Patent Number: 054597684
Section: summary

CROSS-REFERENCE TO RELATED APPLICATION This application is a Continuation of International Application Serial No. PCT/DE93/00180, filed Mar. 2, 1993. BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a safety device against overpressure failure of a nuclear reactor pressure vessel in case of insufficient cooling of the core. If the extremely improbable failure of all of the cooling devices of the reactor core is assumed in a nuclear power station in general, and in a pressurized-water reactor nuclear power station in particular, there is a risk of the reactor core overheating. In a pressurized-water nuclear power station, an unacceptable overpressure in the primary circuit is prevented by the pressurizer system containing spray and pressure relief devices. A pressurizer relief tank serves to condense the steam blown off upon opening of the pressurizer valves, relief valves and safety valves and of the volume control system safety valves. The pressurizer relief tank is filled with water to about two thirds, above which there is a nitrogen cushion. In the case of pressurized-water reactors, the primary circuit is at a pressure of, e.g., 158 bar (normal operation). German Published, Non-Prosecuted Application DE 35 26 377 A1, corresponding to U.S. Pat. No. 4,777,013, describes a high-temperature reactor with a reactor pressure vessel and a safety valve, being constructed as a spring valve, for limiting the pressure in the reactor pressure vessel in case of core heat-up accidents. The reactor pressure vessel is lined on its inside with a liner connected to a liner-cooling system. The valve spring of the safety valve is formed of a material having an elastic force which decreases with increasing temperature. When the safety valve is open, the valve spring is exposed to outflowing gas and is connected to the liner-cooling system for the purpose of cooling. The invention is based on the concept of substantially reducing the popping or blow-off response pressure in the cooling circuit of a nuclear reactor as a function of temperature, especially in the primary circuit of a pressurized-water reactor, so that in the very unlikely case of the reactor core overheating, 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 against overpressure failure of a nuclear reactor pressure vessel, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, which permits the above-mentioned criterion to be met and which thus forms a barrier against overpressure failure of the nuclear-reactor pressure vessel in the case of overheating of the core. With the foregoing and other objects in view there is provided, in accordance with the invention, in a nuclear reactor having an interior, a pressure vessel, a coolant conducting surface exposed to primary pressure, such as a wall or pipeline of the pressure vessel, and a core, a safety device against overpressure failure of the pressure vessel upon insufficient cooling of the core, comprising a differential-pressure-loaded pressure relief valve being set in the coolant conducting surface, the pressure relief valve having a hollow guide cylinder, a closure piece in the form of a differential-pressure piston being constructed as a hollow body and being longitudinally displaceable in the hollow guide cylinder between a closure position and an opening position, and a fusible stop sealing and retaining the differential-pressure piston in the closure position, the fusible stop melting due to a threshold temperature heat flow reaching the fusible stop upon reaching an upper threshold temperature in the interior of the reactor, for permitting the differential-pressure piston to move into the opening position. In accordance with another feature of the invention, the pressure relief valve has seating surfaces, the differential-pressure piston has sealing surfaces, and the fusible stop is disposed between the sealing surfaces and the seating surfaces. In accordance with a further feature of the invention, the differential-pressure piston has peripheral piston surfaces, the guide cylinder has an inner periphery with guide surfaces, and the fusible stop is additionally disposed between the peripheral piston surfaces and the guide surfaces. In accordance with an added feature of the invention, the pressure relief valve has a valve body with a wall and an inner periphery and the guide cylinder has an outer periphery, defining an annular duct remaining free as an overflow duct between the inner and outer peripheries; the pressure relief valve has vanes being disposed in the annular duct and joined to the wall of the valve body for holding the guide cylinder in a centered position in the valve body; and the overflow duct has an inlet cross section being normally sealed by the differential-pressure piston in the closure position and being cleared and released in the opened position. In accordance with an additional feature of the invention, the guide cylinder has an end facing away from the differential-pressure piston, and the end has a bottom with a pressure relief orifice formed therein. The main advantages which can be achieved by means of the invention are that, when a certain threshold temperature in the reactor core is reached, which is distinctly below the failure temperature of the reactor pressure vessel, the fusible stop is caused to melt and the closure piece is thus released. The system pressure (reactor pressure) causes the preferably employed differential-pressure piston to be displaced in its guide cylinder as far as a piston end stop. After the piston end stop is reached, the system pressure is reduced through the relief cross section which is thus opened, to values below 30 bar. In accordance with yet another feature of the invention, the pressure relief valve is set into the wall of a primary coolant pipe near the nuclear reactor pressure vessel. In accordance with yet a further feature of the invention, the pressure relief valve is set into the wall of the pressure vessel at the level of the primary coolant pipe sockets and in wall sections between them. In accordance with yet an added feature of the invention, the pressure relief valve is connected to a pressurizer discharge line opening into a pressurizer relief tank. In accordance with a concomitant feature of the invention, with correspondingly lower cross-sectional dimensions of the pressure relief valve and the lines connected thereto, the pressure relief line is alternatively constructed as a control line for a separate relief valve. Among fusible alloys, silver solder alloys have been found to be particularly advantageous and they are stable and radiation-resistant in a temperature range up to approximately 700.degree. C. 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 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.