On October 16, 1964, our atomic bomb test code-named "Miss Qiu" was successfully detonated at 3 p.m. at the Lop Nur Test Site.
This incident made us the fifth country in the world to possess an atomic bomb. It has laid the foundation for China's future development and greatly enhanced our international status. Seven years after the atomic bombing, on 25 October 1971 we regained our seat at the United Nations and continued to serve as a permanent member. And the rise of China's nuclear capabilities forced the United States and the Soviet Union to sign the Treaty on the Non-Proliferation of Nuclear Weapons in 1968.
But when the first atomic bomb exploded that year, both the United States and the Soviet Union unanimously expressed the view that China's first atomic bomb was a plutonium bomb.
For decades, the uranium-plutonium dispute over China's first atomic bomb has not been interrupted.
Yesterday I did a Q&A "Why can workers go to the uranium ball when making the atomic bomb, and the radiation at such a high concentration is not very strong?" It is also logical to discuss with the fans the question of whether our first atomic bomb is a "uranium ball" or a "plutonium ball".
In fact, the state has not made a precise explanation of this matter in the real official channels and statements. However, in media propaganda, it is often said that it is a "uranium implosion nuclear weapon" with higher technical difficulties. The person closest to this core is the original three knives (formerly Gongpu). But in a large number of materials, it is actually only written that the old man processed the core components of China's first atomic bomb. As for whether the composition is uranium or plutonium, it is basically not written. The uranium ball in the back is only a deduction in the story of the old man.
After all, our nuclear weapons secrecy system is the most stringent secrecy system in the world. And in those days, due to the spies of various enemies, it was difficult for us to make a lot of real information public.
For example, in 1962, we successfully shot down an American-made U-2 high-altitude reconnaissance aircraft using surface-to-air missiles. Let's be the first country in the world to shoot down a U-2. When foreign reporters asked about the situation, Marshal Chen Yi, then foreign minister, told foreign reporters that "we stabbed down with bamboo poles...", of course, the diplomatic joke context also made our surface-to-air missiles successfully secrecy for a long time. At the same time, it also made a moderate mockery of the enemy's "weak chicken".
Many things in history are often just mysteries when the official real data is released.
However, if we carefully analyze the basic data of a nuclear weapon, we will judge for ourselves whether our first atomic bomb was a uranium bomb or a plutonium bomb.
The main knowledge here is the awareness of the critical state.
"Critical" includes the concepts of "critical mass" and "critical diameter". The day before yesterday, some fans said that as long as the critical mass is reached, a nuclear explosion will occur, which is incorrect.
W Jun gave an example at that time: a 2 mm diameter uranium 235 filament with a length of 2 km. The mass of this uranium filament is about 119 kg. This weight far exceeds the critical mass of uranium-235, but no nuclear explosion will occur. The reason is that there is still a critical diameter difference.
In general, a perfect sphere is the shape with the smallest critical volume.
The reason is the problem of neutron utilization efficiency in the chain reaction process.
Spontaneous fission can occur anywhere inside a piece of uranium-235 or plutonium-239 metal. If the neutrons produced by nuclear fission did not collide with any other heavy atoms, they would fly out of the metal surface. Then the neutrons produced by fission are difficult to trigger the fission of the next atomic nucleus. Fission reactions are difficult to sustain.
In order for a fission reaction to be self-sustaining to form a chain reaction, it is necessary to ensure that most of the neutrons can hit the nucleus. Here two methods can be used, the first is to increase the number of nuclei on the neutron flight path, the way is to increase the density and increase the mass - which is also the source of critical mass, and the other is to extend the path of the neutron in the nuclear fission metal, that is, to increase the thickness of the nuclear fission material, and the increase in thickness in different directions finally forms a perfect sphere. This is also the core shape of ordinary nuclear bombs.
The neutron utilization rate produced by fission cannot continuously hit other nuclei in statistical calculations and runs outside is called a subcritical state. When both the mass and radius have increased, it is statistically believed that neutrons can trigger a reaction that lasts until fission is called a critical state.
Without setting up an additional neutron source, as well as a neutron reflective layer on the outside of the core, the critical mass of uranium-235 at 100% purity needs to reach 52 kg, and the diameter of the perfect core sphere also needs to reach 17 cm.
The critical mass of 100% plutonium-239 requires only 10 kilograms and a diameter of only 9.9 centimeters.
Of course, this value is a theoretical calculation value. There is simply no 100% purity of plutonium and uranium. The impurity atoms that fill the middle of the metal uranium and the metal plutonium do not fission when hit by neutrons.
But this value is misleading!!!
May I ask you – what is the weight of a uranium sphere with a specific gravity of 19.1 grams and a diameter of 17 centimeters?
Sphere volume = ⁴⁄₃πr³, the volume of the sphere with a diameter of 17 cm is 2572.44078485 cubic centimeters. The weight is 49133.6189906 grams, huh?! And the critical mass is actually 3 kg difference!!!
In other words, even if a 17 cm diameter uranium ball is made, it cannot explode.
You need to make the uranium ball larger, and you have to meet both the critical mass and the critical diameter to trigger the chain reaction.
Of course, this is in a state without neutron sources and reflective layers.
What about the indicator of plutonium bombs? In fact, the critical diameter has reached the critical mass, the density of plutonium is 19.81 g / cubic centimeter, I do not believe that you can calculate it yourself.
For uranium bombs, not only is the critical mass and critical diameter a crater, but there is also a more interesting problem, which is the impurity problem we just mentioned.
20% concentration of uranium-235, it takes 400 kilograms to reach the critical mass, 15% of uranium-235 already needs 600 kilograms to reach the critical mass. What can you think of when you read this?
Uranyl nitrate
Critical accident in tokai village, Japan. At that time, JBC employees in Japan made a mistake and poured seven barrels of uranyl nitrate solution with an enrichment of 18.8% into the bucket. Many articles write about the critical accident in Tokai Village. In fact, this is only a subcritical accident, the real uranium-245 content is only 46 kilograms, and has not reached the critical value. The blue light seen by witnesses at that time was not "Cherenkov radiation" but blue light emitted by nitrogen and oxygen in the air after being ionized. It's very similar to Cherenkov radiation but it's not a thing.
Why did the nuclear accident in Donghae Village, where only 46 kilograms of uranium caused huge casualties and forced the evacuation of people 10 kilometers around the site of the incident? The reason is that at that time, outside the steel iron barrel dissolved in uranyl nitrate, the workers put on another iron barrel to cool down and injected a large amount of water into the iron barrel. This forms a neutron reflection layer. This resulted in a critical mass reduction throughout the installation, bringing the sediment in the bucket to a subcritical level.
If this is not the case, the uranium metal can be held in the hand.
Speaking of uranium implosion, if our first atomic bomb was a uranium implosion type atomic bomb, then it can be considered that our country is the only country that has mastered the technology of uranium implosion type atomic bomb so far - the reason is that it is too difficult. Apart from what we claim to have done, no other country has yet come up with a nuclear weapon designed for uranium implosion.
Uranium implosion nuclear weapons are difficult to engage in, and the main reason is somewhat similar to the accident in Tokaimura, when uranium is far from reaching critical mass, if external conditions permit, subcritical radiation will be generated. Before the atomic bomb explodes, it can already constitute damage to those around it.
Therefore, when making uranium atomic bombs, the gun structure is often used. How to do it, here is the exquisite.
The gun-type structure atomic bombs that you see are all schematic diagrams. I can only tell you a basic principle, that is, two pieces of uranium 235 that are less than the critical value merge together to cause a nuclear explosion.
As we mentioned earlier, even if the critical diameter reaches the critical mass of uranium, it is still three kilograms. This leads to the need to increase the weight of the nuclear charge on the one hand, and the neutron reflective layer for the nuclear charge on the other hand. The real atomic bomb is a matryoshka structure.
The design of the little boy's atomic bomb is what it looks like above, and of course it is also a schematic diagram. At the front end of the atomic bomb, a small piece of uranium is placed inside a layer of dark gray neutron reflections. But because there is still a large enough gap between the small uranium block and the neutron reflective layer. So this small piece of uranium is not in a subcritical state.
Another large chunk of uranium was placed at the tail of the atomic bomb, note. Two sliders between the tail end of the explosive and the uranium block, one deep and one shallow, are also neutron reflective layers. Although the tail uranium block is large, since the neutron reflective layer is only in the heresy of the uranium block, this large piece of uranium-235 is not in a subcritical state.
When the little boy is activated, the explosive explosion pushes the entire structure and the small uranium block at the front of the atomic bomb into a whole. At this point you will see that the blue neutron reflector layer completely envelops the uranium core. The uranium core reaches a critical value in an instant, triggering a chain reaction.
Let's think about the structure of the implosion atomic bomb:
The neutron reflective layer envelops the entire core of the nuclear bomb. There is no open structure with a gun structure, nor can there be a gap large enough. The entire detonation process is the process of using the energy of the explosives to reforge the inside. In this state. High-purity uranium-235 is activated by the neutron emission layer to reach a subcritical state.
That's why implosion uranium bombs are so difficult to build. It's theoretically hard to achieve on its own.
In order to reduce this subcritical risk, a design with a separate reflective layer and core can be used to maximize the distance between the reflective layer and the core surface. But this also raises engineering problems. After all, the most important way we detonate implosion nuclear weapons is through explosive lenses.
The explosive lens also requires the structure of the core to be as tight as possible without gaps. Only in this way can the blast wave compress the small core as evenly as possible. Otherwise, any slight deviation (necessarily present) of the detonation wave will push the core out of the correct position and cause the detonation to fail.
Seeing this, can you understand why W Jun also thinks that we exploded a plutonium bomb that year?
Or do you have any other ideas? You can also talk about it