Magnetic coupling device for control rod drive

#This invention relates generally to control rod drives used in nuclear reactors and, more specifically, to a control rod drive which eliminates conventional packing-type seal assemblies for the rotating spindle thereof.

#In one type of nuclear reactor, control rods are selectively inserted and withdrawn from a nuclear reactor core for controlling the operation thereof. Each of the control rods is typically positioned by a conventional control rod drive which includes a ball screw or spindle threadably engaging a ball nut for raising or lowering the ball nut as the spindle is rotated either clockwise or counterclockwise respectively. A hollow piston rests upon the ball nut at one end thereof and at its other end is conventionally joined to the control rod. Displacement of the ball nut provides displacement of the piston which in turn inserts or withdraws the control rod in the core.

#In order to achieve faster insertion of the control rod than can be obtained by normal rotation of the ball spindle, for example, during a scram operation, a rapid flow of high-pressure water is injected through the control rod drive past the piston for lifting the piston off the ball nut in a relatively short time for quickly inserting the control rod into the core. The high-pressure water is channeled to the control rod drive through a scram line pipe attached to a high-pressure water accumulator.

#FIG. 1 is a schematic representation of a nuclear reactor pressure vessel including a control rod drive of the type wherein the control rod is raised or lowered in response to rotation of a spindle.

#FIG. 2 is a sectional view, partly schematic, of a control rod drive of the type illustrated in FIG. 1.

#FIG. 3 is a longitudinal sectional schematic view of a concept for a control rod drive including a motor and brake.

#Illustrated in FIG. 1 is a conventional reactor pressure vessel 10 of a BWR having a plurality of fine motion control rod drives 12, only one of which is shown. In one exemplary embodiment, there are 205 fine motion control rod drives 12 extending into the pressure vessel 10 through the bottom thereof. Referring also to FIG. 2, an enlarged, sectional view of one fine motion control rod drive 12 is illustrated. The control rod drive 12 includes a tubular housing 14 extending downwardly and outwardly from the pressure vessel 10 and secured thereto. The housing 14 is connected to a flange or manifold 16 which is in flow communication with a scram line or conduit 18. The scram line is provided with high-pressure scram-line water 20 from a high-pressure water accumulator 22 (see FIG. 1).

#A ball screw or spindle 24 is disposed inside housing 14. The spindle 24 has threads 26. The control rod drive 12 includes a longitudinal centerline axis 28, with the housing 14 and spindle 24 being disposed coaxially therewith.

#A ball nut 30 is threadably coupled to spindle 24 and is restrained from rotating therewith. As a result, when the spindle is rotated in one direction, the ball nut translates downward and away from the pressure vessel 10, and when the spindle is rotated in the other direction, the ball nut 30 translates upward and toward the core. A hollow, elongate piston 32 is disposed coaxial with the spindle 24 and includes a conical base end 34, which rests on the ball nut 30, and a tip end 36 extending through a central aperture 38 in the upper end of housing 14 into the pressure vessel 10. The tip end 36 is coupled to a respective control rod 40 by a bayonet coupling. Therefore, rotation of spindle 24 vertically displaces the control rod.