scram switch

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.
    

scram switch

   <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)