The phrase or term "shut down", "shut-down" or "shutdown" can be used to mean "turning off" something, but most commonly used for machines, especially nuclear reactors and computers. It also has a use in economics, for profit maximization. The verb forms, such as "to shut down", are always separate words, as otherwise would lead to absurd forms as "shutdowning". Note that the phrase "shut up" has quite different meanings. "SD" is a slang term for "shut down".

Nuclear reactors

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In a nuclear reactor, shutdown refers to the state of the reactor when it is subcritical by at least a margin defined in the reactor's technical specifications. Further requirements for being shut down may include having the reactor control key be secured and having no fuel movements or control systems maintenance in progress.

The shutdown margin is defined in terms of reactivity, frequently in units of delta-k/k (where k is taken to mean k-effective, the effective multiplication factor) or occasionally in dollars. Shutdown margin refers to the margin by which the reactor is subcritical when all control rods are in OR the margin by which the reactor would be shut down in the event of a scram. Hence, care must be taken to define shutdown margin in the most conservative way in the reactor's technical specifications; a typical research reactor will specify the margin when in the cold condition, without xenon. Under this specification, the shutdown margin can be simply calculated as the sum of the control rod worths minus the core excess.

Minimum shutdown margin can be calculated in the same way as shutdown margin, except that the negative reactivity of the most reactive control rod is ignored. This definition allows the reactor to be designed so that it remains safely shut down even if that most reactive control way becomes stuck.

See also: scram

Computers

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In Unix, the shutdown command can be used to shut down (turn off) or reboot a computer.

One commonly issued form of this command is shutdown -h now, which will shut down a system immediately.

Another one is shutdown -r now to reboot.

Unlike in many other operating systems, you may also specify an exact time or a delay: shutdown -h 20:00 will turn the computer off at 8:00 PM, and shutdown -r -t 60 will automatically reboot the machine in 60 seconds (one minute) of issuing the command.

The complete syntax from the Linux version of the command is as follows. Usage: shutdown [-t secs time message -a: use /etc/shutdown.allow -k: don't really shutdown, only warn. -r: reboot after shutdown. -h: halt after shutdown. -f: do a 'fast' reboot (skip fsck). -F: Force fsck on reboot. -n: do not go through "init" but go down real fast. -c: cancel a running shutdown. -t secs: delay between warning and kill signal. ** the "time" argument is mandatory! (try "now") **

Please note that you must be the superuser (usually root) to shut the system down. This stops unpriveliged users (eg, schoolchildren) from causing potentially catastrophic damage - by shutting down a server without permission, for example.

See also: halt, reboot – equivalent to shutdown -h now and shutdown -r now respectively.

Economics

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If price is below average variable cost at the profit-maximizing output, the firm is said to be in shutdown. Losses are minimized by not producing at all, since any production would not generate returns significant enough to offset any fixed cost and part of the variable cost. By not producing, the firm loses only its fixed cost (see profit maximization for more information).

Government

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A government shutdown halts all or most services of a government, usually only temporarily. This can occur because of war or civil disruption, although some governments will close non-essential services if budgets cannot be enacted by the legislature. The United States federal government closed its doors briefly in 1995-1996, and this has periodically occurred in U.S. states: most recently, it happened in Minnesota in 2005 and New Jersey in 2006.

Slang

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"Shut down" is a phrase used in speech when someone has been verbally bludgeoned and completely defeated. For example: Tommy says "Mrs. Smith, I think you did that math problem wrong. Shouldn't the answer be 12?" Mrs Smith replies, "No Tommy, if you had been listening instead of talking, you would know how to get the right answer, which is 15." (This is where someone in the vicinity of Tommy would chime in "shut down" or "SD".)

Nuclear technology | Operating system technology | Political terms