Patent Number: 048636720
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows in a schematic view a nuclear reactor power plant 10, comprising a prestressed concrete vessel 12, which in a cavity 14 contains a reactor 16 with a pile of spherical fuel elements 19, referred to briefly as the pebble pile 18. An absorber rod representing rods 20 or 21 immerses into the pebble pile 18. The absorber rod is guided in an armored tube 22 cast into the prestressed concrete 12 and moved by a piston drive 24. The absorber rods 20 and 21 exhibit two sets of elongated slots 26, 27 on their circumference in two axially offset areas. The spacing of the areas exhibiting slots 26, 27 relative to each other and the rod tip 30 is chosen so that cooling gas may enter from the intermediate space 28 located above the pebble pile 18 into the inside of the rod 20, 21 and flow through to the rod tip 30 for cooling upon the immersion of the rod 20, 21 into the pebble pile 18. By observing the geometric conditions, it is possible to assure that an area with slot 26 or 27, is in the intermediate space 28 of the reactor 16, providing access by the cooling gas into the rod. FIG. 2 shows an absorber rod 20 exhibiting an annular gap 38 formed by two concentric cylindrical rod element 32, 34. The absorber material 36 is located with the gap 28. The absorber rod 20 may be constructed in a stepped manner of equal length sections of rod elements 32, 34, joined together by coupling 40. The inner rod element 32 and the outer rod element 34 are welded to an integral tip 30 at the lower end of the rod illustrated at 82 and 83 respectively. The tip terminates in a cone and exhibits a nearly hemispherical recess 46 adapted to the shape of the fuel elements 19. The inner tube 32 has a thicker wall than the outer tube 34 to absorb and transmit the stresses resulting from the operation. A lower end of the inner tube or rod element 32 is welded to the rod tip 30 by a weld 82 or coupling 40 by a weld 84. The center piece of coupling 40 exhibits a plurality of axially extending radial slots 26 distributed over the circumference for the entry of the cooling gas. Only the lower portion of outer rod elements 34 are joined or welded to either the rod tip 30 or the coupling 40 by welds 83 and 42 respectively. The outer rod elements 34 are supported in a sliding fashion at their upper end, i.e., an annular gap remains between the outer rod element and coupling 40. This gap allows for thermal expansion and insures that the outer rod element does not perform an axial support function. Spacers 39 are inserted into the annular gap 38 at both the rod tip 30 and at the coupling 40, maintaining the absorber material 36 at an adequate distance from the joint location 42 (weld) of the outer rod element 34 to the rod tip 30 and coupling 40. A plurality of the outlet slots 25 are circumferentially distributed over the circumference in the cone area of the rod tip 30. A plurality of ventilating bores 45 are provided through the inner tube 32 establishing a connection between the inside 29 of the rod and the gap 38 thereby preventing an unacceptable pressure buildup as a result of swelling of absorber material 36 or the formation of gaseous decomposition products of absorber material 36. FIG. 3 shows a segment of a cross section through the releasable coupling 44 connecting the core rod 20 to a piston rod 61 of an actuating drive, not shown. The coupling 44 is constructed of a claw body 63 screwed into the upper open end of the core rod 20. An insert 62 located under the claw body serves to close off the core rod 20. The tubular claw body 63 has a plurality of elongated slots 64 on its upper end uniformly distributed over the circumference; the slots are provided with a radius at their ends. In this manner uniform, coaxial plates 66 are formed, which each exhibit a claw-like nose 68 at their free end. The lower end of the piston rod 61 exhibits a shoulder 69 as the counter piece for this nose 68 to engage the nose 68. The shape of the plates 66, i.e., their thickness, width, and length, is chosen so that they are capable of moving inward in the manner of a flat spring. It is possible thereby to push a hollow cylinder 70 over the claw body 63, as shown in FIG. 3. The spring action plates 66 snap into the shoulder or collar 69 upon reaching it, thereby establishing a positive axial connection between the piston rod 61 and the core rod 20. To release the coupling, a cylindrical ring 72 is inserted between the noses 68 and the cylinder wall 71, whereby the noses 68 are released from their positive connection with the collar 69, whereupon the piston rod 61 may be released from the coupling 44 by axial displacement. The cylinder ring 72 may be actuated by its own drive, not shown, or by advancing the piston rod 61 and fixing the core rod 20 until the cylinder ring 72 is engaged, as described above. FIG. 4 shows an alternative configuration of an absorber rod 21 according to the invention, where the rod has a modified rod tip 31 and a safety holder 50 described below. The rest of the configuration of the absorber 21 identically conforms to the principle shown in FIG. 2 and is therefore not described in detail. The single piece rod tip 31 tapers conically from the connecting area 58 to the rod elements 33, 35, to a small frontal surface comprising a nearly hemispherical recess 47. An axial central passage bore 56 is provided, which continues in a thin walled, hollow guide cylinder 54 and receives a holding rod 51. This holding rod, which completely fills the bores 56, is fixedly engaged at its free or upper end to a supporting fitting 53, which is part of a safety holder 50. Spacers 60 a reuniformly distributed at approximately one half the length of the guide cylinder 54 on the outer circumference to prevent lateral play by resting against the inner rod element 33. The safety holder 50 is joined to the connecting 40, which is closest to the rod tip 31, by a holding ring 52 or other appropriate means. The safety holder 50 serves to hold the rod tip 31 if, upon a fracture, it is released from the outer rod element 35. The safety holder further prevents a tip from immersion into, and retention by, the pebble pile, which would have severe operational consequences subsequent to a fracture. The safety holder may advantageously exhibit a downwardly converging conical portion which narrows from a region connected to the coupling 40 to a holding rod supporting fitting. The conical portion may advantageously include slots to allow cooling gas to flow from an upper interior area, in a downward direction. The slots may be offset from the connecting piece slots 26 to facilitate appropriate downward flow into a lower interior area. A further advantage of the holding device, is that the cooling gas flowing through the inside of the rod contributes intensively to cooling the inner rod element 33 in the remaining concentric annular space within the guide cylinder 54 and around the holding rod 51 and the support fitting 53. The length of the holding rod 51 is chosen so that it may be introduced free of play into the support fitting 53, and yet does not protrude into the concave recess 47. The length and the number of the individual segments of the rod elements 33, 35 is determined by the geometric conditions of the reactor and the mode of operation intended. Advantageously at least 3 segments may be utilized. It may be advantageous to provide the connecting pieces with inlet slots 26 in a differentiated manner, i.e., optionally alternating them with and without slots 26. FIG. 5 shows an enlarged section A of FIG. 4, illustrating details for the configuration of rod tip 31. A half of the rod tip 31 is shown in a longitudinal section. The rod tip, to be made as a single piece turning, comprises an axially extending center passage bore 56, into which a holding rod 51 is set, said holding rod terminating on the bottom flush with the concave recess 47. The rod tip 31 expands to the diameter of the outer rod element 35 beginning at this frontal surface. The rod tip is fixedly connected by the weld 83 to the outer rod element 35. This weld 83 should be located as high up as possible. A connecting fitting 59 is concentric with the outer rod element. An axially offset end of the inner rod element 33 is welded to the connecting fitting 59. The annular space 38 formed between the two rod elements 33, 35 is bounded by an annular frontal surface let into the rod tip 31 and exhibits a semicircular cross section. The relatively large radius of this annular surface corresponds to approximately one-half of the distance of the rod elements 33, and is primarily intended to prevent notch stresses which are superposed on the stresses generated by the differential thermal expansion caused by the different heat and unequal material thicknesses of the joint section 58 and the wall thicknesses in the weld zones. Similarly the downward extension of annular space 57 formed between the inner rod element 33 and the centrally placed guide cylinder 54 surrounding the holding rod 51 exhibits an inwardly converging cross section. The outer surface of the convergence is advantageously parallel to the outer surface of the rod tip so that the outer wall of the rod tip exhibits a uniform thickness. The lower extension annular space 57 has a triangular cross section with a rounded (circular) point. The absorber rod tip 30 may be advantageously fastened to the holding rod 51 by a holding pin 55. In this manner, the stresses created by the difference in material thicknesses are kept low and notch stresses are avoided. The absorber rods 20 are inserted into the reactor 16 until their tips 30 are located directly above the pile 18 to correct power fluctuations in the operation of the reactor with sufficient speed. The maximum neutron flux is located above the pile 18 in the intermediate space 28 of the reactor 16 when the reactor is operated according to the OTTO principle. Constant exposure to neutron radiation results in a reduction in ductility and load carrying ability of the outer sheathing tubes of the absorber rods by neutron embrittlement. In order to assure the maintenance of regular operations under these conditions, the absorber rod 20 is arranged so that the force transmitting rod elements 32 are arranged in the interior 29 of the rod and surrounded by the layer of the absorber material 36. The outer sheathing tube 34 is intended merely to maintain the absorber material in position. In accordance with this layout it is possible to operate absorber rods over extended periods of time in the intended fashion, without the risk of premature failures resulting from fractures caused by increasing brittleness. The spacing of the cooling gas inlet slots 26 facilitates cooling the inside 29 in any rod position, even upon insertion into the pebble pile 18. A slot area is always disposed in the intermediate space 28 filled with the cooling gas by an appropriately chosen geometric adaptation of the slot areas to the geometry of the reactor. This enables continuous entry of cooling gas to the inside 29 of the rod 20, flow therethrough to cool it, and exit at the tip 30. Cooling gas will flow through the interior of the absorber rods, through entry slots 26 and outlet slots 25 so long as the pressure gradient is equal to or less than the corresponding pressure gradient of the pile. The absorber rods according to the invention may be produced simply and cost effectively. The simple layout enables use of common shapes and tubes .