Patent GB-913405-A (GB)
Title: Control rod drive system
IPC: G21C7/02
Filed: 1961-08-01
Abstract: 913,405. Controlling nuclear reactors. GENERAL DYNAMICS CORPORATION. Aug. 1, 1961 [Aug. 9, 1960], No. 27960/61. Class 39 (4). A control-rod drive system for a nuclear reactor having a prompt negative temperature coefficient, e.g. as described in Specification 852,877, includes means for instantaneously withdrawing the control rod to a first predetermined position, means for further withdrawing the control rod at a predetermined rate, and means for stopping the control rod when it has been withdrawn to a second predetermined position. As shown in Fig. 1, the control rod 10 is connected to the shaft 32 of a piston 14 slidably located in a cylinder 16 which is movable vertically through the aperture 31 of a mounting plate 30 by a drive means 20. This drive means comprises a reversible D.C. motor (not shown) and a ball-screw and nut assembly 42 formed by a plurality of balls 50 disposed in the spiral channel formed by a spiral groove 46 on the outer periphery of the cylinder 16 and a corresponding spiral groove 52 on the inner periphery of a tubular nut 48 concentric with the cylinder 16 and rotatable by means of a worm 82 connected to the drive shaft of the motor. In order to instantaneously withdraw the control rod 10 from the core 12 of the reactor, fluid pressure from a suitable source 94 is passed through the conduits 96, 98 and the 2- way solenoid-operated valve 97 to an inlet 99 at the base of the cylinder 16 to drive the piston 14 to the top of the cylinder 16 as shown in Fig. 1. The drive system may be used for regulating the steady state operation of the reactor in which case the piston 14 is maintained against the upper end wall of the cylinder 16 and the drive means 20 actuated to either withdraw or insert the control rod 10. To drop the control rod under gravity when emergency conditions arise, the valve 97 is closed to release the pressure in the cylinder 16. The reactor may be pulsed by opening and then closing the valve 97, the piston 14 being held at the top of the cylinder 16 for a short period, the amount of reactivity inserted into the reactor depending on the position of the cylinder 16 and being adjustable by the control means 20. An increase in reactivity causes an increase in the temperature of the fuel elements, which increase in the case of a reactor having a prompt negative temperature coefficient causes an immediate decrease in reactivity. The reactivity is reduced further by the delayed temperature coefficient. To compensate for these effects and to provide a square pulse of power, or a rapid and automatic start up of the reactor, the control circuit shown in Fig. 6 may be used. D.C. power for the drive motor of the control rod 10 is provided by a rectifier 146, the field coil 150 and the armature 152 of the motor being connected across its output, the latter in series with a normally-closed contact 154 of a relay 156 and a pair of potentiometers 158, 160. A normally-closed contact of a relay 164 is connected in parallel with the potentiometer 158. The input of the rectifier 146 is connected to the output of a servo amplifier 136 which compares the signals from a power level indicator 138 and a period indicator 140 with the signal from a power demand control 142. To obtain a step function of power, the power demand control is adjusted to the desired level and switches 144, 148 and 172 (cf. Fig. 6) are closed. The predetermined reactivity is inserted into the reactor by the piston 14 being moved instantaneously to the upper end of the cylinder 16 as the solenoidoperated valve 97 is opened, the drive motor then withdrawing the control rod 10 at a rate dependent upon the setting of the potentiometer 160. The additional reactivity is inserted at a rate which compensates for the initial rate of decrease due to the temperature rise of the fuel. The potentiometer 158 is inserted in series with potentiometer 160 when the fuel temperature reaches a predetermined value as determined by a thermocouple 168. When a second predetermined temperature is reached, a thermocouple 166 energizes the relay 156 to open the contact 154 and stop the drive motor. Minor corrections to the rate of reactivity insertion may be made by the use of a regulating rod 132 controlled by the servo amplifier 136 or the control rod 10 may be controlled by a suitable servo amplifier. The control rod 10 includes an elongated, tubular member 24 of low weight, non-corrodable material, e.g. aluminium, provided with a coating 26 of a neutron-absorbing material, e.g. cadmium, boron or boron carbide. A resilient tubular member 106 is provided as a damping means at the bottom of a guide tube 104 for when the control rod 10 is dropped in an emergency and a damping means 110 at the top of the cylinder 16 serves to de-accelerate the piston 14. The mounting plate 30 is mounted on shock absorbers 74.