[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.
