Patent Number: 058898301
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a Continuation of International Application Serial No. PCT/DE95/01823 published as WO96/20485 Jul. 4, 1996. BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The invention relates to a cooling system having a cooling pipe for cooling a containment chamber that serves to receive core melt of a reactor core of a nuclear power plant. In order to provide safe operation, nuclear power plants have numerous diverse and redundant safety systems, including cooling systems, through the use of which operating conditions that deviate from normal operating conditions can be detected early and counteracted. As a result, such safety-critical states as reactor core meltdown are practically precluded. In order to control that kind of accident, which is considered hypothetical, German Published, Non-Prosecuted Patent Application DE 40 41 295 A1, corresponding to U.S. Pat. No. 5,343,506, describes a core retainer and a method for emergency cooling of a nuclear power plant. The core retainer has a catch basin, which is disposed immediately below the reactor pressure vessel that encloses the reactor core. Both the catch basin and the reactor pressure vessel are disposed inside a reactor cavern, which is a concrete structure. Cooling channels extend along the floor and the walls of the catch basin between the catch basin and the concrete structure and coolant water can be carried through the cooling channels. The cooling channels on the floor communicate with a water supply and discharge into a cooling pipe that protrudes in siphonlike fashion into the water supply. The siphonlike cooling pipe includes one part shaped as an inverted U. The apex of the U is located above an operative level of the water supply, and although the cooling pipe does dip into the water supply, in the vicinity of its apex it protrudes out of the water supply. As a result, as long as the level is at the operative level, no coolant water enters the cooling channels. It is not until the water supply is flooded to a level higher than the apex of the U that coolant water enters the cooling channels, resulting in cooling of the outside of the catch basin. Cooling of the interior of the catch basin is carried out through a flood pipe, which is passed from the water supply through the concrete structure into the catch basin. The flood pipe is closed in the catch basin by a meltable stopper that does not melt open until at a high ambient temperature, thus allowing coolant water to flow into the interior of the catch basin. Coolant water is present in the flood pipe even during normal operation of the nuclear power plant, and as a result the meltable stopper is continuously cooled. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a cooling system for cooling a retention or containment chamber constructed for receiving a core melt, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which initiates cooling of a catch basin by passive measures and therefore inherently safely. With the objects of the invention in view, there is also provided a cooling system for cooling a containment chamber for receiving core melt of a reactor core of a nuclear power plant, comprising a flooding container to be filled with coolant fluid; a cooling pipe leading from the flooding container to the containment chamber; and a passively opening closure element closing the cooling pipe in the flooding container and opening as a function of a level of the coolant fluid. Through the use of a closure element that opens as a function of the level of the coolant fluid, it is assured that cooling of the containment chamber will not ensue by feeding of coolant fluid into the flooding container until a safety-critical state exists. The coolant fluid that additionally flows into the flooding container is preferably primary coolant water, which emerges from the primary coolant loop of the reactor core during the safety-critical state. Coolant fluid can optionally be fed into the flooding container through a separate coolant fluid reservoir. The cooling pipe is closed until the closure element opens and therefore is free of water. As a result, during normal operation of the nuclear power plant, coolant fluid and particularly coolant water is kept away from the containment chamber, thus averting such problematic factors as corrosion from coolant water or unintended cooling of a temperature-dependent closure element that closes the cooling pipe. Moreover, the inherent safety of the nuclear power plant is improved by the passively opening closure element, and human error in initiating cooling of the containment chamber is precluded. In accordance with another feature of the invention, the closure element is a float that closes off the cooling pipe. At an operative level of the coolant water, this float has such buoyancy that it sealingly closes the cooling pipe, for instance through a ball seat. The float is preferably movable along a primary axis in a guide, so that unintended slippage of the float from its sealing seat is avoided even upon the occurrence of jarring of the kind that can be caused by earthquakes, for instance. In accordance with a further feature of the invention, the float has an interior that can be filled with coolant fluid. A filler pipe passing into this interior has an inlet opening for coolant fluid, through which the coolant fluid flows in if a flooding level occurs that rises above an operative level. The inlet opening may be located geodetically above the operative level or geodetically below this operative level. In the latter case, the filler pipe is extended from the inlet opening in a U above the operative level, so that an apex of the inverted U is located above the operative level. In this latter case as well, coolant fluid does not flow into the interior of the float until the operative level has been exceeded by a predeterminable amount. Coolant fluid flowing into the interior lessens the buoyancy of the float, so that beyond a certain fill level of the interior, the float leaves its sealing seat, thereby opening the cooling pipe. Cooling of the cooling pipe thus ensues in a passive way. In accordance with an added feature of the invention, the float has a condensed water suction removal device, by which condensed water that occurs can be removed by suction during a normal operative state of the nuclear power plant. As a result, lowering of the float from the occurrence of condensed water and an attendant unintentional initiation of cooling of the containment chamber are reliably avoided. In accordance with an additional feature of the invention, the containment chamber communicates with the flooding container through a return for coolant fluid that extends geodetically above the cooling pipe, and in particular above the operative level. This return is closed in the flooding container by a further closure element that opens as a function of the level. Internal cooling of the containment chamber by a coolant fluid loop is attained through the use of the return. Coolant fluid flowing from the flooding container to the containment chamber flows in natural circulation. This assures that during a safety-critical state of the nuclear power plant, sufficient coolant fluid is returned to the flooding container, and cooling of the containment chamber and in particular of the core melt received in the containment chamber occurs. In accordance with yet another feature of the invention, the closure element that closes the return to the flooding container and which may also be a float, has a ball valve. This ball valve may have a floatable ball, which is held in a sealing position by a guide path. The ball valve protects the closure element from a pressure wave which can occur, for instance, from a temperature increase inside the containment chamber. If coolant water flows out of the containment chamber into the return, the ball of the ball valve floats upward and thereby opens the return to the flooding container. In accordance with yet a further feature of the invention, the cooling pipe is a flood pipe, which discharges into the containment chamber and thereby assures direct cooling particularly of the surface of any core melt that has flowed into the containment chamber. The flood pipe preferably extends horizontally and can be both installed and removed by working from the flooding container. Installing the flood pipe in the containment chamber from the flooding container has the advantage of permitting the mounting to be provided outside the containment chamber, which may be poorly accessible and might be affected by radiation. This is especially favorable in the case of a containment chamber that surrounds the reactor core, since this installation can be carried out after the containment chamber is lined in the usual way with a crucible-like guard and collection layer. In accordance with yet an added feature of the invention, during normal operation of the nuclear power plant, the flood pipe is closed in the containment chamber with a closure element that opens as a function of temperature. During normal operation of the nuclear power plant, it is filled with air, and as a result the closure element that opens as a function of temperature is thermally insulated from the coolant water of the flooding container, and coolant water does not enter the flood pipe until during a safety-critical state of the nuclear power plant, so that the effects of corrosion are reliably avoided. As a result of the thermal insulation of the closure element that opens as a function of temperature, reliable opening, and in particular melting open, in the event of major heat development inside the containment chamber, are assured. The closure element that opens as a function of temperature can therefore be constructed in such a way that it opens the flood pipe only at high temperatures as compared with a closure element that is in direct contact with coolant water. The closure element that opens as a function of temperature is preferably resistant to neutron radiation, which occurs during normal operation of the nuclear power plant in the immediately vicinity of the reactor core and particularly in the reactor cavern that receives the reactor pressure vessel. Moreover, it has the advantage of using only a single melting element (melting screw, melting strip), so that canting and thus belated opening of the closure element as would occur if there were a plurality of elements melting at different times, is averted. The closure element is furthermore adapted to the cross section of the flood pipe, so that installation of the flood pipe with the closure element already assembled is assured. In accordance with yet an additional feature of the invention, the closure element that opens as a function of temperature has a material that melts open at a high temperature, for instance above 900.degree. C. This material may be corrosion-resistant and radiation-resistant and in particular may be silver. The closure element that opens as a function of temperature may have a bale closure with a silver tightening screw. The bale closure presses a cap sealingly into the flood pipe, so that this flood pipe is closed with certainty during normal operation of the nuclear power plant. In accordance with again another feature of the invention, the closure element that opens as a function of temperature can alternatively have a closure cap that is sealingly soldered to the flood pipe. Silver can also be used as the soldering substance. In accordance with again a further feature of the invention, the containment chamber has an external cooling device for externally cooling at least a floor and/or one wall of the containment chamber. The cooling pipe is a supply line connecting the external cooling device to the flooding container. During normal operation of the nuclear power plant, the supply line is closed by a float. The external cooling device preferably has a drain line for the coolant fluid, which returns to the flooding container. As a result, coolant fluid, in particular primary coolant water that has flowed into the flooding container, returns to the flooding container again, so that a coolant loop is provided for the external cooling of the containment chamber. In accordance with again an added feature of the invention, the containment chamber is a crucible-like catch basin disposed below the reactor core. Cooling of the catch basin, which ensues passively by a float disposed in the flooding container, takes place on the outside of the catch basin by the external cooling device and/or in the interior of the catch basin through the use of a flood pipe. Preferably, a flood pipe is extended thermally elastically from the flooding container to the catch basin, discharging into the latter. The flood pipe has a compensator outside the catch basin, in particular between the wall of the catch basin and a concrete structure that forms a reactor cavern. The compensator, which in particular is welded on and has a welded-on spherical flange, seals off the catch basin that has an interior with a temperature of approximately 300.degree. C., for instance, from the external cooling of the catch basin, which has a temperature of 20.degree. C. to 30.degree. C. The compensator serves to compensate for thermal expansions of the catch basin and additionally assures sealing off of the flood pipe from a coolant fluid flow for cooling the outer wall of the catch basin. In accordance with a concomitant feature of the invention, the cooling system is also suitable for cooling a propagation chamber located laterally below the reactor core. The interior of the propagation chamber may be cooled by a flood pipe, which extends from a flooding container into the propagation chamber. External cooling of the propagation chamber by suitably extended cooling channels, which are flooded with coolant fluid through a passively opening closure element, such as a float, inside the flooding container, is also possible. 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 cooling system for cooling a containment chamber constructed for receiving a core melt, 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.