Patent Number: 055263852
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a Continuation of International application Ser. No. PCT/DE93/00181, filed Mar. 2, 1993. BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a safety device protecting against overpressure failure of a nuclear reactor pressure vessel due to overpressure in the event of insufficient core cooling. A relief device for a pressure vessel is described in Australian Patent No. 18,171/67. The relief device includes a fusible plug which is fitted in such a way that it blocks a flow channel leading out of the pressure vessel. The plug is constructed in such a way that it is solid below a critical temperature and breaks up at the critical temperature. If, in a nuclear power station in general and in a pressurized-water reactor nuclear power station in particular, the extremely improbable failure of all of the cooling devices of the reactor core is assumed, there is a risk of the reactor core being overheated. In a pressurized-water nuclear power station, an inadmissible overpressure in the primary circuit is avoided by the pressurizer system with spray devices and blow-off devices. A blow-off vessel serves to condense the steam blown off upon opening of the pressurizer valves, blow-off valves and relief valves and of the volume control system relief valves. The blow-off vessel is filled with water up to about two thirds, and above that is a nitrogen cushion. In pressurized-water reactors, the pressure in the primary circuit is, for example, 158 bar. The invention starts from the consideration of substantially reducing the blow-off response pressure in a cooling circuit of a nuclear reactor, especially in the primary circuit of a pressurized-water reactor, as a function of temperature, in such a way that in the highly improbable case of overheating of the reactor core, the primary circuit pressure is automatically reduced to values below 30 bar. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a safety device protecting against overpressure failure of a nuclear reactor pressure vessel, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which allows the above-stated criterion to be fulfilled and therefore forms a barrier against failure of the nuclear reactor pressure vessel due to overpressure in the case of core overheating. With the foregoing and other objects in view there is provided, in accordance with the invention, in a nuclear reactor having a core and a pressure vessel with a wall and an interior, a safety device protecting against overpressure failure of the pressure vessel upon insufficient cooling of the core, comprising a pressure pipe passing pressure-tightly through the wall and extending into the interior of the pressure vessel, the pressure pipe having at least one pressure compensation opening formed therein in the interior of the pressure vessel and having a fusible sealing body sealing the at least one pressure compensation opening; the fusible sealing body being formed of a melting solder melting at a limit temperature and unblocking the at least one pressure compensation opening, but keeping the at least one pressure compensation opening sealed during normal operation. The limit temperature is preferably within a range of from 600.degree. C. to 700.degree. C. In accordance with another feature of the invention, the pressure pipe is a blow-off pipe and the pressure compensation opening is a pressure relief opening. In this case, the safety device serves to degrade the pressure directly with the pressure pipe leading, as a blow-off line, into a blow-off vessel outside the pressure vessel. In accordance with a further feature of the invention, for purposes of indirect pressure degradation, the pressure pipe is a pressure control pipe, by means of which a blow-off valve provided outside the vessel can be triggered for reducing the system pressure. In accordance with an added feature of the invention, regarding the measurement location and the mounting of the pressure pipe, the pressure pipe is sealingly guided in a suspended configuration through a cover branch of the pressure vessel and extends with a perforated pipe head sealed by means of the fusible sealing body into the interior of the pressure vessel. This perforated pipe head is then located, for example, in the immediate vicinity of the upper core grid plate and, in the event of overheating of the core or of the upper core region, would very rapidly "sense" the increase in temperature. In accordance with an additional feature of the invention, which does not require a penetration of the cover, the pressure pipe is positioned with a perforated pipe head sealed by means of the fusible sealing body underneath the reactor core, in particular inside a lower core structure, and is laid downwards in the interior of a bottom hemisphere and next to it in an annular space (between the core vessel and the pressure vessel wall) upwards up to a pressure-tight penetration located in a wall region between main coolant branches. Since the lower core structure is located in the immediate vicinity of the reactor core and is connected with metallic conductivity to the core vessel, the perforated pipe head is also able to reliably and rapidly "sense" core overheatings at this point. In accordance with yet another feature of the invention, the pressure pipe is constructed as a blow-off pipe, the perforated pipe head is sealed at the end surface by a pipe plug and has a plurality of mutually adjacent pressure compensation openings in its pipe shell wall, and the pressure compensation openings are sealed by a fusible sleeve soldered to the pipe shell wall. In accordance with yet a further feature of the invention, for reasons of redundancy and of achieving a sufficiently large blow-off cross-section, there is provided a plurality of adjacent rings of pressure compensation openings being coaxial with the pipe. In accordance with yet an added feature of the invention, the pipe plug has a conical profile with a rounded tip, in order to minimize the influence upon the normal coolant flow in the interior of the pressure vessel. If the pressure pipe being used is a pressure control pipe, as already stated above, the passage cross-section of the pipe as well as of its pressure compensation openings can be made smaller than in the case of a blow-off pipe. In accordance with yet an additional feature of the invention, the pressure pipe which is constructed as a pressure control pipe is sealed at its end extending into the interior of the pressure vessel, in particular by a cover, and has at least one pressure compensation opening in its shell wall, inside which a spherical metal body is embedded in a fusible sealing body. In accordance with again another feature of the invention, the pressure compensation opening is an oblique opening having an opening axis which is oriented obliquely inwards, so that, in the event of fusion, the spherical metal body drops into the interior of the pressure control pipe. In accordance with again a further feature of the invention, a brazing silver alloy with a relatively high silver content (for example 50%) is used as the melting solder for the fusible sealing body. In accordance with again an added feature of the invention, the desired limit temperature region, which advantageously is at 700.degree. C., for example, can be adjusted by means of the melting solder alloy. In accordance with a concomitant feature of the invention, the silver solder is temperature and radiation-resistant below the response temperature. 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 safety device protecting against overpressure failure of a nuclear 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.