Nuclear fission

In physics, fission is a nuclear process, meaning it occurs in the nucleus of an atom. Fission is when the nucleus splits into two or more smaller nuclei plus some by-products. These by-products include free neutrons and photons (usually gamma rays). Fission releases substantial amounts of energy (the strong nuclear force binding energy).

Critical mass

When fission events occur in a mass of uranium (or other fissile material), neutrons are released. Some of these neutrons are captured by other uranium nuclei and lead to fission; some will escape the mass or be absorbed by some other kind of nucleus. If the expected number of neutrons which trigger new fissions is less than one, a nuclear chain reaction may occur but the size will decrease exponentially. If the expected number of neutrons is greater than one, the chain reaction will increase exponentially. This situation (expected number of neutrons causing fission is one or more) is called criticality, and the configuration is called a critical mass (although strictly speaking the shape is as important a factor as the mass; see below).

Related Topics:
Expected number - Increase exponentially - Critical mass

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While any critical mass will in principle lead to exponential growth, the time this will take depends on several factors. The degree to which the mass is supercritical affects the rate of growth. However, as mentioned above, a fraction of the neutrons that cause fission do so only after a brief delay. This delay slows the process of exponential growth and permits the control of nuclear chain reactions. If there are enough neutrons captured so that the ones causing immediate fission are sufficient to lead to exponential growth, then the mass is called prompt critical and it becomes very difficult to control.

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A simple nuclear weapon relies on this exponential growth to induce fission in a significant fraction of the fissile nuclei it contains. Such a device must not only be prompt critical, it must be highly prompt critical. Moreover, it must be rapidly converted from a subcritical configuration (for storage) to a highly prompt critical configuration upon detonation. This is a difficult procedure; see nuclear weapon design for an overview.

Related Topics:
Nuclear weapon - Nuclear weapon design

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The relative number of neutrons which escape from a quantity of uranium can be minimized by changing the size and shape. In a sphere any surface effect is proportional to the square of the radius, and any volume effect is proportional to the cube of the radius. Now the escape of neutrons from a quantity of uranium is a surface effect depending on the area of the surface, but fission capture occurs throughout the material and is therefore a volume effect. Consequently the greater the amount of uranium, the less probable it is that neutron escape will predominate over fission capture and prevent a chain reaction. Loss of neutrons by non-fission capture is a volume effect like neutron production by fission capture, so that increase in size makes no change in its relative importance..

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