Patent Number: 
Section: description

In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a part of a reactor pressure vessel 1 with upper connecting branches 2 for a primary circulation and a dome 3. Above the upper connecting branches 2, a cross section of the reactor pressure vessel 1 is covered by an upper support plate 4. The support plate 4 has openings 5 for control rods 6. In the reactor pressure vessel 1 with a xe2x80x9chot domexe2x80x9d, an opening 5 is larger than a cross section of a control rod 6. The support plate 4 then takes a form of a lattice plate, for example. In the reactor pressure vessel 1 of the xe2x80x9ccold domexe2x80x9d type the opening 5 is largely identical in size to the cross section of the control rod 6. In order that the medium (coolant) present in the reactor pressure vessel 1 can be exchanged between a lower chamber 1a below the support plate 4 and an upper dome chamber 1b above the support plate 4, an equalization opening (dome bypass) 7 is provided in the support plate 4 in both variants. The equalization opening 7 is smaller in the xe2x80x9chot domexe2x80x9d than in the xe2x80x9ccold domexe2x80x9d. In the xe2x80x9ccold domexe2x80x9d a bypass of coolant through the equalization opening 7 is necessary only in order that the boron concentration may be equalized on both sides of the support plate 4. Otherwise it is desirable in power output operation that as little coolant as possible should pass into the dome chamber 1b, so that the power output of the power station can be maximized. When running the nuclear power station down, however, the rate of cooling in the dome chamber 1b should not be slower than in the lower chamber 1a. A larger temperature difference between the upper and the lower side of the support plate 4 would namely retard the cooling process unnecessarily and lead to problems when opening dome bolts 16, represented in diagrammatic form. Consequently a great exchange of coolant between the lower chamber 1a and the dome chamber 1b is advisable when shutting down. In order that the equalization opening 7 is relatively large when running the nuclear power station down and relatively small in power output operation, a device 8 shown in FIG. 2 is to be fitted into the equalization opening 7 (dome bypass). The device 8 varies the opening cross section and hence the medium flow in the bypass as a function of the temperature. In the event of a temperature drop in the lower chamber 1a to a temperature lower than that in the upper dome chamber 1b, which occurs when running a nuclear power station down, the opening cross section of the equalization opening 7 is enlarged, so that temperature equalization with the still hotter dome chamber 1b is ensured. While the temperature in the lower chamber 1a in power output operation is higher than the temperature in the upper dome chamber 1b, however, the cross section of the equalization opening 7 remains relatively small, sufficient only to ensure the exchange of boronized water between the two chambers 1a and 1b. A power loss as the result of an unnecessarily large dome bypass or an unnecessarily large equalization opening 7 is avoided. The device 8 is shown in detail in FIG. 2. It has a cylinder 9, which is open at both ends. In an upper area, openings 10 are disposed in a side wall of the cylinder 9. In the lower area of the cylinder 9 there is an expansion sleeve 11 known in the art, for example described in Published, Non-Prosecuted German Patent Application DE 196 07 693 A1, which in the event of a temperature rise expands in an axial direction of the cylinder 9. The expansion sleeve 11 is connected to a hollow piston 12. Inside the expansion sleeve 11 there is a duct 18 connected to a lower opening 17 of the device 8, which duct at a lower end opening 19 of the piston 12 opens into the interior of the piston 12. The piston 12, at its upper end, has an upper opening 13, which represents a minimum opening cross section of the device 8. The piston 12 also has additional lateral openings 14, which align with or overlap the openings 10 in the cylinder wall only when the expansion sleeve 11 is contracted. A temperature drop in the lower chamber 1a, which occurs when running the nuclear power station down, leads to a contraction of the expansion sleeve 11, which brings the lateral openings 14 of the piston 12 into contact with the openings 10 in the cylinder wall of the cylinder 9, thereby significantly enlarging the opening cross section of the device 8. The device 8 shown therefore ensures a process for temperature equalization between the chambers 1a and 1b, which varies the flow of medium or coolant through the equalization opening 7 (dome bypass) as a function of the temperature. This affords the advantage that when running the nuclear power station down a large flow of medium is ensured from the lower chamber 1a into the dome chamber 1b for the rapid cooling of the dome 3, while in power output operation only a then sufficiently small flow occurs, so that power losses are avoided.