- published: 22 Jul 2014
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A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (e.g. the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature and its surroundings. The concept is important in nuclear weapon design.
When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a critical state in which there is no increase or decrease in power, temperature or neutron population.
A numerical measure of a critical mass is dependent on the effective neutron multiplication factor k, the average number of neutrons released per fission event that go on to cause another fission event rather than being absorbed or leaving the material. When Failed to parse (Missing texvc executable; please see math/README to configure.): k = 1 , the mass is critical, and the chain reaction is barely self-sustaining.
A subcritical mass is a mass of fissile material that does not have the ability to sustain a fission chain reaction. A population of neutrons introduced to a subcritical assembly will exponentially decrease. In this case, Failed to parse (Missing texvc executable; please see math/README to configure.): k < 1 . A steady rate of spontaneous fissions causes a proportionally steady level of neutron activity. The constant of proportionality increases as k increases.
Europe's plains
Blood and fire spill again.
Warring forces set their cause.
The race for fission had begun.
Powers of destruction sought,
Hidden by the hand of God.
Took six years to conceive,
Construct, deploy, and now destroy.
Burning sky. Say goodbye to the rising sun.
In a mushroom cloud. Heaven knows what we've begun.
Firestorm, earth consumed with a fervent heat.
Power loosed, now we cannot kill this beast.
Pacific isle,
A bird flies out before the dawn.
Oppenheimer's legacy is carried by Enola Gay.
Many sons owe their thanks
To the heroes of that day.
But victory was bittersweet,
As many blameless were swept away.
Burning sky. Say goodbye to the rising sun.
In a mushroom cloud. Heaven knows what we've begun.
Firestorm, earth consumed with a fervent heat.
Power loosed, now we cannot kill this beast.