The physical principles of nuclear weapons are actually not complicated, and we learned chain reactions in high school. But the barrier to preventing the proliferation of nuclear weapons is the separation of two isotopes that differ by only 1%.

The element uranium was discovered by German chemist Martin Heinrich Klaproth in 1789, and humans have only known it for 230 years. At first everyone didn’t take it seriously. It was not until 1938 that the dark potential of uranium was discovered.

That’s it. Natural uranium ore contains two isotopes of uranium: uranium 235 (92 protons, 143 neutrons) and uranium 238 (92 protons, 146 neutrons) .


Diagram of Uranium 235 (left) and Uranium 238 (right)

The uranium 235 was used to make the nuclear bomb, because it would release two neutrons during the fission process. Uranium 235 is a strange element. It is a little different from other elements and its younger brother uranium 238. Even if it absorbs low-energy neutrons, it will fission and burst into a small universe. Therefore, as long as a small amount of uranium 235 is fissioning, other uranium 235 will also fission with the wind, which is a chain reaction.


Chain reaction of uranium 235 @Uchichago magazine

The chain reaction is maintained in a critical state (the infinite medium multiplication factor k ∞  = 1) , which means that the number of neutrons released from generation to generation remains stable and can gently release energy. This is the principle of nuclear power plants. Critical (the infinite medium multiplication factor k ∞  > 1) , and the number of neutrons released from generation to generation continues to increase, then nuclear weapons can be created. This middle school physics class will also take.

In 1941, Enrico Fermi , an Italian-American physicist, used a little bit of uranium 235 to create the first chain reaction in history.


Illustration of the world’s first nuclear reactor made by Fermi on December 2, 1942. @Chicago Historial Society

But then, uranium-238 and uranium-235 is not the same, it is relatively calm, low energy neutrons pretend to see nothing to eat into a uranium 239 (92 protons and 147 neutrons) , and does not continue to spit out neutrons (but Will secretly beta decay) . Uranium 238 cracks and cracks only when it is choked by fast neutrons.


In other words, if the old uranium 238 younger brother near uranium 235, then the neutrons it spit out would be unmanned and unable to produce a chain reaction, and it would not be possible to use it to make weapons or use it as nuclear fuel. On the other hand, the chain reaction is so easy to happen, there is no uranium ore on the earth, is it?

Therefore, we need to separate uranium 235 from brothers and concentrate uranium 235 juice. However, only 0.7% of the earth’s uranium mines are 235 uranium, and the rest are uranium 238. To produce nuclear fuel for nuclear power plants, the purity of uranium 235 must reach more than 3.5%, while nuclear weapons need 80% -90%.


Now the question arises, how to enrich enough 235 uranium?

This is the most difficult step in making nuclear weapons. Because the chemical properties of uranium 235 and uranium 238 are similar, there is no way to separate them by chemical reaction. And most of their physical properties, such as boiling points, are similar, and there is no way to separate the two brothers with simple physical methods.

The slight difference between them is that uranium 235 is lighter than uranium 238 (the atomic weight ratio of uranium 238 to uranium 235 is 1.013) . So in some cases, uranium 235 is a little bit faster than brothers. The current mainstream uranium enrichment method is to take advantage of the fact that uranium 235 is a little faster than uranium 238.

The first method of enriching uranium is also the method of making the first atomic bomb-the gas diffusion method.


The process of gas diffusion method is probably like this: the gas of the uranium compound passes through a perforated tube, and the tube is placed in a lower pressure box.

According to Grignard’s law, the speed of gas diffusion is inversely proportional to the square root of gas density. Therefore, uranium 235 will diffuse out of the pores of the tube earlier than the heavier brother. Collecting this wave of early-stage gas, you can get a higher concentration of uranium 235. By repeating this operation, the uranium 235 content will continue to increase and eventually reach the level required for the manufacture of nuclear power plant fuel or nuclear bombs.

Such a plant is called a gas diffusion plant. The first atomic bomb gas diffusion plant, K-25, was located in the US state of Tennessee, covering an area of ​​nearly 160,000 square meters, with a total length of 150 kilometers. It looks like this–


K-25 gas diffusion plant @wikipedia

In addition to this method, there is another more common method of enriching uranium, which is the gas centrifugation method. Of course, it also uses a slight difference in the density of uranium 235 and uranium 238.

The gas centrifugation method operates as follows: After the uranium compound gas is input, the rollers in the centrifuge rotate in this way.

The denser uranium 238 was dumped to the outer and lower layers, while uranium 235 remained in the upper center. Collecting the upper layer gas near the center, you can get a higher concentration of uranium 235.


But a centrifuge cannot get to the sky in one step. In this way, the centrifuges are connected in series with each other and become uranium plutonium. Shake it a few more times to shake the uranium 235 to a specific level.


The energy consumption of the gas centrifugation method is only 1/60 of the gas diffusion method, and the theoretical concentration efficiency is higher. Of course, it is also because of higher efficiency, and now more countries can do uranium enrichment. In fact, the centrifuge method brought hidden dangers to the proliferation of nuclear weapons, so a powerful country allowed an ancient civilization to throw uranium 235 above 3.5%.


On April 8, 2008, Ahmadinejad, then President of Iran, visited Iran ’s uranium enrichment facility. @AP

So, what is the compound gas of uranium just mentioned?

It is uranium hexafluoride (UF6) , a molecule with six fluorine atoms enclosing a single uranium atom.


Solid UF6 @wikipedia

How did this hairy UFO be made?

After the uranium ore is mined, it must first remove the carbon-containing components, then soak it in nitric acid, and then react with ammonia, hydrogen, hydrofluoric acid and fluorine in order, and finally obtain UF6, which can begin to concentrate.


The concentrated UF6 gas needs to be solid before it can be used. Therefore, calcium must be added to the concentrated UF6 gas, and slag fluorine is more desirable for calcium, and then uranium will be discarded.

After such a wave of operation, the oxide of uranium, a lonely and lonely individual, was finally left, uranium dioxide. Uranium dioxide is baked at a high temperature of 1400 degrees Celsius and then made into such small pieces.


Ceramic-type uranium dioxide pellets for nuclear fuel @science photo library

However, the careful person will still have questions: In fact, after becoming UF6, the mass ratio gap between the compounds formed by uranium 235 and uranium 238 is smaller. Doesn’t this make the separation more difficult? Why choose UF6? Why not use uranium oxide or uranium element to enrich it directly?

There are two main reasons for this. First, the boiling points of uranium oxides and uranium are too high. For example, the uranium element has a boiling point of 4131 degrees Celsius. If it turns into steam, who can withstand the metal container?

In contrast, UF6 has a low boiling point. UF6 is a white solid at normal temperature and pressure, but it only needs 56 degrees Celsius (under normal pressure) to turn it into a gas.

Coincidentally, fluorine has only a stable isotope F19, so gas diffusion and gas centrifugation can come in handy.

Imagine that if fluorine has multiple isotopes like uranium, then even if a lighter gas is separated, we cannot be sure whether the lighter uranium 235 or the lighter fluorine isotope is separated.

OK, now we understand that UF6 is very suitable for enriching uranium. It sounds simple, but in fact, the process of manufacturing UF6 is difficult, and the technical threshold and risk factor are extremely high.

First of all, UF6 is quite difficult to mess with. It is not only very toxic, but also reacts violently with water to produce terrible hydrofluoric acid. As we mentioned last time, when hydrofluoric acid corrodes human hands, you don’t even feel it.

Chicken drumsticks soaked in hydrochloric acid (left), hydrofluoric acid (middle), and sulfuric acid (right) for 18 hours

UF6 can also corrode most metals, so it is 666 to make a centrifuge that can withstand UF6.

In addition, the fluorine element used in the manufacture of UF6 is a big monster in itself. This guy is super lively and can play fiery with anyone. It can react with all elements on the periodic table except neon and helium.

For example, in the darkness of minus 250 degrees Celsius, both fluorine and hydrogen can explode. If glass is used as a simple substance, fluorine can react with a little water vapor on the glass to generate hydrofluoric acid that can penetrate the glass. Martyn Poliakoff, a chemistry professor at the University of Nottingham, says most chemists are scared to death when they see a fluorine element. Its danger can be imagined.



The elemental fluorine gas passing through the tube rotted steel wool through a hole @pediodicvideos

In fact, Henri Moissan , the first person who separated the fluorine element without dying on the spot, won the 1906 Nobel Prize in Chemistry for this.

Before Movasan, although many people also found the fluorine element, but because they did not know how to safely separate, they lost one limb or hung up. In a word, the biggest use of fluorine element in the world is uranium enrichment.


Big nose French chemist Henri Moissan @wikimedia

Finally, gas centrifuges rotate at very high speeds, and centrifuges with enriched uranium can produce centrifugal forces of millions of times gravity.

How fast is this? For example, after the Second World War, the speed of the Z-type centrifuge invented by a German tragedy squad captured by the Soviet Union reached 1500 rpm. For comparison, the average household washing machine is about 12-25 revolutions per second.


Therefore, to make a centrifuge for enriching uranium, a very special metal that will not be blown at high speeds and will not be corroded by UF6. Common metals commonly used to make centrifuges are metals such as aluminum and steel. There is an oxide film on the aluminum surface, so it does not react much with UF6.

These technical difficulties are the technical barriers to preventing the proliferation of nuclear weapons, and the last line of technical defense against human death.

Take a look at the gas diffusion enriched uranium plant in Piketon, Ohio. There are 60,000 gas centrifuges in this plant, but it can only enrich uranium 235 to 30%.


Gas diffusion enriched uranium plant in Piketon, Ohio, USA @wikimedia

Finally, let me tell you a little common sense of life. When you see antique porcelain of this color, please throw away the small carp in your hand and scream to escape, because this stunning orange is made of uranium.

In the 1930s and 1970s, the American porcelain company Fiesta once burned batches of Fanta orange porcelain with uranium. They are still radioactive until now, making the Geiger squeak (uranium 238 will undergo alpha decay, releasing alpha particles) .


Don’t use this porcelain to eat  … because it will fluoresce at night after eating …

This article is from Bringing Science Home (wechat ID: steamforkids)