Each of the 21 control rods has its own drive mounted vertically on the
upper reactor head. Of these, 9 are servo controlled and 12 are of the
nonservo type. The 9 servo rods have variable speed drives and operate
in two groups in a synchronous manner, according to demand signals from
the reactor system. The 12-rod group can be operated manually or in
groups according to predetermined conditions. All of these operate at a
speed determined by their gearing.
The safety considerations are as follows:
1. Each servo loop contains a monitor that will sound an alarm and
initiate a fast insertion if the rod fails to follow its command signal.
2. Another circuit monitors all nine servo monitors, and should any of
the servo monitors malfunction, an alarm will sound and appropriate
corrective action will be taken through the automatic safety system.
3. “Scram” action starts in the safety system and is independent of
operator control. Once started, a “scram” action cannot be stopped.
4. For conditions that do not warrant “scram” action, a fast insertion
serves to reduce power and permit the operator to correct the condition
without a complete shutdown. A manual fast insertion can be made by the
operator.
The electrical circuits controlling the reactor control rods are
monitored, and an electrical failure in one or more circuits will result
in a fast insertion or “scram” action. Should electrical power to the
control rod drives fail completely, the hydraulic drives will be
actuated.