 The Control Rod Drive Mechanism is an electric
stepping motor actuated linear drive equipped with a magnetic
coupler and a positive feedback potentiometer. A five phase
stepping motor drives a pinion gear and a 10-turn potentiometer
via a chain and pulley gear mechanism. The pinion gear engages
a rack attached to the draw tube, housing an electromagnet
attached to the lower end. The electromagnet in turn engages
an iron armature which is attached to a connecting rod assembly
which terminates at its lower end in the control rod itself.
The electromagnet, armature and the upper portion of the
connecting rod are all housed in a tubular barrel that extends
below the water line in the reactor pool.  The
upper portion of this barrel is ventilated to permit unrestricted
rod movement in water, whereas the lower portion has graded
vent holes to restrict movement and to provide a damping
action when the electromagnet is deenergized and the control
rod is released from the drive.
A series of microswitches are provided on the
drive assembly to control the up-down movement of the control
rod and rod drive. Rod up/down motion is controlled from
the control console, by energizing the magnet and stepping
motor, which rotates the pinion gear shaft in turn raising
the magnet draw tube. If the electromagnet is energized,
the armature and the connecting rod will rise with the draw
tube so that the control rod is withdrawn from the reactor
core. In the event of a reactor scram, the magnet is deenergized
and the armature will be released. The control rod will then
drop, reinserting the neutron poison into the reactor core.
The control rod drive speed is adjustable over
a wide range, and its design can accommodate control rods
in lengths of 15, 24, 30 or 36 inches.
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