Patent Number: 041893476
Section: summary

The invention relates to a vessel which is suited for high, varying temperatures, especially for gas-cooled, high-power nuclear reactors. Both in gas-cooled nuclear reactors, the cores of which are formed of fuel element blocks, especially of hexagonal cross section, as well as in gas-cooled pebble bed reactors, the base of the reactor vessel is often made of hexagonal blocks or columns formed of graphite or another high temperature-resistant material, such as carbon. The side walls of these vessels are often formed of two layers of graphite or carbon blocks which are stacked on top of one another and are held together in a suitable manner by dowels or tongue-and-groove joints. As the very high operating temperatures of these nuclear reactors, considerable dimensional variations can be expected when the reactors are started-up and shut down, which stress the vessel, especially in horizontal direction. When heated up, the entire graphite structure of the reactor core and of the vessel expands. In order to avoid stress-producing forces, adequately large expansion gaps have to be provided. With repeated heating and cooling, assurance must be provided that these expansion gaps become neither too large nor too small at any location which, upon renewed heating, could cause local stressing forces and consequent damage. In the construction of blast furnaces, these problems have been solved with comparable temperatures, materials and dimensions in a manner which cannot be applied in reactor technology. In a blast furnace, the hard, inner part of the base, which is formed of graphite, expands outwardly into a yielding support, when heated, and deforms the latter plastically. When cooling, gaps would form which become clogged by the contents of the blast furnace and are no longer available as expansion clearance upon renewed heating. Blast furnaces are therefore heated up only once during their existence and then kept continuously at the operating temperature. In large, gas-cooled nuclear reactors, on the other hand, provision must be made for the required expansion clearance to be available but, in addition, restoring forces should be provided which bring all structural parts of the vessel back to their original position when the nuclear reactor cools down. These restoring forces should always be present in order to prevent loosening of the core components relative to one another under all operating conditions and thereby prevent uncontrollable relative motion. In principle, spring elements could also provide a permanent restoring force, however, they present construction and material problems at high temperatures and under radiation exposure, in the case of nuclear reactors. It is accordingly an object of the invention to provide a vessel subject to high and varying temperatures, especially for gas-cooled pebble-bed reactors, wherein the side wall and the base of the vessel are formed of a multiplicity of stacked blocks of heat-resistant material and are held together by an outer cylindrical or polygonal ring and supported on a foundation, this vessel being capable of retaining the form thereof even after an extended period of operation and repeated heating-up and cooling-down, and of having no excessive forces exerted on the structural parts employed therein. With the foregoing and other objects in view, there is provided, in accordance with the invention, in a reactor vessel for pebble beds at high and varying temperatures having a side wall and a base formed of a multiplicity of stacked blocks of heat-resistant materials and held together by an outer cylindrical or polygonal ring and supported on a foundation, the base and the side wall, respectively, being formed of a plurality of sectors having substantially vertical, radial parting lines therebetween, the sectors being supported with slight friction on the foundation and being braced against the outer ring, the sectors having boundary surfaces with a pebble bed, support surfaces on the foundation and abutment surfaces against the outer ring, the respective surfaces of the boundary surfaces, the support surfaces and the abutment surfaces having a convex mutual inclination whereby each of the sectors is held together in itself by external forces and is forced by its own weight and the weight of the pebble bed into a definite position. The subdivision of the vessel, as just proposed, into coherent sectors or ring sectors with defined parting lines or gaps therebetween has the effect that each of these sectors can freely expand and contact like a monolithic block, although it is formed of several blocks stacked on top of one another. What is avoided thereby is the likelihood that these gaps would add up at some location of the vessel circumference to form excessively large gaps, and would correspondingly become so small at other locations that local restraining forces would occur there. The feature of the sectors being supported and braced with slight friction is realizable with anti-friction or roller bearings or other movable elements. The slightness of the friction has the effect that the forces from the bracing and the supporting tend to act approximately perpendicularly to the respective transmission surfaces on the structural parts of the side wall and the base. Since the sum total of the forces transmitted from the pebble bed to a boundary surface likewise acts approximately perpendicularly to this surface, each sector is in itself pressed together due to the convex disposition of these surfaces, and through the inclined disposition, each sector is forced by the weight thereof and the weight of the pebble bed into a defines position. If the outer ring abutment or bracing is considered as fixed, the fixed point for each sector is located where the support surfaces on the foundation and the abutment or bracing surfaces against the outer ring intersect, because all movements can take place rectilinearly from this point. In actuality, the dimensions of the outer ring abutment or bracing also change with the temperature, so that the fixed points of the sectors follow or participate in these changes in the radial direction. During all expansions caused by temperature, the blocks move, with slight friction and under a calculatable load on the rolling bodies up the inclined planes, and are returned to the original position thereof upon cooling. Since the external dimensions of bodies of materials having high temperature resistence, such as graphite or carbon, are limited, several of these bodies are stacked on top of one another to form a respective column, each of the columns being supported by a rolling body. At the boundary joints between different groups of such columns, overlap can be provided at the marginal or border elements, so that no through-gaps are formed also in the cold condition. This construction is suited, in principle, for bases of any shape, and therefore also for rectangular bases. For high pressures, vessels with circular or polygonal cross sections are advantageously used. In such a case, several ring-segment-shaped groups of columns of trapezoidal cross section are provided, between which radial expansion joints are provided, and the counter-bearing or bracing abutment which absorbs the outwardly directed component of the weight, is formed by an outer, cooled ring or a polygonal wall. The center of the base can serve also as the fixed point for bases with axial symmetry so that no forces act outwardly. In accordance with another feature of the invention, a first group of roller bodies is disposed on the foundation and supports the sectors, and a second group of roller bodies engages the outer ring and braces the sectors thereagainst, the rollers being mounted as the first and second groups thereof in respective planes, and being disposed in each sector in, respectively, two planes inclined convexly to one another, so as to form substantially convex surfaces, the two substantially convex surfaces being inclined at the same angle to a vertical symmetry plane of the sector. The rolling bodies proposed, just referred to, act like balls between two planar surfaces i.e. so that the two participating surfaces can shift in the plane thereof relative to each other in any manner. Since a single ball can transmit only small forces because of the point contact and the limited permissible surface pressure of the material used, and since such a ball, for vertical bearing or support surfaces, can be guided and secured against falling down only at considerable expense, two cylindrical rolling bodies are disposed at the side wall in two different planes with a planar rolling surface therebetween, the axes of the rolling bodies being disposed perpendicularly to one another. The cylindrical rolling bodies, disposed with the intersecting axes, act as a sphere between two plane surfaces i.e. they afford free movement within the rolling plane thereof. By means of an appropriate guide and a non-load-supporting gearing, these rolling bodies can also be disposed in vertical planes without falling down. The last-mentioned feature of the invention of disposing rolling bodies in two planes inclined convexly to one another, has the effect that the bearing forces acting approximately perpendicularly to these planes, hold the supported sectors together. Also, the forces transmitted from the outer ring bracing or abutment onto the side wall sectors have the effect of holding these sectors together. In accordance with a further feature of the invention, the boundary surfaces of at least one of the sectors between the pebble bed and the base are formed of two surfaces of equal size inclined substantially convexly to one another, and the boundary surfaces of at least one of the sectors between the side wall and the pebble bed are formed of two surfaces of equal size inclined substantially convexly to one another, the two substantially convex surfaces being inclined at the same angle to a vertical symmetry plane of the respective sector. The just-proposed feature assumes that the radial boundary surfaces of the individual sectors are not in contact with the boundary surfaces of adjacent sectors and also are not stressed by external forces. This construction assumes further that the forces transmitted from the pebble bed to a boundary surface act approximately perpendicularly to this boundary surface. By the proposed disposition of at least two surfaces of equal size, which are inclined substantially convexly to one another and both inclined at the same angle to a vertical symmetry plane of the respective sector, this sector is in itself held together by the forces transmitted from the pebble bed. The two mutually inclined surfaces need not necessarily intersect with a sharp edge. In view of the generally used graphite material, it would appear advantageous for these two surfaces to merge with one another with a slight rounding. In accordance with an added feature of the invention, the sectors of the side wall have a pentagonal cross section, respectively, and one of the boundary surfaces with the pebble bed and the abutment surface against the outer ring is planar. The side wall feature, just mentioned, is an extrene case of the feature mentioned just before that. In principle, also a side wall sector of triangular cross section is held together by the external forces acting from all three sides on the boundary surfaces in the manner according to the invention. If one cuts the two equiangular corners from such an isosceles triangle, a pentagon is obtained which has only one plane boundary surface either toward the pebble bed or toward the outer abutment. In accordance with yet another feature of the invention, the base is formed with a funnel-shaped opening for discharging fuel pebbles from the pebble bed, and the inclination of the roller-body planes between the base and the foundation is opposite to the inclination of the funnel member which is disposed vertically thereabove. In accordance with yet a further feature of the invention, a plurality of discharge outlets for fuel pebbles of the pebble bed are formed in the base, a parting line extends radially outwardly from each of the discharge outlets, and an annular parting line extending through the middle of all of the discharge outlets, the annular parting line being either substantially circular or polygonal. The last two features mentioned relate ro pebble bed reactors, the base of which has one or more funnel-shaped, mutually penetrating openings or inclined planes for discharging fuel pebbles or balls downwardly. If the inclination of the roller body-planes between the base and the foundation is set always opposite to the inclination of the base part disposed vertically thereabove, assurance is provided that the external forces caused by the pebble bed and the support hold the respective sector together in itself. Since the shape of the base is determined by flow-technology considerations, the inclination of the rolling-body-planes between the base and the foundation must be disposed accordingly. When several discharge outlets are distributed over the circumference, the radial parting lines or joints must be placed so that they pass through the respective discharge centers and, in addition, a ring-shaped or polygon-like parting line or joint passes through all discharge centers distributed over the periphery. The ring-shaped parting line or joint defines a central, cone-like body, which rests on a number of rolling body-planes inclined from the outside toward the center, and has a fixed point at the center of the vessel. This disposition relates especially to pebble bed reactors such as are described in the German Published Non-Prosecuted Applications DT-OS No. 24 08 926.7 and DT-OS No. 24 56 405.4. As described therein, the cone disposed with an upwardly directed apex in a funnel-shaped base gives the fuel pebbles or balls a vertical velocity which is approximately constant over the cross section and, therefore, has considerable nuclear-physical advantages. All fuel balls or pebbles are burned up nearly uniformly over the cross section to a final state which is economically sensible, so that they only have to pass through the reactor once. This is made possible by a flow cone which is formed of several sectors in polygon-fashion. However, contrary to the rest of the funnel-shaped base, the joints between these sectors are always closed. By supporting this cone in a funnel-shaped depression or valley of the foundation, which is formed by several rolling body-planes, this cone is held radially together by its own weight and by the weight of the pebble bed resting thereon and is centered at the lower end thereof relatively to a central fixed point. The support of the outer ring on cylindrical rolling bodies has the effect that, one the one hand, this ring can expand and contact without stressing forces during temperature changes, and that, on the other hand, this ring remains centered relative to the center of the vessel, both of which is necessary for the undisturbed functioning of the reactor vessel. These cylindrical rolling bodies, since they can move only in the radial direction, furthermore, prevent the outer ring from turning relative to the support thereof, which is necessary for the trouble-free functioning of the pipelines penetrating therethrough. Although the invention is illustrated and described herein as embodied in base for vessel subject to high temperature, 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.