The SCRAM button controls the electromagnetic couplers holding the control rods up. When this button is pressed, the control rods drop and the reactor shuts down. It's for emergencies only.
SCRAM stands for Safety Cut Rope, Axe Man. It comes from the days when Fermi was messing around with his experimental reactors in the early 40's. Back then, there was a guy holding an axe ready to cut the cable holding up the control rods, and his signal to cut the cable would be when somebody yelled "SCRAM!"
On a little side note, It takes about a half an hour for the reactor to start up every morning.
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