| 摘要 |
<p>A centrifugal brake is disclosed for a control rod drive within a boiling water reactor (BWR). Control rods within reactors have dual insertion mechanisms including a motor drive and a hydraulic drive. These drives are concentrically mounted within a cylindrical enclosed pressure chamber. The motor drive is typically of the ball screw type wherein a motor drive rotating threaded shaft through the bottom of the cylindrical chamber. The rotating threaded shaft drives a ball screw nut within a guide tube which urges a hollow piston upwardly. The control rod, connected to the upper end of the hollow piston is in turn urged upwardly into the reactor. The hydraulic drive utilizes hydraulic pressure to move the hollow piston and control rod independently from the ball screw drive upwardly for insertion into the reactor. Where loss of hydraulic system pressure occurs, both the weight of the control rod and pressure from the reactor can cause reversal of the ball screw drive and an undesired retraction of the control rod from the reactor. Accordingly, a centrifugal brake is installed in between the motor and ball screw threaded shaft concentric to a conventional coupling. The brake includes at least the one disc rotatably attached to the ball screw shaft having at least one and preferably two arcuate pawls pivoted on the disc. The pawls are maintained in a retracted position with respect to the disc by conventional detents. A stationary housing with lugs for engagement with the pawls are provided surrounding the disc. When the shaft of the ball screw rotates, responsive to pressure induced control rod withdrawal, the brake disc commences to rotate with increasing angular acceleration. This angular acceleration continues until the centrifugal force acting on the pivotal arcuate pawls of the brake overcome the force of the detents. When the force of the detents is overcome, the pawls pivot outwardly with respect to the disc. The pawls engage the stationary lug and immediately arrest ball nut shaft rotation and therefor control rod withdrawal. Upon motor controlled rod insertion, the stationary lugs reset the pawls to their respective detents and normal control rod insertion can occur. Consequently, unintentional control rod withdrawal under reactor pressure is prevented during hydraulic casualties to the control rod drive while the capability of motor and the ability to reset the brake following inadvertent actuation insertion is preserved.</p> |
