LOSS OF POWER TO CONTROL ROD DRIVES
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.