The requirements for the CEDM are fine position control, lubrication by primary coolant, easy access to electrical parts, and high seismic resistivity. Soluble boron free reactivity control and load following operation over its full power range require a fine positioning capability of the control rod. The investigation of the failure frequency of operating CEDMs has revealed that the major source of trouble was in the electrical components requiring an easy access for maintenance. The long protruded part for the extension shaft stroke outside the reactor pressure vessel leads to high level of seismic excitation and reduces the margin to design stress limit. The current magnetic jack type CEDM used in the Korean Standard PWR is considered to be inadequate to meet the fine control requirements because it has only a step wise positioning. In addition the adoption of a self-pressurizer which occupies the upper plenum of the reactor vessel introduces difficulties in lubricating the moving part with the primary coolant since the latch mechanism would be located in the steam-gas region of the self pressurizer. Therefore, new concept of CEDM was proposed. The proposed design consists of position encoder, brushless DC servo motor, lift magnet coil, rare earth permanent magnet rotor, driving tube and split ball nut assembly. The rotor, driving tube and ball nut assembly are all connected into a single piece and lodged within the pressure housing which forms the pressure boundary. The encoder, DC servo motor, and lift magnetic coil are installed outside the pressure housing for easy maintenance. The use of a brushless DC servo motor with rare earth permanent magnet rotor allows a maintenance free operation of the motor. The fine control capability of the

CEDM is assured by the use of ball nut — lead screw mechanism and its lubrication with primary coolant is provided by placing this part below the water steam interface surface in the pressurizer. The ball nut assembly is of three pieces split type. Lift magnet located below the DC servo motor engages the ball nut to the lead screw by lifting the driving tube and the rotation of the rotor induces linear motion of the control element assembly up and down. The lead screw is part of extension shaft at the bottom of which a control element assembly is attached. When the scram signal is issued, the current supply to the lift magnet coil is cut off and the split ball nut releases the lead screw while dropping down by gravity and spring forces.