Does water have different levels of light and heavy? Why is heavy water needed to make nuclear weapons?

Does water have light and heavy water? Of course, the water we drink on a daily basis is "light water", and the opposite is "heavy water".

What is heavy water? As the name suggests, it is heavier than ordinary water. The molecular mass of the light water we drink daily is 18.0153, while the molecular mass of heavy water is 20.0275. It can be seen that the mass of heavy water is about 11% higher than that of light water. Where does this extra mass come from? In fact, it is the mass of neutrons. There are various elements in nature, such as hydrogen, oxygen, carbon, and iron. Each element is composed of corresponding atoms, and atoms are composed of atomic nuclei and electrons. There are protons and neutrons in the atomic nucleus. The number of protons determines the type of element. For example, a hydrogen atom has one proton, and a helium atom has two protons.

In addition to many different elements in nature, the same element also has several different isotopes, which are determined by the number of neutrons.

For example, hydrogen has three isotopes, namely protium, deuterium and tritium. These three isotopes all have the same number of protons, that is, one proton, but the protium nucleus has only one proton and no neutrons, deuterium has one neutron, and tritium has two neutrons. The molecular formula of a water molecule composed of two protium atoms and one oxygen atom is the familiar H2O, which is the most common and ordinary light water, while the molecular formula of a water molecule composed of two deuterium atoms and one oxygen atom is written as D2O, which is heavy water. Because heavy water has two more neutrons than light water, its mass is naturally larger.

Although heavy water only has two more neutrons than light water, it is no longer the "source of life". Not only is it not the source of life, drinking heavy water can also lead to death.

But we don't need to worry, because in nature, the proportion of heavy water is very small. It is mixed in light water, but the proportion is less than 0.02%. Although heavy water cannot be drunk, it has other uses, such as the manufacture of atomic bombs. An atomic bomb is a weapon that uses nuclear fission reactions to produce huge destructive power. Its power mainly comes from the "charge" inside. There are currently two elements that can be used for atomic bomb charges, namely uranium 235 and plutonium 239. No matter which element is used, it must be purified. Take uranium 235 as an example. Its proportion in uranium ore is only 0.7%, and the rest is uranium 238. To make nuclear weapons, the concentration of uranium 235 must be purified to at least 85%, which is only the theoretical minimum. The purity required in the actual process of making atomic bombs is even higher.

In order to increase the concentration of uranium 235, a large number of advanced gas centrifuges must be used for repeated enrichment, which is an insurmountable threshold for many countries.

If uranium 235 is used as the charge of an atomic bomb, heavy water is not needed, but if plutonium 239 is used, heavy water is indispensable. Unlike uranium 235, uranium 235 exists in nature. What we need to do is to try to separate it from uranium 238, but plutonium 239 does not exist in nature. It must be made artificially. How to do it? This starts with uranium 238. The atomic bomb is so powerful because it uses the principle of "chain reaction".

The so-called chain reaction, in simple terms, means that after uranium 235 absorbs neutrons, it will undergo fission, releasing energy and throwing out two neutrons. These two neutrons will then be absorbed by other uranium 235, causing a chain reaction.

However, uranium 238 mixed with uranium 235 will also absorb neutrons, but it will not fission after absorbing neutrons, but will directly become uranium 239. The neutrons produced by the fission of uranium 235 are silently eaten by uranium 238, and the chain reaction cannot be achieved. Uranium 238 is redundant for uranium 235, but it is very useful in the production of plutonium 239. Uranium 238 will become uranium 239 after absorbing neutrons. Uranium 239 is unstable and will become neptunium 239 through β decay. Neptunium 239 can produce plutonium 239 by decaying again. Atomic reactors are needed to produce plutonium 239, and atomic reactors can be divided into light water reactors and heavy water reactors according to the coolant used.

Both light water reactors and heavy water reactors can produce plutonium 239, but plutonium 239 produced by light water reactors cannot be used to make atomic bombs.

This is because the plutonium 239 produced in the light water reactor is also mixed with a large amount of plutonium 240. Plutonium 240 is an extremely unstable element that will undergo spontaneous fission from time to time. If there is too much plutonium 240 mixed in plutonium 239, you and I know what will happen, that is, unpredictable self-explosion at any time. Therefore, if the plutonium 240 content in plutonium 239 exceeds 19%, it is a waste material that must be landfilled. Between 7% and 19%, it can be used as fuel. If it is used to make atomic bombs, the plutonium 240 content must be controlled below 7%, which is absolutely impossible in light water reactors. Therefore, if you want to make atomic bombs, you must use heavy water reactors to produce plutonium 239. Why must plutonium 240 content exceed the standard and must be landfilled instead of being purified? There is only a difference of 1 neutron between plutonium 239 and plutonium 240, which is like a few cows in a herd that have one less hair than the others. It is impossible to pick them out.

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