Patent Number: 052767191
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The three drive versions A1 to A3 illustrated in the FIGS. 1 to 7 belong to hydraulic drives for control rods in a nuclear reactor, particularly in a heating reactor. Referring now to the figures of the drawing in detail and first, particularly, to FIGS. 1 and 2 thereof, there is seen an upwardly and downwardly movable hollow cylinder 3 as a part of a control rod 4 which is coaxially disposed relative to a stationary hollow piston 1 defining an annular gap 2 therebetween. The hollow cylinder 3 forms a carrier tube for the control elements 5, here embodied as so-called absorber blades. The hollow cylinder 3 serves as a drive cylinder for the control rod 4 and it cooperates with the hollow piston 1 for that purpose. The working fluid (the coolant of the nuclear reactor), i.e. chemically processed (deionized) water, is supplied to the lower end 1.1 of the hollow piston 1 which is mounted in a lower core carrier plate 6, for lifting, lowering or holding the hollow cylinder 3. The working fluid flows into the reactor plenum 7 at the lower (narrower) end 2b of the annular gap 2. The flow arrows f21 indicate that flow. A venting channel configuration 8 is provided at the top region of the hollow cylinder 3 which is a throttling venting opening for the working medium. From the venting configuration partial venting flows according to arrows f22 exit into the reactor plenum 7. These partial flow currents may carry gas bubbles. The channel configuration 8 will be dealt with in more detail in the following. A cylindrical disk-shaped ultrasound reflector 10 is placed on an attachment or adaptor part 9 which is cross-shaped in a top plan view and which is hat or plate-shaped as seen in elevation; the outer diameter of the ultrasound reflector 10 corresponds approximately to that of the hollow cylinder 3. The reflector 10, together with an ultrasonic transducer 12, which is rigidly mounted above the control rod 4 in alignment with the longitudinal axis 11, forms a position measuring system for the control rod 4. The distance of the reflector 10 and thus the control rod location are determined by sending ultrasonic signals with the transducer 12 in the direction of the reflector 10 and by receiving the signals after they have been reflected. As mentioned, a venting cross section through the channel configuration 8 is provided which serves to distribute the partial venting flow f22 of the working fluid onto outlet locations 13, located below the reflector 10 at a greater axial distance a1 and at a greater radial distance a2 with respect to the reflector (or its outer circumference). In the illustrated hydraulic drive A1 with cross-shaped absorber plates 5 (control elements) and with corresponding cross-shaped gaps 16 within the core cell formed of four fuel elements, a cross-shaped adapter part 9 is provided which is mounted at the upper end of the hollow cylinder and whose radial channels emanating from a central channel part 8a and disposed in cross legs or extensions 17 exit into the reactor plenum 7 approximately in the region of the absorber plate tips 5a. The core carrier plate 6 has internal hydraulic channels 14 and external hydraulic lines 15 for supplying working fluid into the lower end 1.1 of the hollow piston 1 which is closed at that lower end. As indicated by an arrow f1, the working fluid flows from here upwardly through an interior or inner space 1a of the hollow piston 1 and, depending on the mass flow, causes a pressure which is less or greater than that in the surrounding reactor plenum 7. The control rod 4 with the hollow cylinder 3 is thus either held in its current position (when the mass flow remains constant), the control rod 4 is lowered by one or several steps (when the mass flow is reduced), or it is lifted up by one or several steps (when the mass flow increases). Changes in position take place until a new hydraulic equilibrium is reached in the lifted or lowered position. For the purpose of achieving a defined hydraulic throttling and for stabilizing a respective control rod position, the hollow piston 1 is provided with annular protrusions 18 and recesses 19 on its outer circumference, and the hollow cylinder 3 is provided with corresponding annular recesses 20 and protrusions 21 on its inner periphery. The control rod drive is fully integrated in the reactor pressure vessel; no conduct of movable parts through the wall of the reactor pressure vessel is necessary. The hydraulic control is provided such that for the case of an immediate shut-down, all control rods (only the control rod 4 is illustrated) fall to the lower position in the core under their own weight. This can be the position shown in FIG. 1. The control rod may in its lower position, however, be yet slightly closer to the lower core carrier plate 6. The absorber plates 5 are preferably filled with boron carbide. They move in their own (non-illustrated) guides within the gaps 16 (FIG. 2). The length of the control rod guides is chosen such that guiding of the control rods 4 is still ensured when they are fully driven out of the core (see position of FIG. 5). The fuel elements 22 which are indicated only by their contours (FIG. 2) form the control rod guide with the gaps of the four-fold core cell configuration; the control rod guide extends from the core carrier plate 6 upward, whereby about half of the axial length is occupied by the fuel elements 6 and the axial length above that is provided with a (non-illustrated) sheet-metal box structure which serves as a control rod guide. Referring now to FIGS. 3 to 5 for providing an additional explanation of the functionality of the inventive drive configuration, whereby identical parts in these FIGS. are also provided with the same reference numerals as in FIGS. 1 and 2. The embodiment according to FIGS. 3 to 5 differs from that of FIGS. 1 and 2 in that a head plate 90, whose diameter approximately corresponds to the hollow cylinder, is provided at the top of the hollow cylinder. The head plate 90 is provided with a channel configuration 8a, 8b which is T-shaped in an axial section. Outlet pipes 8c are inserted in the channel openings and are bent downwardly and outwardly, approximately in S-shape. The pipes 8c allow the partial venting flows f22 to exit from their outlet openings 13 in the region of the upper edges 5b of the absorber plates. The channel system 8a, 8b and including the outlet pipes 8c is also arranged in a cross-shaped plan-view, in correspondence with the cross-shaped gaps 16 for the absorber plates 5 (compare FIG. 4). Working medium (reactor water) is pressed into the hollow piston 1 through a pump and a (non-illustrated) hydraulic control unit. According to arrows f1, the working medium flows through the hollow body 1 into the upper part of the hollow cylinder 3 (inner space 3a). A (smaller) portion flows through the channel system 8a, 8b and the outlet pipes 8c. Most of the working medium, however, is pressed through the gaps between the hollow piston 1 and the hollow cylinder 3; it emerges at the foot of the hollow cylinder 3. These pressure losses result in excess pressure in the upper part of the hollow cylinder 3 as compared to the pressure in the plenum 7. Accordingly, with sufficient pressure, a hovering or holding of the control rod 4 is effected. In dependence on the position of the protrusions 18, 21 and the recesses 19, 20, a variable flow cross section results, which makes it possible to hold the control rod 4 in stable positions through a great range of mass throughput. Increasing or decreasing the hydraulic control flow attains a new positional level of the control rod 4. The hydraulic drive system as described is self-secured because, should the pump stop or the necessary working medium pressure cease for any reason, the drive system dives automatically into the core and assumes its shut-down position. FIG. 5 shows an upper control rod position as compared to FIG. 3. The embodiment of a hydraulic drive A3 according to FIGS. 6 and 7 differs from that of FIGS. 3 to 5 in that the outlet pipes 8c are extended to the region of the absorber plate tips 5a. Accordingly, the partial currents f22 emanate from the openings of the outlet pipes 8c at locations which lie in the region of or outside the absorber plate tips 5a. In all three embodiments with the drives A1 to A3 it is assured by the enlarged axial and/or radial distances a1 and a2 that the partial venting flows f22 flow into the reactor plenum 7 at a security distance far from the ultrasonic measurement path 12-10, which distance is great enough so that the ultrasonic distance measurement is virtually unaffected by the density fluctuations in the working fluid. In the case of a hydraulic pressure in the hollow piston 1 which is greater than that in the reactor plenum 7 by at least about 1.5 to 3 bar, the downwardly bent outlet pipes 8c have no disadvantageous effect, because possibly existing air bubbles are necessarily dragged along.