(a) Magnetic Jack

This CRDM is used widely in pressurized-water reactors. The magnetic jack has the advantage that it does not require mechanical or electrical penetrations of the reactor pressure boundary. Electrical coils around the CRDM housing are capable of supplying magnetic forces that can engage appropriate latches and can lift or drop the control rod between stops. The motion of a rod is accomplished by a series of lifts or drops with appropriate latching operations between moves. A typical step size is about in. and the minimum time between moves is 1 sec. The move time is virtually instantaneous compared to the time constants of interest in most reactor dynamics tests. Adequate reactivity worth can usually be obtained by applying the electrical signal for rod motion simultaneously to several CRDMS. This was demonstrated on the magnetic jack CRDMs used in the dynamics tests in the Halden reactor and several pressurized-light-water reactors. For a minimum time between moves of 1 sec, the highest frequency that one would expect to be able to measure is 0.5 Hz. Since this is within the range of important frequencies for PWRs, we conclude that the speed of the magnetic jack is satisfactory. Also, since the transition is so rapid, the position indication is no problem. The rod may be assumed to move instantly between two known positions at the time of the move command.