What is Critical Mass?

Critical mass refers to the mass of fissile material needed to maintain a nuclear chain reaction. Different fissile materials are affected by the nature of the nucleus (such as the fission cross section), physical properties, material shape, purity, whether it is surrounded by neutron reflecting materials, whether there are neutron absorbing materials, etc. Critical mass. Factors affecting critical mass include nuclide properties, purity (commonly referred to as enrichment).

In a mass of uranium 235, assuming the shape of a cube, then the critical volume of uranium 235 can be obtained by mathematical and physical equations. We assume that u is the density of neutrons. Then, within a certain time, the density of neutrons varies with The time is again diffused outward to satisfy the diffusion equation. At the same time, the density u of the neutron is continuously increasing due to the chain reaction. The rate of increase is proportional to the concentration of the neutron, that is, the more neutrons, The faster the neutron density increasesthat is, the exponential growth that is usually called. Therefore, under the action of neutron diffusion and proliferation, there will be a critical volume. When the volume is greater than the critical volume, the neutron concentration will diverge exponentially with time, and the atomic bomb will explode.

Take CPR1000, a typical domestic large commercial

The hydrogen bomb itself is an inaccurate name and should actually be a fusion-enhanced fission bomb, but for convenience, it is still called a hydrogen bomb. Traditional atomic bombs simply use radioactive material> critical mass, or radioactive material close to critical mass is compressed under high-energy explosives, thereby triggering a chain reaction. However, due to the instant of the explosion, the chain reaction was far from complete, the explosive material was disintegrated, the fragmented uranium or plutonium was again below the critical mass, and the gap became larger, neutrons escaped, and only poor utilization efficiency ( Take fat and little boys as an example, their efficiency is 17% and 1.4% respectively. The principle of the hydrogen bomb is that hydrogen (deuterium and tritium) with atomic weights of 2 and 3 is placed in the center of the fissile material. When the fissile material undergoes a chain reaction, deuterium and tritium undergo a fusion reaction at high temperature and pressure, and He3 or He4 is released A large number of high-energy neutron flows (in fact, the total energy generated by fusion is not high, mainly neutrons). The neutrons generated will intensify the chain reaction and greatly improve the efficiency of the chain reaction. Based on this principle, an additional uranium or plutonium core can be wrapped in the fusion material to further improve the chain reaction efficiency of the core part. There are currently no pure fusion bombs. The radiation of nuclear weapons is divided into two parts, one is the radiation during the explosion, and the other is the radioactive contaminated radiation after the explosion. Compared with hydrogen bombs of atomic bombs made of the same mass of plutonium, the explosion of hydrogen bombs is more complete and the chain reaction is more complete, so there must be more radiation. The fission of uranium 235 and plutonium 239 can produce a variety of materials with different half-lives. Such a substance produced by the hydrogen bomb explosion and its contamination range are also larger, so its harmful radiation is still greater than that of the atomic bomb.

The essence of the critical mass is the mass of the nuclear material needed to allow a nuclear chain reaction to occur, and this depends on the average number of new neutrons that can be produced by impacting the nucleus before escaping, this number is greater than one In general, nuclear reactions can occur.

Therefore, as long as the probability that the neutron effectively collides with the nucleus is increased, or the effective distance of the neutron is extended, the amount of nuclear material required can be reduced. Factors affecting the former include purity, density, and nucleus properties. Factors affecting the latter include shape, whether there are neutron reflection or absorption devices. So in theory, as long as there is sufficient technology, the critical mass can be made smaller and smaller (of course, if the required equipment is too heavy, it will be worth the loss) and the nuclear bomb will be miniaturized. Moreover, the weight of other equipment can also reduce the size of nuclear bombs. ^[1]