Patent Number: 039403092
Section: summary

The present invention relates to a safety device for the control and safety rods of a nuclear reactor with vertical rods to the control drive mechanism that is to the mechanism for inserting into or withdrawing from the reactor the power control rods. This type of rods in addition to control functions, perform also a safety function in as much as the release thereof from the drive mechanism can be commanded which, in the case of vertical rods, results in the dropping of the rods into the reactor and as a consequence the shutdown of it. In fact those rods which are intended for performing the twofold task of control and safety rods are affixed to the drive mechanism through an electromagnetic coupling. This coupling comprises a portion which is attached to the drive mechanism and contains a core with the related energizing winding -- that is an electromagnet -- and a portion attached to the rod -- electromagnetic armature. While operating normally or even abnormally but within certain safe limits, the magnet is permanently energized and the rod is maintained suspended at a level which is determined by the drive mechanism. On the other hand, when particular conditions occur of the reactor operation, whereby the reactor must be shutdown, the power supply to the electromagnet is cut off and the rod drops into the reactor and causes that kind of shutdown which is called scram. Each of the two functions -- control and safety -- of the rod are dependent on a set of data different and independent from one of the others. In fact the data on which the operation of the drive mechanism is dependent are different by their own nature and by their magnitude from the data on which the release device is operated. Because this invention is intended for improving the means for achieving a safety control of the reactor with an increased safety factor, the drive mechanism and the related control means will be dealt with briefly. It should be noted that a failed shutdown of a reactor may bring about a disastrous situation, the untimely shutdown may also bring about undesirable consequences. From this the requirement ensues of providing the electromagnet coupling and the related controls with the highest possible reliability. The means which control the electromagnetic coupling include a number of sensing units or transducers sensitive to changes in various quantities such as the coolant temperature, the ratio of the reactor power and the coolant flow rate, the fuel temperature etc. The sensed data are fed to a series of combinational circuits which take into account the magnitude of the signals from the sensing units and evaluate all together to find out whether the conditions exist which impose the reactor shutdown. The input from the combinational circuit or logic lines are supplied to a feeder of the suspension electromagnet of the rod. In general in the existing plants, in order to improve the reliability of the safety control system, the number of sensing units of the logic lines of the feeders have been multiplied; however no such system is presently available wherein the magnet for suspending the rods is multiple. In other words, in the current plants, upstream of the electromagnet a high safety level is attained which is proportional to the number of logic lines connected in parallel. For example, of these logic lines, one can be assumed to be excluded due to a failure or for checking purposes without discontinuing the electromagnet supply. When determining the safety level of the lines by which the coupling is controlled, two aspects are to be considered, that is the requirement of ensuring the coupling effectiveness in the case also of a failure of a portion of said lines whereby it no longer supplies the coupling and the requirement of ensuring the release of the rod in the case of a failure of said portion of lines in a sense opposite to the former one, that is in the case that, although the conditions occur for the reactor shutdown, the safety rods still remain suspended because a portion of the coupling remains energized sufficiently to support the rod. In general the current safety systems are provided with three safety lines of which two at least must operate for obtaining the desired action. With this criterion, which hereinafter will be referred to as "two out of three logics" one of the three lines can suffer a failure or can be excluded for overhaul purposes without impairing the system effectiveness. However, with such systems, the servicing of a whole protective line to begin from the sensor of the reactor condition up to the electromagnet inclusive can be effected. This is a major limitation of the reliability level of the system. The main object of this invention is therefore to provide a suspension coupling for a control and safety rod which coupling comprises a plurality of electromagnets that have a plurality of electromagnet cores and related windings which are independent from one of the others. The magnetic fluxes of said plurality of electromagnets sum on a single armature which is attached to the rod. The total attractive force of the electromagnets is redundant with respect to the force required for ensuring a safe connection between the rod and the drive mechanism. Concurrently the redundant fraction of electromagnets should not be greater than that strictly required in as much as a failed de-energization of the redundant fraction when a release of the rod is required must not prevent such release. A coupling according to this invention permits the redundant fraction of electromagnets to be cut off for check and overhaul purposes without impairing the safety of the reactor.