Patent Number: 054426677
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a reactor pressure vessel for a nuclear reactor. Such a reactor pressure vessel typically includes a lower part which is constructed as a dome, and an adjoining upper part which is constructed cylindrically. As a rule, the reactor pressure vessel is restrained by so-called fastening claws. Today, the raw material used is typically ferritic steel material which was developed for that special application and is known by the designation 20MnMoNi55. In operation, the reactor pressure vessel is at a high internal pressure, which can amount to as much as approximately 170 bar. In order to withstand that pressure, a lower part is constructed with a wall thickness of approximately 15 cm and an upper part with a wall thickness of approximately 25 cm. In the nuclear power plant, the reactor pressure vessel is surrounded by a reactor protection building (containment), which is substantially formed of concrete. Heretofore, the conventional thinking on safety considerations assumed that there was no need to fear reactor pressure vessel failure, because of the materials and dimensions selected. However, more-intensive safety studies of nuclear energy utilization have also considered the possibility, however unlikely, of a "failure" of a reactor pressure vessel. In particular, one new reactor type, the European Pressurized water Reactor or EPR, is based on such considerations. In contrast to the earlier philosophy of safety, in that reactor type the possibility of a core meltdown accident, a so-called MCA (Maximum Credible Accident), is not rejected out of hand. Some thought has also been directed to whether steam explosions might not occur during a core meltdown, and whether in such a critical phase, suddenly produced water vapor might not cause the pressure vessel to burst. There is no question that control must be gained over such accidents, however theoretical they may be. The point of departure of theoretical studies is this: in an overload of a thermal nature (overheating) or of a mechanical nature (overpressure), and in particular in the event of a core meltdown accident, a crack that is propagated at high speed could occur locally in the homogeneous wall of the reactor pressure vessel. The crack can then spread, out of control, to relatively large regions. The possibility exists that a relatively large region could break up, for instance the entire dome in the lower part. Something similar could happen if the lower part of the reactor pressure vessel fills with core melt, given the high internal pressure. If a part that large were to break up, a reaction surge would occur within far less then one second, and in that surge the upper part of the reactor pressure vessel could be torn from the fastening claws and be spun like a rocket upward against the inner wall of the containment. The containment must withstand such an impact. The containment must also offer sufficient resistance if the reactor pressure vessel "explodes", or in other words breaks apart into a number of relatively large or small pieces. Even with relatively heavy concrete construction, it is difficult to absorb such explosion-like effects. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a reactor pressure vessel for a nuclear reactor with limited failure zones, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which has further increased safety. In other words, the safety of the nuclear power plant should be assured, even in the unlikely event of a crack in the vessel wall due to thermal and/or mechanical overload (overheating, overpressure). This is intended to apply especially in the event of a core meltdown in the reactor pressure vessel. The invention is based on the concept that a spontaneous global failure of the reactor pressure vessel must be averted or limited. This refers in particular to a breakup of a relatively large part, for instance of the entire lower dome-shaped part, in the vessel wall. If this is successfully averted or limited, then in the event of a credible accident, a reactor surge high enough to be uncontrollable cannot occur. In other words, in that case the forces of reaction acting upon the restraint or fastening (fastening claws) of the reactor pressure vessel can be controlled constructively. The invention is also based on the concept that the reaction surge can be kept small by lengthening the time over which a cracking event may proceed, however unlikely. With the foregoing and other objects in view there is provided, in accordance with the invention, a reactor pressure vessel for a nuclear reactor, comprising an upper part and a lower part, at least the lower part having first subregions with a lesser wall thickness and second subregions with a greater wall thickness, the lesser wall thickness being selected for a rated operation. In accordance with another feature of the invention, the first subregions with the lesser wall thickness are recesses in the outer surface of at least the lower part, which is a comparatively simple construction structurally. Experience shows that the wall of a reactor vessel can be machined more easily from outside than from inside. Moreover and especially significantly, the ease of checking the reactor pressure vessel from outside is assured, because then the inside region can have smooth surfaces. Moreover, fixtures on the inside for draining a hypothetically occurring core melt can then be dispensed with. In accordance with a further feature of the invention, the recesses are rounded on the inside and in particular are dome-shaped. In accordance with an added feature of the invention, the recesses have a symmetrically configuration, when viewed in the direction toward the outer surface of the reactor pressure vessel, which is also advantageous for production purposes. Accordingly they may be round, elliptical or hexagonal in form, for instance. With respect to the number and size of the recesses, there is a wide range of variation, depending on the intended application and the credible accident. However, in accordance with an additional feature of the invention, for production purposes, all of the subregions or recesses are of the same size. In accordance with yet another feature of the invention, with respect to number, observations have shown that depending on the size, between 8 and 30 first subregions or recesses in the lower part of the reactor pressure vessel are adequate. In accordance with yet a further feature of the invention, the first subregions and[in particular the recesses on the outer surface are distributed symmetrically. In accordance with yet an added feature of the invention, in a reactor pressure vessel, the upper part is preferably cylindrically constructed and the first subregions having the lesser wall thickness are provided in the upper part of the reactor pressure vessel. In accordance with yet an additional feature of the invention, the upper and lower parts are formed of a ferritic steel wall material, the lesser wall thickness in the first subregions is approximately 15 cm, and the greater wall thickness in the second subregions is approximately 25 cm. In accordance with again another feature of the invention, each of the recesses has a diameter being approximately in a range of from 0.5 to 2 m. In accordance with a concomitant feature of the invention, the upper and lower parts have an outer surface, and there is provided a ring on the outer surface between the upper and lower parts. 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 reactor pressure vessel with limited failure zones, 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.