Two aspects of position indication are essential for the operation of control rods in a reactor the measurement of control-rod position with respect to the core and the determination of rod position with respect to the drive mechanism The first is far more important. The second is relevant to systems where the rod can be separated from

FLEXIBLE ROD BUNDLES

ROD Fig. 7.8—Magnetic-jack configuration.

the drive, as is often the case in drive systems that satisfy both the shim or control and safety requirements.

Pickup or release of a control rod and full insertion or full withdrawal are usually indicated by panel lights controlled by relays or sensing switches actuated by magnetic or mechanical coupling to the control rod. These switches or relays generally have auxiliary contacts used in the control logic circuits. An example is the logic that initiates movement of a bank of shimming control rods when the automatic regulating rod has exceeded its normal control range. Another is a permissive logic circuit that often takes the form of withholding control power at start-up until all rods are completely down or fully inserted in the reactor and all drives are down and engaged to their respective rods.

Whenever more than one group of shim rods is used on reactor start-up, limit-sensing switches can be used to permit or initiate movement of a second bank of rods when the first has reached a programmed “full-out” position.

Once the condition of major rod position and attach­ment to drives has been satisfied, knowledge of the position of rods with respect to the core is essential to reliable control of a reactor. Both absolute position and relative movement must be measured with an accuracy sufficient to establish safe conditions during start-up and power adjust­ment and to establish predetermined power distribution patterns in the core. The position of rods relative to each other can also be used to diagnose reactivity anomalies within the core. A study of the history of the reactor operating characteristics makes it possible to predict rod position under various operating conditions. If an indica­tion differs markedly from its expected value, it is usually a sign of anomalous behavior in the core, control system, instrumentation, or rod-drive system. Analysis can often pinpoint the malfunctioning item.

Position indication may be direct, when the control rod actuates the sensing device, or indirect, when the position sensor is coupled to the drive mechanism. The control system is arranged so that after rod release the drives are automatically returned to zero position.

In all reactor operation, indication of rod position is displayed at the control console. Panel-mounted dial indicators driven by synchro-receiver units with the synchro-transmitters directly geared or coupled to the driven mechanisms, synchro-driven bar indicators, tapes, or digital counters are conventional. The bar or tape indicators give an immediate and graphic nonambiguous indication of absolute rod position, thereby minimizing possible operator error.

In conjunction with the previously mentioned “all-in” and “all-out” position indication and indication of relative position, position switch sensing of intermediate position is valuable for interlocking with rod-programming circuits and for giving the operator an independent indication of where the rods are under all circumstances. Intermediate position measurements can be used to signal definite rod positions on a bar or tape indicator, thereby providing a backup measurement that also prevents operator misinterpretation of absolute rod position.

Rod-position sensing in sealed systems, in which the drive as well as the rod is in a pressurized high-tem­perature coolant environment, is usually accomplished magnetically. Here extreme in/out positions and also intermediate points may be indicated by having magnetic coupling to coils located at the various points along the pressure-seal housing around a control-rod extension. Ac­curate and continuous sensing of rod position along the entire length of travel, however, is more difficult.

An example of rod-position sensing and indication in a sealed system is the magnetic-jack control rod in the San Onofre Atomic Power Plant (see Sec. 7-4.2). Lights on the control panel indicate rod position. Thirty transformers are mounted around the control-rod-extension pressure hous­ing. Each of the 30 secondary windings is connected to its own individual light on the control panel. As the magnetic portion of the control-rod extension passes each trans­former, the coupling between primary and secondary windings is increased to the point where each lamp in the row is successively lighted and stays lit as rod movement progresses. Although these lights give only approximate position, there is no ambiguity. A secondary scheme for securing indirect position indication uses synchro-repeater indication of the rotation of the cam shaft that actuates the jacking mechanism from the power supply.

Another magnetic readout device for a sealed system uses a winding distributed around the thimble (pressure housing) around the rod extension. The winding is part of an induction bridge with readout on a panel meter. As the rod is withdrawn, the magnetic portion enters the solenoid and changes the inductance of the winding as it progresses. Through suitable design of the coil, its spacing and number of turns, the indication can be made approximately linear with rod displacement.

CONTROL- ROD DRIVE MECHANISM

CONTROL- ROD SHROUD ASSEMBLY-

CONTROL- ROD SHAFT ASSEMBLY-

OUTLET NOZZLE-

CONTROL-ROD ASSEMBLY-

BLANKET ASSEMBLY-

FMI PIPING

PRESSURE — VESSEL HEAD

-FMI CONDUIT

-HOLD-DOWN BARREL

-CORE-SUPPORT SPRING

-CORE CAGE

-SEED CLUSTER

INLET NOZZLE

Fig. 7.9—Shippingport Pressurized Water Reactor, center­lines section. (From The Shippingport Pressurized Water Reactor, p. 62, Addison—Wesley Publishing Company, Inc., Reading, Mass,, 1958.)