Nuclear chain reaction Totally Explained

Everything about Nuclear Chain Reaction totally explained

A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (for example ²³⁵U) or the fusion of light isotopes (for example ²H and ³H). The nuclear chain reaction is unique since it releases several million times more energy per reaction than any chemical reaction.

History

The concept of a nuclear chain reaction was first realized by Leó Szilárd in 1933. He filed a patent for his idea of a simple nuclear reactor the following year.
   In 1936, Szilárd attempted to create a chain reaction using beryllium and indium, but was unsuccessful. In 1939, Szilárd and Enrico Fermi discovered neutron multiplication in uranium, proving that a chain reaction was indeed possible. This discovery prompted the letter from Albert Einstein to President Franklin D. Roosevelt warning of the possibility that Nazi Germany might be attempting to build an atomic bomb.
   Enrico Fermi created the first artificial self-sustaining nuclear chain reaction, called Chicago Pile-1 (CP-1), in a racquets court below the bleachers of Stagg Field at the University of Chicago on December 2, 1942. Fermi's experiments at the University of Chicago were part of Arthur H. Compton's Metallurgical Laboratory facility, which was part of the Manhattan Project.
   In 1956, Paul Kuroda of the University of Arkansas postulated that a natural fission reactor may have once existed. Since nuclear chain reactions only require natural materials (such as water and uranium), it's possible to have these chain reactions occur where there's the right combination of materials within the Earth's crust. Kuroda's prediction was verified with the discovery of natural self-sustaining nuclear chain reactions at Oklo in Gabon, Africa in September 1972.

Fission chain reaction

Fission chain reactions occur because of interactions between neutrons and fissile isotopes (such as ²³⁵U). The chain reaction requires both the release of neutrons from fissile isotopes undergoing nuclear fission and the subsequent absorption of some of these neutrons in fissile isotopes. When an atom undergoes nuclear fission, a few neutrons (the exact number depends on several factors) are ejected from the reaction. These free neutrons will then interact with the surrounding medium, and if more fissile fuel is present, some may be absorbed and cause more fissions. Thus, the cycle repeats to give a reaction that's self-sustaining. Nuclear power plants operate by precisely controlling the rate at which nuclear reactions occur, and that control is maintained through the use of several redundant layers of safety measures. Moreover, the materials in a nuclear reactor core and the uranium enrichment level make a nuclear explosion impossible, even if all safety measures failed. On the other hand, nuclear weapons are specifically engineered to produce a reaction that's so fast and intense it can't be controlled after it has started. When properly designed, this uncontrolled reaction can lead to an explosive energy release.

Nuclear fission fuel

Nuclear fission weapons must use an extremely high quality, highly-enriched fuel exceeding the critical size and geometry (critical mass) in order to obtain an explosive chain reaction. The fuel for a nuclear fission reactor is very different, usually consisting of a low-enriched oxide material (for example UO₂). It is impossible for a nuclear power plant to undergo an explosive nuclear chain reaction. Chernobyl was a steam explosion, not a nuclear explosion. Furthermore, all power plants licensed in the United States require a negative void coefficient of reactivity, which completely eliminates the possibility of the accident that occurred at Chernobyl (which was due to a positive void coefficient).

Fission Reaction Products

When a heavy atom undergoes nuclear fission it breaks into two or more fission fragments. Also, several free neutrons, gamma rays, and neutrinos are emitted, and a large amount of energy is released. The sum of the masses of the fission fragments and ejected neutrons is actually less than the mass of original atom and incident neutron. The mass difference is accounted for in the release of energy according to the equation E=mc²:
ť

frac

, where t is the elapsed time. Nuclear weapons are designed to operate under this state. There are two subdivisions of supercriticality: prompt and delayed. In an infinite medium, the multiplication factor is given by the four factor formula.

Prompt and delayed supercriticality

Not all neutrons are emitted as a direct product of fission, some are instead due to the radioactive decay of some of the fission fragments. The neutrons that occur directly from fission are called "prompt neutrons," and the ones that are a result of radioactive decay of fission fragments are called "delayed neutrons." The fraction of neutrons that are delayed is called β, and this fraction is typically less than 1% of all the neutrons in the chain reaction. The delayed neutrons allow a nuclear reactor to respond several 100 times more slowly than just prompt neutrons would alone. Without delayed neutrons, changes in reaction rates in nuclear reactors would occur at speeds that are too fast for humans to control.
The region of supercriticality between k = 1 and k = 1/(1-β) is known as delayed supercriticality. It is in this region that all nuclear power reactors operate. The region of supercriticality for k > 1/(1-β) is known as prompt supercriticality, which is the region in which nuclear weapons operate.
The change in k needed to go from critical to prompt critical is defined as a dollar.

Neutron multiplication in nuclear weapons

Nuclear fission weapons require a mass of fissile fuel that's prompt supercritical.
   For a given mass of fissile material the value of k can be increased by increasing the density. Since the probability per distance traveled for a neutron to collide with a nucleus is proportional to the material density, increasing the density of a fissile material can increase k. This concept is utilized in the implosion method for nuclear weapons. In these devices, the nuclear chain reaction begins after increasing the density of the fissile material with a conventional explosive.
   In the gun-type fission weapon two subcritical pieces of fuel are rapidly brought together. The value of k for a combination of two masses is always greater than that of its components. The magnitude of the difference depends on distance, as well as the physical orientation.
   The value of k can also be increased by using a neutron reflector surrounding the fissile material
   Once the mass of fuel is prompt supercritical, the power increases exponentially. However, the exponential power increase can't continue for long since k decreases when the amount of fission material that's left decreases (for example it's consumed by fissions). Also, the geometry and density are expected to change during detonation since the remaining fission material is torn apart from the explosion.

Predetonation

Detonation of a nuclear weapon involves bringing fissile material into its optimal supercritical state very rapidly. During part of this process, the assembly is supercritical, but not yet in an optimal state for a chain reaction. Free neutrons, in particular from spontaneous fissions, can cause the device to undergo a preliminary chain reaction that destroys the fissile material before it's ready to produce a large explosion, which is known as predetonation. To keep the probability of predetonation low, the duration of the non-optimal assembly period is minimized and fissile and other materials are used which have low spontaneous fission rates. In fact, the combination of materials has to be such that it's unlikely that there's even a single spontaneous fission during the period of supercritical assembly. In particular, the gun method can't be used with plutonium (see nuclear weapon design).

Fusion chain reaction

In a more generalized sense, a nuclear fusion reaction can be considered a nuclear chain reaction: it occurs under extreme pressure and temperature conditions, which are maintained by the energy released in the fusion process.

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