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scram switch
(foldoc) | scram switch
(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)
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scram switch
(jargon) | 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.)
“Scram” was in origin a backronym for “Safety Cut Rope Axe Man” coined by
Enrico Fermi himself. 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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