Patent Number: 052326560
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to the operation of fast acting nuclear reactor control devices and more particularly, to a device for providing primary drive to a safety control rod in a nuclear reactor to provide for control of the nuclear reaction, and as conditions warrant, for shutdown of the nuclear reactor. Nuclear reactors typically employ control rods which are inserted into the core of the reactor to control the level of the nuclear reaction. Control rods are commonly used during normal operation of the reactor to maintain a desired level of neutron flux in the core, and additionally, some of the safety rods provide a means for shutting down the reactor in emergency situations ("SCRAM"), or normal shutdown All such rods have a neutron absorbing portion containing a substance, such as hafnium, to control the flow of neutrons. Movement of the rods in or out of the core controls the nuclear reaction. When it is necessary to shut down the reactor during emergency situations the entire neutron absorbing portion of the safety control rod must be inserted as rapidly as possible into the reactor core. Various fast acting nuclear reactor control devices have been employed in attempting to provide a reactor control system. Most nuclear reactor facilities use spring drives held in a strained position. For example, one design employs a compression spring and a recirculating ball lead screw arrangement. The spring provides torque to the drive system mainshaft by driving the ball lead screw. This design has several limitations. The spring provides a decreasing torque resulting in less overall energy input and requires a high retaining torque. This encumbers the fast release capabilities of an electrical clutch also employed in this design, because high currents are required in the clutch to resist this torque. Another imitation is that the highly loaded lead screw is prone to galling and lacks efficiency. Additionally, the spring and ball screw SCRAM-assist system provides torque only during about the first one-third of the safety control rod's downward stroke as it is positioned in the reactor core. The remainder of the downward stroke is effected by the weight of the rod under the influence of gravity, and water pressure if so configured. Accordingly, it is an object of the present invention to overcome the inefficiencies of present nuclear reactor safety control rod drive units to improve the safety and reliability of nuclear reactor operation. It is a further object of the present invention to provide a fast acting nuclear reactor control device which improves the safety performance and reliability of nuclear reactor operation. Another object of the present invention is to provide a nuclear reactor control device which better maintains power and force to compel safety-rod insertion over the full length of the control rod. Yet another object of the present invention is to provide a nuclear reactor control device which provides for easy adjustment and control of the control rod. An additional object of the present invention is to provide a nuclear reactor control device which allows the safety control rod system to freely travel toward a safe position in the event of a partial drive system failure. SUMMARY OF THE INVENTION This invention provides a fast-acting nuclear reactor control device for controlling and positioning a safety control rod within the core of a nuclear reactor, the nuclear reactor being controlled by a reactor control system. The device includes a primary safety control rod drive means operatively connected to the safety control rod for driving and positioning the safety control rod within or without the reactor core. The safety control rod is oriented in a substantially vertical position to allow the safety control rod to fall into the reactor core under the influence of gravity and water pressure during shutdown of the reactor. The safety control rod is connected to a rack, and the primary drive means can be a safety control rod drive shaft having a pinion driving the rack for allowing the safety control rod to be positioned within or without the reactor core. The primary drive means is further operatively connected to a hydraulic pump such that operation of the primary drive means simultaneously drives the safety control rod to desired positions within the reactor core and operates the hydraulic pump such that a hydraulic fluid is forced into a pressurized accumulator, charging the accumulator with compressed gas for the storage of potential energy. A solenoid valve is interposed between the hydraulic pump and the accumulator, the solenoid valve being a normally open valve actuated to a closed position when the safety control rod is out of the reactor core during reactor operation. The solenoid valve opens in response to a signal from the reactor control system calling for shutdown of the reactor with rapid insertion of the safety control rod into the reactor core, such that the opening of the solenoid valve releases the potential energy in the accumulator to cause hydraulic fluid to flow back through the hydraulic pump, converting the hydraulic pump to a hydraulic motor having speed and power capable of full length insertion and high speed driving of the safety control rod into the reactor core. The insertion of the safety control rod will now be powered by the combined effects of high pressure gas, gravity, an differential water pressure. This will ensure rapid and reliable operation. The primary drive means or safety control rod drive shaft can include an electromagnetic clutch co-axial with the drive shaft for positioning the safety control rod in a run position. Further, the primary drive means includes an overrunning clutch co-axial with the drive shaft and located intermediate the hydraulic motor and the electromagnetic clutch. This overrunning clutch is capable of allowing the speed of the primary drive means to rotate at a speed greater than the speed of the hydraulic motor during shutdown of the reactor to provide for rapid insertion of the safety control rod into the reactor core. A reservoir of hydraulic fluid is connected to the hydraulic pump. The primary drive means further includes a drive motor driving through the electromagnetic clutch to position the safety control rod in a run position while simultaneously driving the hydraulic pump through the overrunning clutch. In this manner, hydraulic fluid is transferred from the reservoir to the accumulator.