7- 4.1 PWR Power Plant at Shippingport

The Shippingport reactor is a pressurized-water reactor with 32 vertical hafnium control rods, both manually and automatically controlled, to adjust the power level (Figs. 7.9 and 7.10). Since the reactor coolant system is completely sealed, the control rod and the driving element must be within the pressure barrier of the coolant system. For this application the “canned” motor shown in Fig. 7.6 is used. The rod-drive mechanism is a roller nut attached directly to the motor rotor; it operates on the lead-screw portion of a control-rod extension as shown in Fig. 7.11. Since the coupling between the rotor and roller nut is direct, i. e., no reduction gearing is used, a special very slow (22 rpm) motor is required.

The motor torque is applied by magnetic coupling through the pressure barrier (the thin “can” between the rotor and stator). For the control-rod lead screw to disengage quickly from the roller-nut mechanism, the nut is split into halves that are held together by a magnetic flux generated in the stator winding. Thus cutting the power to the motor also scrams the rod. The split roller nut must be kept from reengaging until the rod has fully inserted to prevent damage to the mechanism.

The motor winding is designed for a three-phase power supply to produce a rotating field similar to that of the ordinary 60-Hz induction motor, except that it operates on alternating current with a frequency variable from zero (direct current) to a few hertz. The d-c power (zero frequency) is required, as pointed out earlier, to keep the nut engaged to the lead screw and to maintain the rod at the selected position. Even though the frequency can be reduced to zero to stop rod motion, the voltage must be kept applied to maintain the latch of the roller nut to the rod. This implies the requirement that the alternating current be changed to direct current without any change in value. Another requirement is for phase sequence to reverse the direction of rod motion.

A low-frequency d-c to a-c converter is used to meet these unusual power supply requirements (see Fig. 7.12). Basically, this converter is a circuit of series-connected resistors formed into a closed ring and mounted on an insulated commutator disk. Each junction point of the resistors is connected to one of the commutator segments arranged in a ring. Two diametrically opposite points on the ring of resistors are permanently connected to a d-c power supply. A rotating brush structure, with three insulated brush segments 120° apart, picks off a three-phase voltage from the commutator segments. Shorting-type brushes are used to prevent circuit interruption as the brushes move from one segment to the next. The a-c voltage is taken from the brushes through slip rings on the rotating brush structure. The frequency of the rod-motor voltage is determined by the speed of a small d-c motor that drives the brush structure. The peak a-c voltage is determined by the value of the d-c voltage applied to the series-resistor rings. Stopping the brush rotation automatically results in applying a d-c voltage to the motor stator fields so that the motor is held stationary.

In the Shippingport reactor two sets of brushes are incorporated into each resistor-commutator assembly. Each set of brushes supplies power to two rod-drive motors in
parallel. With this arrangement four control rods are moved simultaneously whenever a commutator assembly is ro­tated. If four rods from different portions of the core are selected to operate off each commutator assembly, the core power and the desired symmetrical distribution of neutron flux can be more easily maintained at the various power levels. For the 32 rods 8 inverters are used; 2 spare inverters are available.

In addition to four rods being controlled by one inverter, the rod programming plan for this reactor divides the rods into two or more groups, each having subgroups of four rods. Group 1 is necessary to bring the reactor from shutdown to initial criticality. Sixteen rods were selected for this group. They are moved individually in multiples of four in sequence such that any subgroup is never more than 3 in. beyond the position of the remainder of the group. This procedure, together with a maximum limit on rod speed, meets the criteria for the maximum rate of insertion of reactivity. The remaining 16 rods are programmed in groups of 8, 4, and 4. The rod-programming equipment is designed to place 16, 20, 24, 28, or 32 rods in the first group, leaving any remaining rods grouped in multiples of

4.

Versatile combinations of rod speed and grouping are necessary to provide for a rapid and uniform burnout of 13SXe after a scram or a power reduction. Although a scram causes all rods to drop, there are some situations demanding a fast or “safety” insertion in which power must be rapidly reduced a relatively small amount without a full-scale scram. (The latter, incidentally, also demands a correlative reduction or shutdown of the power-plant output.)

Each rod position is displayed on the control console by an individual column of indicator lights. Each light is connected to a small transformer on the control-rod — extension housing. As the magnetic material of the control — rod extension passes through each transformer, the coupling between the windings increases, and the lamp is lit.

Automatic control of reactor power is provided by a power and temperature control system. The reactor is inherently self-regulating through a negative temperature coefficient of reactivity that compensates for reactor coolant-temperature variations caused by variations in steam load at the turbine. The accumulation of fission — product poisons makes gradual adjustment necessary. As in

most reactors, rod movements must be fairly frequent immediately after start-up or a load change.