Patent Number: 048636720
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to absorber rods for nuclear reactors with spherical fuel elements which are exhausted after a single passage through the core and more particularly absorber rods inserted directly into the pile in order to affect the prevailing neutron flux in the reactor by absorber material located in an annular gap between two concentrically arranged cylindrical rod elements. 2. Description of the Related Technology Absorber rods are used in nuclear reactors to control the reactor output, the startup and shutdown processes, to equalize burnup, and to shut down the reactor. For this reason, they contain a neutron absorbing substance, i.e., an absorber material. The absorber material reduces the neutron flux and thus reactivity of the reactor depending on the immersion depth of the rod into the reactor filled with fuel elements based on its neutron capture cross section. The neutron flux in the reactor attains its maximum flow density as a function of the burnup state of the fuel elements at different heights of the reactor. If the radiation intensity, i.e., the radioactivity of the fuel elements varies, the maximum neutron flux also changes. In nuclear reactors having piles of spherical fuel elements, in contrast to nuclear reactors with block or rod shaped fuel elements, it is possible to replace fuel elements continuously, without interrupting the operation of the reactor, and thereby to affect the burnup state of the fuel elements in nuclear reactors having piles of spherical fuel element . The reactor may be adjusted so that the fuel elements are used up after a single passage and replaced by new ones. This operational principle is also called the OTTO principle (OTTO=once through then out). In reactors operated by the OTTO principle, the maximum of the neutron flux is located in an intermediate space between the pebble pile and the reactor cover. The absorber rods are inserted through this space into the reactor. For reactor specific reasons the absorber rods project in their rest position into the reactor space above the pebble pile and are therefore constantly exposed to the reactor atmosphere. In the case of nuclear reactors according to the OTTO principle, the absorber rods in this area are exposed to additional neutron irradiation stresses. Additional mechanical stresses appear upon rod insertion into the pile as there are no guide installations in nuclear reactors with piles of spherical fuel elements. These additional stresses result from forces against the fuel elements which oppose the insertion of the absorber rod. This resistance of the pile to insertion increases with the depth of the insertion. Absorber rods are only supported in a guide area in armored tubes in a fashion similar to a cantilever beam. Depending on its free length and section modulus the free end immersing into the peeble pile may be deflected from its immersion axis. Accordingly, the absorber rod is exposed to a lateral force producing a bending moment in addition to the force acting in a direction opposing its penetration. It is therefore necessary to take these types of operational mechanical stresses into account in the design of absorber rods for a nuclear reactor They must be correlated with the already present stressing of the rods. Absorber rods are stressed thermally upon their immersion in the reactor in two respects. The radiation heat emitted by the fuel elements leads to a heating of the rod and heat is generated in the absorber material of the absorber rod as the result of neutron absorption. An unacceptable increase in the temperature of the absorber rod due to these heat sources, i.e., an increase in temperature to a value at which the rod would lose its minimum mechanical strength, must be safely excluded. The same is true for the case in which the absorber rod would lose its necessary elasticity and ductility due to neutron embrittlement. Exposure to neutron radiation is, as set forth above, dependent on the layout and the mode of operation of the reactor, i.e., the position of the maximum neutron flux density in the reactor. Mechanical stresses are functions of geometrical parameters, such as the rod cross section, core diameter or core height and thermal stresses are determined by the fuel element inventory. SUMMARY OF THE INVENTION In view of relationships set forth above, it is an object of the invention to provide absorber rods which may be manufactured simply and cost effectively; the design configuration of which enables long term use free of incidents in a reactor having a spherical fuel element pile. The absorber rod, according to the invention, includes concentrically arranged pairs of cylindrical rod elements. The inner rod element performs the support function, i.e., absorbing and transmitting the forces and moments resulting from the movement of the rod upon the insertion of the rod into the pebble pile. The inner rod is dimensioned to escape damage by deformation or fracture. The outer element serves as a protective shield for the inner rod element against excessive thermal and radioactive stresses. The absorber material serves to shield the inner rod element from radioactive stress. The inner cylindrical rod element is advantageously designed as a support element to absorb mechanical stresses. It is protected by the absorber material against the constantly acting neutron radiation. The outer rod element may in this fashion have a function limited to maintaining the absorber element in its position and shielding the inner supporting tube. All of the outer mechanical forces and moments are introduced by the rod tip attached by welding and absorbed and transmitted by the inner tube. The tip of the rod is generally an integral head piece welded to the adjacent inner and outer rod elements. The inner and outer rod elements are joined to the tip by respective annular weld joints axially offset relative to each other. For thermal and neutron physical reasons the weld is appropriately located as far as possible from the tip of the rod, preferably at a distance corresponding to the diameter of the rod, from the frontal plane of the rod tip. The head piece may exhibit a central bore with a cylindrical piece, which in turn supports a holding device for the head piece. In the event of a fracture of the absorber tip the holding device functions to retain the tip, thereby preventing its irreversible immersion in the pebble pile. In this manner even damaged absorber rods may be retrieved completely from the fuel element pile, without any potentially interfering residues remaining therein. As mentioned above, gas flows through the absorber rods for cooling. It is advantageous to use part of the flow of cooling gas passing through the fuel element pile. This partial flow is separated above the fuel element pile where relatively low cooling gas temperatures are prevailing and guided through axially placed cooling gas slots distributed over the circumference of the outer rod element. By the appropriate choice of the size, number and axial positions of the slots on the absorber rod, adequately low material temperatures may be obtained in both the inner and the outer rod elements. It is possible in this manner to use the absorber rods which are designed according to the invention in high temperature reactors with cooling gas outlet temperatures of up to 750.degree. . It is further possible to use them in nuclear reactors combined with gas turbine machines. In nuclear reactor plants of this type having closed gas loops, such as so-called single loop installations, the gas temperature in the reactor core may attain values of 750.degree. to 950.degree. C. At least two inlet parts are provided in the outer rod element in axially different locations for introduction of the cooling gas. The inlets operate as a function of the position of the rod relative to the pebble pile. In principle, the part of the rod located in the pile is always cooled. The two inlet parts exhibit a large number of axially arranged inlet slots distributed over the circumference of the outer rod elements, thereby insuring uniform flow and cooling throughout the zone or area of the rod below the inlets. The cooling gas outlet is provided in the tip of the rod and are in the form of slots which have the advantage over bore holes in that they reduce the loss of pressure and improve cooling. As the result of the aforedescribed cooling, both a radial and an axial temperature gradient are created in the absorber rod. In order to prevent the additional loading of the absorber rod, by stresses generated by restricted thermal expansion, the rod elements without a support function are mounted on one end only and are slidingly guided in a defined slot. To further enhance cooling, the inner rod element may be in the form of a ribbed tube. In order to prevent the occurrance of notch effects and to minimize thermal stress peaks, care must be taken in the design of the single piece head portion or rod tip to provide a rounded structure terminating the annular gap between the outer and the inner connecting cylinders to which the outer and inner rod elements are joined. This measure prevents permanent damage such as cracking by radial deflections of the outer rod element transmitted to the outer cylindrical connecting piece of the head piece. The absorber material, which may be absorber rings of boron carbide, is annular in shape and manufactured with uniform dimensions (diameter and height) for simplification and the reduction of costs. Spacers or rings are inserted between the inner and outer rod elements in order to keep the absorber material away from the joint location (weld) of the outer and inner rod element to the rod tip. This prevents undesirable carburization and carbide formation in the rod elements during welding and in operation. Furthermore, in this manner temperature and stress peaks are kept away from the weld joints. The spacers also serve to maintain annular gaps between the inner and the outer rod elements and the absorber material in order to prevent harmful reactions between the rod elements and the absorber material upon swelling of the absorber material under radiation. An axial gap between the inner and the outer rod element is not occupied to equalize absorber material axial swell under radiation thereby avoiding additional axial stresses in the absorber rod. The absorber rod geometry is a smooth, cylindrical welded structure with a constant outer diameter. The outer surface of the rod is provided with an abrasion resistant layer, which is conveniently applied by means of flame spraying to prevent abrasion and the resulting variation in diameter, particularly in the area of bearing locations and barking devices. Coating layers produced in this manner are characterized by above average adhesion and a high density. Coating with chromium carbide has been found to be especially advantageous. Both the weakening of the outer rod element and abrasion on reactor components and fuel elements are prevented by maintenance of a smooth rod surface. The absorber rod has a screwed-on coupling on its upper end to establish the connection with the associated rod drive. The coupling exhibits a spring elastic claw coupling and may be released by remote control. The coupling has two parts, a claw body with coaxially arranged circumferential claws and a cylindrical counterpart with a collar engaged by the claws. The two coupling halves are easily separated and axially displaced by a release device, which may be a circular, axially displaceable part pressuring the claws from its clamping position. Bores are provided on the word circumference, to be engaged by a holding tool for securing the rod during installation and dismantling. This and further advantageous embodiments and improvements of the invention are set forth in the claims. A greatly different layout of a shutdown rod for nuclear reactors with a pile of spherical fuel elements is shown in DE-2 066 109. It consists of a rod with two concentric cylindrical rod elements connected to a common tip. The compressive stress on the contact surface of the rod tip and fuel elements is reduced by an increased support surface. The solution consists of a recess or hollow in the rod tip adapted to the contour of the fuel element similar to recess 46. No further characteristics relevant to the present invention are disclosed. An embodiment of the invention is presented below with reference to the drawings, wherein the invention is shown in more detail, together with advantageous configurations and improvements.