Patent Number: 053435060
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a Continuation of International Application Ser. No. PCT/DE91/00993 filed Dec. 18, 1991. BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a nuclear reactor installation, in particular for light water reactors, with a reactor pressure vessel having a reactor core and a core catcher device. Such a nuclear reactor installation is known from U.S. Pat. No. 3,607,630. In addition, that known nuclear reactor installation has the following features: a supporting and protective structure delimits a reactor cavern with a bottom region and a circumferential wall, and the reactor pressure vessel, disposed in the reactor cavern at vertical and lateral distances in relation to the bottom region and the circumferential wall, is seated in the supporting and protective structure. In that case the core catcher device has a collecting basin for the core melt, which can be cooled by means of a coolant and is embedded within the reactor cavern and below the reactor pressure vessel in the bottom region of the supporting and protective structure. The collecting basin, which is also known as a "core catcher", is flat, pan-shaped and water-cooled internally. It is connected through an ascending pipe to a flood container disposed at a higher level. The wet steam forming in a hypothetical case of a core melt, i.e. when the core melt is distributed in the collecting basin, is blown off through outlet lines into the containment vessel or into condensation devices (steam separators). The condensed cooling water is returned to the flood container. The collecting basin is formed of a plurality of parallel tubes connected at the respective inlet and outlet ends to a common distribution or collection tube. However, the relatively good cooling properties of such a known collecting basin can be impaired, particularly in the case of nuclear reactors with higher output, if the tube structure of the collecting basin is deformed by large falling masses and the cooling cross sections are reduced or blocked by thereby. The core catcher according to Published UK Application No. GB 2 236 210 A, has a collecting basin in "multilayer sandwich construction", with a bearing, downwardly arched steel pot, cladding of interlocked zirconium blocks and a steel skin covering the cladding at the top, which melts through in the case of an impacting core melt, i.e. it is sacrificed. Since the catch volume of the core catcher is relatively small and, alternatively to gas cooling, is only provided with a standing water column in the shield pit or reactor cavern, effective continuous cooling of the core melt (which in the beginning may have temperatures above 2000.degree. C.) would only be possible with small reactor outputs, since otherwise film boiling could occur at the outer steel jacket of the core catcher, along with the danger of considerably reduced heat transfer. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a nuclear reactor installation with a core catcher device and a method for exterior cooling of the latter by natural circulation, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and in which it is possible, by means of its collecting basin construction and support, to assure sufficient cooling channel cross sections and cooling of a possible core melt, even with increased reactor output and reactor core weights, without having to fear impairment of the structure defining the cooling channels by the deformation forces of impacting masses. In addition, within the framework of subordinate and coordinate objects, it is intended in connection with a nuclear reactor installation according to the invention to provide requirements for cooling the collecting basin in accordance with the principle of natural circulation with a liquid and, in addition, to provide a dual cooling system (air cooling and water cooling) in such a way that, in the case of emergency cooling, air cooling is replaced, at least partially, by water cooling without special switching commands. Another subordinate object is effectively keeping away the radioactive radiation emanating from the bottom of the collecting basin in case of a core melt, from the wall sections located above the collecting basin of the supporting and protective structure. A further subordinate object is integrating a heat insulation surrounding the reactor pressure vessel into the system being formed of the collecting basin and dual cooling device. Heretofore there has been no lack of suggestions for eliminating an incident of an accidental core melt through special safety measures. The recently developed safety philosophy assumes that it is better from the viewpoint of safety technology to include a core melt incident in the considerations, even though the possibility of its occurrence might be infinitely small. The invention is based on this viewpoint. A particularly effective protective barrier for preventing undesirable consequences of a core melt incident is intended to be provided by means of the invention. Additional subsidiary objects connected with the above defined general objective ensue from the following considerations. It is desirable in connection with light water nuclear reactors in general, and with pressurized water nuclear reactors in particular, that the integrity of the containment be maintained in all assumed incidents, i.e. also in case of a core melt, regardless of whether it is a beginning partial core melt or a complete melt-through of the core. The following requirements in particular are set forth for controlling such an incident: a. no fission products must be allowed to escape in large amounts from the core melt into the containment; instead, the core melt must remain covered by continuously cooled water (or another suitable liquid coolant), or a crust must be formed by means of cooling in order to achieve a catching effect; b. the core melt must not be allowed to come into interaction with the concrete of the support structure of the safety container, at least not during the initial days of the event which exceeds the design specifications. That is also true because otherwise hydrogen, water vapor, non-condensable gases and other reaction products could be released; c. cooling of the core melt over a long period of time must be assured, by means of which the post-decay heat is transferred to a heat sink and the melt is caused to harden and is maintained in a solid aggregate state over a long period of time; and d. large-size steam explosions, which can occur when large amounts of core melt mass fall or "plop" into a water bath, must be prevented. With the foregoing and other objects in view there is provided, in accordance with the invention, a nuclear reactor installation, in particular a light water reactor installation, comprising a reactor pressure vessel, a reactor core in the reactor pressure vessel; a supporting and protective structure supporting the reactor pressure vessel and surrounding the reactor pressure vessel on the bottom and laterally, the supporting and protective structure having a bottom region and a circumferential wall; a core catcher device for the reactor core having a collecting basin for a core melt, which can be cooled by means of a cooling liquid and which is installed below the reactor pressure vessel, the collecting basin having a bottom wall and a jacket wall being respectively separated from the bottom region and the circumferential wall of the supporting and protective structure by a spacing gap; cooling channels disposed in the spacing gap at the bottom and on the sides for exterior cooling of the collecting basin with a cooling liquid; and turbulence bodies disposed in a surface region of the bottom wall for generating a turbulent flow of the cooling liquid flowing from the inside to the outside over the bottom wall toward the jacket wall. Advantageous further embodiments of the subject of claim 1 are recited in dependent claims 2 to 21. The advantages which can be attained by means of the invention are to be found mainly in the following: the collecting basin has such a height (at least approximately 3 m) that the minimum height for forming a naturally circulating flow is provided in liquid-filled cooling channels on the bottom and the sides (outer cooling system). The collecting basin protects the concrete of the supporting and protective structure (biological shield) not only with its bottom wall, but also with its upwardly extending jacket wall, against the effects of heat and radiation emanating from the reactor pressure vessel or a core melt. In this case the inner width (inner diameter) and the depth of the reactor cavern is suitably made sufficiently large so that, even with a sufficiently great spacing gap (=gap width of the outer cooling system), the collecting basin encloses a volume which permits the lining of the interior of the base body of the collecting basin, preferably a crucible made of a temperature-resistant steel alloy, with a protective layer and with masonry of shielding concrete blocks, while still providing sufficient receiving space for the possible core melt case. The crucible-like base body and the support by the turbulence bodies on its underside, in which case the turbulence bodies are in the form of turbulence-generating flow guidance bodies, can be easily constructed sufficiently strong and with a load-bearing capacity distributed over the base so that sufficiently large cooling cross sections can be maintained, even under great dynamic and static loads. Based on the large flow-through cross sections of the exterior cooling system, the naturally circulating flow with a corresponding coolant flow rate which can be generated and the generated turbulent flow, it is possible to make the exterior cooling of the collecting basin so effective that film boiling at the exterior cooling surfaces of the collecting basin can be prevented even under the greatest thermal loads. In accordance with another feature of the invention, the cooling channels at the bottom are connected through an inlet channel configuration, and the cooling channels on the jacket are connected through an outlet channel configuration, to a cooling water reservoir provided outside of the supporting and protective structure and forming a reactor housing sump or being connected therewith with such lift that, with a hot collecting basin and water-filled cooling channels, a naturally circulating flow through the cooling channels is generated. The collecting basin can be seated while being suspended from the supporting and protective structure. For this purpose it can be provided, similar to a core container seated and suspended within a reactor pressure container, with a support flange, by means of which it is seated on corresponding support surfaces of the supporting and protective structure. However, the collecting basin is preferably seated on the bottom part of the supporting and protective structure by means of the turbulence bodies (which are then also support bodies), and a dual function (support and turbulence generating) can be achieved in this way. In order to allow for the unhindered heat expansion in the radial direction, the bottom wall of the collecting basin can be seated glidingly and/or resiliently on these support bodies, or the latter can be seated in this manner on the bottom region of the supporting and protective structure. In accordance with a further feature of the invention, the collecting basin is a crucible and for this purpose its bottom wall is curved towards the bottom or the outside, wherein its bottom wall merges into the jacket wall through a rounded-off edge area, and the jacket wall preferably tapers slightly conically from the rounded-off edge area to the upper edge of the collecting basin. In accordance with an added feature of the invention, the bottom wall of the collecting basin widens in the shape of a flat envelope of a cone from the lowest central area to the edge area, and the intersecting surfaces of which, located in axial-radial intersecting planes, extend with a slight angle of slope relative to the horizontal. This slight inclination and the rounding-off in the edge area make the bathing of the bottom and jacket walls of the collecting basin with cooling liquid, especially with water, easier in accordance with the natural circulating principle and in this way make effective cooling possible. In accordance with an additional feature of the invention, in order to provide dynamically balanced even cooling of the collecting basin, an inlet channel configuration discharges into the cooling channels in the central area of the bottom wall of the collecting basin through an inlet chamber, the cooling channels on the bottom extend outwardly from the inlet chamber as far as the edge area of the collecting basin, and an upwardly extending cooling channel adjoins the edge area on the jacket side and terminates in the outlet channel configuration. In accordance with yet another feature of the invention, the inlet channel configuration penetrates through the bottom region of the supporting and protective structure and extends from the bottom wall of a chamber forming the outer cooling water reservoir as far as the central area of the bottom wall of the collecting basin. Accordingly, the outlet channel configuration penetrates through the circumferential wall of the supporting and protective structure, forms a continuation of the cooling channel on the jacket and terminates in the area of the upper level of the cooling water reservoir. In accordance with yet a further feature of the invention, in connection with the protective barrier function of the collecting basin, the base body of the collecting basin is formed of a crucible being formed of a non-corroding, temperature-resistant steel alloy, the interior bottom and jacket surfaces of the crucible are lined with a protective shell used for protecting the crucible material against attacks by the melt, and a sacrificial material deposit follows the protective shell as a second protective layer on the crucible, the amount of which is sufficient for reacting with the maximally possible volume of core melt entering the collecting basin in case of a possible incident. In accordance with yet an added feature of the invention, the protective shell is formed of one of the following alloys, either singly or in combination: MgO, UO.sub.2 or ThO.sub.2. In accordance with yet an additional feature of the invention, the sacrificial material deposit is a masonry structure of shielding concrete blocks. Lining with a deposit of sacrificial material in the form of a granulate or an even more large-grained bulk material or preferably in the form of a masonry facing of shielding concrete blocks serves the purpose of altering the material values of the mixture in a directed way, for example for the purpose of: protecting the wall of the collecting basin against high temperatures immediately following the penetration of the core melt into the collecting basin; using up energy through melting the sacrificial material, with the result of delaying heating-up of the melt and thus being in a position of expecting lower values for the post-decay heat for cooling; making the core melt more fluid; increasing its heat conductivity; increasing its surface; improving the heat transfer from the core melt to the cooling surfaces; preventing steam explosions by displacing volumes of water; providing defined calculation bases by means of the known properties of the sacrificial material; and lowering the melting point of the mixture and the temperature of the melt. In accordance with again another feature of the invention, the channel bodies mentioned above with regard to their property as flow guidance bodies for generating a turbulent flow in the exterior cooling system, are shaped as so-called delta wings in the form of prisms with three-sided surfaces and are fastened at least on the bottom of the supporting and protective surface located opposite the bottom wall with the cooling gap of the collecting basin. Such delta wings have proven themselves to be particularly effective for generating a turbulent flow in the cooling gap. The delta wings assist in preventing a film of steam on the underside of the plate through which the heat transfer number, which is decisive for heat transfer from the heated plate to the cooling water flow, would be undesirably reduced. The natural circulation in the cooling gap can be intensified by means of the generated turbulent flow in such a way, that it is possible to maintain a sufficient safe distance from the so-called critical heating surface load. In accordance with again a further feature of the invention, the channel bodies are also used for support if they are constructed as pipe sockets, and the pipe sockets are provided on their ends facing the bottom wall portion of the collecting basin with channel recesses for generating partial cooling water flows, so that the latter also bathe the bottom wall in the area of the pipe sockets. These pipe sockets can be constructed either as simple flow guidance bodies or as turbulence-generating flow guidance bodies. In accordance with again an added feature of the invention, when the pipe sockets are constructed as turbulence-generating flow guidance bodies, two respective U-shaped channel recesses aligned in the flow direction are provided per pipe socket and the ends thereof are made angular to increase turbulence. As was already explained, the collecting basin also has a radiation shielding function. In accordance with again an additional feature of the invention, the radiation shielding system provided thereby is completed in an advantageous manner by installing a shielding ring above the collecting basin and adjoining it in the annular chamber between the circumferential wall of the supporting and protective structure and the outer periphery of the reactor pressure vessel. In particular, the shielding ring assumes the function of the biological shield in the circumferential area of the reactor core at those places where the circumferential wall (biological shield) is penetrated by outlet channels, so that the radioactive radiation emanating from the reactor core is kept away from the spaces outside the supporting and protective structure. Suitably the shielding ring is formed of shielding concrete, which is also called leca-concrete. The shielding ring has a wall thickness closely approximating the wall thickness of the biological shield (supporting and protective structure) and preferably its extent in height is somewhat greater than its wall thickness. It is also advantageous to incline the top of the shielding ring, so that a larger annular surface is provided as the outlet cross section for air cooling channels. In accordance with another feature of the invention, the shielding ring is anchored or braced on the circumferential wall of the supporting and protective structure. In accordance with a further feature of the invention, the shielding ring is formed of prestressed concrete in particular, and its steel reinforcement is preferably combined into a uniform steel reinforcement system together with the steel reinforcement of the supporting and protective structure which also is formed of prestressed concrete. The shielding ring can be poured on site, in which case an appropriate masonry work has to be provided, or it can be assembled from individual ring segments which are pre-fabricated. In the latter case the ring segments of the shielding ring are advantageously interlocked with each other and with the circumferential wall of the supporting and protective structure. In accordance with an added feature of the invention, the exterior cooling system of the collecting basin is constructed as a dual air and water cooling system which, during the normal operation of the nuclear reactor installation, i.e. when the exterior cooling system is dry, is used for air cooling of the nuclear reactor pressure vessel or of the outside of a thermal insulation enclosing it. For this purpose the inlet channel configuration is connected with a cooling air source and the outlet channel configuration is connected with a cooling air sink. A thermal insulation adapted to the collecting basin and the shielding ring as well as to the dual cooling system is preferably put together of austenitic all-metal cassettes. In accordance with an additional feature of the invention, there is provided a further air cooling system in addition to the exterior dual cooling system which is advantageously used for ventilating an upper air cooling chamber, that is disposed above the collecting basin and is limited at its inner periphery by the thermal insulation enclosing the reactor pressure vessel with an annular gap. In accordance with yet another feature of the invention, the collecting basin is penetrated in the upper half of its jacket wall by at least one melt cooling tube which, with a multi-layer construction of the collecting basin, extends through its crucible wall, protective layer, sacrificial material deposit and thermal insulation, is sealed on its inner end by means of a melting plug, extends with a gradient from the outside to the inside and is attached on the inlet side to a cooling liquid reservoir, so that with a core melt present in the collecting basin, the melting plug is heated to its melting temperature and caused to melt and in this way a flow channel to the surface of the core melt for cooling liquid is opened. These features substantially assist in meeting the requirement made under a., above, as well as the requirement c., because surface cooling of the core melt can be achieved thereby. Such surface cooling is not problematic from a safety viewpoint, because the steam, which is not generated suddenly but instead develops continuously, can escape upward through the gaps and cooling gaps that are present and can also condense on the containment walls and the additionally installed recooling heat exchanger heating surfaces, so that the condensed water can again flow into the cooling water reservoir (sump water). Advantageously, the inlet of the melt cooling tube is located outside the supporting and protective structure and is connected with the cooling water reservoir, in which case the melt cooling tube therefore penetrates the circumferential wall of the supporting and protective structure and the spacing gap of the exterior cooling system. With the objects of the invention in view, there is also provided a method for starting and maintaining exterior cooling of a core catcher device of a nuclear reactor installation, which comprises maintaining a cooling water level of the cooling water reservoir at a low water level, during normal operation of the nuclear reactor installation, at which no cooling water can reach the inlet channel configuration of the collecting basin cooling system; feeding emergency cooling water, when a leak occurs in the primary circuit, from the pressure reservoirs to be activated as a function of pressure of the primary circuit into the main coolant lines of the reactor pressure vessel, by feeding the emergency cooling water through the leak location and, if necessary, parallel thereto through further feed locations into the cooling water reservoir; and maintaining a sufficient water volume in the pressure reservoirs to lift the cooling water level of the cooling water reservoir up to a high water level for causing cooling water from the cooling water reservoir to reach the inlet channel configuration and from there the spacing gap of the collecting basin cooling system for filling the cooling system up to the level of the outlet channel configuration, for starting a naturally circulating flow, when the collecting basin is hot, from the cooling water reservoir through the inlet channel configuration to the cooling channels at the bottom wall and the jacket wall of the cooling system and from there through the outlet channel configuration back to the cooling water reservoir. Through the use of this method, according to the object of the invention, the possibility is attained in the case of design or a layout incident to take prepared steps for initiating the naturally circulating cooling of the collecting basin and to employ them. 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 nuclear reactor installation with a core catcher device and a method for exterior cooling of the latter by natural circulation, 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. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.