scram switch

Slang Dictionary

scram switch

n. [from the nuclear power industry] An emergency-power-off switch (see Big Red Switch), esp. one positioned to be easily hit by evacuating personnel. In general, this is _not_ something you frob lightly; these often initiate expensive events (such as Halon dumps) and are installed in a dinosaur pen for use in case of electrical fire or in case some luckless field servoid should put 120 volts across himself while Easter egging. (See also molly-guard, TMRC.)

A correspondent reports a legend that "Scram" is an acronym for "Start Cutting Right Away, Man" (another less plausible variant of this legend refers to "Safety Control Rod Axe Man"; these are almost certainly both backronyms). The story goes that in the earliest nuclear power experiments the engineers recognized the possibility that the reactor wouldn't behave exactly as predicted by their mathematical models. Accordingly, they made sure that they had mechanisms in place that would rapidly drop the control rods back into the reactor. One mechanism took the form of `scram technicians'. These individuals stood next to the ropes or cables that raised and lowered the control rods. Equipped with axes or cable-cutters, these technicians stood ready for the (literal) `scram' command. If necessary, they would cut the cables, and gravity would expeditiously return the control rods to the reactor, thereby averting yet another kind of core dump.

Modern reactor control rods are held in place with claw-like devices, held closed by current. SCRAM switches are circuit breakers that immediately open the circuit to the rod arms, resulting in the rapid insertion and subsequent bottoming of the control rods.
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Computing Dictionary

scram switch definition

jargon
(From the nuclear power industry) An emergency power-off switch (see Big Red Switch), especially one positioned to be easily hit by evacuating personnel. In general, this is *not* something you frob lightly; these often initiate expensive events (such as Halon dumps) and are installed in a dinosaur pen for use in case of electrical fire or in case some luckless field servoid should put 120 volts across himself while Easter egging.
SCRAM stands for Safety Control Rod Ax Man. In the early days of nuclear power, boron moderator rods were raised and lowered on ropes. In the event of a runaway chain reaction, a man with an axe would chop the rope and drop the rods into the nuclear pile to stop the reaction.
See also molly-guard, TMRC.
[Jargon File]
(2003-05-17)

The Free On-line Dictionary of Computing, © Denis Howe 2010 http://foldoc.org
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