method many, many times. In order to obtain uranium suitable for the manufacture of fuel from a reactor in a power plant, the gas diffusion treatment system is repeated several hundred times.
Expert reviews about this method are mixed. On the one hand, the gas-diffusion separation method is the first to provide the United States with high-quality uranium, making them temporarily a leader in the military sphere. On the other hand, gas diffusion is thought to produce less waste. The only thing that fails in this case is the high price of the final product.
This is the development of Soviet engineers. At present, in addition to Russia, there are a number of countries where uranium is enriched by the method discovered in the USSR. These are Brazil, Great Britain, Germany, Japan and some other states. The method is similar to gas diffusion technology in that it uses the mass difference of 235 and 238 isotopes.
Uranium gas spins in a centrifuge to 1,500 rpm. Due to different densities, isotopes are affected by centrifugal forces of different sizes. Uranium 238, as heavier, accumulates near the walls of the centrifuge, while the 235th isotope gathers closer to the center. The gas mixture is pumped to the top of the cylinder. Having passed the way to the bottom of the centrifuge, the isotopes have time to partially separate and are selected separately.
Despite the fact that the method also does not provide 100% separation of isotopes, and to achieve the necessary degree of enrichment it must be used repeatedly, it is much more economically efficient than gas diffusion. Thus, enriched uranium in Russia using centrifuge technology is about 3 times cheaper than that obtained on American membranes.
Enriched Uranium Application
Why is all this complicated and expensive red tape with purification, metal separation from oxides, separation of isotopes? One washer of enriched uranium 235, of those used in nuclear energy (from such “pills” are assembled rods - fuel rods), weighing 7 grams replaces about three 200-liter barrels of gasoline or about a ton of coal.
Enriched and depleted uranium are used differently depending on the purity and ratio of 235 and 238 isotopes.
Isotope 235 is a more energy-intensive fuel. Enriched uranium is considered when the content of 235 isotopes is more than 20%. This is the basis of nuclear weapons.
Enriched energy-saturated raw materials are also used as fuel for nuclear reactors in submarines and spacecraft due to the limited mass and size.
Depleted uranium, containing mainly 238 isotopes, is a fuel for civilian stationary nuclear reactors. Natural uranium reactors are considered less explosive.
By the way, according to the calculations of Russian economists, while maintaining the current production rate of 92 elements of the periodic table, its reserves in explored mines around the world will already be depleted by 2030. That is why scientists are looking forward to fusion as a source of cheap and affordable energy in the future.