Patent Number: 054426677
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a conventional reactor pressure vessel for a nuclear power plant which has a spherical lower part 2 and a cylindrical upper 4. A line of symmetry is shown at reference numeral 5. Both parts 2, 4 have walls that are substantially smooth, both inside and out. The parts 2, 4 are intimately joined together in the region of a transition point 6. Today, the parts 2, 4 are usually made of a ferritic steel material. The lower part 2 has a relatively thin wall with a thickness a'. The thickness a' is selected for a rated internal pressure p.sub.i, which may ,amount to 160 to 170 bar, for instance. A wall thickness A' of the cylindrical upper part 4 is greater by a factor of approximately 2. It too is selected for the rated internal pressure P.sub.i, plus a safety margin. Typical values are a'=15 cm and A'=25 cm. It is significant to note that until now, the walls of the parts 2, 4 have been constructed homogeneously, and each with a uniform thickness. The reactor pressure vessel of FIG. 2 is a departure from the structure of FIG. 1. As will be described in further detail below, this pressure vessel includes limited failure zones or regions in the lower part 2 and optionally in the upper part 4 as well. FIG. 2 shows a reactor pressure vessel according to the invention, which is intended for a pressurized water reactor and which again has a dome-shaped lower part 2 and a cylindrical upper part 4. The transition region is again indicated by reference numeral 6. The lower part 2 has first subregions 8 with a lesser wall thickness a and second subregions 10 with a greater wall thickness A. The lesser wall thickness a is selected in this case for rated operation of the reactor pressure vessel at the internal pressure P.sub.i. Correspondingly, in the upper part 4, the reactor pressure vessel has first subregions 12 of lesser wall thickness b and second subregions 14 of greater wall thickness B. Once again, the lesser wall thickness b is selected for rated operation. In particular, the wall thickness a may be equal to the wall thickness a' (that is a=a'), and the wall thickness b may be equal to the wall thickness A' (that is b=A'). The first subregions 8 in the lower part 2 are formed by recesses 18 in the outer surface 16. Correspondingly, the subregions 12 of lesser wall thickness b in the upper part 4 are formed by recesses 22 in the outer surface 16. The recesses 18, 22 are rounded on the inside, and in particular have a dome-shaped construction. The recesses 18, 22 have a symmetrical configuration, as seen in the direction towards the outer surface 16. They can accordingly be round, elliptical or hexagonal in appearance. All of the recesses 18, and therefore all of the first subregions 8, in this case are of the same size. Correspondingly, all of the recesses 22, and therefore all of the first subregions 12, are of the same size. A ferritic steel material is also preferentially used as the wall material in this case in the lower and upper parts 2, 4. The least wall thickness a in the aforementioned first subregions 8 in the lower part 2 is approximately 15 cm, and the remaining wall thickness A in this case is approximately 25 or more. In the upper part, the least wall thickness b in the aforementioned first subregions 12 is approximately 25 cm, and the remaining wall thickness B is more than 25 cm. In this embodiment example, b=A' has accordingly been chosen. The diameter of each of the recesses 18, 22 is approximately in the range from 0.5 to 2 m. In the transition region 6 between the lower part 2 and the upper part 4, an additional annular wall thickening or ring 26 is also provided or machined onto the outer surface 16 of the reactor pressure vessel. This ring 26 is additionally intended to serve as a safety measure and to absolutely prevent the spherical lower part 2 from tearing away from the upper part 4. In the lower part 2, at least eight first subregions 8 of lesser wall thickness a are provided in this case. If the number of recesses 18 is increased from 8 to 15, then one can still assume that the reactor pressure vessel can be tested relatively easily from the outside by the so-called recurrent testing method. If the number of first subregions 8 is increased even further, for instance to 30, then the theoretically assumed accident scenario in this case becomes even easier to control. The first subregions 8 having the lesser wall thickness a are preferably symmetrically distributed in the lower part 2. This is expressed by the line of symmetry 5 shown in the drawing. It is important above all that the first subregions 8 which have a lesser wall thickness a but are adapted to the rated operation, be provided in the dome-shaped lower part 2. However, as is additionally shown, such first subregions 12 with a lesser wall thickness b may also be disposed in the cylindrical upper part 4. The following can be said about the function of the device: The wall of the reactor pressure vessel is structurally developed in such a way that the first subregions 8, 12 provided there fail first if there is a thermal or mechanical overload. One can refer to these first subregions 8, 12 as rated breaking points. In other words, in the generally substantially thicker wall (with the thickness A or B), the first subregions 8, 12 having the lesser wall thickness a and b are distributed spatially. These subregions 8, 12 can also be called "mesh eyelets", or spaces The lesser wall thickness a, b is selected for the loads in rated operation. The "thicker" zones, that is the second subregions 10, 14 having the greater wall thickness A and B, then represent a reinforcement of the vessel wall. This can be considered to be an additional "load-bearing framework". For the sake of simplicity, only the lower part 2 will be considered below. In the case of local thermal overheating, for instance in the case of being moistened by a falling core melt 30, the thinner wall regions 8, that is the mesh spaces, will fail first. This is because they are heated much more rapidly than the thicker wall of the load-bearing framework, in which a substantially lesser mechanical strain also prevails because of the internal pressure P.sub.i. In the event of a failure, a crack reaching all the way through first arises in the mesh space or first subregion 8. At the existing internal pressure P.sub.i this crack can rapidly spread to the load-bearing framework formed by the two second regions 10. When it reaches them, it is either stopped or deflected. The entire first subregion 8, or in other words the entire area of the mesh space, can then open and fall out. The vessel medium (or coolant such as water vapor, or the liquid core melt 30) can flow out through the thus-formed opening and through other mesh space openings that may also be present. This causes the internal pressure P.sub.i to fall, and as a result further propagation of the vessel failure is in turn stopped. The second regions 10 having the greater wall thickness A accordingly assure that in the event of a crack, the destruction of the vessel wall will proceed relatively slowly. As a result, a reaction surge of high amplitude cannot develop, which in turn means a reduction in the reaction force exerted on the restraint, mounting or fixation of the reactor pressure vessel. It is accordingly important to ensure that a global pressure vessel failure, from ripping off of the entire lower part 2, for instance, will not immediately ensue. Instead, at most, one mesh space after the other can fail, which means a considerable gain in time and thus a lengthening of the time during which the vessel medium flows out.. In this way, the reaction forces that arise can be limited. Even if the reactor pressure vessel rips away from its restraint, mounting or fixation, a threat to the surrounding concrete building should not occur. In closing, it will also be noted that the provisions according to the invention can in principle be employed with any pressure vessels in which additional safety is to be assured, with examples being gas vessels or containers for the chemical industry.