A little more than two months have passed since the end of the worst war in the history of mankind. And on July 16, 1945, the first nuclear bomb was tested by the US military, and a month later, thousands of residents of Japanese cities die in atomic hell. Since then, nuclear weapons, as well as the means of delivering them to targets, have been continuously improved for more than half a century.
The military wanted to have at their disposal both super-powerful ammunition, sweeping entire cities and countries off the map with one blow, and ultra-small ones that fit in a briefcase. Such a device would bring the sabotage war to an unprecedented level. Both with the first and with the second there were insurmountable difficulties. The reason for this is the so-called critical mass. However, first things first.
Such an explosive core
To understand how nuclear devices work and understand what is called critical mass, let's go back to the desk for a while. From the school physics course, we remember a simple rule: charges of the same name repel each other. In the same place, in high school, students are told about the structure of the atomic nucleus, consisting of neutrons, neutral particles andpositively charged protons. But how is this possible? Positively charged particles are so close to each other, the repulsive forces must be colossal.
Science is not fully aware of the nature of intranuclear forces that hold protons together, although the properties of these forces have been studied quite well. Forces act only at very close range. But as soon as the protons are even slightly separated in space, the repulsive forces begin to prevail, and the nucleus shatters into pieces. And the power of such expansion is truly colossal. It is known that the strength of an adult male would not be enough to hold the protons of just one single nucleus of the lead atom.
What was Rutherford afraid of
The cores of most elements of the periodic table are stable. However, as the atomic number increases, this stability decreases. It's about the size of the cores. Imagine the nucleus of a uranium atom, consisting of 238 nuclides, of which 92 are protons. Yes, protons are in close contact with each other, and intranuclear forces securely cement the entire structure. But the repulsive force of protons located at opposite ends of the nucleus becomes noticeable.
What was Rutherford doing? He bombarded atoms with neutrons (an electron will not pass through the electron shell of an atom, and a positively charged proton will not be able to approach the nucleus due to repulsive forces). A neutron entering the nucleus of an atom causes its fission. Two separate halves and two or three free neutrons flew apart.
This decay, due to the enormous speed of the flying particles, was accompanied by the release of enormous energy. There was a rumor that Rutherford even wanted to hide his discovery, afraid of its possible consequences for humanity, but this is most likely nothing more than a fairy tale.
So what does the mass have to do with it and why is it critical
So what? How can one irradiate enough radioactive metal with a stream of protons to produce a powerful explosion? And what is critical mass? It's all about those few free electrons that fly out of the "bombed" atomic nucleus, they, in turn, colliding with other nuclei, will cause their fission. A so-called nuclear chain reaction will begin. However, launching it will be extremely difficult.
Check the scale. If we take an apple on our table as the nucleus of an atom, then in order to imagine the nucleus of a neighboring atom, the same apple will have to be carried and put on the table not even in the next room, but … in the next house. The neutron will be the size of a cherry seed.
In order for the emitted neutrons not to fly away in vain outside the uranium ingot, and more than 50% of them would find a target in the form of atomic nuclei, this ingot must have the appropriate size. This is what is called the critical mass of uranium - the mass at which more than half of the emitted neutrons collide with other nuclei.
In fact, it happens in an instant. The number of split nuclei grows like an avalanche, their fragments rush in all directions with speeds comparable tothe speed of light, ripping open air, water, any other medium. From their collisions with environmental molecules, the area of the explosion instantly heats up to millions of degrees, radiating heat that incinerates everything in an area of several kilometers.
Suddenly heated air instantly expands in size, creating a powerful shock wave that blows buildings off the foundations, overturns and destroys everything in its path … this is the picture of an atomic explosion.
How it looks like in practice
The device of the atomic bomb is surprisingly simple. There are two ingots of uranium (or other radioactive metal), each of which is slightly less than the critical mass. One of the ingots is made in the form of a cone, the other is a ball with a cone-shaped hole. As you might guess, when the two halves are combined, a ball is obtained, in which the critical mass is reached. This is a standard simple nuclear bomb. The two halves are connected using the usual TNT charge (the cone is shot into the ball).
But do not think that anyone can assemble such a device "on the knee". The trick is that uranium, in order for a bomb to explode, must be very pure, the presence of impurities is practically zero.
Why there is no atomic bomb the size of a pack of cigarettes
All for the same reason. The critical mass of the most common isotope of uranium 235 is about 45 kg. An explosion of this amount of nuclear fuel is already a disaster. And to make an explosive device with lessamount of substance is impossible - it just won't work.
For the same reason, it was not possible to create super-powerful atomic charges from uranium or other radioactive metals. In order for the bomb to be very powerful, it was made from a dozen ingots, which, when detonating charges were detonated, rushed to the center, connecting like orange slices.
But what actually happened? If, for some reason, two elements met a thousandth of a second earlier than the others, the critical mass was reached faster than the rest would “arrive in time”, the explosion did not occur at the power that the designers expected. The problem of super-powerful nuclear weapons was solved only with the advent of thermonuclear weapons. But that's a slightly different story.
How does a peaceful atom work
A nuclear power plant is essentially the same nuclear bomb. Only this "bomb" has fuel elements (fuel elements) made of uranium located at some distance from each other, which does not prevent them from exchanging neutron "strike".
Fuel elements are made in the form of rods, between which there are control rods made of a material that absorbs neutrons well. The principle of operation is simple:
- regulating (absorbing) rods are inserted into the space between the uranium rods - the reaction slows down or stops altogether;
- control rods are removed from the zone - radioactive elements actively exchange neutrons, the nuclear reaction proceeds more intensively.
Indeed, it turns out the same atomic bomb,in which the critical mass is reached so smoothly and is regulated so clearly that it does not lead to an explosion, but only to heating the coolant.
Although, unfortunately, as practice shows, not always the human genius is able to curb this huge and destructive energy - the energy of the decay of the atomic nucleus.
