Patent Number: 059075880
Section: summary

BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a device for collecting and cooling core melt from a reactor pressure vessel (RPV). It is, in particular, usable in the case of an EPR pressurized water reactor. German Published, Non-Prosecuted Patent Application DE 43 19 094 A1, corresponding to International Publication WO 94/29876 and U.S. application Ser. No. 08/569,676, filed Dec. 8, 1995, now U.S. Pat. No. 5,867,548, discloses a device for collecting and cooling core melt from an RPV, in which a prechamber that is connected through a channel to a spreading chamber, is disposed below the RPV. On the reactor side, the channel has a barrier wall or separating wall which is destroyed by the core melt within a predetermined time after the core melt has arrived. The prechamber is constructed approximately frustoconically and is bounded at the bottom by a refractory concrete base which serves somewhat as a crucible. The concrete base should be made of a refractory ceramic or special bricks. In that case, when the material alloys with the core melt the effect of the material is preferably to reduce the melting point, with the result that it makes the core melt less viscous. The bottom of the spreading chamber is lined with a heat-resistant material. German Patent DE 43 06 864 C2 discloses a safety configuration for a nuclear reactor, in which a collection device for the core melt is disposed below the reactor pressure vessel. In that case the collection vessel is a structure made of cast elements of cast iron and/or cast steel, which forms an outflow system for the core melt. The collection device in that case corresponds, in comparison to the above-mentioned prior art, to the spreading chamber located therein, which is disposed laterally next to the RPV in that device. In that configuration, no prechamber is provided. The purpose of the collection device is clearly to cool the collected core melt in an accumulation chamber. In the case of a core melt gradually flowing downwards, plugging of the supply channels can possibly occur as a result of excessive cooling. Additional cooling is prevented in that way, as a result of which further damage can possibly occur. German Published, Prosecuted Patent Application DE-AS 22 34 782 discloses a reactor core vessel, in which a receptacle for collecting the core in the event of a core meltdown is disposed below fuel elements. In order to provide cooling, the receptacle is constructed in such a way as to be ribbed in the manner of a heat sink on the side remote from the core and is in contact with the water of a coolant circuit. The core melt can be cooled in this way. The receptacle is disposed inside the RPV in this case. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a device for collecting and cooling core melt from a reactor pressure vessel, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which achieves improved flow of the core melt into its spreading chamber. With the foregoing and other objects in view there is provided, in accordance with the invention, a device for collecting core melt from a reactor pressure vessel, comprising a prechamber disposed below the reactor pressure vessel; a spreading chamber disposed laterally next to the reactor pressure vessel for receiving the core melt; a channel connecting the spreading chamber to the prechamber; and a base unit forming a bottom region at least of the prechamber, the base unit made of a material having such a high thermal conductivity that a crust forms after arrival of the core melt on the base unit. In this way, the core melt first of all forms a crust on the base unit which acts as an autogenous crucible. The base unit is thereby initially protected from damage. This is achieved by virtue of the good heat dissipation of the base unit. The autogenous crucible has an insulating effect on the remaining core melt, so that the latter at first maintains its consistency (liquid). After a barrier wall has been opened, the core melt then flows continuously or constantly into the spreading chamber. The intention in this case is for all of the core, as far as possible, to flow at once out of the prechamber into the spreading chamber. In accordance with another feature of the invention, the base unit is made of metal. The desired thermal conductivity is thereby produced, which leads to good crust formation. In accordance with a further feature of the invention, the base unit is composed of subunits. The device is thereby easy to transport and simple to install. In accordance with an added feature of the invention, the base unit is constructed on the prechamber side in the manner of a crucible. A container structure is thereby obtained which allows favorable thermic handling of the core melt. In accordance with an additional feature of the invention, the base unit has a cooling device. Melting of or damage to the base unit is thereby prevented. Long-term thermal overloading is additionally prevented. In accordance with yet another feature of the invention, at least the prechamber, and optionally the channel, is filled with packing units. Additional stress on the prechamber due to a steam explosion is thereby likewise avoided. In accordance with yet a further feature of the invention, the bottom surfaces of the channel and/or the spreading space are formed by cooled bottom elements. Good dissipation of the residual heat is possible in this way. Such bottom elements can be laid easily and additionally provide equalization in the event of temperature fluctuations. In accordance with yet an added feature of the invention, in order to provide good sealing with regard to the coolant, the bottom elements have tongue and groove connections or a stepped connection at their connection points or locations. In accordance with a concomitant feature of the invention, there is provided a flexible seal interposed between the bottom elements. This provides a good mechanical connection and liquid leak-tightness. 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 device for collecting core melt from a reactor pressure vessel, 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.