Patent Number: 053435060
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 reactor building R shown in section, which includes a safety vessel 1 that is also referred to as a containment and is formed by a spherical steel sealing skin 3, a reinforced concrete foundation 2 having an appropriate spherical receiver 2.1, and a nuclear reactor installation KA being disposed inside the safety vessel 1 and including installation components and connecting lines of tubes, electrical lines and building structures which are enclosed in a gas-tight manner by the steel sealing skin 3. The latter is enclosed, including a gap, by a non-illustrated concrete containment shell connected with the reinforced concrete foundation 2 and protects the safety vessel 1 from effects from the outside ("Eva"). A concrete structure 4 of the safety vessel 1 is adapted by means of a downwardly oriented convex receiver 4.1 thereof to the convex steel sealing skin 3 and the correspondingly concave receiving surface 2.1 of the concrete foundation 2. At connecting points 5.1 and 5.2 thereof, the concrete structure 4 is connected with the reinforced concrete foundation 2 by means of anchor bolts sealingly penetrating the steel sealing skin 3. A reactor pressure vessel of the pressurized water type, which is identified as a whole by reference numeral 6, is surrounded at a distance in the lateral and vertical direction by a supporting and protective structure 7. This supporting and protective structure 7, with a bottom or bottom region 7.1 and a circumferential wall 7.2 thereof is a component of the concrete structure 4 within the containment 1. A reactor cavern 8 is formed by the bottom region 7.1 and the circumferential wall 7.2, within which the reactor pressure vessel 6 is disposed. A central, recessed bottom part 7.10 of a preferably central inlet chamber 33 which will be described below, is also part of the bottom region 7.1. The essentially hollow-cylindrical reactor pressure vessel 6, having a vertical axis z and being formed of a lower part 6a with a bottom cup 6.1 and an upper part 6b with a top receiver 6.2, is suspended by its lower part 6a in a support ring structure 9. The support ring structure 9 is seated and secured against lifting and twisting, in an annular recess in the circumferential wall 7.2 of the supporting and protective structure 7. The reactor pressure vessel 6 is seated on the support ring structure 9 and is secured against twisting and lifting, within a circular recess with a flange of its lower part 6a and/or suitable lug supports, in a manner which is not shown in FIG. 1. A reactor core 10 is indicated by dashed lines. A steam generator DE of the primary circuit components of the nuclear reactor installation KA which is also shown, is connected to the reactor pressure vessel 6 through a so-called hot leg 11 of main coolant channels HL. The respective hot leg 11 (this is a multi-loop installation) guides hot coolant to a primary chamber 12 of the steam generator DE. The primary chamber 12 is separated from a secondary chamber 13 of the steam generator DE by a tube sheet 14 and U-shaped heat exchanger tubes indicated at reference numeral 15. In addition, the primary chamber 12 is divided by a separating wall 16 into two chamber halves. Thus, the primary coolant flows from the hot leg 11 over one half of the primary chamber 12 into the heat exchanger tubes 15, transfers its heat there to the secondary medium which turns into steam, and is fed back into the interior of the reactor pressure vessel 6 in a circuit through the other half of the primary chamber 12, a so-called cold leg 17 connected therewith, a non-illustrated primary coolant pump disposed in this cold leg 17, and the remainder of the cold leg 17. This can be a so-called two-loop installation, i.e. a pressurized water reactor with two steam generators and a pair of main coolant lines each. This would be the case in the exemplary embodiment of FIG. 1 if a cold leg 17 (only one is shown) were assigned to each of the two hot legs 11. However, it can also be a three-loop or four-loop installation, if it is contemplated that further pairs of legs are added in FIG. 1, or as can be seen from the illustrations in FIGS. 2 and 3. The steam generators DE are seated on the concrete structure 4 by means of support rings 18 in their tube sheet area. A bottom wall 20 of a coolable collecting basin 19 of a core catcher device CC is disposed inside the reactor cavern 8 and below the reactor pressure vessel 6 and a jacket wall 21 of the collecting basin 19 extends upward from the bottom wall 20. The circumferential wall 7.2 of the supporting and protective structure 7 which is vertical or is slightly inwardly inclined as is illustrated, is also referred to as a biological shield, because it constitutes a protective shield against neutrons and gamma radiation. The circumferential wall 7.2 is clad on its inner periphery with a steel liner 22, as are inside surfaces of the bottom region 7.1. Thus, the bottom region 7.1 and the circumferential wall 7.2 are outside of this liner 22 and at a vertical and lateral distance from the collecting basin 19 and are connected with the remainder of the concrete structure 4. The latter is built as a chambered structure, and a reactor sump in the form of a cooling water reservoir 24 with a normal level P1 is disposed in a chamber 23, which should be imagined to be in the approximate shape of a rotational solid and which surrounds the circumferential wall 7.2 (biological shield). A ceiling 25 of this chamber 23 is supported by steel walls 26. A separating wall 27, together with a U-shaped ascending pipe 30, constitutes an inlet structure of an inlet channel configuration 31. The main coolant lines (hot legs) 11 extend through appropriate wall openings 7.3 in the circumferential wall 7.2, and the cold legs 17 extend through similar openings which cannot be seen in FIG. 1. Preferably, the jacket wall 21 of the collecting basin 19 extends approximately at least as far as the lower edge of the reactor core 10, as is illustrated. In this case, a spacing gap 28 is defined between the bottom wall 20 and the jacket wall 21 of the collecting basin 19 relative to the bottom 7.1 and the circumferential wall 7.2 of the supporting and protective structure 7. A cooling system 29 on the outside of the collecting basin 19 which has cooling channels 29.1, 29.2 at bottom and jacket sides is provided inside the spacing gap 28 for the purpose of exterior cooling of the collecting basin 19. The invention is not limited to the spherical containment 1 of FIGS. 1 to 3, but instead it can also be employed with a cylindrical containment, wherein the transition from the concrete structure 4 of the safety vessel 1 to the foundation 2 does not take place through spherical surfaces (as in the embodiment according to FIG. 1), but instead through level transition surfaces. Reference is made below to the detailed illustration in accordance with FIGS. 2 to 6 for further explanation. Parts which are the same as in FIG. 1 have the same reference numerals. The cooling channels 29.1 at the bottom of the exterior cooling system 29 are connected through the inlet channel configuration 31, and the cooling channels 29.2 on the jacket are connected through an outlet channel configuration 32, to the cooling water reservoir 24 which is provided outside of the supporting and protective structure 7 and which forms a reactor housing sump or is connected therewith with such a lifting height that, with a hot collecting basin 19 and a water-filled cooling system 29, a naturally circulating flow in the cooling system 29 through the cooling channels 29.1 and 29.2 is generated. The inlet channel configuration 31 flows into the exterior cooling system 29 of the spacing gap 28 in the central area of the bottom wall 20 of the collecting basin 19 through the inlet chamber 33. The cooling channels 29.1, which are delimited by turbulence bodies 34 and the bottom wall 20 as well as the bottom region 7.1 of the supporting and protective structure 7, extend outward from the inlet chamber 33 as far as a rounded-off edge area 19.1 of the collecting basin 19. Following this, the upwardly leading cooling channel 29.2 on the jacket side extends from the edge area 19.1 as far as the outlet channel configuration 32. As can be seen from FIGS. 2 to 4, the inlet channel configuration 31 extends through the bottom region 7.1 of the supporting and protective structure 7. Inlet channels 31a extend in a star pattern or radially-horizontally from a short vertical inlet channel piece 31b to the inlet chamber 33. A vertical inlet channel piece is constructed as a pump sump chamber 31c of a non-illustrated pump, as is seen in the lower left part of FIGS. 2 and 3. An inlet chamber 35 is placed upstream of the inlet channel piece 31b and in normal operation is separated by the separating wall 27 from the chamber 23 of the cooling water reservoir 24. It is only when the normal level P1 of the cooling water rises, namely to a high water or minimum water level P2, that cooling water reaches the inlet chamber 35 and the remainder of the inlet channel configuration 31 through the ascending pipe 30, as will be described further below. The outlet channel configuration 32 penetrates the circumferential wall 7.2 of the supporting and protective structure 7, forms a continuation of the cooling channel 29.2 on the jacket side and empties into the cooling water reservoir 24 in the area of the upper level P2 of the reservoir 24, which can only be seen in FIG. 1. FIG. 4 shows that outlet channels 32a of the outlet channel configuration 32 are distributed over the circumferential wall 7.2. Six of the outlet channels 32a are shown. Four of the outlet channels 32a are in an axial crossing configuration and two additional outlet channels are in first and third quadrants of parts of the circumferential wall 7.2. As can be seen in FIGS. 2 and 3 (as well as FIG. 1), the collecting basin 19 is constructed in the shape of a crucible, and in order to achieve this, its bottom wall 20 is curved downwardly or toward the exterior. The bottom wall 20 makes a transition into the jacket wall 21 through the rounded-off edge area 19.1. A base body 19a of the collecting basin 19 is formed as a crucible which is preferably formed of a temperature-resistant steel alloy. Interior bottom and jacket surfaces of the crucible 19a are clad with a protective shell 19b, which is used to protect the crucible material against an attack by the melt. Preferably, this protective shell 19b is formed of one of the following alloys: MgO, UO.sub.2 or ThO.sub.2. A sacrificial material deposit 19c follows the protective shell 19b as a second protective layer for the crucible 19a. This sacrificial material deposit 19c is preferably formed of shielding concrete blocks 36, which are connected with each other and the protective shell 19b for forming a masonry facing. A distance between the sacrificial material deposit 19c in the form of the masonry facing and the bottom cup 6.1 of the reactor pressure vessel, is sufficiently great to enable the surfaces of the masonry facing oriented toward the bottom cup to be clad with a heat-insulating shell W1. This heat-insulating shell W1 is the lower portion of a heat insulation which is indicated as a whole by reference symbol W, for the reactor pressure vessel 6. The lower insulating portion W1 has an approximately cup-shaped form. This lower insulating portion W1, as well as a central insulating portion W2 at the interior periphery of a shielding ring 37 and an upper insulating portion W3 extending from the shielding ring 37 to the region of a cover portion gap 38 of the reaction pressure vessel 6, all enclose the reactor pressure vessel 6 with a sufficient gap, so that an air chamber 39 is formed. The collecting basin 19 therefore is a cup or crucible-like multi-layered structure, with the base body 19a in the shape of a crucible which can have a wall thickness of 50 mm, for example, and the protective shell 19b lining the interior surface of the crucible with a wall thickness three times that of the crucible. The wall thickness of this protective shell 19b is preferably increased in a central area 19.0 of the collecting vessel 19, because the greatest temperature stresses can occur in this area in the case of a possible core melt. As mentioned above, the sacrificial material deposit 19c, which is adapted to the contour of the crucible, follows the protective shell and the appropriately adapted lower insulation portion W1. Preferably, the jacket wall 21 of the crucible 19a, that is of the collecting vessel 19, extends from the rounded-off edge area 19.1 to an upper edge 21.1 seen in FIG. 1, in a conically tapering manner. Due to this structure, the contour of the crucible 19a or of the collecting vessel 19 is adapted to the contour of the outer periphery of the circumferential wall 7.2 of the supporting and protective structure 7, and a desired cross section for a spacing gap 28 or the cooling channels 29.2 of cooling system 29 on the jacket side is attained. The bottom wall portion 20 of the crucible 19a of the collecting basin 19 widens in the shape of a flat envelope of a cone from the lowest central area 19.0 to the edge area 19.1, and intersecting surfaces thereof that are located in axial-radial intersecting planes, extend with a slight angle of slope .alpha. relative to the horizontal. This slight inclination of the bottom wall 20 which is present from the central area 19.0 to the edge area 19.1 results in defined flows of cooling water in the channel system 29, in which no air bubbles are formed or maintained (leading to an avoidance of so-called dead cooling zones). Instead, this slight inclination aids the natural circulation. Thus, there is a slight slope in the interior of the collecting basin 19 from the edge area 19.1 to the central area 19.0, so that a possible core melt will always collect in a centered manner in the collecting basin 19 (provided it is in the liquid state). In accordance with a preferred embodiment, the collecting basin 19 is seated in the bottom region 7.1 of the supporting and protective structure 7 by means of the turbulence bodies 34. This does not preclude an additional support, where needed, by means of non-illustrated support bodies. It is also possible to provide turbulence bodies 34d which are only used for turbulence generation (and not for support), as will be described below by means of FIG. 5. The turbulence bodies 34 are inserted in the exterior cooling system 29 between the bottom wall 20 of the collecting basin 19 or the base body or crucible 19a and the bottom region 7.1 and are used for supporting the collecting basin 19 on the bottom region 7.1 and for generating a turbulent flow of the cooling liquid. In FIGS. 2 and 3, only turbulence bodies 34 are shown in the cooling channel 29.1 on the bottom which are not only used for flow guidance and turbulence generation, but also for support. This is also true for central turbulence bodies 34a disposed in the central area 19.0. These turbulence bodies 34a are supported on the central, recessed bottom part 7.10 which is part of the bottom region 7.1 and is located at the level of the lower wall 4.2 of the inlet channels 31. The turbulence bodies 34a are longer than the turbulence bodies 34, because they have to bridge a greater channel height of the inlet chamber 33. The turbulence bodies 34, 34a are distributed within the cooling channels 29.1 on the bottom and inside the inlet chamber 33, in such a way that an even weight transfer into the bottom region 7.1 of the supporting and protective structure 7 is assured, and cooling flow paths 40 seen in FIG. 5 can be formed along a path from the inside, i.e. from the central inlet chamber 33, radially outward to the edge area 19.1 and directed from there into the cooling channel 29.1. The latter is an annular channel. The cooling flow paths 40 can extend in their main direction along radii, i.e. they can be star-shaped or in the form of involutes, for example, in the course of which the turbulence bodies 34, 34a create a turbulence flow, particularly within the cooling channels 29.1 on the bottom when the naturally circulating flow is started in the cooling system. FIGS. 5 and 6 show closer details of the structure and disposition of the turbulence bodies 34 (the same is correspondingly true for the turbulence bodies 34a). Flow arrows for cooling liquid, particularly cooling water, are generally indicated by reference symbol fl and shown in dashed lines. Flow arrows for cooling air are generally indicated by reference symbol f2 and are shown in solid lines (also see FIGS. 1 to 3). Either only cooling air (solid arrows f2) or cooling water (dashed arrows fl) can flow in the cooling channels 29.1 and 29.2, which will be explained below. Heat flow arrows in FIG. 5 for the heat flow emanating from the reactor pressure vessel 6 or a possible core melt and penetrating the collecting basin 19, in particular its base body or crucible 19a and the bottom wall 20 of the crucible, are generally indicated by reference symbol f3 and shown in heavy solid lines. The arrows fl therefore symbolize an emergency cooling water flow in the cooling system 29. FIG. 5 is a diagrammatic, perspective view which shows a section of the cooling system 29, namely in the area of the bottom wall portion 20 of the collecting basin 19 and of the bottom 7.1 with the liner 22 of the supporting and protective structure 7 located opposite a cooling gap al. The turbulence body 34 illustrated therein is constructed as a pipe socket (this embodiment preferably applies to all of the turbulence bodies 34 in FIGS. 1 to 3). In order to distinguish the pipe sockets from the other general turbulence bodies 34, these pipe sockets are designated by reference symbol 34r and in order to distinguish delta wings which generate turbulence and are still to be explained from the other general turbulence bodies 34, the delta wings are indicated by reference symbol 34d. The pipe sockets 34r are provided with channel recesses 41 on ends thereof facing the bottom wall portion 20 of the cooling basin 19. In particular, two U-shaped recesses 41 per pipe socket 34r are provided which are in alignment in the flow direction (main direction of the flow arrow fl) and have edges 41.1 thereof which are made angular to increase turbulence. Partial cooling water flows f11 are generated by the channel recesses 41 with their edges 41.1, which in the area of the turbulence bodies 34 in general and in the area of the pipe sockets 34r in particular are forced into contact with the cooling surfaces of the bottom wall portion 20. It is important to generate a sufficiently large turbulence of the flow within the cooling water flow paths 40 and the cooling paths 40a of the partial cooling water flows f11, to ensure that intimate mixing of the partial cooling water flows is achieved and the formation of a steam film on downwardly pointing cooling surfaces 20.0 of the bottom wall portion 20 is prevented. The so-called delta wings 34d which are in the shape of prisms with triangular surfaces F1 to F4 and which have the shape of tetrahedons, are provided for this purpose. These are fastened at least on the bottom 7.2 located opposite the cooling surface 20.0 within the cooling gap al, or on the liner 22 of the bottom 7.2. The delta wings 34d or flow guidance bodies in general are preferably manufactured of corrosion-resistant steel which in composition is the same as the steel alloy of the liner 22 or is similar to it, so that they can be attached by welding, as is seen by weld beads that are indicated at reference numeral 42. For the sake of clarity, only two delta wings 34d are shown in FIG. 5, and the effect that these flow guidance bodies in the form of the delta wings 34d have on the otherwise mostly laminar flow is schematically indicated by spiraling flow lines f12. Strong turbulence is generated, which increases a safety distance against film boiling on the cooling surfaces 20.0. It can be seen in FIG. 6, that the collecting basin 19 is supported on the bottom 7.1 through its pipe sockets 34r and in a spring-elastic manner with the interposition of a spring element 43. The pipe socket 34r is welded on the bottom wall 20 of the base body or crucible 19a of the collecting container 19 by weld beads 44, in which case again the steel alloys of the pipe sockets 34r and the crucible 19a are adapted to each other in such a way that a compatibility in relation to welding is assured. The spring elements 43 can be helical pressure springs, which are supported by a lower spring plate 43a on the bottom parts 7.1 and through another non-illustrated spring plate at the upper end thereof on the pipe socket 34r. In place of helical pressure springs it is also possible to employ non-illustrated plate springs or plate spring packets, wherein helical pressure or plate springs are appropriately pre-stressed because of the heavy weight to be supported. The underside of the lower spring plate 43a is preferably finely worked, i.e. smoothed, so that the coefficients of friction relative to the adjoining surface of the steel liner 22 becomes as low as possible. By making a sliding movement possible, even if only over small distances, it is possible to prevent constraining forces during heating in the course of a hypothetical case of a core melt. The spring elements 43 can also be constructed as non-illustrated spring rods permitting spring-elastic yielding in the lateral direction to a limited extent. The delta wings 34d shown in FIG. 5 can be used with advantage for the generation of a turbulent flow within a cooling channel, not only within the purview of the exemplary embodiment shown, but also in all places where a liquid coolant flows through the cooling channel and is delimited in the vertical direction by two channel walls disposed above each other and at a distance from each other, namely by an upper first channel wall heated by the heat to be dissipated, and a lower second channel wall provided on the inside with the delta wings 34r. Referring again to FIGS. 1 to 3, it is noted that it would be possible in principle to suspend the collecting basin 19 with its crucible 19a from the supporting and protective structure 7. In this case the jacket wall 21, for example, could be upwardly extended and seated by means of a support flange on its upper end on a support ring inserted into an annular recess in the wall portions 7.2 in the supporting and protective structure 7, in a non-illustrated manner. In such an embodiment too, the turbulence bodies 34, 34a can be used, at least in part, as support bodies, i.e. not only as flow guidance bodies, or they can be disposed in a narrow gap below the bottom wall 20 as a safety measure against a crash. However, the embodiment shown with the seating of the collecting basin 19 on the channel bodies 34, 34a is more advantageous, because in this case the circumferential wall 7.2 (biological shield) is not additionally loaded. Instead, the transmission of the seating forces takes place over the considerably larger surface area of the concrete structure 4 and the bottom region 7.1. Preferably, the collecting basin 19 extends at least as far as about the lower edge of the reactor core 10 seen in FIG. 1 and, as explained at the start, the required lifting height of at least approximately 3 m needed for the natural circulation of the cooling liquid through the cooling system 29 is therefore achieved. In this way, the collecting basin 19 encloses the entire bottom cup 6.1. With the extent in height of the collecting basin 19 as shown, it is a preferred feature of the embodiment to provide the shielding ring 37 seen in FIGS. 2 to 4, which is installed above the collecting basin 19 and adjoins it in an annular chamber 45 seen in FIGS. 2A and 3A between the circumferential wall 7.2 of the supporting and protective structure 7 and the outer periphery of the reactor pressure vessel 6. The shielding ring 37 assumes the function of the biological shield in the area of the core 10 seen in FIG. 1 in the places where the circumferential wall 7.2 (biological shield) is penetrated by outlet channels 32. The shielding ring 37 is preferably made of shielding concrete. Suitable compositions for such shielding concrete can be found in Table XXIV on page 701 of the book entitled: "Nutzenergie aus Atomkernen" [Useful Energy from Atomic Cores] by Dr. K. R. Schmidt, Vol. II, Walter D. Gruyter & Co., publishers, Berlin 1960, so that a detailed description thereof can be omitted herein. The shielding ring 37 is anchored on the circumferential wall 7.2 of the supporting and protective structure 7. Wedge-shaped bars 46 that are evenly distributed over the outer periphery of the shielding ring 37 can be provided for this purpose, as is shown in FIG. 4. It is also possible, such as is indicated by dashed lines in FIG. 3, to provide wedge-shaped support surfaces 47 on the circumferential wall 7.2, in which the shielding ring 37 is interlocked by means of wedge-shaped counter-surfaces 37a. It is advantageous for installing the shielding ring 37 if it is composed of non-illustrated individual ring segments. The ring segments then must be interlocked with each other and the circumferential wall 7.2 of the supporting and protective structure 7 as is seen in FIG. 3, or wedged against it as is seen in FIG. 4. According to another advantageous embodiment, the shielding ring 37 is made of shielding, prestressed concrete, and its steel reinforcement is combined into a uniform steel reinforcement system with the steel reinforcement of the supporting and protective structure 7 also being formed of prestressed concrete. Reinforcing steel cables 48' for such an embodiment are indicated by dashed lines in FIG. 2A. It is possible to provide additional ring clamping cables inside the shielding ring 37, by means of which the individual ring segments, which are interlocked with each other, are clamped together in the circumferential direction in a non-illustrated manner. In order to minimize heat losses of the reactor pressure vessel 6 during normal operation, its heat insulation W seen in FIGS. 2 and 3, is of great importance. Of equal importance is the ventilation of this heat insulation on its exterior by the cooling air flows, which are symbolized in their totality by flow arrows f2. The heat insulation W includes the heat insulating portions W1 to W3 for the lower part 6a of the reactor pressure vessel 6, a movable or removable heat insulating hood W4 extending over the upper part 6b of the reactor pressure vessel 6, and additional heat insulating portions W5 for the main coolant lines HL. Essentially, three insulating portions that merge into each other are provided for the lower part 6a, namely: the lower insulating portion W1 which lines the sacrificial layer of the collecting basin 19 and encloses the bottom cup 6.1 of the reactor pressure vessel 6, the central insulating portion W2 lining the interior periphery of the shielding ring 37, and a ring-shaped connecting piece W21, which extends around the bottom of the shielding ring 37 and provides a connection between the lower insulating portion W1 and the upper insulating portion W3 extending from the shielding ring 37 to the area of the cover portion gaps 38 of the reactor pressure vessel 6 and which is penetrated by a main coolant connection 48. The main coolant connection 48, as well as the adjoining main coolant lines HL are enclosed by the additional insulating portions W5, as was mentioned above. The heat insulation W is preferably constructed of all-metal cassettes which are made of austenitic, i.e. corrosion-resistant steel. Appropriate fastening structures made of lightweight materials for securing these individual cassettes, which can be aligned to form a complete heat insulating shell, are not shown. The exterior cooling system 29 of the collecting basin 19 is constructed as a dual air and water cooling system which, in the normal operation of the nuclear reactor installation KA, i.e. with the exterior cooling system 29 dry, is used for air-cooling of the reactor pressure vessel 6, or for air-cooling of the exterior of the heat insulation W in general and the individual insulating portions W1 to W3 and W5 in particular. For this purpose, the inlet channel configuration 31 is connected to at least one cooling air source. In FIGS. 2A and 3A this source is schematically indicated as a cooling air blower 49. This blower represents a plurality of blowers which convey the cooling air in accordance with the arrow f2 into the inlet channel configuration 31 in the area of the pump sump chamber 31c. FIG. 2 shows the cooling air paths of the cooling air, which are superimposed on one another as is seen by the solid flow arrows f2, and the paths of the cooling water as is seen by the dashed flow arrows f1. In case of a hypothetical accident, the air cooling in the cooling system 29 smoothly transitions into water cooling of the collecting basin 19, which is still to be described. The outlet channel configuration 32 terminates in the containment and in this way provides a cooling air sink for the cooling air coming out of the cooling system 29, which therefore is used for indirect cooling of the outside of the lower insulating portion W1. A further air cooling system which is superimposed on the dual cooling system indicated by the flow arrows f1 and f2 seen in FIG. 2, is indicated by flow arrows having reference symbols f21 to f23 in FIGS. 2 and 3. The entirety of the first air cooling system in accordance with the flow arrows f2 is identified by reference symbol ZL1, and the additional air cooling system in accordance with the flow arrows f21 to f23 is identified by reference symbol ZL2. In order to provide the air supply for this additional air cooling system ZL2, inlet channels 50 which penetrate the circumferential wall 7.2 of the supporting and protective structure 7 and the shielding ring 37 terminate in the upper air cooling annular chamber 45. This annular chamber 45 extends outside of the upper insulating portion W3 as far as a support ring structure 51 of the reactor pressure vessel 6 and is delimited on the exterior by the inner periphery of the circumferential wall 7.2. The upward-flowing cooling air is guided in a plurality of partial flows along the following cooling surfaces: at the outer periphery of the upper insulating portion W3 and the inner periphery of the circumferential wall 7.2. In this case the flow of cooling air f22 comes from the inlet channels 50. The inlet channels 50 are formed of two channel parts: a first channel part 50a which penetrates the circumferential wall 7.2 and extends at a slight incline in the flow direction, and a second channel part 50b which penetrates the shielding ring obliquely upward at an angle of inclination of approximately 45.degree.. The channel parts 50a, 50b, or the entire inlet channel 50, can be formed by brickwork channels 52, seen FIG. 4. In a mouth opening region of the inlet channels 50, the shielding ring 37 is provided with an inclined surface 37a, and a flow guidance sheet 53 covers each of the mouths of the inlet channel 50 and permits the cooling air to exit while being distributed over the cross section of the cooling air chamber 45 through non-illustrated outlet openings; the cooling air flow f21 comes from the first cooling air system ZL1. The flow f21 is upwardly guided on the inner periphery of the circumferential wall 7.2 and forms a cooling air veil distributed over the circumference of the biological shield, which unites with the cooling air flows f22 above the cooling air chamber 45, forming the cooling air flow f23 that is also seen in FIG. 2, and flows along the exterior surfaces of a support ring structure 51, particularly along support arms 51a which support lug supports 54 of the reactor pressure vessel and along a seat or support 55 of the support ring structure 51; furthermore, in accordance with FIG. 2A, outlet ring channels 7.4. are provided for the cooling air flows f23, in which case these outlet ring channels are formed between the main coolant lines HL and the inner periphery of wall openings 7.3 of the supporting and protective structure 7. From there the cooling air reaches the containment or the interior of the safety vessel 1 and from there it travels into a non-illustrated exhaust air filter installation. An additional water cooling system for the surface of a possible core melt, which is suitably integrated into the air cooling systems ZL1 and ZL2 and the exterior cooling system 29 for water cooling, is located in the collecting basin 19 and has at least one melt cooling tube 56 shown in FIG. 2. For this purpose, the collecting basin 19 is penetrated in the upper half of its jacket wall by the at least one melt cooling tube 56 which, in the multi-layer construction of the collecting basin 19 as shown, extends through its crucible wall 19a, the protective layer 19b, the sacrificial material deposit 19c and the lower thermal insulation W1. An inner end of this melt cooling tube 56 is sealed by means of a melting plug 56a. As is shown, the melt cooling tube 56 extends with a gradient (for example, an angle of inclination of 20.degree.) from the outside to the inside and is attached on the inlet side to a cooling liquid reservoir, which can be identical to the cooling water reservoir 24 of FIG. 1. With a core melt present in the collecting basin 19, the melting plug 56a is heated to its melting temperature (the melting temperature lies above the temperature reached in the air chamber 39, but far below the melting temperature of the core melt, for example at 600.degree.). The melting plug 56a is caused to melt and in this way opens a flow channel for cooling liquid to the surface of the hypothetical core melt, so that it is additionally shielded upwardly by a water film and is cooled, and the evaporating coolant, particularly water vapor, can escape upwards through the cooling channels provided for air cooling. An inlet end 56.1 of the melt cooling tube 56 is located outside of the circumferential wall 7.2. It may be connected with the separately ascending pipe 30 shown in FIG. 2B or FIG. 1, so that when the cooling water enters the inlet channel configuration 31 and thus the cooling system 29 through the normal ascending pipe 30 in the course of a rising level, the melt cooling tube 56 is also correspondingly supplied with cooling water. Therefore, the illustrated embodiment is particularly advantageous, since the inlet 56.1 of the melt cooling tube 56 is located outside of the supporting and protective structure 7 and accordingly the melt cooling tube 56 penetrates the circumferential wall 7.2 of the supporting and protective structure 7 and the spacing gap 28 of the exterior cooling system 29. Anchors 57 are used for anchoring the liner 22 and the entire supporting and protective structure 7 in the concrete structure 4. Although only two anchor points are shown, the anchors 57 connect the supporting and protective structure 7 with the concrete structure 4 at such a large number of anchor points that all forces and moments which are transmitted by the reactor pressure vessel 6 through the support structure or ring 51 shown in FIG. 3 on the supporting and protective structure 7 and vice versa are assuredly controlled. Besides the weight forces, these can also be lifting forces, tangential forces, tilting moments or lateral forces which may occur in case of an earthquake or structure disrupting event. In order to provide a more rapid reduction of overpressure which might build up in the collecting basin 19 during steam and gas generation, it can be practical to provide the shielding ring 37 with additional non-illustrated relief openings or overflow openings. It is furthermore recommended to fasten the heat insulation W or W1 to W3 on a relatively thin-walled insulation support container of stainless steel and to suspend this insulation support container on the support arms 51a of the support ring 51 through suitable protrusions or annular flanges and to fix it in place. In this way a particularly earthquake-proof and accident-proof fastening of the heat insulation W is assured. Such a non-illustrated insulation support container is advantageously provided with one or more inspection ports which can be closed by covers. In this way, installation of the insulation support container is made easier. The support ring or the support ring structure 51 is connected to the liner 22 and therefore additionally to the circumferential wall 7.2 by clamping elements 66 for the liner. The support ring structure 51 can be welded or screwed together from forged ring segments with a sufficient number of sturdy lug support segments, for example eight, on which the support arms 51a are formed. Additional non-illustrated anchors are provided for the steel sealing skin 3 of the safety vessel 1. A base plate 59 is fastened by means of an anchor device 58 on the lower wall or channel bottom surface 4.2 which supports the turbulence bodies 34a and on which further flow guidance bodies 60 are fastened. An upper region of FIG. 3B shows a so-called cover compensator 61 between the concrete structure of the circumferential wall 7.2 and the support ring 51. The latter is upwardly fixed by an upper counter seat 62, namely against a cover 63a for an annular recess 63 in the circumferential wall 7.2. A plug for a repeat test opening 64a in the support ring structure 51 is indicated by reference numeral 64. As was mentioned above, it is possible by means of the invention to provide a method for initiating and maintaining an exterior emergency cooling of the collecting basin 19 of the nuclear reactor installation KA. Referring to FIGS. 1 and 2, the individual method steps are as follows: during normal operation of the nuclear reactor installation KA, the cooling water level of the cooling water reservoir 24 is at the low water level P1, at which no cooling water but rather only cooling air in accordance with the flow arrow f2 can enter the inlet channel configuration 31 of the collecting basin cooling system 29, as was already explained; for the continued course of the method it is assumed that an event exceeding the structural limitations is imminent or has already occurred. Such an event can be the result of an LOCA, for example, which will first be described below. In case of an LOCA (loss of coolant accident) it is postulated that a crack in one of the main coolant lines HL or a detachment of such a line has occurred. When such a leakage occurs in the primary circuit, emergency cooling water is pumped from the pressure reservoirs which can be activated as a function of the primary circuit pressure into the main coolant lines HL of the reactor pressure container 6, such as has been described in German Patent DE-PS 23 57 893. This is accomplished due to the fact that check valves react to the pressure decrease in the primary system (normally the pressure in the pressure reservoirs is lower than in the primary system). If this pressure reduction occurs as a result of a leak, the check valves open and the pressure reservoirs supply their contents to the main coolant lines HL on the cold as well as on the hot side. Through the use of this step the reactor core 10 is supplied with a sufficient amount of cooling water. Emergency cooling water then exits from the leak into the reactor sump or the cooling water reservoir 24, which has a level that slowly rises as a result. During this emergency cooling situation in the form of an LOCA, naturally all control rods have been inserted into the core ("scram"), i.e. the normal output operation of the nuclear reactor has been shut off, and only the so-called post-decay heat is generated in the core 10, which amounts to approximately 5% of the rated output of the nuclear reactor. Then, if the emergency cooling systems function satisfactorily, it is possible to sufficiently cool down the primary circuit and the secondary circuit after some time, so that a repair of the ripped or damaged main coolant line becomes possible. The non-illustrated volume of water in the pressure reservoirs is sufficient to raise the cooling water level of the cooling water reservoir 24 to the high water level P2 shown in dashed lines. Once this high water level P2 has been reached, cooling water is conveyed through the ascending pipe 30 (a plurality of such ascending pipes 30 can be distributed over the circumference of the separating wall 27) into the inlet chamber 35, and the cooling water flows from this inlet chamber 35 through the inlet channels 31b, 31a to the inlet chamber 33 and from there into the exterior cooling system 29. In accordance with the principle of communicating pipes, the exterior cooling system is filled with cooling water. However, there is no natural circulation yet, because the effect of heat on the collecting basin 19 due to a core melt is lacking. If the water rises in the ascending pipe 30 (which can also be described as a syphon), a check valve 65 opens. If the water should fall from the water level P2 to the water level P1 or lower, in accordance with the syphon principle water would still be conveyed through the ascending pipe 30 into the inlet chamber 35, because the check valve 65 is closed. The exterior cooling system 29 has been filled with cooling water because of the above-described course of the events as a preventive measure. Then, if the emergency cooling system which supplies emergency cooling water to the reactor pressure vessel through its main coolant lines HL should break down for any reason, so that the water level in the reactor pressure vessel 6 begins to fall, finally the core 10 seen in FIG. 1 will no longer be covered by cooling water and the remaining cooling water in the reactor pressure vessel 6 will also evaporate without a replacement being possible, if the hypothetical event of a core melt occurs. The collecting basin 19 with its external cooling system then is ready for such an event on its own and without any control commands, as described above. In other words, a core melt which following melting of the bottom cup or receiver 6.1 would at first drip and then flow into the collecting basin 19, would mix with the sacrificial material deposit 19c (after having melted through the heat insulation W1) and would be distributed inside the collecting basin 19. The heat flow would heat the crucible 19a correspondingly, along with the cooling water (that is still stationary) contained in the exterior cooling channels 29.1, 29.2. Due to the supply of heat to this cooling water column, a natural circulation could then develop, i.e. the heated cooling water could rise in accordance with the flow arrow f1 and leave the cooling system 29 through the outlet channel configuration 32. A portion of the cooling water would evaporate and condense on recoolers disposed inside the containment or on containment walls. The condensate would drip or flow back into the cooling water reservoir 24 and be available again for the circuit or the natural circulation cooling. After a defined amount of core melt has penetrated into the collecting basin 19, the radiation heat is so great that the melting plug 56a melts away. Then cooling water can flow through the melt cooling tube 56 to the surface of the core melt and can cool it also from above. In this way the core melt is intensely cooled from below through the crucible 19a and from above by means of the cooling water film. Since the protective material 19b also mixes with the core melt and forms an alloy with it, the melting point of which has been preferably lowered so that a liquifying effect is exerted on the melt, the heat dissipation from the core melt and its internal rolling cell flow is also favored by this process. Since the cooling water is available in sufficient amounts, the core melt is caused to set after some time, which can take several days. Some time will elapse after solidification until the core melt is completely cooled, and in this state the repair of the nuclear reactor installation can be begun. It is required for this purpose to decontaminate the nuclear reactor installation and to replace the damaged nuclear reactor pressure vessel 6, together with the collecting basin 19 containing the solidified core melt, with corresponding new components.