Patent Number: 058898301
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The figures of the drawings show an exemplary embodiment of a containment chamber of the invention that is a crucible-like catch basin but is analogously applicable to a containment chamber constructed as a propagation chamber. Referring now in detail to the figures of the drawings, in which identical reference numerals identify identical components, and first, particularly, to FIG. 1 thereof, there is seen a fragmentary, longitudinal section through a nuclear power plant having a cooling system 1 for cooling a containment or retention chamber 2 constructed to receive core melt. A reactor pressure vessel 3 which is largely rotationally symmetrical about its primary axis 5 is disposed in a reactor cavern 48 formed by a supporting structure 36. The reactor pressure vessel 3 contains a reactor core 4. The containment chamber 2 is formed in the reactor cavern 48 below the reactor pressure vessel 3 through the use of a catch basin 28 for a core melt. The catch basin 28 has a floor 24 and a wall 25. A free space remains between the support structure 36 on one hand and the wall 25 and the floor 24 on the other hand, for an external cooling device 23 of the catch basin 28. In the interior of the catch basin 28, the floor 24 and the wall 25 are adjoined by a lining 38, for instance of zirconium oxide (ZrO.sub.2) tiles. A layer of sacrificial concrete 27, especially for lowering the melting point of a core melt, is disposed on the lining 38 toward the floor 24. A cooling pipe 6 for coolant fluid 7 is constructed as a flood pipe 31 which passes from a flooding container 8 through both the wall 25 and the adjoining support structure 36, with a slight inclination from the horizontal, into the catch basin 28. In the catch basin 28, the flood pipe 31 is closed by a closure element 15, in particular a closure element that opens as a function of temperature. In the flooding container 8, the flood pipe 31 is closed by a closure element 9 that opens as a function of a fluid level and in particular has a float 10. A compensator 29 which surrounds the flood pipe 31 between the wall 25 and the support structure 36, seals off the wall 25 from the external cooling device 23 and absorbs thermal expansion of the catch basin 28. The float 10 that seals the flood pipe 31 has an interior 11. A filler pipe 12, which has an inlet opening 13 geodetically above the flood pipe 31, is introduced into the interior 11. The inlet opening 13 is likewise located above an operative level 14 of the coolant fluid 7, in particular coolant water, that is located in the flooding container. The external cooling device 23 of the catch basin 28 communicates with the flooding container 8 through a supply line 26 that extends through the support structure 36 substantially horizontally below the reactor cavern 48. In the flooding container 8, the supply line 26 is likewise closed by a closure element 9 having a float 10. The closure element 9 of the supply line 26 also has a filler pipe 12 which extends into the interior 11 of the float 10, leads out of the coolant fluid 7 above the operative level 14 and is bent back in a U to enter the coolant fluid 7 again, where it ends in an inlet opening 13. A return 20 for internal cooling which is disposed above the operative level 14 and thus above the flood pipe 31, extends from the reactor cavern 48 into the flooding container 8. Inside the flooding container 8, this return 20 is closed by a further closure element 21, which has a further float 10 that is immersed approximately half-way into the coolant water 7. A ball valve 22 with a float ball is disposed between the further closure element 21 and the return 20. Each of the closure elements 9, 21 has a respective condensed water suction removal device 19. The return 20 extends in the reactor cavern 48 above the catch basin 28 through both the support structure 36 and an insulation 37 adjoining the support structure 36. The return 20 communicates with the interior of the catch basin 28. During normal operation of the nuclear power plant, the cooling system 1, which includes the external cooling device 23, the flood pipe 31, the return 20 and the closure elements 9, 21, 15, is closed. In particular, both the external cooling device 23 and the flood pipe 31 are filled with air. During normal operation of the nuclear power plant, the external cooling device 23 serves the purpose of operative air cooling, which prevents heating up of the support structure. Cooling air is fed from below through airshafts that are located outside the support structure 36, into the supply line 26, which is constructed as an annular channel and communicates with eight horizontal channels, to the outside of the catch basin 28. The cooling air rises on the outside of the catch basin 28 and the support structure 36 as it heats up and can escape into a non-illustrated reactor building of the nuclear power plant. The annular channel likewise communicates through eight pipes with the flooding container 8. During an accident involving melting of the reactor core 4, the flooding container 8 is flooded with additional coolant fluid, in particular coolant water 7, so that the level rises from the operative level 14 to an elevated level that is located above the inlet opening 13 of the float 10. The additional coolant fluid in this case is primary coolant water emerging from the primary coolant loop of the reactor core 4. The additional coolant fluid can optionally be fed from a separate, additional coolant fluid supply. The floats 10, which close the flood pipe 31 and the external cooling device 23, are filled with coolant water 7 and sink downward because of the decreasing buoyancy. As a result, both the flood pipe 31 and the external cooling device 23 are filled with coolant water. When the operative level 14 is exceeded, the external cooling 23 comes into operation first. A return of coolant water 7 through the external cooling device 23 takes place through six horizontally extending, non-illustrated channels above the operative level 14 into the flooding container 7. The return through the external cooling device 23 and the return 20 of the internal cooling are separate from one another. The core melt that emerges as the reactor core 4 melts down leads to heat development in the catch basin 28, as a result of which the closure element 15 of the flood pipe 31 likewise opens, since it opens as a function of temperature. As a result, the coolant fluid 7 flows into the interior of the catch basin 28 to cool the core melt. The elevated level inside the flooding container 8 thereupon drops, for instance by 30 cm to 60 cm, to a flooding level 32, so that the level of the coolant water 7 is at the same height in both the reactor cavern 48 and the flooding container 8. The coolant fluid 7 flowing into the catch basin 28 through the flood pipe 31 is heated and rises by natural circulation as is indicated by flow arrows 30 and flows back through the return 20 into the flooding container 8, as is also represented by the flow arrows 30. Opening of the closure element 9 of the external cooling device 23 causes the coolant water 7 to pass out of the flooding container 8 through the supply line 26, as is represented by flow arrows 44, so that it can reach the outside of the catch basin 28. The coolant water 7 evaporates there and is returned into the flooding container 8 through non-illustrated channels. As a result of the evaporation, cooling of the catch basin 28 from the outside occurs as well. The evaporated coolant water 7 rises inside the nuclear power plant, condenses, and passes back into the flooding container 8. Effective cooling of any core melt occurring in the catch basin 28 is assured over a long period of time through the use of the closure elements 9 for both the flood pipe 31 and the external cooling device 23, which elements open upon a rise of the level in the flooding container 8. In FIG. 2, the further closure element 21 of FIG. 1, having a float 10 and a ball valve 22 with a floatable ball, is shown on a larger scale. At the operative level 14, the float 10 is immersed approximately halfway in the coolant water 7. The floatable ball of the ball valve 22 rests on a ball position holder 33 that extends downward from the return 20 to the float 10. Even in the event of a pressure wave arising in the reactor cavern 48 and propagating through the return 20, the ball valve 22 seals off the float 10, so that the float remains protected. The float 10 is guided in guides 35, and it is thus displaceable along an axis 49. The ball valve 22 has a vent 34. During a normal operating state of the nuclear power plant, the return 20 is dry and in particular is filled with air. If the level inside the flooding container 8 rises from the operative level 14 to a flooding level 32, which is located geodetically above the further closure element 21, then the coolant water 7 reaches the ball valve 22 through the return 20. After the entry of the coolant water 7 into the ball valve 22, the floatable ball rises and uncovers an opening 50, through which the coolant water 7 can flow out of the return 20 into the float 10. As a result of the inflowing coolant water 7, the buoyancy of the float 10 decreases, and it sinks along the axis 49 in the flooding container 8, and therefore the coolant water 7 can flow out of the return 20 into the flooding container 8 in natural circulation. The flood pipe 31 of FIG. 1 is shown on a larger scale in FIG. 3. Inside the catch basin 28, the flood pipe 31 is closed by the closure element 15 that opens as a function of temperature and has a bale closure 16. The flood pipe 31 is surrounded between the support structure 36 and the catch basin 28 by the compensator 29, which rests sealingly on the catch basin 28 in a ball sealing seat 38. On a larger scale, FIG. 4 shows the closure element 15 of FIG. 3 that opens as a function of temperature. The bale or hoop closure 16 acts through a bale or hoop 42 to press a cap 40 firmly into a ball sealing seat 39 of the flood pipe 31. The bale 42 is firmly connected to the flood pipe 31 through a tightening screw 17, which has a melting bolt 43. The melting bolt 43 is formed of silver with a melting temperature of about 960.degree. C. A splash protector 41 between the melting bolt 43 and the cap 40 is disposed parallel to the flood pipe 31, to protect the melting bolt 43 against escaping coolant water 7. As a result, it is assured that melt-through of the melting bolt 43 is not delayed by evaporating coolant water 7, even if the ball sealing seat 39 should leak. FIG. 5 shows an alternative embodiment of a closure element 15, which opens as a function of temperature, for the flood pipe 31. The closure element 15 has a closure cap 18, which is soldered to the flood pipe 31 at two solder strips 45 through a silver strip 46 that surrounds the flood pipe 31. An insulator 47 having an air cushion is introduced between the silver strip 46 and abutting portions of the flood pipe 31 and the closure cap 18. If high heat develops in the containment chamber 2, the solder strips 45 and if applicable the silver strip 46 melt open, so that the closure cap 18 falls off and the flood pipe 31 opens. The closure elements 15 shown in FIG. 4 and FIG. 5 each have only one melting element 43, 46. As a result, the danger of unequal melting open of two melting elements that close the closure element, with the possibility of belated opening of the closure element, is averted. The invention is distinguished by a cooling system with a cooling pipe for cooling a containment chamber constructed to receive a core melt. The cooling is tripped through the use of a passive closure element. The closure element opens as a function of the level of coolant water in a flooding container, so that coolant water flows into the containment chamber or along its outside surfaces. The closure element preferably has a float which due to its buoyancy closes off the cooling pipe. The float is constructed in such a way that when a level of cooling water that is above an operative level is reached, the float is filled with coolant water through a filler pipe, and the cooling pipe sinks downward into the flooding container, thereby opening. The cooling system has a return that is extended above the fluid pipe that feeds coolant water into the containment chamber. Through the use of the return and the fluid pipe, a natural circulation of the coolant water develops, thereby assuring effective cooling of the containment chamber and the core melt caught therein.