The Dresden plant has a boiling-water reactor system using a dual steam cycle and forced circulation. Figures 7.17 and 7.18 depict two views of the reactor vessel. Eighty cruciform-shaped control rods are interspersed through the array of fuel assemblies. Figure 7.19 shows how the control rods are arranged in the core.

The control rods were originally made of 2% boron- steel alloy but were replaced by stainless-steel sheet packed

Table 7.3—Detailed Description of San Onofre Control-Rod
Actuation [18]

Control-Rod Withdrawal: Sequence of Operations

1. Movable gripper coil—On.

2. Stationary gripper coil—Off.

3. Lift coil—On. The У — in. gap between the lift armature and the lift magnet pole closes, and the drive rod rises one step length.

4. Stationary gripper coil—On. The stationary gripper armature rises and closes the gap below the stationary gripper magnet pole. The three links, pinned to the stationary gripper armature, swing the stationary gripper latches into a drive-shaft groove. The latches contact the shaft and lift it Уз2 in. In this manner, the load is transferred from the movable to the stationary gripper latches.

5. Movable gripper coil—Off. The movable gripper armature separates from the lift armature under the force of three springs and gravity. Three links, pinned to the movable gripper armature, swing the three movable gripper latches out of the groove.

6. Lift coil—Off. The gap between the lift armature and lift magnet pole opens. The movable gripper latches drop in. to a position adjacent to the next groove.

7. Movable gripper coil—On. The movable gripper armature rises and swings the movable gripper latches into the drive-shaft groove.

8. Stationary gripper coil—Off. The stationary gripper latches, and the armature moves downward by gravity until the load of the drive shaft is transferred to the movable gripper latches. It then swings out of the shaft groove.

The sequence described above, where the control rod moves У in.

for each cycle, is termed “one step” or “one cycle.” The sequence

is repeated at a rate of up to 40 steps per minute. The control rod

is thus withdrawn at a rate of up to 15 in. per minute.

Control-Rod Insertion: Sequence of Operations

1. Stationary gripper coil—On.

2. Movable gripper coil—Off.

3. Lift coil—On. The movable gripper latches are raised to a position adjacent to a shaft groove.

4. Movable gripper coil—On. The movable gripper armature rises and swings the movable gripper latches into a groove.

5. Stationary gripper coil—Off. The stationary gripper armature moves downward and swings the stationary gripper latches out of the groove.

6. Lift coil—Off. Gravity separates the lift armature from the lift magnet pole, and the control rod drops down in.

Fig. 7.17—Longitudinal section of the Dresden reactor showing control-rod drives mounted below the reactor. (From Andrew W. Kramer, Boiling Water Reactors, p. 461, Addison—Wesley Publishing Company, Inc., Reading, Mass., 1959.)

the reactor in the “up” position. Reactivity is increased by lowering the control rod below the core. The control-rod

drive mechanism is basically a hydraulic cylinder. Normal controlled movement of the rod is attained by applying reactor feedwater regulated at 200 psia above reactor pressure to either the top or bottom surface of the piston rod and simultaneously connecting the opposite side to a vent tank held at 30 psia above reactor pressure. This movement, however, cannot be effected until the locking ball located in a slot between the control-rod piston and the cylinder wall is released by the unlocking piston (Fig. 7.20). For rod “up” motion (direction toward decreasing reac­tivity) pressure is applied on the bottom of the piston rod. The control-rod piston, secured by the ball to the spring — loaded locking piston, moves upward a small but sufficient distance against the spring to release the ball into the annulus in the unlocking piston and free the rod. The rod continues to move upward as long as pressure is kept applied to the “up” inlet port. When the inlet pressure is shut off, reactor pressure is applied through the shuttle valve, and upward movement continues until the next notch in the piston reaches the ball. At this point, the spring loading of the locking piston is enough to lock both pistons together and prevent further movement.

For “down” motion, the high pressure is applied to the “down” inlet port and the “up” inlet port is switched to the vent tank. In this condition, the high pressure is applied to the top surfaces of both the rod piston and the unlocking piston. The unlocking piston moves down against a spring load and exposes an annulus that frees the locking ball, allowing the main rod piston to be moved. If the down pressure is maintained, rod down movement continues. If the pressure is applied only momentarily, the ball is freed long enough to allow the piston to move to the next slot where it again engages the locking piston and stops.

There are 12 slots along the 8.5-ft travel of the control rod, and rod movement in 8-in. steps can be obtained. The maximum rate of reactivity insertion from this control rod is 1.3 X 1СГ4 6k/sec. The interlocking circuits allow move­ment of only one rod at a time.

For scramming, a 1400-psi accumulator tank is supplied for every three drives. For each drive two solenoid valves provide pressure to the “up” inlet port and open the “down” inlet port to a dump tank. The rod movement for scram is completed in 2.5 sec.

Rod position is indicated by a series of magnetically operated switches located inside an inner cylinder along the drive stroke. A magnet built into the rod piston actuates the switches as it passes them, and corresponding indicating lights on the control panel show the rod position.

The effect of scram-system failure is minimized by the use of independent systems for every three rods. If a large-scale failure occurs, however, a chemical poison — injection system containing sodium pentaborate at a pres­sure much higher than that of the reactor is actuated.

7- 4.4 Gas-Cooled Reactors

Gas-cooled reactors can be considered as being either low-temperature or high-temperature gas-cooled reactors.

The low-temperature gas-cooled reactors are typified by the natural-uranium graphite-moderated C02 — cooled units in the United Kingdom and similar installations in Italy, France, and Japan. These reactors are very large structures owing to the low excess reactivity available from natural — uranium fuel. For example, 40- to 70-ft diameter and 40-ft height are typical. The number of control rods is corre­spondingly large, 100 or more. The control rods (usually boron steel) are generally mounted and operated vertically. They are suspended by steel cables wound around a drum which is driven by a low-speed motor equipped with induction braking. A typical control-rod drive mechanism is shown in Fig. 7.21.

The control problems in low-temperature gas-cooled reactors arise chiefly from spatial variations in fission — product (especially 135Xe) poisoning. To cope with these problems, for example, the Hunterston reactor in Scotland is divided into one central zone and eight radial zones, with the control rods distributed throughout. The central zone

HEAD LIFTING LUG (4 REQ-AZIMUTH 45*- l35*-225*- 315*)

HEAD STUDS (56 REQ)

CHANNEL SUPPORT BRACE — 14 REQ)

PRESSURE DIFFERENTIAL TUBE HEADER AND OUTLET

Fig. 7.20—Control-rod drive mechanism, Dresden Nuclear Power Plant.

GRID (36 RODS) Fig. 7.19—Dresden reactor fuel-element and control-rod array. Fuel elements are 36-rod assemblies of U02 contained in Zircaloy-2 tubes. Cruciform control rods fit in spaces between the elements. (From Andrew W. Kramer, Boiling Water Reactors, p. 464, Addison—Wesley Publishing Com­pany, Inc., Reading, Mass., 1958.)

has the largest number of rods per unit cross section area The rods in each zone are grouped for control purposes and each rod can be independently operated.

The normal rate of addition of reactivity is very low in the low-temperature gas-cooled reactors. In the Berkeley (England) reactor, for instance, the rate is 2 X l(f6 5k/sec Start-up from a cold shutdown condition to critical takes 6 or 7 hr, to increase from critical to full power takes 11 hr because of the negative temperature coefficient.

High-temperature gas-cooled reactors (HTGR’s) use enriched-uramum fuel and are much smaller in size than the low-temperature reactors. Graphite is used as moderator A variety of coolant gases may be used, including N2, C02, H2, He, and air In the United States, helium is used, elsewhere, carbon dioxide is the favored coolant for gas-cooled power reactors

The Peach Bottom HTGR and the Fort St Vrain HTGR, two U. S nuclear power stations, are described in some detail in Vol. 2, Chap. 18. In Chap. 18, Sec. 18-6, there is a discussion of their control-rod drive and position — indicating systems The brief description of the Peach Bottom control-rod drive mechanism presented here over­laps the Chap. 18 material to some extent. However, the emphasis is different. Specifically Unit 1 of the Peach Bottom Power Station in Pennsylvania is considered here The core, located near the bottom of a 25-ft-high cylindri­cal pressure vessel, is approximately 9 ft in diameter and 7.5 ft high There are 36 control rods, each worth 0.007 6k (average), mounted below the reactor where the drive mechanisms are in a mild environment (^200°F and ~10 R/hr gamma flux) The location provides for easy access to maintain the drives and leaves the top of the pressure vessel free for fuel-handling operations. The control rods are stainless-steel tubes containing boron carbide.

The basic drive mechanism (Fig. 7.22) is an axial piston-type hydraulic motor that turns a ball screw and produces linear motion of a ball-nut assembly The ball-nut moves a push rod, which, in turn, raises or lowers the control rod Each control-rod port (extension of the pressure vessel) contains the entire drive mechanism includ­ing the hydraulic motor, regulating and scram valves, position transmitters, rotary-to-linear motion ball-nut screw device, scram-energy accumulator, and scram-action snubber

Hydraulic turbine oil is used for the motor and is supplied to the drive from two header connections One is a low-pressure supply that produces the normal operating speed of the control rod. The high-pressure line maintains the fluid level in a scram accumulator, pressurized by helium, which drives the rod at the scram speed of 10 ft/sec. A return header is supplied for the effluent from the motor The regulating and scram speeds of the motor are estab­lished by two sets of solenoid valves. One set, the regulating valves, admit low-pressure hydraulic oil for rotating the motor in either direction. The other set, scram valves, apply high-pressure oil to the motor for one direction of rotation only, namely, that for driving the rod upward into the core.

Total rod movement is 7 ft. The regulating or control speed is 0.06 ft/sec or 0.72 in./sec, corresponding to a rate of change of reactivity of 1.1 X 10-4 5k/sec (maximum) Except for rod-removal operations, the drive is never detached from the rod. Monitoring of the coupling between the drive and the rod depends on an electrical circuit between the two, which, if broken, trips an annunciator at the operator’s console. Downward motion of the rod increases core reactivity, and scram motion is upward. Control of rod direction is by manual activation of solenoid valves through remote switches or from an automatic “on-off” control circuit.

A clutch-brake on the hydraulic-motor shaft prevents downward drift of the rod when no hydraulic pressure is being applied to the motor. This is accomplished by means of a friction brake and an over-running clutch that allows completely free rotation of the shaft to produce rod motion upward (decreasing reactivity) but applies a reverse torque at 1.75 to 2.5 times that produced by the deadweight of the rod and attached drive parts acting vertically. The operation of the hydraulic motor, however, easily overcomes this friction when rod-down motion is desired. A mechanical latch holds the rod in the “up” (full-in) position after insertion. The latch has to be actuated when the rod is withdrawn down past the latch Mechanically, the latch cannot be withdrawn if the rod weight is on the latch. Safety requires that no more than three rods at one time be in a position intermediate between latched in and full out.

There are two independent emergency shutdown sys­tems. One is a group of 19 electrically driven emergency shutdown rods. These rods are operated by conventional Acme thread screw-nut mechanisms driven by simple and rugged d-c motors. Batteries, located in an extension of the rod port housing, supply power to drive the motors in the scram direction External d-c supplies are used to provide withdrawal power to these drives. Operation of these rods is by operator manual control only. They are used only in the event that normal scram operation fails. Complete rod insertion is attained in 24 sec, and the design provides very large torques that can overcome a resistive force as high as 10,000 lb, in which case insertion takes 1 min. Such operation was provided for the remote possibility that reactor damage (warping or debris) could restrict the normal movement of the rods The other emergency shutdown system, located in the top section of the guide tubes for each of the control and emergency shutdown rods, is a set of 5 5 gravity-drop neutron absorbers that are thermally released by abnormally high core temperature. This system is intended to operate in the event the other rod systems fail to act

Rod-position indication is provided by four selsyn receivers and by limit lights Each control-rod drive mechanism contains a selsyn transmitter geared to 170° rotation for full rod travel. Each position indicator has a graphical display (column type,~5V2-m scale corresponding to 0 to 84-in. rod travel, position accuracy ±2%) and a

CONTROL ROD — GUIDE SLEEVE

f a I

BALL SPLINE

-CONTROL ROD

, V : : ■

GRID PLATE

ROTARY FACE-TYPE DIFFERENTIAL PRESSURE SEAL-~-Ulj^

COOLANT FLOW

ROD BACKSTOPPING DEVICE-

HELIUM PURGE

PRESSURE VESSEL

POSITION AND CONTROL TRANSDUCERS’

-SCRAM DECELERATION VALVE -SCRAM EMERGENCY SNUBBER

HYDRAULIC MOTOR AXIAL PISTON TYPE.

BIOLOGICAL SHIELD

——— FLUID LEAKAGE DETECTOR

PRESSURE

RETURN

‘SOLID PUSH ROD

Fig. 7.22—Control-rod drive for Peach Bottom gas-cooled reactor.

digital display (three digit counter calibrated in inches, indication of rod position ±0 12 full scale, full-scale slewing time <6 sec)

When operating at power, the reactor is controlled by a group of three rods in the center region of the core The particular rods in the group are continuously monitored by the synchro position indicators at the reactor operator’s console The fourth indicator is switched to the individual rod being moved and thus serves as a check on the synchro-receiver normally on that particular rod. The remainder of the rods not being used for control are either full in or full out, as indicated by a green or red background light in the particular control-rod window on the operating console, the lights are operated by upper and lower limit switches In addition, a precision potentiometer is driven from each rod-drive gear train to provide for testing rod response and operation and for checking rod-position indication if a synchro-transmitter malfunctions

7- 4.5 Fast Reactors

The control systems for fast reactors (EBR-2 and Fermi) are described in Vol 2, Chap 17, Sec 17-2

REFERENCES

1 United States of Amenta Standards Institute, USA Standard Glossary of Terms m Nuclear Science and Technology USAS N1 1 1967, USA Standards Institute New York, 1967

2 M A Schultz, Control of Nuclear Reactors and Power Plants, 2nd ed, McGraw Hill Book Company, Inc, New York, 1961

3 W H Esselman and W H Hamilton, Position Control in Sealed Systems, in Proceedings of the 1953 Conference on Nuclear I nergy University of California Press, 195 3

BIBLIOGRAPHY

Argonne National Laboratory, The EBWR Experimental Boiling Water Reactor, in AFC Nuclear Technology Series, USAEC Report ANL 5607, May 1957

Freund, George A, Materials for Control Rod Drive Mechanisms Rowman and Littlefield, Inc, New York, 1963

Harrer, Joseph M, Nuclear Reactor Control Fngineermg, D Van Nostrand Company, Inc, Princeton, N J , 1963

Hutter, E., Fast Reactor Control Mechanisms, AEC, Washington, D C,1964

International Atomic Energy Agency, Directory of Nuclear Reac tors, Vol I VI, International Atomic Energy Agency, Vienna, Austria

Lipinski, W C, J M Harrer, and R L Ramp, in Reactor Handbook Vol IV, Engineering, Chap 8, Interscience Pub­lishers, a division of John Wiley & Sons, Inc, New York, 1964

Loftness, Robert L, Nuclear Power Plants Design, Operating Fxpenence and hconomics D Van Nostrand Company, Inc, Princeton, N J, 1964

Nuclear Power pp 282 286, November 1956

Nuclear Power p 5 3 ff, May 1962

Nuclear Power pp 90 97, April 1962

Weaver, Lynn E, Systems Analysis of Nuclear Reactor Dynamics, Rowman & Littlefield, Inc, New York, 1963