Patent Number: 048266493
Section: summary

A water-cooled nuclear reactor, especially a heating reactor that operates on the principle of internal natural circulation, is known from German Published, Non-Prosecuted Application DE-OS No. 33 45 099, corresponding to U.S. Application Ser. No. 005,289, filed Jan. 16, 1987 and particularly FIGS. 1-3 thereof. Such a reactor has a reactor pressure vessel and a reactor core supported therein, including a fuel element cluster disposed upright, control rods supported in such a way that they are insertable and retractable in interspaces between the fuel elements, as well as a supporting structure for fuel elements, control rods and the drives thereof. FIG. 3 of that publication shows the principle of a hydraulic control rod drive for the reactor, wherein the control rods are retractable against the force of gravity and are insertable in the direction of the force of gravity. Drive rods of the control rods having hydraulic piston/cylinder systems are provided, to which the cooling water can be supplied from below as working fluid through non-illustrated fluid lines that are internal to the reactor. The cooling water reservoir serves as a hydraulic drain. European Published, Non-Prosecuted Application No. 0 173 767 furthermore shows that it is known to provide a fluid pump external to the reactor, which communicates through pressure, suction lines and associated line ducts with the piston/cylinder systems on the pressure side and with the coolant reservoir in the reactor pressure vessel on the suction side. It is also known to vary the quantity of fluid on the pressure side of the fluid pump, by means of a control valve assembly disposed on the outside of the reactor pressure vessel, in order to adjust the control rods in the "raising" or "lowering" direction, or for the sake of maintaining the control rod position that has been reached. In the prior art control rod drive according to the above-mentioned European Application, the influence upon the fluid quantity on the compression side of the fluid pump is effected not only by means of the control valve assembly in the form of a pump bypass valve, but also, and above all, by adjusting the rpm of the fluid pump. However, in the prior art devices, a finely metered adjustment of the control rods is not possible if the pump is operating as a constant pump in its optimal rpm range and operation cannot be maintained if a pump malfunction or a disruption in the branch of the control valve assembly provided for the operation of raising the control rods occurs. It is accordingly an object of the invention to provide a hydraulic control rod drive for water-cooled nuclear reactors, especially heating reactors, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, with which a finely metered adjustment of the control rods is possible even if the pump is operating as a constant pump in its optimal rpm range, and in which both the disposition of the fluid pump external to the reactor and the control valve assembly connected to the pump are intended to be inherently safe, in the sense that in the event of either a pump malfunction or a disruption in the branch of the control valve assembly provided for the operation of raising the control rods, operation can be maintained. With the foregoing and other objects in view there is provided, in accordance with the invention, in a water-cooled nuclear reactor, especially a heating water reactor including: a reactor pressure vessel; PA1 a reactor core supported the reactor pressure vessel including a fuel element cluster disposed in an upright position, the fuel element cluster including fuel elements spaced apart in the reactor core defining interspaces inside and between the fuel rods, control rods, drive means for inserting the control rods in the direction of the force of gravity and retracting the control rods against the force of gravity in the interspaces, and a supporting structure for the fuel elements, the control rods and the drive means; PA1 the control rods having guide rods with hydraulic piston/cylinder systems, fluid lines inside the reactor feeding a working fluid in the form of cooling water to the guide rods from below, and a hydraulic drain in the form of a cooling water reservoir in the reactor pressure vessel; and PA1 a fluid pump assembly having a pressure side and a suction side, suction lines, pressure lines and associated line ducts connecting the pressure side of the fluid pump assembly with the piston/cylinder systems and connecting the suction side of the fluid pump assembly with the cooling water reservoir; PA1 an improvement comprising a hydraulic control rod drive assembly having at least one control rod drive, including: PA1 a control valve assembly disposed outside the reactor pressure vessel and connected to the fluid lines and the pressure lines, for influencing the fluid quantity on the pressure side of the fluid pump assembly in order to adjust the control rods in a raising and a lowering direction and to maintain a control rod position; PA1 the control valve assembly including the following control branches for actuating each of the control rods: PA1 a holding branch having an inlet side connected to the pressure line and an outlet side connected to the pressure side of the piston/cylinder system, a first fluid throttle, and a bypass fluid throttle disposed upstream of the first fluid throttle relative to a drop of the first fluid throttle for discharging into the drain, the first fluid throttle and the bypass fluid throttle having throttle cross sections dimensioned for allowing a fluid flow therethrough sufficient to hold a given control rod in a given position with the fluid pump assembly running; PA1 a raising branch having ends respectively connected to the holding branch upstream and downstream of the first fluid throttle, and a series circuit having a raising valve assembly and a second fluid throttle; and PA1 a lowering branch having one end connected to the holding branch downstream of the first fluid throttle relative to the drop of the first fluid throttle, another end discharging into the cooling water reservoir, and a series circuit of a lowering valve and a third fluid throttle; PA1 and means for automatically opening the lowering valve in the lowering branch if the raising valve in the raising branch sticks in an open position. The raising valve assembly may also include at lest two raising valves connected in series with one another for reducing a fluid flow through the raising branch if the raising branch is malfunctioning, at least enough to prevent further raising of a control rod being controlled. The advantages attainable with the invention are above all that the inherent safety of the control valve assembly from further retraction or from undesirable sticking of a control rod in the retracted position in the event that a raising valve sticks in its open position is increased considerably; that is, the safety that already exists without reducing the rpm or shutting off the fluid pump is considerably increased, so that taking total safety into account, that is, including the option of an emergency shutdown of the fluid pump, operator error can be considered to be precluded. The invention is particularly suitable for heating reactors that operate on the principle of a pressurized water reactor with internal natural circulation, or for heavy water moderated pressurized water reactors (pHWR=Pressure heavy Water Reactor). Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a hydraulic control rod drive for water-cooled nuclear reactors, especially heating reactors, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.