Patent Number: 047568682
Section: summary

FIELD OF THE INVENTION The invention relates to a mechanism for longitudinally moving and for securing a drive rod of a control bar of a nuclear reactor comprising a tank containing a pressurized coolant, said mechanism having an enclosure coaxial with the rod and forming a hydraulic cylinder slidably receiving the rod with which it defines a decompression chamber, and means for mechanically securing the rod in the high position thereof. The invention is particularly suitable for use in control bar operating mechanisms which are adapted to locate the bars, during a life cycle of the reactor, either in a low position in which they are inserted in the core of the reactor, or in a high position where they are outside the core. This is typically the case of control bars comprising elements containing fertile material for modifying the neutron energy spectrum in a reactor as the fuel therein is progressively exhausted. BACKGROUND OF THE INVENTION Mechanisms of the above-defined type are already known. For example, there is disclosed in French Pat. No. 2,496,193 a mechanism whose decompression chamber is connected to a low pressure discharge reservoir by pipe means having a solenoid or electromagnetic valve. The control bar is lifted by opening the electromagnetic valve so as to create a pressure differential which applies an upwardly directed force to the rod. The means for mechanically securing the rod in the high position are formed by a bi-stable toggle device whose condition changes position upon each opening and closure cycle of the electromagnetic valve. An advantage of such a mechanism is simplicity in design; on the other hand, it has drawbacks. A first drawback is that it is not possible to control, at will and independently, the force tending to lift the bar and the rising speed of the bar. In fact, both values are increasing functions of the degree of opening of the electromagnetic valve and a decreasing function of the leaks between the rod and the cylinder. It is, for example, not possible to increase the lifting force while reducing the speed. In addition, the relationship which exists between the degree of opening of the electromagnetic valve and the force (or the speed) varies over time, especially due to wear or clogging. Another drawback is related to the fact that reactors typically include several tens of control bars. If each drive mechanism comprises pipe means and an electromagnetic valve connected to a low pressure dump reservoir, a number of pipe lengths must be provided which, since they convey a flow of primary coolant, must comply with very strict safety rules, and must be protected against earthquakes and possible missiles. The pipe lengths and their electromagnetic valves are potential sources of leaks likely to cause uncontrolled lift of control bars. SUMMARY OF THE INVENTION It is an object of the invention to provide a drive and securing mechanism which associates the simplicity in design of control by hydraulic coolant discharge with control of the lifting speed. It is a more specific object of the invention to simplify the control of the mechanisms in a reactor having a large number of control bars, some of which must be displaced simultaneously so as to maintian symmetrical distribution of the neutron flux in the reactor core. It might be thought that this result can be obtained, when the drive mechanism is of the type having discharge pipes and electromagnetic valves, by connecting the decompression chambers of all mechanisms which are to operate simultaneously to a same manifold through a common electromagnetic valve. But such a design must be avoided when the securing means are of the bi-stable type, controlled at each operating cycle of the electromagnetic valve since, if for any reason correcting securing or release does not occur in one of the mechanisms, the latter will later operate in opposition of phase with respect to the other mechanisms connected to the same manifold. According to a first aspect of the invention, a mechanism of the above-defined type comprises first electromagnetically controlled means placed outside the enclosure, hydraulic means controlled by said electromagnetically controlled means for reducing the pressure of the collant in the decompression chamber so as to cause step by step lift of the rod with a predetermined step and second electromagnetically controlled means, different from the first ones, also placed outside the enclosure, for controlling hydraulic means communicating the decompression chamber with the dump tank so as to cause downward movement of the rod and release of said securing means, said hydraulic means being placed in the enclosure. According to another aspect of the invention, a mechanism of the above-defined type comprises a positive displacement reciprocating pump inserted between the decompression chamber and the inside of the tank, whose piston is actuated by first electromagnetically controlled means and, upon reciprocation, causes a lift of predetermined amplitude of the rod, and means for securing the rod in the high position releasable by second electromagnetically controlled means. With this arrangement, the lifting force and the lifting speed may be controlled independently. The lifting force depends mainly on the force exerted by the first electromagnetically controlled means. The lifting speed depends essentially on the rate of actuation of these means. The downward speed of the bar for insertion thereof into the core may be controlled independently, by adjusting the head loss in the flow path between the decompression chamber and the tank, said path being open when the second electromagnetically controlled means are energized. It will be appreciated that the mechanism of the invention does not comprise pipe lengths conveying the primary coolant outside the tank, which simplifies the overall design and removes the risks of leaks which may cause uncontrolled withdrawal of the control bars or the need to permanently make up for these leaks. Finally, because the function of lifting the rod and the function of securing the rod are separate, the first electro-magnetically controlled means of several bars may be controlled by a same generator without the risk of out of phase operation should the securing system of one of the mechanisms fail during one operation. Bar fall may be caused by gravity, by opening a coolant flow path between the inside of the tank and the decompression chamber. In another embodiment, the pump is used for accelerating the fall speed.