The San Onofre Atomic Power Plant in San Clemente, Calif., has a pressurized-water reactor similar to that at Shippingport except for the method of rod drive and control. Each of the 45 control rods for this reactor (Fig. 7.13) consists of a 5-in.-square spider-like cluster of 16 absorbing rods, 126-in. long, each fabricated from an alloy of silver, indium, and cadmium and hermetically sealed within a stainless-steel sheath. The cluster of absorber rods fits into guide thimbles in a fuel-rod assembly (Figs. 7.14 and 7.15) and is attached to a driven vertical shaft extending through the top of the reactor to the rod-drive mechanism. Boric acid is added to the coolant water to control reactivity during both operating and shutdown periods to reduce the required number of control rods and to achieve uniform neutron absorption throughout the core.

Whereas each rod drive of the Shippingport reactor uses a canned motor to rotate the nut mechanism attached to a lead-screw extension of the control rod, the San Onofre reactor rod-drive mechanism uses a form of the magnetic jack. There is a similarity between the two in that motion is produced magnetically through the pressure barrier. The motion is rotary in the Shippingport reactor; it is linear with the magnetic jack.

The rod-drive mechanism (Fig. 7.16) consists of a latch assembly and a rod-drive assembly that operate within a thimble, all at reactor pressure. A stack of operating coils surrounds the rod-drive portion of the thimble. A 120-in.- long coil stack over the upper part of the thimble, into which the control-rod extension travels as the control is raised, is used for rod-position indication. The reactor coolant water fills the pressure-containing parts of the mechanism and cools and lubricates the moving parts of the drive.

The drive shaft has circular grooves, spaced in. apart, machined into its surface along its entire length. Mag­netically operated gripper latches lock into the grooves to hold the drive shaft stationary with the drive. The operating coil stack consists of the lift coil, movable gripper coil, and stationary gripper coil. These are energized in a fixed sequence by cam switches actuated by a rotating cam shaft. The coils induce magnetic flux through the pressure housing and operate the latch components. Within the pressure housing, two sets of latches lift or lower the grooved drive shaft. Turning the cam shaft one revolution causes the lifting mechanism to cycle once and quickly moves the rod out in one %-in. step. Reversing the rotation of the cam shaft reverses the direction of rod motion. Latch actuation is produced by pole piece motion for the three magnets.

Since the magnetic jack develops a lifting force of 400 lb and the total drive shaft weight is 144 lb, there is

SNAP ACTION CAM SWITCH

— CONTACT OF ROD TRANSFER DEVICE

TO GROUP 1 PROGRAMMING CIRCUITRY

BRUSHES MOUNTED ON ROTATING PLATE FACE-PLATE COMMUTATOR

Fig. 7.12—Shippingport Pressurized Water Reactor, schematic diagram of rod-control d-c to a-c power inverter. (From The Shippingport Pressurized Water Reactor, p. 282, Addison—Wesley Publishing Company, Inc., Reading, Mass., 1958.)

ample capacity to overcome friction in the system. For rod insertion, gravity furnishes the driving force and overcomes the friction load.

The upper set of latches operates to raise or lower the drive shaft in %-m. steps. After one lift step the lower latches are engaged in a shaft groove and raise the rod У32 in. to unload the upper latches so they may be reset to lift the rod another increment. The latches are actually linked to sliding armature or pole pieces that move when their respective magnet coils are energized or deenergized.

Table 7.3 gives a detailed description of the sequence of steps in control-rod actuation at the San Onofre plant.